a r X i v :a s t r o -p h /0012181v 1 8 D e c 2000
A&A manuscript no.
(will be inserted by hand later)
ASTRONOMY
AND
ASTROPHY SICS
1.Introduction
Low-and intermediate-mass stars (with masses 0.8M ⊙<～M <～5?8M ⊙,depending on model details)play an impor-tant role in galactic chemical evolution,thanks to the ejec-tion into the interstellar medium (ISM)of material con-taining newly synthesized nuclear products,mainly 4He,12
C,and 14N,and possibly 16O.
The interest in the nucleosynthetic history of 12C and 14
N,in particular,has recently increased thanks to the observations of high red-shift systems.For instance,it is possible to measure the nitrogen abundance and the N/O ratio in the damped Ly-αsystems (e.g.Pettini et al.1995;Lu et al.1998),whereas carbon is detected in the Ly-α
2P.Marigo
sets of stellar yields presented by Renzini&Voli(1981) have been extensively used in chemical evolution models of galaxies,providing almost the only available data source up to recent years.
It is important to remark that in Renzini&Voli(1981) synthetic AGB model the fundamental parameters(essen-tially mass-loss and dredge-up)were speci?ed according to the indications from the current knowledge of the involved physical processes and available complete stellar calcula-tions.In this sense the model was uncalibrated.That ap-proach had the merit of supplying a testing tool for com-plete stellar models,as it pointed out at some fundamen-tal inadequacies in the predictions(e.g.too low e?ciency of the third dredge-up),and assumed prescriptions(e.g. too low mass-loss rates),which led to clear discrepancies between theory and observations(see,for instance,Iben 1981;Iben&Renzini1983;Bragaglia et al.1995).
With awareness of that,the later synthetic AGB mod-els(e.g.Groenewegen&de Jong1993;Marigo et al.1996, 1999a)have made the next step ahead,that is update the input prescriptions and calibrate the model parameters in order to reproduce fundamental observables(e.g.the carbon star luminosity functions,the initial-?nal mass re-lation).Basing on the results of these model calibrations, various sets of stellar yields from low-and intermediate-mass stars have been presented in recent years as an al-ternative to Renzini&Voli(1981),namely:Marigo et al. (1996,1998),van de Hoek&Groenewegen(1997);see also Forestini&Charbonnel(1997),and Boothroyd&Sack-mann(1999).
To the above reference list we will now add the new homogeneous sets of stellar yields presented in this work. With respect to our previous calculations(Marigo et al. 1996,1998),the present yields are derived from stellar models with updated input prescriptions and improved treatment of the relevant processes involved(i.e.the third dredge-up,see Marigo et al.1999a for all details).
Speci?cally,we follow the evolution of low-and intermediate-mass stars,coupling the results of complete stellar models(Girardi et al.2000)–that cover the evolu-tion from the zero age main sequence(ZAMS)up to the onset of the thermally pulsing asymptotic giant branch (TP-AGB)–with synthetic TP-AGB models(Marigo 1998ab;Marigo et al.1999a)that extend the calculations up to the end of this phase.Relevant model prescriptions are brie?y recalled in Sect.2.
With the aid of these evolutionary calculations,we then derive the stellar yields(H,He,and main CNO ele-ments;refer to Sects.3and4)for a dense grid of initial stellar masses(in the range0.8M⊙–5M⊙)and various metallicities(Z=0.004,0.008,0.019).For the most mas-sive stars,experiencing hot-bottom burning(hereinafter HBB,or envelope burning)during the TP-AGB phase, the corresponding yields are given for three values of the mixing-length parameter,i.e.α=1.68,α=2.00,and α=2.50.This parameter mainly a?ects the predicted
production of14N and4He due to HBB(See Sect.4.1).
The?nal part of this work(Sect.5)is dedicated to compare our results with the yields calculated by other au-thors,i.e.Renzini&Voli(1981),and van de Hoek&Groe-newegen(1997).In the attempt to single out the causes of the main di?erences,we analyse the e?ect of di?erent model prescriptions(e.g.mass-loss,dredge-up,HBB)on the predicted yields,testing at the same time the capa-bility of a model to satisfy basic observational constraints (e.g.initial-?nal mass relations,white dwarf mass distri-bution,carbon star luminosity functions,chemical abun-dances of planetary nebulae).
2.Evolutionary models
2.1.Some de?nitions
Let us?rst de?ne some quantities which will be often used throughout this paper to indicate critical masses for the occurrence of particular physical processes.These quanti-ties are:M HeF,M up,M min
HBB
,M min
dred
,and M min
c
.
The?rst one,M HeF,denotes the maximum initial mass for a star to develop a degenerate He-core,hence expe-rience the He-?ash at the tip of the Red Gian Branch (RGB),and is comprised between1.7–2.5M⊙depend-ing on metallicity and model details.
The second one,M up,is de?ned as the critical stellar mass over which carbon ignition occurs in non-degenerate conditions,marking the boundary between intermediate-mass and massive stars.It is worth recalling that this mass limit is usually comprised within5–8M⊙,being signif-icantly a?ected by the adopted treatment of convective boundaries,as discussed in Sect.5.
The third one,M min
HBB
,corresponds to the minimum initial mass for a star to undergo HBB during the TP-AGB phase.Stellar evolution calculations indicate that
M i>～M min
HBB
～3.5?4.5M⊙,depending on metallicity (see,for instance,Marigo1998b).
The fourth one,M min
dred
,denotes the minimum initial mass for a star to experience the third dredge-up during the TP-AGB phase.Observations of carbon stars suggest
that M min
dred
～1.1?1.5M⊙,decreasing with the metallicity (see,for instance,Marigo et al.1999a).
Finally,we recall the basic parameters of the third
dredge-up:λand M min
c
.The parameterλis intended to measure the e?ciency the third dredge-up,being de?ned as the fraction of the increment of core mass over an inter-pulse period which is dredged-up to the surface at the sub-sequent thermal pulse.The actual values forλare still a matter of debate among theoreticians,and can range from λ～0toλ>～1,depending both on stellar properties(e.g. mass and metallicity)and model details(e.g.treatment of convective boundaries).
The parameter M min
c
refers to the minimum core mass for the occurrence of the third dredge-up,and is a?ected
Yields from low-and intermediate-mass stars3 by several factors as well.In general,theoretical models
would predict that M min
c decreases–so that the third
dredge-up is favoured–at increasing mass and mixing-length parameter,and decreasing metallicity(see,for in-stance,Wood1981;Marigo et al.1999a).
2.2.Input physics
In this work we consider low-and intermediate-mass stars,
i.e.those with initial masses in the range from about
0.8M⊙to M up～5.0M⊙and and three choices of the original compositions(i.e.[Y=0.273,Z=0.019],[Y= 0.250,Z=0.008],[Y=0.240,Z=0.004]).
Their evolution from the ZAMS up to the beginning of the TP-AGB phase is taken from the Padua stellar mod-els(Girardi et al.2000),that include moderate overshoot from core and external convection(Chiosi et al.1992; Alongi et al.1993).The reader should refer to Girardi et al.(2000)for more details of the adopted input physics.
The models by Girardi et al.(2000)provide the ex-pected changes in the surface abundance of several chem-ical elements(H,3He,4He,12C,13C,14N,15N,16O, 17O,18O,20Ne,22Ne,25Mg)caused by the?rst and second
dredge-up episodes,i.e.prior to the onset of the TP-AGB phase.It should be also remarked that these models do not assume any ad-hoc“extra-mixing mechanism”,e.g. the so-called cool-bottom process(Wasserburg et al.1995; see also Charbonnel1995;Boothroyd&Sackmann1999; Weiss et al.2000),which is invoked to reconcile discrepant predictions of surface abundances with those measured in ?eld Population II stars,galactic globular clusters,and Magellanic Clouds clusters(see,for instance,the results by Gratton et al.2000in their chemical analysis of?eld metal-poor stars).
Mass loss by stellar winds su?ered by low-mass stars (with M i≤M HeF～1.7?2.2M⊙)on the ascent of RGB,is analytically included applying the classical Reimers(1975) formula to the evolutionary tracks calculated at constant mass by Girardi et al.(2000).An e?ciency parameter η=0.45is adopted to ful?l the classical observational con-straint provided by the morphology of horizontal branches in Galactic Globular Clusters.
Finally,once the the?rst signi?cant thermal pulse is singled out in each evolutionary sequence calculated by Girardi et al.(2000),that point is assumed to de-?ne the starting conditions for synthetic calculations of the TP-AGB phase(following the model prescriptions de-scribed in Marigo1998ab,and Marigo et al.1999a),which are carried on up to the complete ejection of the enve-lope by stellar winds.Mass loss is included according to the semi-empirical formalism developed by Vassiliadis& Wood(1993).Nucleosynthesis occurring in the innermost envelope layers of TP-AGB stars with HBB is followed adopting the Caughlan&Fowler(1988)compilation of reaction rates for CNO and p-p reactions.The electron-screening factors are those of Graboske et al.(1973).3.Derivation of the wind contributions
Following the classical de?nition by Tinsley(1980),the stellar yield,p k(M i),of a given chemical element k,is the mass fraction of a star with initial mass M i that is con-verted into the element k and returned to the ISM during its entire lifetime,τ(M i).Tables A1–A12give the quan-tities:
M y(k)=M i p k(M i)(1) expressed in solar masses,for all the chemical elements considered,as function of the initial stellar mass M i, metallicity Z,and mixing-length parameterα.
According to the de?nition of stellar yield we can write: M y(k)= τ(M i)0[X(k)?X0(k)]d M
4P.Marigo the pulse cycles,the generic j th one consisting of a ther-
mal pulse–when the j th dredge-up possibly occurs–fol-lowed by the inter-pulse period,during which the mass
?M j,ej
TP?AGB is ejected.
It should be noticed that this approximation holds for TP-AGB stars which experience only the third dredge-up.
For more massive TP-AGB stars(with M i>M min
HBB )also
su?ering HBB,the changes in the surface chemical com-
position and mass loss are concomitant processes,so that the calculation of stellar yields requires the adoption of in-
tegration time steps shorter than the inter-pulse periods.
In general,negative M y(k)correspond to those elemen-tal species which are prevalently destroyed and diluted in
the envelope,so that their abundances in the ejected mate-
rial are lower with respect to the main sequence values.On the contrary,positive M y(k)correspond to those elements
which are prevalently produced so that a net enrichment
of their abundances in the ejecta is predicted.
Figures1and2show the quantities M y(k)for all the
chemical elements considered,as a function of M i and Z. For stars experiencing HBB results are given for three val-
ues of the mixing-length parameter.
Finally,for the sake of clarity,we remind that the CNO cycle does not change the total number of CNO nuclei
involved as catalysts in the conversion of H into4He,i.e.
Y CNO= k X CNO k/A k=constant.It follows that the ?rst and second dredge-up,though a?ecting the surface
abundances of the CNO isotopes,do not alter their total abundance by number.In fact,the material injected into the envelope has experienced the CNO cycle involving only isotopes already present in the original composition.On the contrary,the constancy of Y CNO breaks down as soon as the dredge-up of primary carbon and oxygen,produced byα-capture reactions at thermal pulses,occurs.
However,in all cases the total abundance by mass of
the CNO isotopes is somewhat changed because of the conversion of these elements mainly into14N,so that a small positive CNO yield(in mass fraction),is expected, for example,from the RGB phase(see Tables A1–A3). The quantity,M y(CNO),referring to the total net yield of all CNO isotopes,is shown in Fig.3.
4.Stellar yields as a function of M i,Z,andαModel predictions can be understood more easily consid-ering that the stellar yield of a given element is essentially determined by the e?ciency and duration/frequency of:–the nucleosynthesis/mixing processes(e.g.dredge-up events,HBB)which alter its abundance in the surface layers;
–the mass-loss process which ejects the surface layers into the ISM
According to the physical prescriptions adopted in this work for the TP-AGB phase(see also Sect.5.1)we can summarise the following points:
–The third dredge-up determines the surface enrich-ment mainly of4He,12C,and16O.The adopted in-tershell abundances are[X(4He)=0.76;X(12C)=0.22;
X(4He)=0.02]according to Boothroyd&Sackmann (1988b).The process is more e?cient(i.e.higherλ)at lower Z;
–HBB operates via the CNO-cycle,hence essentially in-creasing the surface abundances of4He,and14N.The process is more e?cient at higher M(provided that M>M min
HBB
),lower Z,and largerα.
–mass loss is,in general,less e?cient(i.e.lower˙M)at decreasing Z and increasingα.In fact,both factors tend to produce hotter tracks,and generally˙M anti-correlates with T e?.As a consequence,lower mass-loss rates correspond to longer TP-AGB lifetimes,hence greater number of third dredge-up events and a longer duration of HBB.
The expected trend of M y(k)for the elements un-der consideration as a function of the stellar initial mass, metallicity,and mixing-length parameter is shown in Figs.1-3.We can notice the following:
–H and4He
The net yields of these elements have mirror-like trends,being negative for H,and positive for4He.
The maximum of4He production at around2?3M⊙(depending on metallicity)is explained considering the e?ect of the increase of the number of thermal pulses(hence dredge-up episodes)with stellar mass for
0.8M⊙ pronounced at lower metallicities due to longer TP-AGB duration(Fig.5)and larger number of thermal pulses(Fig.6)for given stellar mass.The subsequent increase of4He production towards higher masses, 4M⊙<～M i≤5M⊙,is caused by the occurrence of HBB in addition to the third dredge-up.The yield of 4He is larger for lower metallicities and higher values of the mixing-length parameter,re?ecting the greater e?ciency of both the third dredge-up and HBB. –3He The net yield of this element presents a pronounced peak at very low masses,say at M i～1M⊙,and de-creases at higher masses.This trend is explained con-sidering that the main contribution to the yield is due to the?rst dredge-up,and that both the related surface enrichment of3He and the amount of mass lost during the RGB phase are inversely proportional to the stel-lar mass in the low-mass domain.At higher masses, (M i>～3M⊙),the net yield becomes even slightly neg-ative because of HBB. –12C The net positive yield increases with the initial mass, up to a maximum located at about2?3M⊙,corre-sponding to largest number of dredge-up episodes suf-fered during the TP-AGB phase,provided that HBB has not operated. Yields from low-and intermediate-mass stars 5 https://www.doczj.com/doc/783521882.html, yields M y (k )(in M ⊙)for each indicated chemical element (H,3He,4He,12C,and 13C)as a function of the initial mass (in M ⊙)of the star.The solid,dashed,and dotted lines correspond to the metallicity sets Z =0.019,Z =0.008,and Z =0.004,respectively.Panels along each column refer to the same value of the mixing-length parameter. The subsequent decline towards higher masses is ini-tially due to fewer dredge-up events,and then to the prevailing e?ect of HBB.It follows that no substantial enrichment of 12C is provided from the most massive AGB stars. This general trend is more marked at lower metallic-ities,because of the longer TP-AGB phases,and the greater e?ciency of the third dredge-up and HBB.–13C The ?rst dredge-up causes an increase of the 13C sur-face abundance in stars of all masses,and the resulting yields are positive.A further contribution is provided 6P. Marigo https://www.doczj.com/doc/783521882.html, stellar yields M y (k )(in M ⊙)for each indicated chemical element (14N,15 N, 16 O, 17 O,and 18 O)as a function of the initial stellar mass (in M ⊙).The notation is the same as in Fig.1.by mild HBB,as long as creation of 13C via the re-action 12C(p,γ)13C prevails over destruction via the reaction 13C(p,γ)14N.The results also depend on the interplay between the strength of HBB and mass loss.The most favourable cases correspond to the mod-els [e.g.(4M ⊙,Z =0.019,α=2.00),(3.5M ⊙,Z =0.004,α=1.68)],in which the e?ciency of reactions allows the synthesis of 13C for a long time before the drastic reduction of the envelope causes the extinc- tion of nuclear burning.The spikes of 13C production for these models would suggest that a proper tuning of HBB is required:If nuclear reactions are somewhat too weak 13C is not signi?cantly created,else if somewhat too strong 13C is quickly destroyed in favour of 14N.–14N It turns out that HBB plays the dominant role for the synthesis of 14N.The positive yield as a function of the stellar mass depends on both the e?ciency and dura- Yields from low-and intermediate-mass stars 7 https://www.doczj.com/doc/783521882.html, stellar yields of all CNO elements as a function of the initial stellar mass.The notation is the same as in Fig.1. tion of nuclear burning.It follows that lower metallic-ities and higher values of the mixing-length parame-ter concur to favour nitrogen production.The contri-bution from low-and intermediate-mass stars to the galactic enrichment of nitrogen may result important,as suggested by chemical evolutionary models of galax-ies (e.g.Portinari et al.1998).–15N The net yield of this element is mostly negative.For stars with initial masses in the range,0.8M ⊙<～M i <～ 3.5M ⊙,the depletion of 15 N is due to the e?ect of the ?rst and second dredge-up.For higher mass stars,the results are a?ected by HBB,depending on the degree of CNO cycling attained in the burning regions.This element has the shortest nuclear lifetime,after that of 18 O,so that it quickly attains nuclear equilibrium with 14 N.We note that the depletion of 15N is much more ef-?cient at higher metallicities due to the increase of the CNO-cycling,which implies a more e?cient destruc-tion of this element.The predicted trend of the 15N yield mirrors,to some extent,that of the secondary component of 14N (cf.Sect.4.1).–16O The net yield of this element is positive for stars with 0.8M ⊙<～M i <～3.5M ⊙thanks to the dredge-up events during the TP-AGB phase,whereas it becomes nega-tive at higher masses because of HBB. The synthesis of fresh 16O occurs via the reaction 12 C(α,γ)16O during thermal pulses,so that the yield of this element depends,among other factors,on its abundance in the dredged-up material.According to recent TP-AGB calculations –which include deep overshooting from all convective boundaries –Herwig et al.(1997)?nd that the 16O abundance in the con-vective intershell is roughly ten times higher,X(16O)～0.25,than previously predicted,i.e.X(12C)～0.02by Boothroyd &Sackmann 1988b (standard case).Then, adopting Herwig et al.indications,oxygen production by low-and intermediate-mass stars may be favoured with respect to the standard case,but it is worth re-calling that,in general,the ?nal yields are crucially a?ected by various other factors (e.g.number and ef-?ciency of dredge-up episodes;see also Sect.6). Anyhow,regardless of its abundance in the dredged-up material,the trend of 16O,as a function of M and Z ,is expected to be qualitatively similar to that of 12 C.Moreover,according to the present calculations,a notable dependence on metallicity comes out.The increasing positive trend with decreasing metallicities essentially re?ects the longer duration of the TP-AGB phase,and hence the greater number of dredge-up episodes,in combination with their larger e?ciency.–17O A certain production of this element is provided by TP-AG B stars with HBB,as long as the chain of reactions 16 O(p,γ)17F(β+ν)17O prevails over nuclear destruc-tion via the reactions 17O(p,γ)18F and 17O(p,α)14N.The behaviour resembles that of 13C,in the sense that under some ?ne-tuned conditions –for particular com-binations of the stellar mass and metallicity –a pro-duction spike of 17O can result.–18O This element has the shortest nuclear lifetime against proton captures,so that it easily burns even at mild temperatures,quickly attaining the nuclear equilib-rium with 17O.The net yield of 18O is negative for all masses,with a trend mirroring that of 17O.More-over,as in the case of 15N,the depletion of 18O is more pronounced at higher metallicities due to the more ef-?cient CNO cycling. 8P.Marigo 4.1.Secondary and primary components As far as the CNO nuclei are concerned,we can distinguish for each element k the secondary,M S y(k),and primary, M P y(k),components of the stellar yield M y(k)=M S y(k)+M P y(k)(7) calculated with(see Eq.(2)) M S y(k)= τ(M i)0[X S(k)?X S,0(k)]d M d t d t(9) where X S(k)and X P(k)denote the secondary and pri-mary current surface abundances,respectively.Moreover, we have X S,0(k)=X0(k)and X P,0(k)=0,as follows from the de?nitions of secondary and primary abundances. We notice that M P y(k)can be only≥0,whereas M S y(k) can be either≥0or<0,in the respective cases that the mass-averaged secondary abundance of the element in the ejecta is greater,equal or smaller than its original value. A few basic remarks should be made at this point. Both the?rst and second dredge-up a?ect(by increasing or decreasing)only the secondary components of the CNO surface abundances.In fact,in these episodes the envelope is polluted by material which has undergone CNO-cycling, with a net change in the relative abundances of the CNO isotopes synthesized from metal seeds originally present in the star.On the contrary,the third dredge-up enriches the chemical composition of the envelope with12C and16O of primary origin(synthesized byα-capture reactions).Fi-nally,HBB a?ects the abundance distribution of the CNO isotopes of both secondary and primary synthesis. Keeping in mind these concepts,it turns out that: –stars with initial masses M i dred can produce CNO yields of secondary origin only; –for stars with initial masses M min dred <～M i <～M min HBB the yields of12C and16O should include a primary component,as a consequence of the third dredge-up during the TP-AGB phase;and –for stars with initial masses M i>～M min HBB undergoing HBB during the TP-AGB phase,the yields of the CNO isotopes should all display primary components,be-cause of the injection of primary12C and16O nuclei (at each dredge-up event)into regions where the CNO cycle is operating. Comparing the secondary and primary components of the CNO yields four cases can be met(Tables A1–A12; see also Marigo1998b): 1.M S y=0and M P y=0so that M y=M S y 2.M S y>0and M P y>0so that M y>0 3.M S y<0and M P y>0so that M y>0 4.M S y<0and M P y>0so that M y<0 The?rst(1.)case applies to low-mass stars with M i< M min dred ,i.e.never experiencing the third dredge-up dur- ing the TP-AGB phase.No primary component of stellar yields is expected. The second(2.)case corresponds to both primary and secondary production.In stars with M i>M min HBB it ap-plies,for instance,to13C,17O,and14N.In general,for these elements a positive secondary contribution may be provided by the?rst(and possibly second)dredge-up and HBB,the latter process being also responsible for the pri-mary synthesis of these elements(starting from primary 12C and16O injected by the third dredge-up). As far as13C is concerned(see also Sect.4)we notice that a suitable interplay between the strength of HBB and mass loss can occasionally result in very favourable conditions for the production of13C,giving a peak of the related yields(for instance,at the model4M⊙in the case Z=0.019,α=2.0). Concerning the yields of14N,it should be remarked that the contribution from intermediate-mass stars may be relevant in view of interpreting,with the aid of chem-ical evolutionary models of galaxies,the observed trend in the log(N/O)vs.log(O/H)diagram(see,for instance, Vila-Costas&Edmunds1993;Henry et al.2000b),where the large scatter of data points towards lower metallicities would imply the existence of a signi?cant primary compo-nent in the measured nitrogen abundances. The third(3.)possibility corresponds to a dominant primary production.In stars with initial masses M min dred <～ M i<～M min HBB ,this case applies to12C surface abundance, which is?rst decreased by the negative secondary con-tribution from the?rst and second dredge-up,and subse-quently increased by the third dredge-up injecting primary nuclei into the envelope.A similar situation occurs for the yield16O in the same range of stellar masses,as the e?ect of the third dredge-up prevails over that of the previous mixing episodes(?rst and second). Finally,the fourth(4.)case corresponds to a dominant secondary depletion.This refers to15N,18O for stars of all masses,and to16O for stars with M i>～M min HBB if the re-duction of the original abundance caused by the?rst and second dredge-up dominates over the injection of primary oxygen via the third dredge-up(even possibly partially destroyed by HBB).Under these circumstances,no en-richment of the interstellar medium is expected for these elemental species. Tables A10–A12give the net yield for each element of the CNO group(T entry),together with the secondary(S entry)and primary(P entry)components,for stars with initial masses3.5M⊙≤M i≤5M⊙,for various values of the original metallicity and mixing length parameter. Yields from low-and intermediate-mass stars9 Table1.Summary of the main prescriptions adopted in the synthetic TP-AGB models here considered for comparison. PROCESS/QUANTITY MODEL PRESCRIPTION HG97 M up7M⊙ M c?L relation,M c?T ip relation with metallicity dependence mass loss Reimers(1975)η=5 3D.up:e?ciencyλ0.75 function of M c,any Z0.55for Z=0.008,any M 0.60M⊙from envelope integrations for any Z,M HBB:overluminosity no HBB:nucleosynthesis parameterised approx. 10P. Marigo Fig.4.Theoretical core mass -luminosity relations.The well-known Iben &Truran (1988)relation is extrapolated for small core masses and assuming a total mass of 1.5M ⊙.The predicted e?ect of the chemical composition of the en-velope on the luminosity is shown according to Boothroyd &Sackmann (1988a). stance,the quiescent TP-AGB luminosity for Z =0.02is about 20%higher than for Z =0.001(see Fig.4).5.1.3.Mass loss on the AGB The adopted prescription for mass loss on the AGB cru-cially in?uences the predictions of stellar yields,as it af-fects the total number of thermal pulses (hence dredge-up episodes)su?ered by a TP-AGB star,hence the growth of its core mass and AGB lifetime. In this work we adopt the semi-empirical formalism presented by Vassiliadis &Wood (1993)who couple results of pulsation theory with observations of variable AGB stars.In RV81the classical Reimers’law (1975)is assumed with the e?ciency parameter ηset equal to 1/3or 2/3.HG97as well use the Reimers’law,but with η=5,which they ?nd as the best value to ful?l basic observational constraints (see Sects.5.2.2and 5.2.1). To this regard the following remark should be made.As a matter of fact,the Reimers’prescription was origi-nally designed to describe mass loss su?ered by low-mass stars climbing up the RGB,and it is usually calibrated in view of reproducing observations of stars in the subsequent horizontal branch phase of quiescent core He-burning (see Sect.2). However,as already shown by RV81,the straightfor-ward extension of the Reimers’formula to the AGB evo- Fig.5.Expected number of thermal pulses (and inter-pulse periods)as a function of the initial stellar mass for di?erent values of the initial metallicity (see legenda in Fig.6).Results of the present work (M2K)are compared with those of Renzini &Voli (1981;RV81). lution does not suit important constraints.In fact,the Reimers’prescription with η<～1cannot produce the “super-wind”mass-loss rates (˙M ～10?4?10?5M ⊙yr ?1)measured in stars close to the AGB-tip luminosities,and consequently it cannot account for the typical values of masses and radii of planetary nebulae at the observed luminosities. These di?culties have been overcome by later mass-loss prescriptions –speci?cally designed for AGB stars (e.g.Bowen 1988;Fleisher et al.1992;Vassiliadis &Wood 1993;Bl¨o cker 1995)–which are all characterised by a more rapid increase of mass-loss rates during the AGB evolution,then naturally leading to the development of the superwind regime. The e?ect of di?erent laws for mass loss is signi?cant with respect to the expected number of thermal pulses experienced on the TP-AGB evolution.As an example (see Fig.5),we can notice that for a (5M ⊙,Z =0.02)model our calculations yield N p =117?153(depending on α),whereas RV81predict N p =8941(1631)with the e?ciency parameter η=1/3(η=2/3).In general,it turns out that the largest di?erences in the number of thermal pulses show up for models with higher stellar masses (i.e.M i >～2.5M ⊙),that are expected to experience the super-wind regime according to the the Vassiliadis &Wood’s prescription. Moreover,signi?cantly di?erent results are obtained by RV81and M2K as far as the TP-AGB duration is Yields from low-and intermediate-mass stars 11 Fig.6.Predicted TP-AGB lifetimes as a function of the initial stellar mass and metallicity according to the present work (M2K)and that of Renzini &Voli (1981;RV81)with the mass-loss parameter η=1/3. concerned (see Fig.6).First of all,we can notice that our TP-AGB lifetimes present a pronounced trend with the stellar mass,showing a maximum at around 2–2.5M ⊙(depending on metallicity).In particular,a drastic drop of the TP-AGB duration is expected in the high-est mass domain,i.e.for stars with HBB.In RV81the mass-dependence is much less marked,and such a strong reduction of the TP-AGB lifetimes of the most massive models is not predicted.This latter point is relevant for the interpretation of the high-luminosity wing of the ob-served carbon star luminosity functions (see Sect.5.2.3).We also notice that with the Vassiliadis &Wood’s for-malism both the total number of the thermal pulses and the TP-AGB lifetimes are notably sensitive to the metal-licity,i.e.increase with decreasing Z .This feature re?ects consequently on the predicted yields from stars with the same initial masses but di?erent initial metallicities. Finally,it is worth remarking that the e?ciency of mass loss on the AGB crucially a?ects the masses of the bare C-O cores left at the end of this phase.It follows that the empirical initial-?nal mass relation sets impor-tant constraints to the theoretical prescriptions for stellar winds (see Sect.5.2.1).It is important also to recall that according to RV81in stars with M i >～(5?8)M ⊙the C-O degenerate core grows up to the Chandrasekhar limit (～1.4M ⊙;see Fig.8),leading to explosive carbon ignition (Type I-1/2Supernova event;see Iben &Renzini 1983).On the contrary,according to M2K and HG97this cir- cumstance is always prevented by the earlier removal of the whole stellar envelope. 5.1.4.The treatment of the third dredge-up This process is expected to critically a?ect the actual yields of many elements,mainly 4He,12C,and 16O.More-over,the reduction of the core mass caused by the third dredge-up concurs to determine the ?nal mass of the rem-nant left at the end of the AGB. The largest di?erences in the analytical treatment of this process,in the models here considered,can be sum-marised as follows.In RV81model the onset of the third dredge-up in low-mass stars possibly occurs later (higher M min c )and with a lower e?ciency (lower λ)than in the HG97and M2K calculations,that are carried out with similar values of the parameters.(see Table 1). This can be explained considering the di?erent usage of the quantities M min c ,an d λin th e models.As already men-tioned in Sect.1,in RV81these parameters were derived according to complete stellar models currently available at that epoch.The subsequent comparison between the pre-dictions o f synthetic models and observations pointed out at the so-called “carbon star mystery”,as denominated by Iben (1981),i.e.too few faint and too many bright carbon stars expected than observed. Di?erently,the later analyses carried out by HG97and M2K move from another perspective,that is to consider M min c and λas free parameters which should be calibrated in order to reproduce observations of carbon stars.The aim is to provide indications on the average characteris-tics of the third dredge-up,so as to remove the theoreti-cal discrepancy related to the “carbon star mystery”(see Sect.5.2.3). 5.1.5.The treatment of hot bottom burning The most notable e?ect of this process on stellar yields involves 14N and 4He,which are newly synthesised at the expense of hydrogen and,in general,of the other CNO catalysts.It is important to stress that the occurrence of HBB in the most massive AGB stars (M >3.5?4.5M ⊙)has not only a direct e?ect on stellar yields –via changing the chemical abundances in the envelope –but also deter-mines an indirect action,a?ecting the energetics of these stars,hence their evolutionary properties. In fact,as a consequence of HBB,the M c ?L relation breaks down,a feature clearly shown by complete AGB stellar calculations (e.g.Bl¨o cker &Sc¨o nberner 1991;see also Fig.7).In these massive AGB models the luminosity evolution is characterised by a steeper increase with the core mass (above the M c ?L relation)up to a maximum,followed by a decline as soon as the envelope mass is sig-ni?cantly reduced by mass loss.Eventually the M c ?L re-lation is recovered (e.g.Vassiliadis &Wood 1993;Marigo 1998b).This behaviour is exempli?ed in Fig.7where we 12P.Marigo report the results of complete evolutionary calculations performed by Bl¨o cker(1995)for a7M⊙model with solar metallicity(triangles). Actually,the e?ect of the predicted luminosity evolu- tion on stellar yields is at least two-fold.In fact,the stellar luminosity is closely related to the temperature at the base of the envelope,which the nuclear reaction rates crucially depend on.Moreover,the overluminosity of AGB stars with HBB can trigger high mass-loss rates,thus favour- ing the onset of the super-wind regime with consequent reduction of the TP-AGB lifetimes. Such overluminosity e?ect caused by HBB is included neither in RV81nor in GdJ93,where the luminosity evo-lution is assumed to follow the M c?L relation by IT78 (with some revision for the composition dependence in the GdJ93work).In Fig.7we show the behaviour of the luminosity for the7M⊙model(solid line)as it would re- sult adopting the IT78relation with the the same values for current M c and M as in the Bl¨o cker(1995)model se- quence.The discrepancy is notable.To overcome this lim-itation of synthetic models Marigo et al.(1998;see also Marigo1998ab)developed a solution scheme based on en-velope integrations,so that the overluminosity produced by HBB is taken into account and the results of complete stellar calculations are recovered(dashed line in Fig.7). 5.2.Observational constraints In the following we will examine the AGB synthetic mod-els under consideration(RV81,HG97,M2K(this work)) in relation to their capability of reproducing important observational constraints,which are closely related to the stellar yields. 5.2.1.The initial-?nal mass relation The initial-?nal mass relation(IFMR)of low-and intermediate-mass stars is intimately linked to the chem-ical yields,as it determines the total amount of matter ejected by a star during its entire evolution.In a com-plementary way,it gives information on the reservoir of stellar remnants,irreversibly lost by the star-forming gas. Moreover,assessing the upper mass limit for WD progen-itors(M WD)is an important point,since it a?ects the expected rate of type II supernovae.All these aspects are fundamental issues for chemical evolutionary models. Figure8shows a few empirical calibrations of the IFMR for the solar neighbourhood.The?rst striking point is that the more recent determinations signi?cantly di?er from the earlier work by Weidemann(1987).For instance, the revised relation by Herwig(1996)presents a?atter slope up to Hyades location(M i～3M⊙,M f～0.7M⊙), followed by a steeper rise,and a?nal?attening towards higher initial masses(M i>4M⊙).The presence of an in?ection point at the Hyades mean location seems to be Fig.7.Quiescent luminosity as a function of the core mass during the TP-AGB phase.The predictions for the 7M⊙,Z=0.021model with HBB according to full evo-lutionary calculations by Bl¨o cker(1995)are compared to the those of synthetic calculations.The luminosity evolu-tion for a2.5M⊙model is also shown(Marigo et al1999a). The reference M c?L relation is taken by Wagenhuber& Groenewegen(1998).The adopted mass-loss prescription is that by Baud&Habing(1983).See text for further explanation. con?rmed also by Reid(1996),as discussed by Weidemann (1997). The second point to be made deals with the critical mass M WD1,that is the maximum initial mass of WD progenitors.At present,this limiting value is still rather uncertain(most likely in the range5–8M⊙),since it heavily depends on model details.In particular,as already discussed by Weidemann(1987),the de?nition of convec-tive boundaries–via either the Schwarzschild criterion or an overshooting scheme–plays a crucial role.It turns out that with the latter choice M WD is lower than assuming the former classical assumption.However,other param-eters may a?ect the predictions for M WD.For instance, the recent metallicity re-determination(i.e.half-solar)of the young open cluster NGC2516by Je?ries(1997;see Fig.8)has lead to assign it a younger age.As a conse-quence,Je?ries(1997)derives M WD around5–6M⊙, that is considerably lower than M WD～7?8M⊙as esti- Yields from low-and intermediate-mass stars 13 Fig.9.WD mass distribution for solar metallicity.The observed data in the solar neighbourhood are taken from the sample of bright DA WDs by Bragaglia et al (1995).The shaded area beneath the ob-served histogram is set equal to unity.The middle and right pan-els on the top show the WD mass distributions (solid line)derived by adopting the semi-empirical IFMRs by Weidemann (1987)and Herwig (1996),respectively.The bottom panels display the distributions ob-tained by assuming the theoretical IFMRs from the quoted works.All the solid line histograms are nor-malised to the fraction of observed WDs with masses larger than 0.5M ⊙.See text for more details. Fig.8.Initial-?nal mass relation for low-and intermediate-mass stars with solar metallicity.Semi-empirical calibrations for the solar neighbourhood are taken from Weidemann (1987),Herwig (1996),and Je?ries (1997).Solid lines refer to theoretical predictions.When the Reimers’prescription for mass-loss is adopted,the corresponding e?ciency parameter ηis indicated.See text for more details. mated in previous studies (e.g.Weidemann 1987;Koester &Reimers 1996).However,it should be noticed that in a more recent paper Je?ries et al.(1998)still do not exclude that the metallicity of NGC 2516might be nearly solar. Figure 8displays the theoretical IMFRs as derived by RV81,HG97,and M2K for low-and intermediate mass-models with initial solar metallicity.We can notice that the both HG97and M2K are satisfactorily consistent with the trend of the most recent observational relations,whereas RV81is far from reproducing the empirical data.In particular,the RV81relation shows a quick divergency of the ?nal mass at increasing initial mass,with the most massive stars being able to build C-O cores up to the Chandrasekhar limit of 1.4M ⊙.The ?nal fate of these stars would correspond to the occurrence of type I-1/2su-pernova events,which seems not to be supported by the observations. 5.2.2.The white dwarf mass distribution The white dwarf mass distribution (WDMD)is closely related to the IFMR from which it can be theoretically derived,provided that assumptions on the initial mass function (IMF)and star formation rate (SFR)are made to perform the integration over mass and time.The observed WDMD in the solar neighbourhood (Bergeron et al.1992;see Fig.9)is narrowly peaked in the mass range 0.5–0.6M ⊙(adopting a mass bin of 0.1M ⊙),which contains more than 45%of the observed objects. We notice that,with a ?ner bin sampling (i.e.0.05M ⊙),the location of observed peak would fall between 0.5and 0.55M ⊙,which may be di?cult to be theoreti-cally reproduced.In fact,according to stellar evolutionary models,these values would be consistent with the mini-mum remnant mass produced by progenitor stars as old as the age of the Galaxy (～15Gyr corresponding to ini- 14P. Marigo Fig.10.Luminosity functions of?eld carbon stars in the Magellanic Clouds.The observed data(shaded his- tograms)are taken from Costa&Frogel(1996)for the LMC,and Reiberot(1993)for the SMC.Theoretical dis- tributions(thick solid line)are shown for comparison.Top left-hand side panel:Iben&Renzini(1983)calculations for the LMC–based on a synthetic AGB model very sim- ilar to that of Renzini&Voli(1981;RV81)–with the pa- rameter set(η=2/3,α=2,?=0.1).Top right-hand side panel:Groenewegen&de Jong(1993;GdJ93)best?tting distribution for the LMC carbon stars.Bottom panels: Marigo et al.(1999a)best?ts to the CSLFs in the LMC and SMC.See text for further explanation. tial masses～0.9M⊙),provided that their C-O cores do not grow in mass during the TP-AGB phase.In fact,these stars are predicted to enter the TP-AGB phase with a core mass of already～0.52M⊙(Girardi et al.2000).Then,the location of the observed peak in the range0.5–0.55M⊙ might be explained by theory only if assuming i)that mass loss su?ered by AGB low-mass stars is so strong that they leave this phase as soon as they enter it,or ii)very e?- cient dredge-up prevents the growth in mass of the core (Herwig et al.1997).On the other hand,as suggested by Bragaglia et al.(1995),the origin of such discrepancy be- tween theory and observations would be most likely due to a systematic underestimation of the surface gravities derived from WD models. Given this point of uncertainty and considering that WDs with M<0.4M⊙are probably helium WDs de- rived from binary evolution,in this work both theoretical and observed WDMDs(see Fig.9)are derived adopting a mass bin of0.1M⊙,and normalising them to the observed fraction of WDs with M≥0.5M⊙. The predicted fraction of WDs contained in the k th mass bin(from M k f to M k+1 f )is calculated with: N k∝ (M k+1 i ?M k i) Yields from low-and intermediate-mass stars15 discussion on the predicted IFMRs(Sect.5.2.1),in RV81 there is a sizable overproduction of WDs more massive than0.6M⊙,a feature already pointed out by Bragaglia et al.(1995). 5.2.3.The carbon star luminosity function The carbon star luminosity function(CSLF)is a funda- mental observable as it gives indications on,at least,two basic processes occurring in TP-AGB stars with di?erent masses,namely:the third dredge-up–that determines the increase of the surface carbon abundance–,and mass loss by stellar winds,that a?ects the duration,hence the luminosity excursion during this phase. Iben(1981)?rst pointed out the so-called“carbon star mystery”,corresponding to a long-standing discrepancy between theory and observations,i.e.current stellar mod- els predicted a de?cit of faint carbon stars,accompanied by an excess of bright carbon(in general AGB)stars.This situation is exempli?ed in Fig.10,where we report the pre- dicted CSLF for the LMC,according to the calculations performed by Iben&Renzini(1983),with model prescrip- tions very similar to RV81.For this particular case,the authors adopt the following set of parameters:e?ciency parameterη=2/3in the Reimers(1975)mass-loss for- mula;mixing-length parameter,α=2;minimum core for the third dredge-up to occur,M min c =0.5M⊙;an d e?- ciency of the third dredge-up,λ,as a function of the core mass. The CSLF in the LMC is instead very well?tted (Fig.10)by the other two AGB synthetic models here con-sidered,namely:Groenewegen&de Jong(1993,GdJ93; top right-hand side panel)and Marigo et al.(1999a, MGB99;bottom left-hand side panel).We remind again that,di?erently from RV81,in these studies the third dredge-up is suitably calibrated in order to reproduce the CLSF in the LMC.Finally,it should be remarked that Marigo et al.(1999a)have extended the analysis to the CSLF in the SMC(see bottom right-hand side panel of Fig.5.2.3),so as to include a metallicity-dependent treat-ment of the third dredge-up in their synthetic AGB model. 5.2.4.The chemical abundances of planetary nebulae The observed chemical composition of planetary nebulae represents another crucial constraint for stellar models,as it is the record of the nucleosynthetic and mixing processes occurred during the previous stellar evolution. As far as HG97predictions are concerned,a detailed discussion is given in Groenewegen&de Jong(1994)and it will not be repeated here.We restrict here to the results of RV81and M2K which are compared in Fig.11with the measured abundances of He,C,and O in galactic plane-tary nebulae.Predicted PN abundances can be found in Tables A13–A15.Since a full analysis is beyond the pur-pose of this work,we simply consider the most relevant aspects. Both RV81and M2K results shown here are derived from calculations of stellar models with initial solar com-position.Therefore,they cannot reproduce the data points with He/H<～0.10?0.11,since these latter most likely correspond to progenitor stars with initial subsolar metal-licity. The expected paths of PN abundances as a function of the initial mass of the progenitor star re?ect the e?ciency and duration of the involved processes.For instance,in both models the C/O ratio?rst increases at increasing mass due to the third dredge-up,and then(for M>3?4M⊙)it starts to decline because of HBB. An interesting point is the anticorrelation between the N/O ratio and the C/O ratio exhibited by observed PNe with the highest helium content(He/H>0.125;the so-called type I PNe according to the classi?cation intro-duced by Peimbert(1978)).This trend is reproduced by M2K synthetic calculations,tracing the signature of HBB in the most massive AGB stars(M>～3.5M⊙),where car-bon in the envelope is quickly converted into nitrogen. Note that theoretical results are notably sensitive to the adopted value for the mixing-length parameter,i.e. the largerαis,the more e?ciently HBB operates,yield-ing higher N/O and lower C/O ratios.Limiting to the observed sample of PNe,we might deduce that HBB has operated in the most massive progenitors of solar metal-licity,but with a rather mild e?ciency,in agreement with the conclusion already mentioned by Henry et al.(2000a). In fact,as we can see from Fig.11,predictions for the case(α=2.5)–which correspond to strong HBB–lead to N/O ratios quite higher than observed. Hower,these indications should be considered with some caution,as the considered sample of PNe might not cover the whole relevant mass range of the progenitors(see Henry et al.2000a),and predictions of PNe abundances are derived under very simple assumptions(see Appendix A).A much better approach will be adopted with the aid of a detailed synthetic model of PN evolution,which is being developed(Marigo et al.2001,in preparation;see Marigo et al.1999b for a preliminary presentation). Finally,we would remark that RV81results for the most massive stellar models–shown in Fig.11with dotted lines–do not correspond to PN abundances,but rather to surface abundances just prior the progenitor stars ex-plode as SNe I-1/2.Therefore,attention must be paid not to use these data for a comparison with observed PN abun-dances. 5.3.Yields from simple stellar generations We will compare here the stellar yields presented in this work with those calculated by RV81and HG97.Possi-ble di?erences are then discussed on the basis of di?erent 16P. Marigo Fig.11.Abundance ratios of galactic planetary nebulae.Observed data (squares)are taken from Kingsburgh &Barlow (1994),and Henry et al.(2000a).Filled squares correspond to PNe with log(N /O)>?0.5and He /H >0.125.Predicted PN abundances are shown as a function of the initial mass of solar-metallicity progenitor stars,as derived in this work (left-hand side panel),and in Renzini &Voli (1981,right-hand side panel).Lines (solid and dashed)connect predicted abundances at increasing stellar mass (a few values are indicated nearby)for two values of the mixing length parameter.In the case of RV81model,surface abundances just prior Chandrasekhar carbon explosion are also shown (dotted line). model prescriptions and observational constraints,already mentioned in the previous sections. We choose not to make a direct comparison between yields produced by models with the same initial mass,be-cause di?erent sets of yields cover di?erent mass-ranges in the domain of low-and intermediate-mass stars (see Sect.5.1.1).For this reason,it is more meaningful to com-pare the whole chemical contribution provided by low-and intermediate-mass stars belonging to a given simple (i.e.coeval)stellar population.To this aim,we recall that ac-cording to the standard de?nition (Tinsley 1980),the yield from a stellar generation ,y k ,is the mass converted into the chemical element k and ejected by all stars per unit mass locked into stars:y k =(1?R ) M u M l M i p k (M i )φ(M i )d M i M u M l M i φ(M i )d M i (12) where φ(M )=d N/d M i is the IMF (by number)de-?ned between the lower (M l )and upper (M u )mass limits;W (M i )is the remnant mass. In order to weigh the sole contribution from the gener-ation of low-and intermediate-mass stars,let us consider the quantity: y lims k = M up M l Mp k (M i )φ(M i )dM i Yields from low-and intermediate-mass stars 17 Fig.12.Integrated yield contribu-tions from low-and intermediate-mass stars as a function of the metallicity,as de?ned by Eq.(12).The mixing-length parameters (α)adopted by the authors are indi-cated. tial di?erences in the adopted physical prescriptions as already illustrated in Sect.5.1. Di?erences essentially show up both in metallicity trends and absolute values of y lims k .Compared to previ-ous calculations,M2K yields show a pronounced depen-dence on the metallicity,i.e.positive yields increase with decreasing Z .Conversely,the RV81and HG97sets present weak trends with Z . The metallicity dependence can be explained as fol-lows.On one side,AGB lifetimes of low-mass stars in-crease at decreasing metallicities,as mass-loss rates are expected to be lower.This fact leads to a larger num-ber of dredge-up episodes.Moreover,both the onset and the e?ciency of the third dredge-up are favoured at lower metallicities.These factors concur to produce a greater enrichment in carbon.On the other side,HBB in more massive AGB stars becomes more e?cient at lower metal-licities,leading to a greater enrichment in nitrogen.The combination of all factors favours higher positive yields of helium at lower Z . As far as the single elemental species are concerned,we can notice: –M2K yields of 4He are larger than those by HG97and RV81towards lower Z ,likely due to the earlier activa-tion and larger e?ciency of the third dredge-up in our models.With respect to RV81and HG97predictions,our yields of 4He present a signi?cant trend with Z .–M2K yields of 12C are systematically higher than those of RV81and HG97because of the earlier onset (and average greater e?ciency than in RV81)of the third dredge-up. –The dominant contribution to the yields of 14N comes from HBB in the most massive AGB stars.Di?erences in the results re?ect di?erent e?ciencies of nuclear re-actions and AGB lifetimes.In particular,according to M2K the production of 14N,mainly of primary synthe-sis,is favoured at lower Z ,and is sistematically lower than RV81and HG97results.This latter di?erence can be explained considering the drastic reduction of TP-AGB lifetimes for the most massive AGB models with HHB (see Fig.6)in M2K models with respect to RV81.In general,our expected dependence of chemical yields on metallicity is far for being linear,and much caution should be used when extrapolating these quantities with respect to Z in chemical evolutionary models.We cannot verify whether such a non-linear relation with metallicity was displayed also in the RV81models,since just two val-ues of metallicities were considered there.However,the very high number of thermal pulses su?ered by stars with 18P.Marigo HBB regardless of the metallicity,according to the RV81 models,might already explain the apparent lower sensi-tiveness of their yields to the metallicity. 6.Final remarks We would like to outline brie?y the aims and?ndings of the present work. In the?rst part we have presented new homogeneous sets of stellar yields ejected from low-and intermediate-mass stars,in view of providing updated ingredients for modelling the chemical evolution of complex stellar sys-tems.Thanks to the updated input physics employed in the calculations,and the improved treatment of both the third dredge-up and hot-bottom burning,the present es-timation of the stellar yields from low-and intermediate-mass stars has led to new results and developments. In particular,a pronounced trend of the yields with the metallicity is expected.Speci?cally,at given stellar mass positive yields of4He,12C,14N,and16O are larger at decreasing metallicity.This feature is the result of con-curring factors:at lower Z both the third dredge-up and hot-bottom burning are more e?cient,and TP-AGB life-times are,on average,longer because of lower mass-loss rates. Moreover,it is interesting to notice that low-mass stars may produce positive yields of16O,which is brought up to the surface by the third dredge-up.The entity of this con-tribution(as well as for that of12C)depends crucially on the e?ciencyλ,number of dredge-up episodes,and chemi-cal composition of the convective intershell(for this latter, see Boothroyd&Sackmann1988b,and Herwig2000for di?erent model results). The possible production and ejection of newly synthe-sised oxygen from low-and intermediate-mass stars may be an interesting prediction to be tested through its con-sequences in various possible applications,mostly in rela-tion to the chemical composition of planetary nebulae(e.g. P′e quignot et al.2000),and galactic chemical evolutionary models. The second part of the paper is meant to examine sev-eral aspects concerning the stellar models which the chem-ical yields are derived from.To this aim,we have analysed how the main model prescriptions(i.e.treatment of con-vective boundaries,mass loss,analytical relations,third dredge-up parameters,treatment of hot-bottom burning, etc.)may a?ect the predictions of stellar yields. Finally,the third?nal part is dedicated to compare our new yields with other available data sets of large usage,in the attempt of explaining the existing di?erences as the result of particular assumptions.To do this,we have con-sidered basic observational constraints which are closely related to the stellar yields–namely:i)the initial-?nal mass relation;ii)the white dwarf mass distribution;iii) the carbon star luminosity function;and iv)the chemical composition of planetary nebulae–and tested the capa-bility of di?erent models in reproducing them through a direct comparison between predictions and observations. In particular,it has been shown how much the choice of calibrating fundamental e?ciency parameters(e.g.of the third dredge-up and mass loss)in recent works has changed the predictions of stellar yields compared to ear-lier studies. To conclude,we wish this work has somehow con-tributed to clarify a few important general points on the-oretical stellar yields,in view of stimulating an aware and critical usage of them. Acknowledgements.I would like to thank L′e o Girardi,Laura Portinari,and Cesare Chiosi for their important professional advice and support,and the referee,Dr.R.B.C.Henry,for his helpful remarks on this work.This study has been?nancially supported by the Italian Ministry of University,Scienti?c Re-search and Technology(MURST)under contract“Formation and evolution of Galaxies”n.9802192401. 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Concerning low-mass stars(with M i<～M HeF),we give separately the yields ejected during the RGB(Ta-bles A1–A3)and AGB phases(Tables A4–A6).The quantity,M TP,0,corresponds to the mass at the onset of the TP-AGB phase,which is smaller than M i by the amount of mass lost during the previous RGB phase,?M ej(RGB).The mass lost during the AGB is denoted with?M ej(AGB). Total yields produced by stars in the whole mass range (0.8M⊙<～M i<～5M⊙)are presented in Tables A7–A12. The total amount of ejected mass is denoted with?M ej. Total yields from stars with M i≥3.5M⊙are given in Tables A10–A12for three values of the mixing-length parameterα,and distinguishing between the secondary (S entry)and primary(P entry)components in the case of the CNO elements. Tables A13–A15present the predicted abundances ratios(He/H,C/H,N/H,O/H)in planetary nebulae,as a function of the initial stellar mass(M i),metallicity Z,and mixing-length parameter.The PNe chemical abundances (by number,in mole gr?1)are calculated by averaging the abundances in the wind ejecta over the last stages(i.e.a time periodτPN=3×104yr)on the AGB,weighted by the masses of the ejecta. For the sake of simplicity,we do not consider the ques-tion on the actual observability of PNe(which depends on both dynamical and ionisation properties),and the fact that evolutionary timescales of PNe(hence the timeτPN) may largely vary according to the mass of the progenitor star.These points deserve a more complex study,which is currently in progress and presented in a preliminary form by Marigo et al.(1999b). All data are available in electronic format at the web site address:http://pleiadi.pd.astro.it. 20Appendix A:TABLES OF STELLAR YIELDS 数据中心电源解决方案及选型 摘要：随着互联网应用技术的迅速发展，作为互联网载体的数据中心建设规模 日益变大。巨大的用电容量需求给数据中心的建设和运营带来了空前的压力。文 章根据数据中心常用的供电解决方案提出了三种电源产品解决方案，并根据安全 可靠、节能环保、管理维护、建设成本四个方面提出的电源产品解决方案进行对 比和分析，作为数据中心建设的配套电源产品选型参考。 关键词：数据中心；电源系统；节能环保 近年来，随着移动互联网时代的到来，基于互联网技术提供的各种服务已融 人到人类的社会生活中，数据储存容量需求高速发展。作为互联网应用服务载体 的数据中心，其建设需求也在同步不断增加。近几年来，随着互联网服务、电子 商务行业的高速发展，我国的IT服务巨头B.A.T（百度、阿里、腾讯）、三大运 营商（移动、联通、电信）都投人了大量的资金用于数据中心的建设。截止目前，全国已经建成了规模不一但数量庞大的数据中心。各大数据中心运营商为了节约 成本、便于管理、形成规模化效益，数据中心的建设规模逐渐变得越来越大，同时，巨大的用电容量也给数据中心的建设和运营都带来了巨大的成本压力。如何 合理配置资源，提高数据中心供电的安全性，降低运营成本，成为各大数据中心 运营商在机房建设中重点考虑的因数。 一、数据中心对供电的要求 主要应包括以下几方面。 1.1安全可靠 一般要求电源供电系统的可用度A!99.999M。因此，必须合理进行配置，达 到系统供电安全最优化。 1.2节能环保 在能源紧缺、重视环保的今天，“绿色、节能、高效”是新一代数据中心建设 的主流标准。不间断电源（UPS）在绿色、节能、高效方面的表现，主要体现在 输人功率因数、输人电流谐波以及整机效率等方面。 1.3可维护性 不间断电源（UPS）供电系统应当具备维护方便，便于管理的设计需求。在节省系统的维护成本的同时，更为重要的是可以尽量避免因人为维护或者管理不当，而引发的系统故障。 1.4建设成本 从数据中心容量需求在应用过程中会逐步增大的情况，要求电源系统配置可 扩容设计。这样能有效地控制系统建设初期投资费用，同时，又能使UPS供电系 统在数据中心的建设过程中，始终保持安全高效的工作状态，提高了系统的性价比。因此，数据中心供电系统的合理设计，并非一味追求某一项指标的最优化， 而是应该根据实际应用的需求，在保证安全可靠、节能环保、运维方便、成本合 理这些要点中寻求一个平衡点，提供一个节能环保，安全可靠，经济适用的供电 系统解决方案。 二、数据中心用电特点 2.1保证的目标。 在数据中心配电系统中除了正常配电系统中采用的双回路独立电源供电之外，还在数据中心配备事故备用柴油发电机、大容量UPS等。同时在数据中心配电系 统故障处理机制方面，会考虑供电电源失电、母线故障、开关跳闸和开关拒动等 CPU的技术参数的意思1 CPU（Central Processing Unit) 也就是我们常说的中央处理器,就一般的用户来说,它不是装机配件中最昂贵的,但它是电脑当中最核心的配件,一台电脑的性能如何跟CPU的性能有着最直接的关系.而且CPU的选择也同时关系到主板和内存的搭配问题!! 为了让大家更清晰地了解CPU,我们先来了解CPU的一些基本的概念. CPU重要参数介绍： 1）前端总线:英文名称叫Front Side Bus,一般简写为FSB.前端总线是CPU跟外界沟通的唯一通道,处理器必须通过它才能获得数据,也只能通过它来将运算结果传送出其他对应设备.前端总线的速度越快,CPU的数据传输就越迅速.前端总线的速度主要是用频率来衡量,前端总线的频率有两个概念:一就是总线的物理工作频率（即我们所说的外频）,二就是有效工作频率（即我们所说的FSB频率）.由于INTEL跟AMD采用了不同的技术,所以他们之间FSB频率跟外频的关系式也就不同了.现时的Inter是:FSB频率=外频X4;而AMD的就是:FSB频率=外频X2.举个例子：P4 2.8C的FSB频率是800MHZ,由那公式可以知道该型号的外频是200MHZ了;又如BARTON核心的Athlon XP2500+ ,它的外频是166MHZ,根据公式,我们知道它的FSB频率就是333MHZ了.目前的前端总线频率,这一点Intel还是有优势的. 2）二级缓存:也就是L2 Cache,我们平时简称L2.主要功能是作为后备数据和指令的存储.L2容量的大小对处理器的性能影响很大.因为L2需要占用大量的晶体管,是CPU晶体管总数中占得最多的一个部分,高容量的L2成本相当高!!所以INTEL和AMD都是以L2容量的差异来作为高端和低端产品的分界标准! 3）制造工艺:我们经常说的0.18微米、0.13微米制程,就是指制造工艺.制造工艺直接关系到CPU的电气性能.而0.18微米、0.13微米这个尺度就是指的是CPU核心中线路的宽度.线宽越小,CPU的功耗和发热量就越低,并可以工作在更高的频率上了.所以0.18微米的CPU 能够达到的最高频率比0.13微米CPU能够达到的最高频率低,同时发热量更大都是这个道理. 4）流水线:流水线也是一个比较重要的概念.CPU的流水线指的就是处理器内核中运算器的设计.这好比我们现实生活中工厂的生产流水线.处理器的流水线的结构就是把一个复杂的运算分解成很多个简单的基本运算,然后由专门设计好的单元完成运算.CPU流水线长度越长,运算工作就越简单,CPU的工作频率就越高,不过CPU的效能就越差,所以说流水线长度并不是越长越好的.由于CPU的流水线长度很大程度上决定了CPU所能达到的最高频率,所以现在INTEL为了提高CPU的频率,而设计了超长的流水线设计.Willamette和Northwood核心的流水线长度是20工位,而如今上市不久的Prescott 核心的P4则达到了让人咋舌的30(如果算上前端处理，那就是31)工位.而现在AMD的Clawhammer K8,流水线长度仅为11工位,当然 CPU的主要性能参数 主频 通常所说的某某CPU是多少兆赫的，而这个多少兆赫就是“CPU的主频”。主频也叫时钟频率，单位是GHZ，用来表示CPU的运算速度。CPU的主频＝外频×倍频系数。 有人以为认为CPU的主频指的是CPU运行的速度，实际上这个认识是很片面的。CPU的主频表示在CPU内数字脉冲信号震荡的速度，与CPU实际的运算能力是没有直接关系的。当然，主频和实际的运算速度是有关的，但目前还没有一个确定的公式能够定量两者的数值关系，因为CPU的运算速度还要看CPU的流水线的各方面的性能指标（缓存、指令集，CPU的位数等等）。由于主频并不直接代表运算速度，所以在一定情况下，很可能会出现主频较高的CPU实际运算速度较低的现象。因此主频仅仅是CPU性能表现的一个方面，而不代表CPU的整体性能。 外频 外频是CPU与主板上其它设备进行数据传输的物理工作频率，也就是系统总线的工作频率。它代表着CPU与主板和内存等配件之间的数据传输速度。单位也是MHz。CPU标准外频主要有66MHz、100MHz、133MHz、166MHz、200MHz几种。 外频也是内存与主板之间的同步运行的速度，在这种方式下，可以理解为CPU的外频直接与内存相连通，实现两者间的同步运行状态。 倍频 倍频系数是指CPU主频与外频之间的相对比例关系。在相同的外频下，倍频越高CPU的频率也越高。但实际上，在相同外频的前提下，高倍频的CPU本身意义并不大。这是因为CPU与系统之间数据传输速度是有限的，一味追求高倍频而得到高主频的CPU就会出现明显的“瓶颈”效应——CPU从系统中得到数据的极限速度不能够满足CPU运算的速度。 理论上倍频是从1.5一直到无限的，但需要注意的是，倍频是以以0.5为一个间隔单位。 倍频一般是不能改的，现在的CPU基本都对倍频进行了锁定。 CPU的其它参数 电力电子技术课程设计报告 题目高频开关稳压电源 专业电气工程及其自动化 班级 学号 学生姓名 指导教师 2016年春季学期 起止时间：2016年6月25日至2016年6月27日 设计任务书11 高频开关稳压电源设计√ 一、设计任务 根据电源参数要求设计一个高频直流开关稳压电源。 二、设计条件与指标 1.电源：电压额定值220±10%，频率：50Hz； 2. 输出:稳压电源功率Po=1000W,电压Uo=50V； 开关频率:100KHz 3.电源输出保持时间td=10ms(电压从280V下降到250V)； 三、设计要求 1.分析题目要求，提出2~3种电路结构，比较并确定主电路 结构和控制方案； 2.设计主电路原理图、触发电路的原理框图，并设置必要的 保护电路； 3.参数计算，选择主电路及保护电路元件参数； 4.利用PSPICE、PSIM或MATLAB等进行电路仿真优化； 5.撰写课程设计报告。 四、参考文献 1.王兆安，《电力电子技术》，机械工业出版社； 2.林渭勋等，《电力电子设备设计和应用手册》； 3.张占松、蔡宣三，《开关电源的原理与设计》，电子工业 出版社。 目录 一、总体设计 (1) 1．主电路的选型（方案设计） (1) 2.控制电路设计 (4) 3.总体实现框架 (4) 二、主要参数及电路设计 (5) 1.主电路参数设计 (5) 2.控制电路参数设计 (7) 3.保护电路的设计以及参数整定 (8) 4.过压和欠压保护 (8) 三、仿真验证（设计测试方案、存在的问题及解决方法） (9) 1、主电路测试 (9) 2、驱动电路测试 (10) 3、保护电路测试 (10) 四、小结 (11) 参考文献 (11) 商业银行数据中心供配电系统解决方案 商行数据中心的基础设施系统主要分电源、环境控制和机房监控管理系统。由于数据中心承载商行的核心业务，重要性高，不允许业务中断。因而数据中心一般根据TIA942标准的Tier4标准建设，可靠性要求99.99999％以上，以保证异常故障和正常维护情况下，数据中心正常工作，核心业务不受影响。 1、电源系统： 选用两路市电源互为备份，并且机房设有专用柴油发电机系统作为备用电源系统，市电电源间、市电电源和柴油发电机间通过ATS（自动切换开关）进行切换，为数据中心内UPS电源、机房空调、照明等设备供电。由于数据中心业务的重要性，系统采用双母线的供电方式供电，满足数据中心服务器等IT设备高可靠性用电要求。双母线供电系统，有两套独立UPS供电系统（包含UPS配电系统），在任一套供电母线（供电系统）需要维护或故障等无法正常供电的情况下，另一套供电母线仍能承担所有负载，保证机房业务供电，确保数据中心业务不受影响。在UPS输出到服务器等IT设备输入间，选用PDM（电源列头柜）进行电源分配和供电管理，实现对每台机柜用电监控管理，提高供电系统的可靠性和易管理性。 对于双路电源的服务器等IT设备，通过PDM直接从双母线供电系统的两套母线引人电源，即可保证其用电高可靠性。对于单路电源的服务器等IT设备，选用STS（静态切换开关）为其选择切换一套供电母线供电。在供电母线无法正常供电时，STS将自动快速切换到另一套供电正常的母线供电，确保服务器等IT设备的可靠用电。 供配电系统拓扑图 ATS ATS 柴油机发电 第一路市电 第二路市电动力配电柜 第二级配电UPS 配电柜 UPS1 UPS2 PDM1 PDM2 列头柜 STS 机柜P D U 1机柜P D U 2 机柜P D U 1机柜P D U 2 机柜机柜P D U 1 机柜P D U 2 机柜机柜P D U 1机柜P D U 2 机柜P D U 2 机柜P D U 1机柜机柜 第一级配电机柜 第三级配电 空调新风 双母线供电方案 机柜内走线 图示双母线供电系统可确保供电可靠性高达99.99999％以上 2、机房智能配电系统三级结构 数据中心三级配电系统是对机房配电的创新，机房三级配电系统有利于配电系统的设计和运维管理 第一级：机房配电接入层。主要包括大楼地下配电室到机房输入端电缆的部分及机房市电配电部分。 第二级：机房配电管理层。主要包括机房UPS 配电部分。通过使用模块化配电柜，实现机房的模块化配电，并将设备用电和辅助设备用电分开； 第三级：机柜排及机柜配电层。主要包括列头柜PDM 配电、STS 配电到负载部分； 3、 供配电系统的智能化管理 供配电系统的智能化管理：列头柜的智能监控系统可对配电系统开关状态与负载情况进行监测、告警、统计。 监控的输入部分电气参数有：电量、有功功率、无功功率、视在功率、功率因数、三相电压、电流、频率等。 监控的输出支路电气参数有：额定电流，实际电流、负载百分比、负载电流谐波百分比、负载电量、功率因数等。 这些监测信息能让值班人员掌握各设备的运行情况，及时调整负载分布，清楚了解每一个机柜的耗电量，对设备电源部分的潜在故障、对能效管理、降低能耗提供可靠依据。 模块化设计智能管理：本方案配电系统遵循以可靠性设计为核心，专 3.1中介语理论产生的历史背景 60年代是对比分析的兴盛时期。70年代初开始衰落,反映了一种历史的必然,因为这种理论方法无论在理论上还是实践上都面临着严重的危机。 因此,语言学家们为语言教师勾画了这样一幅图景:首先,语言学家们通过两种语言系统(L1和L2)的对比,为语言教师提供一个详细的菜单。这个菜单包括两种语言的相同点与不同点。然后,语言教师便依据这些不同点来预测学习者的难点,并据此来编写教学大纲和教材。但是后来的教学研究和实践证明,语言学家的许诺仅仅是一幅理想的图画而已。70年代初,对比分析遭到激烈的批评。如果第二语言学习者产生的错误完全可以通过两种语言的对比来预测。由此推论,语言的差异等于学习的难点，学习的难点必然导致语言表达的错误。问题是,语言差异是语言学上的概念,学习的难点则是心理学上的概念。学习的难点无法直接从两种语言差异的程度来推测。教学实践也证明,依据对比分析确认的难点事实上并不完全导致错误的产生。对比分析的理论方法存在的致命弱点,如果归结为一句话,那就是,人们试图用简单的语言学的方法去解决复杂的心理学的问题。语言习得涉及到学习的主体和客体的方方面面,对比分析却仅仅局限于语言系统的对比,忽略了学习者这一主体以及作为学习客体的学习过程。由于对比分析在理论与实践上的危机,人们呼吁一种新的理论的诞生,并要求这种新的理论把目光投向学习的主体和客体。早期的中介语理论正是在这种历史背景下产生的。 3.2中介语的概念 于根元、鲁健骥等是在中国应用语言学领域，最早进行了介绍、评述和研究中介语的意义、特点和研究方法。我们来看他们是怎么界定中介语的。于根元认为，所谓中介语就是介于习得语和目的语之间的独立的语言系统，他是第二语言习得者创造的语言系统。鲁健骥认为，中介语指的是由于学习外语的人在学习过程中对于目的语的规律所做的不正确的归纳与推论而产生的一个语言系统,这个语言系统既不同于学习者的母语,又区别于他所学的目的语。 3.3中介语出现的根源 我们着重重复一下鲁健骥对中介语的定义：中介语指的是由于学习外语的人在学习过程中对于目的语的规律所做的不正确的归纳与推论而产生的一个语言 高频开关电源技术规范书 2011年主网技改工程 通讯系统电源 (高频开关电源、免维护电池) 技术规范书 1、概述 1.1本技术规范书仅适用于2011主网技改工程。 1.2本规范未对一切技术细节做出规定，也未充分引述有关标准和规范，卖方应提供符合本规范书和遵照国际电工委员会标准(IEC)、国际公制(SI)及国家标准的符合国家电力行业标准的优质产品。技术指标应符合YD/T731《通讯用高频开关整流器》的规定。 1. 3本规范未尽事宜，双方协商解决。 2、主要技术要求 2.1 供货数量: 序号名称规格及型号单位数量备注 1 高频开关电源屏 - 48V/120A(4*30A) 面 4 2 蓄电池屏 48V/300AH，每组24只，-2V 面 8 3 电池巡检仪每套可接48只电池套 4 高频开关电源:-48V/120A (4*30A) 4套 蓄电池: -48V/300AH 4组(包括电池柜) 每套两组每组24只，2V/只 2. 2设备电气性能 3. 单套高频开关电源配置: 1) 具备交流输入配电单元、整流单元、直流输出配电单元、监控单元，并为一体化机 柜，应能至少接入2组蓄电池。 2) 具备完善的故障告警、保护功能(交流输入故障、直流输出故障、整流单元故障、 监控单元故障等自动保护功能)，且部分状态具有自动恢复功能(交流过压、欠压、 直流过流、过温)。 3) 交流配电单元: 输入2路，输入电压:三相五线制:380V?20%,50?10%HZ，2路交流输入电源能自动切换，且互为主备用。 输出:三相输出分路(2路):2路32A，2路25A 单相输出分路(14路):2路20A，8路10A，4路6A。 4) 直流配电单元: 输入:整流4路:48V/4*30A,可调整为(10A-20A-30A-60A，最大能达到120A。 第 2 页共 8页 蓄电池: -48V?300AH 蓄电池保护值:43V?1% 输出电压:-48V(-40V,-58V)输出总容量为120A。输出电流规格:40A 6路、20A 40路、10A 14路直流输出采用端子接线。 5) 整流模块单元: 整流模块采用N+1方式工作 由多个整流模块组成的整流单元，当其中一个(或几个)模块故障时，不得造成整个 系统停止工作。 输入电压:304,456V(45,66HZ) 中介语简介中介语理论是二语习得中的一个重要理论，它产生于20世纪70年代初并于80年代初被介绍到我国，对我国的外语教学产生了巨大的推动作用，人们也逐步认识到中介语在外语教学中的积极作用。Selinker在其论文Language Transfer （1969）中首次使用了“interlanguage”一词，并于1972发表了题为Interlanguage的研究论文。Selinker认为，中介语是第二语言学习者独立的语言系统，在结构上处于母语和目的语的中间状态（1972）。 中介语在英语学习中的干扰作用 一、中介语定义及特点 中介语（Interlanguage, 简称IL）一词是英国语言学家Selinker 1969 年首次提出. 现在又被译为过渡语、中间语、中继语、语际语等。它是指学习者在某一段时间内所创建的内在语言体系或学习者在整个学习过程中所构建的相互关联的语言体系。学习者在学习和使用第二语言时，不断接受和理解新的语音、语法知识，在此基础上逐渐形成自己的语言结构。并不断对其进行系统的预测调整，通过归纳和推论产生中介语。中介语的语言系统在结构上处于母语（Native Language）和目的语（Target Language）之间，具有独立性，并兼有两者的特点。综合国内外近来的研究,中介语有如下一些特征: 1．独立性 中介语是一个独立的语言系统，它既不同于母语, 又区别于外语, 也不能单纯的把它地看作外语学习过程中由于受母语的干扰而形成的混合体。中介语有其独特的语言规则，这些规则常常被学习者用来解释外语中固有而不规则的语言现象。 2．阶段性 中介语在逐渐进化的过程中，具有一定的阶段性。它是一个开放的体系，不是固定的一成不变的。这个体系在不断被新知识渗透的同时，修正原有知识，逐渐接近目的语。 3．动态性 在外语学习过程中，学习者的中介语在不断的发展变化。虽然它充满了错误，但由于新的语言规则有及强的扩展能力，它们处于不断的组合和变化中，因此中介语随着学习者的努力和交际需要而不断变化，由简而繁，由低而高，逐渐离开母语而接近目的语。如果我们设在母语与目的语之间的中介语为一个连续体，那么，在某一特定阶段，学习者的中介语可以用连续体上的某一点。中介语越接近目的语，说明学习者的外语水平越高。 4．系统性 中介语在每个阶段都表现出较强的系统性和内部一致性。它也是一个由内部要素构成的系统，就是说它有语音的、词汇的、语法的规则系统，而且自成体系。学习者对中介语的使 CPU的簡介及主要性能指標 什麽是CPU？ CPU是英語※Central Processing Unit/中央處理器§的縮寫， CPU一般由邏輯運算單元、控制單元和存儲單元組成。在邏輯運算和控制單元中包括一些寄存器，這些寄存器用於CPU在處理資料過程中資料的暫時保存。 CPU主要的性能指標有： 主頻即CPU的時鐘頻率(CPU Clock Speed)。 這是我們最關心的，我們所說的233、300等就是指它，一般說來，< 主頻越高，CPU的速度就越快，整機的就越高。 時鐘頻率: CPU的外部時鐘頻率，由電腦主板提供，以前一般是66MHz，也有主板支援75各83MHz，目前Intel公司最新的晶片組BX以使用100 MHz的時鐘頻率。另外VIA 公司的MVP3、MVP4等一些非Intel的晶片組也開始支援100MHz的外頻。精英公司的BX主板甚至可以支援133 MHz的外頻。 內部緩存（L1 Cache）： 封閉在CPU晶片內部的快取記憶體，用於暫時存儲CPU運算時的部分指令和資料，存取速度與CPU主頻一致，L1緩存的容量單位一般爲KB。L1緩存越大，CPU 工作時與存取速度較慢的L2緩存和記憶體間交換資料的次數越少，相對電腦的運算速度可以提高。 外部緩存（L2 Cache）： CPU外部的快取記憶體，PentiumPro處理器的L2和CPU運行在相同頻率下的，但成本昂貴，所以 PentiumII運行在相當於CPU頻率一半下的，容量爲512K。爲降低成本Inter公司生産了一種不帶L2的CPU命爲賽揚，性能也不錯。 MMX技術是※多媒體擴展指令集§的縮寫。 MMX是Intel公司在1996年爲增強Pentium CPU在音像、圖形和通信應用方面而採取的新技術。爲CPU增加57條MMX指令，除了指令集中增加MMX指令外，還將CPU晶片內的L1緩存由原來的 16KB增加到32KB（16K指命+16K資料），因此MMX CPU 比普通 CPU在運行含有MMX指令的程式時，處理多媒體的能力上提高了 60％左右。 通讯系统电源 （高频开关电源、免维护电池）技术规范书 1、概述 1.1本技术规范书仅适用于2011主网技改工程。 1.2本规范未对一切技术细节做出规定，也未充分引述有关标准和规范，卖方应提供符 合本规范书和遵照国际电工委员会标准（IEC）、国际公制（SI）及国家标准的符合国家电力行业标准的优质产品。技术指标应符合YD/T731《通讯用高频开关整流器》的规定。 1.3本规范未尽事宜，双方协商解决。 2、主要技术要求 2.1 供货数量： 序号名称规格及型号单位数量备注 1 高频开关电源屏-48V/120A（4*30A）面 4 2 蓄电池屏48V/300AH，每组24只，-2V 面8 3 电池巡检仪每套可接48只电池套 4 高频开关电源：-48V/120A （4*30A） 4套 蓄电池： -48V/300AH 4组（包括电池柜） 每套两组每组24只，2V/只 2.2设备电气性能 3.单套高频开关电源配置： 1)具备交流输入配电单元、整流单元、直流输出配电单元、监控单元，并为一体化机 柜，应能至少接入2组蓄电池。 2)具备完善的故障告警、保护功能（交流输入故障、直流输出故障、整流单元故障、 监控单元故障等自动保护功能），且部分状态具有自动恢复功能（交流过压、欠压、直流过流、过温）。 3)交流配电单元： 输入2路，输入电压：三相五线制：380V±20%,50±10%HZ，2路交流输入电源能自动切换，且互为主备用。 输出：三相输出分路（2路）：2路32A，2路25A 单相输出分路（14路）：2路20A，8路10A，4路6A。 4)直流配电单元： 输入：整流4路：48V/4*30A,可调整为(10A-20A-30A-60A，最大能达到120A。 中介语理论研究与第二语言教学 [摘要]中介语理论是第二语言习得研究领域中的一个认知理论。分析和研究中介语产生的根源和特点有助于了解第二语言习得机制,揭示第二语言习得的发展过程和规律。对提高教学效果有重大意义。[关键词]中介语;特点;教学 第二语言习得研究在近40年间取得了令人瞩目的成就。随着研究的不断深入和发展,人们越来越重视第二语言习得的心理过程。中介语理论把第二语言学习者的语言看作是一个内在的语言行为。因此,中介语的研究对外语教学方法论有着重要的意义。 一、中介语的概念 中介语(interlanguage)，也有人译为"过渡语"或"语际语",指的是第二语言学习者特有的一种目的语系统。是指在第二语言习得过程中,学习者通过一定的学习策略,在目的语输入的基础上所形成的一种既不同于其第一语言也不同于目的语,随着学习的进展向目的语逐渐过渡的动态的语言系统. 中介语理论认为，第二语言学习者在学习过程中所掌握和使用的目的语是一种特定的语言系统，这种语言系统在语音、词汇，语法、文化和交际等方面既不同于自己的第一语言，也不同于目的语，而是一种随着学习的进展向目的语的正确形式逐渐靠拢的动态的语言系统。由于这是一种介乎第一语言和目的语之间的语言系统，所以称之为“中介语”。 与lnterlanguage (中介语)相近的概念最早由Corder在论文《学习者错误的意义》中提出,他把学习者尚未达到的目的语语 言能力的外语能力称为过渡能力( transitional competence)。美国语言学Selinker于1969年在论文《语言迁移》中首先提出中介语假说(interlanguage)的概念。1971年,W. Nemsers在《外语学习者的相似系统》中提出了“approximative system”的概念。1972年Selinker在其著名论文《中介语》中提出的中介语假说, 对“中介语”这一概念进行较详细的阐述,是试图探索第二语言习得者在习得过程中的语言系统和习得规律的假说,在第二语言习得的研究史上有重大意义。从而确立了它在第二语言习得研究中的地位。Selinker指出:“中介语是一个独立的语言系统,它产生于学习者试图掌握第二语言所做的努力。”根据塞格林的定义,中介语既可是指第二语言学习者在学习过程中某一特定阶段中认知目标语的方式和结果的特征系统,即一种特定、具体的中介语言,也可以指反映所有学习者在第二语言习得整个过程中认知发生和发展的特征性系统,即一种普遍、抽象的中介语语言体系interlanguage continuum塞格林还指出中介语本身是一个阶段到过程的双重系统和庞大体系,即母语→中介语→目标语系统中的一个必然成分和过程。在这个系统中二语学习者从母语出发经过中介语到达目标语。并指出要到目标语必须经过中介语,中介语是第二语言认知中的必经之路。 二、中介语的产生 应用语言学领域中产生了对比分析方法(20世纪中期)。它通过对人们的母语以及所要学习的第二语言的语音、语法、词法、 计算机性能指标 (1)运算速度。运算速度是衡量计算机性能的一项重要指标。通常所说的计算机运算速度（平均运算速度），是指每秒钟所能执行的指令条数，一般用“百万条指令／秒”（mips，Million Instruction Per Second）来描述。同一台计算机，执行不同的运算所需时间可能不同，因而对运算速度的描述常采用不同的方法。常用的有CPU时钟频率（主频）、每秒平均执行指令数(ips)等。微型计算机一般采用主频来描述运算速度，例如，Pentium/133的主频为133 MHz，Pentium Ⅲ/800的主频为800 MHz，Pentium 4 1.5G的主频为1.5 GHz。一般说来，主频越高，运算速度就越快。 (2)字长。计算机在同一时间内处理的一组二进制数称为一个计算机的“字”，而这组二进制数的位数就是“字长”。在其他指标相同时，字长越大计算机处理数据的速度就越快。早期的微型计算机的字长一般是8位和16位。目前586（Pentium， Pentium Pro， PentiumⅡ，PentiumⅢ，Pentium 4）大多是32位，现在的大多数人都装64位的了。 (3)内存储器的容量。内存储器，也简称主存，是CPU可以直接访问的存储器，需要执行的程序与需要处理的数据就是存放在主存中的。内存储器容量的大小反映了计算机即时存储信息的能力。随着操作系统的升级，应用软件的不断丰富及其功能的不断扩展，人们对计算机内存容量的需求也不断提高。目前，运行Windows 95或Windows 98操作系统至少需要 16 M的内存容量，Windows XP则需要128 M以上的内存容量。内存容量越大，系统功能就越强大，能处理的数据量就越庞大。 （4)外存储器的容量。外存储器容量通常是指硬盘容量（包括内置硬盘和移动硬盘）。外存储器容量越大，可存储的信息就越多，可安装的应用软件就越丰富。目前，硬盘容量一般为10 G至60 G，有的甚至已达到120 G。 (5)I/O的速度 主机I／O的速度，取决于I/O总线的设计。这对于慢速设备（例如键盘、打印机）关系不大，但对于高速设备则效果十分明显。例如对于当前的硬盘，它的外部传输率已可达20MB／S、4OMB／S以上。 （6）显存 网络教育学院《电源技术》课程设计 题目：高频开关电源的干扰及抑制 学习中心： 层次： 专业： 年级： 学号： 学生：程剑 指导教师： 完成日期：年月日 目录 设计简介及要求 (1) 1 高频开关电源的干扰原理分析 (1) 1.1 高频开关电源工作原理 (1) 1.2 高频开关电源干扰的来源 (2) 1.3 高频开关电源干扰的存在形式及危害 (2) 2 高频开关电源干扰的抑制技术 (2) 2.1 滤波技术 (2) 2.2 屏蔽技术 (3) 2.3 软开关技术 (3) 2.4 扩频调制技术 (3) 2.5 PCB 设计技术 (3) 2.6 接地技术 (3) 3 高频开关电源及滤波器设计 (3) 3.1 高频开关电源设计要求 (3) 3.2 高频开关电源设计方案 (3) 3.3 电源滤波器设计 (3) 3.3.1 EMI滤波器的基本形式 (4) 3.3.2 EMI滤波器的设计原则 (4) 3 总结 (4) 设计简介及要求 现代社会中，人类生活的各个方面都离不开电子设备的发展。电子设备大多数都依赖于开关电源来提供稳定的电力供应。开关电源以其高效率、低损耗、小体积等特点，近年来快速发展，在通信设备、医疗设备以及信息处理设备等不同领域中广泛应用，取得了巨大成就。由于开关电源工作在高频开关状态，内部会产生很高的电流、电压变化率(即高dv/dt和di/dt)，导致开关电源产生较强的电磁干扰(EMI)。在有限的空间及频谱资源条件下，随着电子设备密集程度不断增加，空间的电磁环境越来越复杂。为了适应对电子产品电磁兼容性能指标的高要求，需要对电磁兼容采取重视;同时，要研究开发电磁兼容新技术，采取有效的防护措施。所以，对于开关电源来说，电磁兼容问题的研究是十分必要的。EMI信号既具有很宽的频率范围，又有一定的幅度，它不仅对电网造成污染，直接影响到其他用电设备的正常工作，而且作为辐射干扰闯人空间，对空间也造成电磁污染。目前，抑制开关电源的EMI提高开关电源的质量使之符合EMC标准已成为开关电源设计者越来越关注的问题。 本次设计就此问题展开分析，主要要求有以下几点： （1）围绕开关电源的工作原理，分析开关电源工作过程中产生电磁干扰的原因及抑制措施。 （2）介绍开关电源的基本原理、干扰来源及抑制措施。 （3）分析开关电源产生电磁辐射干扰的原因及造成的危害。 （4）论述开关电源电磁干扰的抑制方法。 1 高频开关电源的干扰原理分析 由于开关电源具有效率高、容易实现小型化的优点，所以目前被广泛应用在电子设备中。但是开关电源本身就是噪声源，在工作时会产生干扰，这就需要采取措施对其产生的噪声进行抑制。目前开关电源的体积不断追求小型化，开关频率也随之提高，导致噪声不断增加。要保证开关电源设备的正常工作，就需要对噪声的抑制加以重视。 1.1 高频开关电源工作原理 开关电源将市电直接整流滤波成为直流高压，然后通过逆变器转换成低压的高频交流电压，再经过二次整流和滤波变成所需要的直流低电压。考虑到目前大量应用的开关电源都是采取AC/DC-DC/DC级联的形式，因此，图1所示的开关电源结构具有较强的代表性。 数据中心电源解决方案及选型 发表时间：2019-11-06T11:30:58.777Z 来源：《基层建设》2019年第23期作者：苏建伟 [导读] 摘要：随着互联网应用技术的迅速发展，作为互联网载体的数据中心建设规模日益变大。 中国通信建设集团设计院有限公司第四分公司河南郑州 450052 摘要：随着互联网应用技术的迅速发展，作为互联网载体的数据中心建设规模日益变大。巨大的用电容量需求给数据中心的建设和运营带来了空前的压力。文章根据数据中心常用的供电解决方案提出了三种电源产品解决方案，并根据安全可靠、节能环保、管理维护、建设成本四个方面提出的电源产品解决方案进行对比和分析，作为数据中心建设的配套电源产品选型参考。 关键词：数据中心；电源系统；节能环保 近年来，随着移动互联网时代的到来，基于互联网技术提供的各种服务已融人到人类的社会生活中，数据储存容量需求高速发展。作为互联网应用服务载体的数据中心，其建设需求也在同步不断增加。近几年来，随着互联网服务、电子商务行业的高速发展，我国的IT服务巨头B.A.T（百度、阿里、腾讯）、三大运营商（移动、联通、电信）都投人了大量的资金用于数据中心的建设。截止目前，全国已经建成了规模不一但数量庞大的数据中心。各大数据中心运营商为了节约成本、便于管理、形成规模化效益，数据中心的建设规模逐渐变得越来越大，同时，巨大的用电容量也给数据中心的建设和运营都带来了巨大的成本压力。如何合理配置资源，提高数据中心供电的安全性，降低运营成本，成为各大数据中心运营商在机房建设中重点考虑的因数。 一、数据中心对供电的要求 主要应包括以下几方面。 1.1安全可靠 一般要求电源供电系统的可用度A!99.999M。因此，必须合理进行配置，达到系统供电安全最优化。 1.2节能环保 在能源紧缺、重视环保的今天，“绿色、节能、高效”是新一代数据中心建设的主流标准。不间断电源（UPS）在绿色、节能、高效方面的表现，主要体现在输人功率因数、输人电流谐波以及整机效率等方面。 1.3可维护性 不间断电源（UPS）供电系统应当具备维护方便，便于管理的设计需求。在节省系统的维护成本的同时，更为重要的是可以尽量避免因人为维护或者管理不当，而引发的系统故障。 1.4建设成本 从数据中心容量需求在应用过程中会逐步增大的情况，要求电源系统配置可扩容设计。这样能有效地控制系统建设初期投资费用，同时，又能使UPS供电系统在数据中心的建设过程中，始终保持安全高效的工作状态，提高了系统的性价比。因此，数据中心供电系统的合理设计，并非一味追求某一项指标的最优化，而是应该根据实际应用的需求，在保证安全可靠、节能环保、运维方便、成本合理这些要点中寻求一个平衡点，提供一个节能环保，安全可靠，经济适用的供电系统解决方案。 二、数据中心用电特点 2.1保证的目标。 在数据中心配电系统中除了正常配电系统中采用的双回路独立电源供电之外，还在数据中心配备事故备用柴油发电机、大容量UPS 等。同时在数据中心配电系统故障处理机制方面，会考虑供电电源失电、母线故障、开关跳闸和开关拒动等一系列非正常情况下如何最大限度地满足设备用电可靠性的要求。 2.2数据中心配电系统较一般建筑配电系统复杂。 与一般建筑配电系统相比，数据中心配电系统其结构更为复杂，对配电管理的要求更加严格。复杂性表现在电源输入及其控制策略上，一般建筑配电多采用双回路一主一备供电，其控制策略为备用电源自动投切。而数据中心配电系统除双回路单独供电之外，自身还配备至少满足全负荷设备容量的柴油发电机，在不同失电故障场景发生时，通过供电策略的改变实现数据中心设备的持续供电。 三、数据中心供配电系统解决方案分析 数据中心在运行的时候，往往是二十四小时不间断运行，其本身具有用电量比较大和可靠性较高以及对电源品质要求比较高的现象，其中比较常见的是电力系统在具体运行的过程中，对电能实施发、输、配、用中的配和用等内容，促使这些方面能够在电力系统正常运行的基础上具体实施。在供配电系统中，功率通常情况下是单向流动的，也就是根据电源端向用户端的方向流动，通过一定的分配手段的基础上，使得供配电的目的得以实现，将电力系统中的电能改变成用户所使用的用电设备可以利用的电能。根据当前的数据而言，其电压等级主要处于35KV或以下。因此供配电系统在设计处理的时候，需要明确其电能负荷的性质和周围区域电量供应的具体情况等。 3.1热备份串联供电的相关方案 串联备份技术和其他技术相对比而言，是比较成熟的，其发展的阶段是比较早的，使用范围相对广泛，其中多种关于UPS技术的相关资料中串联也可称为热备份，多数人都将其称为串联。供电方案中串联的UPS是比较完整的，其也具有自己的旁路在线类型的UPS单机。这些单机的连接媒介仅仅为电源线，没有其他信号连接。通常情况下主机进行全面供电，从机基本上没有对其加以负载处理。 这一方案在具体应用的时候，其优点是结构相对简单，在实施安装处理的时候，比较快捷，相对价格也合理，多个不同公司的UPS能够串联使用。这一方案的具体实施缺陷是需要不间断进行负载用电的扩容处理，就必须持续带电工作，而这一过程中的危险程度也会增加。 3.2直接并机供电方案 直接并机供电方案的形式主要是将多个同类型型号和功率的“不间断电源”在并机柜或并机板等基础上，将输出端连接在一起而形成的。这一方案的主要功能是多台机器对负载功率共同承担处理，其中比较显著的原理是在一般情况下，多个UPS都具有逆变器的输出分担负载及电流，在其中任何一个“不间断电源”出现问题的时候，其余的都会再次对全面的负载进行承担，在促使并联冗余实现的时候，其基础始终是对以下相关内容有效处理。 每个UPS逆变器所输出的波形之间的相位和频率等方面需要是相同的。UPS逆变器在输出电压的时候，这些电压也需要保持一致。每个 中介语是指第二语言学习者特有的一种目的语系统，这种语言系统在语音、词汇、语法、文化和交际等方面既不同于学习者自己的第一语言，也不同于目的语，而是一种随着学习的进展向目的语的正确形式逐渐靠拢的动态语言系统。（吕必松）自然界和人类社会中都存在着大量的中间状态，人类的语言也是如此。人们学习语言的过程中，以及语言接触融合的过程中，都有所谓的中间状态。现代应用语言学理论把语言中的这些中间状态称为中介现象。语言中存在着大量的“中介物”语音方面,词语方面,语法方面还有御用方面都存在着大量的中介语，在语言规范的过程中，也存在着中介状态。 中介语的特点 1.是一个独立的语言系统 中介语在其发展过程中的任何一个阶段都是学习者创造的一种介于第一语言和目的语之间的独特的语言系统。它有一套自身的规律，在语音、词汇、语法等系统方面都有表现。学习者有意识地使用这套规则去生成或理解他们从未接触过的话语。中介语具有人类其他语言所具有的特点和功能，可以用作交际的工具。 2，,是一个动态的语言系统，新知识和新规则不断注入；原有的尚未学好的规则和结构也在不断修正调整。随着学习者语言水平的提高和交际需要的增长，中介语不断发展，并呈一定的阶段性，由简单到复杂、由低级到高级、逐渐离开第一语言向目的语靠拢。 3.反复性。 在对目的语的学习过程中，随着水平的提高，中介语是在逐步地向目的语的规范运动的，但这并不是说，这种接近是直线前进的，而是有反复、有曲折的，这就是中介语的反复性。已经纠正了的偏误还可能有规律地重现。 .具有顽固性（“化石化”） 语言的某些具体形式上学到了一定程度就停滞不前了。比如在语音方面，有的学生学了很长时间，到了高年级，还是掌握不了某几个音。我们把这种现象也称作“化石化”。 中介语理论在对外汉语教学中的运用主要体现在以下方面: (一)研究不同母语的留学生的中介语情况及其特点 (二)对外汉语教师要善于发现留学生母语和汉语的相同和相异之处,运用迁移理论恰当引导 (三) 采用多种教学方法,运用适当的教学策略,使留学生的中介语不断接近目的语 CPU，全称“Central Processing Unit”，中文名为“中央处理器”，在大多数网友的印象中，CPU只是一个方形配件，正面是金属盖，背面是一些密密麻麻的针脚或触点，可以说毫无美感可言。但在这个小块头的东西上，却是汇聚了无数的人类智慧在里面，我们今天能上网、工作、玩游戏等全都离不开这个小小的东西，它可谓是小块头有大智慧。 作为普通用户、网友，我们并不需要解读CPU里的所有“大智慧”，但CPU既然是电脑中最重要的配件、并且直接决定电脑的性能，了解它里面的部分知识还是有必要的。下面笔者将给大家介绍CPU里最重要的基础知识，让大家对CPU有新的认识。 1、CPU的最重要基础：CPU架构 CPU架构： 采用Nehalem架构的Core i7/i5处理器 CPU架构，目前没有一个权威和准确的定义，简单来说就是CPU核心的设计方案。目前CPU大致可以分为X86、IA64、RISC等多种架构，而个人电脑上的CPU架构，其实都是基于X86架构设计的，称为X86下的微架构，常常被简称为CPU架构。 更新CPU架构能有效地提高CPU的执行效率，但也需要投入巨大的研发成本，因此CPU 厂商一般每2-3年才更新一次架构。近几年比较著名的X86微架构有Intel的Netburst （Pentium 4/Pentium D系列）、Core（Core 2系列）、Nehalem（Core i7/i5/i3系列），以及AMD的K8（Athlon 64系列）、K10（Phenom系列）、K10.5（Athlon II/Phenom II 系列）。 Intel以Tick-Tock钟摆模式更新CPU 自2006年发布Core 2系列后，Intel便以“Tick-Tock”钟摆模式更新CPU，简单来说就是第一年改进CPU工艺，第二年更新CPU微架构，这样交替进行。目前Intel正进行“Tick”阶段，即改进CPU的制造工艺，如最新的Westmere架构其实就是Nehalem架构的工艺改进版，下一代Sandy Bridge架构将是全新架构。AMD方面则没有一个固定的更新架构周期，从K7到K8再到K10，大概是3-4年更新一次。 制造工艺： CPU性能指标排名 高端CPU - 更新至2018年1月19日 Processor CPU Mark Price (USD) Intel Core i9-7980XE @ 2.60GHz278191982.15 Intel Xeon Gold 6154 @ 3.00GHz27789$3,543.00* Intel Xeon W-2195 @ 2.30GHz26470$2,553.00* Intel Core i9-7960X @ 2.80GHz264471634.99 Intel Xeon E5-2679 v4 @ 2.50GHz25236$2,702.00* Intel Core i9-7940X @ 3.10GHz251001386.6 Intel Core i9-7920X @ 2.90GHz234791110.99 Intel Xeon E5-2699 v4 @ 2.20GHz233623010 Intel Core i9-7900X @ 3.30GHz22570959.99 Intel Xeon E5-2696 v3 @ 2.30GHz22542$2,498.00* Intel Xeon E5-2699 v3 @ 2.30GHz22358$3,805.02* AMD Ryzen Threadripper 1950X22031899 Intel Xeon E5-2698 v4 @ 2.20GHz21789$3,226.00* Intel Xeon E5-2697 v4 @ 2.30GHz217292805 Intel Xeon E5-2673 v4 @ 2.30GHz21625NA Intel Xeon W-2150B @ 3.00GHz21581NA Intel Xeon E5-2696 v4 @ 2.20GHz215193005.95 Intel Xeon E5-2697 v3 @ 2.60GHz21502$2,661.90* Intel Xeon E5-2690 v4 @ 2.60GHz21323$2,133.99* Intel Xeon Gold 6136 @ 3.00GHz21313NA Intel Xeon E5-2698 v3 @ 2.30GHz211493424.02 Intel Xeon E5-2695 v3 @ 2.30GHz20297$2,499.99* Intel Xeon E5-2695 v4 @ 2.10GHz20258$2,682.50* Intel Core i7-6950X @ 3.00GHz199931574.96 Intel Xeon E5-2687W v4 @ 3.00GHz19979$2,211.95* Intel Xeon E5-2680 v4 @ 2.40GHz19953$1,762.90* Intel Xeon Gold 6144 @ 3.50GHz19833$2,925.00* Intel Xeon E5-2689 v4 @ 3.10GHz19521NA Intel Xeon E5-2686 v3 @ 2.00GHz19255NA Intel Xeon E5-2690 v3 @ 2.60GHz192402359.99 AMD EPYC 7551190033699.99 Intel Xeon W-2145 @ 3.70GHz18968$1,113.00* Intel Xeon E5-2680 v3 @ 2.50GHz187151909.95 Intel Core i7-7820X @ 3.60GHz18656569 Intel Xeon Gold 6130 @ 2.10GHz18587$1,969.99* Intel Core i7-7900X @ 3.30GHz18539NA AMD Ryzen Threadripper 1920X18475674.99 AMD EPYC 7401P18465NA Intel Xeon E5-1681 v3 @ 2.90GHz18238NA Intel Xeon Gold 6134 @ 3.20GHz18083$2,348.26* Intel Xeon E5-4660 v3 @ 2.10GHz18007$4,800.00* AMD EPYC 7351P17991799.99 Intel Xeon E5-2687W v3 @ 3.10GHz178432509.95 Intel Xeon E5-2676 v3 @ 2.40GHz17795NA Intel Xeon E5-2660 v4 @ 2.00GHz17786$1,498.77* Intel Core i7-6900K @ 3.20GHz176971020.89 Intel Xeon E5-2683 v3 @ 2.00GHz17407$1,984.99* Intel Xeon E5-2697 v2 @ 2.70GHz174052235.02 Intel Xeon E5-2673 v3 @ 2.40GHz16982$700.00* Intel Xeon E5-1680 v4 @ 3.40GHz16979NA Intel Xeon E5-2678 v3 @ 2.50GHz16905NA Intel Xeon E5-1680 v2 @ 3.00GHz16769NA Intel Xeon E5-2696 v2 @ 2.50GHz16681NA Intel Xeon E5-1680 v3 @ 3.20GHz16673NA数据中心电源解决方案及选型
CPU的技术参数的意思
CPU的主要性能参数
高频开关电源的设计与实现
机房数据中心供配电系统解决方案
中介语演讲稿
高频开关电源技术规范书
中介语
cpu的简介及主要性能指标
高频开关电源技术规范书
中介语理论研究
计算机性能指标
高频开关电源的干扰及抑制
数据中心电源解决方案及选型
中介语是指第二语言学习者特有的一种目的语系统
CPU主要参数
CPU性能指标排名(截至2018年1月)
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