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Mass loss on the Asymptotic Giant Branch

Mass loss on the Asymptotic Giant Branch
Mass loss on the Asymptotic Giant Branch

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Proceedings Planetary Nebulae in Our Galaxy and Beyond Proceedings IAU Symposium No.234,2006M.J.Barlow &R.H.Mendez,eds.c 2006International Astronomical Union DOI:00.0000/X000000000000000X Mass loss on the Asymptotic Giant Branch Albert A.Zijlstra Jodrell Bank Centre for Astrophysics,School of Physics &Astronomy,University of Manchester,P.O.Box 88,Manchester M601QD email:a.zijlstra@https://www.doczj.com/doc/cb16144085.html, Abstract.Mass loss on the Asymptotic Giant Branch provides the origin of planetary nebulae.This paper reviews several relevant aspects of AGB evolution:pulsation properties,mass loss formalisms and time variable mass loss,evidence for asymmetries on the AGB,binarity,ISM interaction,and mass loss at low metallicity.There is growing evidence that mass loss on the AGB is already asymmetric,but with spherically symmetric velocity ?elds.The origin of the rings may be in pulsational instabilities causing mass-loss variations on time scales of centuries.

2 A.A.Zijlstra

The pulsation equation is given by:

log P=1.949log R?0.9log M?2.07fundamental mode Wood1990)

log P=1.5log R?0.5log M+log Q?rst overtone(Fox&Wood1982),

(2.1) where in the latter case the pulsation constant Q≈0.04;the period P is in days and the radius R and mass M are in solar units.Mira variables are fundamental-mode pulsators, whilst SR variables show overtone pulsations.Fundamental-mode models including tur-bulent viscosity(Olivier&Wood2005)reproduce Mira light curves fairly well.

A well-de?ned Mira period–luminosity relation was found from stars in the LMC.The Macho,Ogle and Sirius surveys have shown that this relation is only one of several parallel sequences.Magellanic Cloud stars show?ve sequences(e.g.Kiss&Bedding2003,Ita et al.2004),two of which are identi?ed as overtones,one is the Mira fundamental mode,one is an RG

B sequence tracing semi-detached binaries,and the last sequence shows periods much longer than the fundamental mode which are still unexplained(Wood et al.2004). Long-term monitoring of Mira variables,mostly by amateur astronomers,has given light curves covering,in some cases,several centuries.These extended light curves have shown that most Miras are stable,but a number of stars show evolving periods.The best known case is that of R Hya,for which historical records(Zijlstra et al.2020indicate a continuous decline from about495days before1800,to385days in1950.The period has remained stable since1950,and the earlier longer period also may have been stable. Templeton et al.(2005)has analyzed the full set of AAVSO records available,and out of 547Mira variables?nd8cases of long-term period changes,at6sigma signi?cance.The most dramatic case is T UMi,which showed a constant period for over60years,before a sudden,sharp decrease in period by25per cent over the next20years.

Zijlstra&Bedding(2002)de?ne three types of period instability:sudden changes as in T UMi,continuous changes as in R Hya,and meandering periods.The latter appears to be common among the longest period Mira variable,and involve variations up to10 per cent on time scales of~50years.

Several explanations exist for the period changes.Thermal pulses involve large,sudden variations in luminosity(and therefore radius and period).This?ts stars such as T UMi well,where there was no pre-indicator of a developing instability.The rate of thermal pulses is of the order of0.5per cent per century,or about~2per century among the known Miras.The second possiblity is a non-linear pulsation(Olivier&Wood2005), where the change in amplitude causes a change in period(Ya’ari&Tuchman1996). Following a large amplitude change,e.g.the onset of pulsation,they?nd a large period decrease of the fundamental mode over a time scale of centuries.The observed correlation between period and amplitude changes(e.g Zijlstra&Bedding2002)provides some support for this model.Icket et al.(1992)have pointed out that the surfaces of long-period stars are susceptible to weak chaos.Zijlstra et al.(2004)discuss strongly variable periods among SC stars,where the C/O ratio is very close to unity and the chemical equilibrium is highly sensitive to temperature changes:they suggest that in SC stars, rapid changes in molecular abundances may trigger weakly chaotic period changes.

3.Mass loss

3.1.Proposed relations

AGB mass loss is a multi-step process.The pulsations extend the atmosphere,and allow molecules to form.At large distances the temperature drops below the condensation tem-

AGB stars3 peratures and a number of solids form:MgSiO-bearing grains in O-rich environments,and SiC and amorphous carbonaceous particles around carbon-rich stars.Radiation pressure pushes the grains out,and by collisions carries the gas with it.Pulsation can support mass-loss rates~10?7M⊙yr?1,but the radiation pressure on dust(and molecules) greatly increases the mass loss.The di?erence corresponds to the case A and case B mass-loss regimes of Winters et al.(2000).

Dynamical models of the extended envelopes do not yet convincingly predict mass-loss rates.Instead,formulistic mass-loss rates are used.The following relations are in general use:

(i)log˙M=a P+b Vassiliadis&Wood1993

(ii)˙M=a M?2.1

i L3.1R M?1Bloecker1995

(ii)˙M=a L2.47T?6.8M?1.95Wachter et al.2002

(iv)˙M=a L1.05T?6.3van Loon et al.2005.

(3.1)

Relation(ii)is derived by?tting the Bowen pulsation models to observed initial–?nal mass relations;(iii)is derived for carbon-rich stars only,using dynamical models,whilst (iv)comes from mass-loss determinations in the LMC,including both AGB and RSG stars,and is observationally the best established.

A very strong dependence on stellar temperature is indicated.However,the optical spectral type used to assign a temperature measures theτ=1layer in the molecular envelope,far above the real stellar surface:it underestimates both the surface and the ef-fective temperature.The strong dependence is also https://www.doczj.com/doc/cb16144085.html,ing L∝R2T4,relations (iii)and(iv)give˙M∝T3.6and T?2.3,respectively.The?rst one is physically unlikely, and Wachter et al.(2002)may overestimate the e?ect of luminosity.The Bloecker rela-tion su?ers from the same problem.Relation(iv)excludes mass as a relevant parameter, because this parameter is not known for the LMC stars.However,we may adopt the suggestion that the mass loss depends on the surface binding energy,M/R.In this case, relations(iii)and(iv)suggest that the total dependence is along the lines of

˙M~ M

4 A.A.Zijlstra

sharply enhanced mass loss event,occuring every104to105yr,depending on the core mass.Mass loss variations on a time scale of102–103yr are implied by the multiple rings seen around many PNe.Finally,extinction variations in both carbon-rich and oxygen-rich stars indicate changes within decades.

Mass loss spikes during the TP are the accepted explanation for the detached shells seen around TT Cyg,U Cam and DR Ser,among others(e.g.Olofsson et al.2000);the thin shells can be explained by a brief burst of100-fold stronger mass loss,sweeping up the previous,weaker and slower wind(Sch¨o ier et al.2005).The TP is followed by a subsequent phase of quiescent helium burning when L is a factor of3lower than during the hydrogen burning phase,with a weaker wind.

The multiple,thin rings seen around PNe(Corradi et al.2004)still lack a commonly accepted explanation.First,an instability involving the dust driving was suggested by Simis et al.(2001):dust formation leads to increased dust density,until dust-gas cou-pling empties the dust shell.This leads to mass-loss episodes at the correct time scales. However,this instability is not correlated between di?erent directions,and does not lead to spherical structures.Cooling due to existing dust strongly enhances the local dust for-mation rate,leading to near-chaotic mass loss Woitke(2005).Second,a nuclear-burning instability was suggested by Van Hron et al.(2003).Third,the period instability found for long-period Miras,interpreted as due to a non-linear pulsation,could cause the rings (Zijlstra&Bedding2002,via period-induced mass-loss variations.The hypotheses need to explain the slight irregularity in the ring spacings,and the decrease in spacings for rings closer to the star(i.e.ejected later).The model involving the non-linearity in the pulsations can explain both.However,the issue is still far from settled.

The decadal(extinction)variations are most easily explained by the dust clumping as found in the Woitke models.Extinction variations are mainly seen in carbon-rich stars(e.g.Whitelock et al.2006)but a few oxygen-rich stars also show variations due to circumstellar extinction,most noticeably L2Pup(Bedding et al.2002).

4.Morphology

Strong asymmetries are present in the morphologies of almost all post-AGB envelopes. These structures must have their origin during the AGB mass loss.But remarkably little evidence for the precursors of the post-AGB morphologies has been found.In fact,the current consensus is that AGB envelopes are spherically symmetric,and that asymmetries form during a run-away process at the very tip of the AGB.The argument that such a fundamental change happens during a phase which is unobservable only by its brevity, is far from convincing.

Two observations provide evidence for spherical AGB shells.One comes from the OH maser pro?les of OH/IR stars,with strong peaks at the extreme velocities.For asym-metric envelopes,the emission at intermediate velocities becomes stronger and multiple peaks may develop(Chapman1988).The second observation is that the outer structures around PNe and post-AGB stars are often round,especially so for the rings.The regular detached shells around,e.g.,TT Cyg should also be mentioned.

However,both can be questioned.Maser emission cannot be used to determine a density structure.Instead it is an exponential function of the column density at constant velocity,in competition between all directions.OH pro?les are far more dependent on the velocity structure than on the densities.An asymmetric envelope with a spherically symmetric velocity structure will give rise to symmetric pro?les.

The rings appear to provide stronger evidence.However,any mass-loss changes leading to such rings are likely to be accompanied by changes in the expansion velocity.The

AGB stars5

Figure1.NTT V-band image of RZ Sgr,showing its bipolar re?ection nebula

rings then become the swept-up interface between the more massive,faster wind and the slower,weaker wind:Zijlstra et al.(2001)show that the swept-up shell will move at constant velocity,with the location at anyone time given by:

r ring=v ring t=V slow 1

μ/ξ ,(4.1)

where

μ=˙m fast

V slow

.

If the two winds are self similar,and speci?cally if they show spherically symmetric velocities and show the same ratio of mass-loss rates in all directions,v ring will be direction independent.The swept-up interface will be spherical,even if the underlying mass loss is not.

Thus,in both cases there is strong evidence for spherically symmetric velocity?elds, but the mass loss itself can be asymmetric.

There is in fact growing evidence for bipolar shells on the AGB.The bipolar enve-lope of X Her was mapped by Nakashima(2005).The re?ection nebula around RZ Sgr Whitelock(1994)is bipolar in our NTT image(Fig.1).Linear polarization is present in integrated photometric data of some OH/IR stars(Jones&Gehrz1990).Finally,the core of IRC+10216shows several components,possibly in a bipolar con?guration(Osterbart et al.2000).(The last two points can also be expained by Woitke-type blobs(Woitke 2006),however.)

5.Binarity

Whereas binarity is now thought to be very common among post-AGB stars and PNe, and the likely cause of the asymmetries of the envelopes,very little is known about binarity among AGB stars.The extreme brightness and low temperature of the AGB star make a binary comnpanion di?cult to detect,except where the companion interacts with the extended atmosphere or the wind.Direct evidence for binarity among AGB stars will be needed to make evolutionary connections between AGB and post-AGB stars.

A distinction can be made between four di?erent types of interaction.

(a)Common envelope systems,where the companion triggers the mass loss.These system will rarely reach the AGB,as the common envelope is likely to develop already on the RGB.Orbital separations are of the order of1AU.Perhaps10per cent of’proto planetary nebulae’may derive from such systems,although fewer would evolve into PNe.

6 A.A.Zijlstra

(b)Wider systems,where the geometry of the mass loss is a?ected,but not the mass-loss rate itself.These systems may include symbiotic stars and the RV Tau progenitors. Orbital separations are a few to tens of AU,and the systems exhibit circumbinary disks and accretion.These may account for~25per cent of the PN birth rate.The companion may ionize part of the stellar wind,as in OH231.8+4.2.

(c)Systems with only minor e?ects on the mass loss geometry.Typical separations are~100AU.The geometry may show the movement of the mass-losing star,as in the spiral nebula around AFGL3068(Mauron&Huggins2006,see also Morris et al,these proceedings).Mira itself has a white dwarf companion~100AU away.

6.ISM interaction

The study of the interaction between the AGB wind and its local ISM was pioneered by Villaver et al.(2002).Previously,the interaction had only been considered for old, ionized PNe.The interaction begins in fact with the onset of the AGB wind,and leads to a swept-up shell surrounding the AGB star.Once the swept-up mass exceeds the mass lost in the wind,the shell slows down and eventually becomes near-stationary.The typical radius of this shell is1–2pc,depending on local conditions.An example of such a shell,called a’wall’,was found by Zijlstra&Weinberger(2002).The wall may wipe out any memory of long-term mass-loss?uctuations.Once the PN fast wind reaches the outer wall,the PN rebrightens;this increases the PN life time.

If there is a signi?cant proper motion of the star with respect to the ISM,the wall becomes one-sided and at higher velocities,becomes a bow shock.This is modelled by Wareing et al.(2006),based on the IPHAS morphology of the PN Sh2-188.The Spitzer image of R Hya(Speck,these proceedings)show another example of such a bow shock.) Evolutionary calculations for a large range of parameters are presented by Wareing(these proceedings).

7.Metallicity

The e?ect of metallicity on AGB mass loss is only just beginning to be explored. Bowen&Willson(1991)?rst showed that for[Fe/H]

Mass-loss rates in the LMC are as high as those in the Galaxy Van Loon et al.(1999). However,it is possible that these values are reached at higher luminosities than for Galac-tic stars.(The metallicity of the LMC is comparable to the outer Galactic disk.)Evidence for reduced mass-loss rates at much lower metallicity comes mainly from the speci?c fre-quency of PNe,which is signi?cantly lower in metal-poor dwarf galaxies compared to more metal-rich systems.This is illustrated in?g.2.

Expansion velocities will be much lower at lower metallicity,limited by the dust-to-gas ratio.Evidence is sparse,as expansion velocities outside of our Galaxy have only been measured for some LMC OH/IR stars.These indeed indicate a signi?cantly lower expansion velocity in the LMC(Marshall et al.2004).

Dredge-up of carbon will lead to higher C/O ratios at lower metallicity,because of the lower oxygen abundance.This leads to the counter-intuitive e?ect that carbon molecules are more abundant at lower metallicity.The C2H2infrared bands were indeed found to be

AGB stars

7

-2-1.5-1-0.500.5

-1

-0.50

0.5[Fe/H]3210

-1-0.5

0.5

C/M3+Figure 2.Left:The ratio between number of planetary nebulae and luminosity of the parent stellar population (y -axis),as function of metallicity (x ),for nearby galaxies,from Magrini et al.(2003).Encircled points indicate spiral galaxies a?ected by internal extinction.Right:The same ratio,as function of the ratio of carbon over M-type stars on the AGB,a tracer of the metallicity of the AGB population

very strong in carbon stars in the Magellanic Clouds (e.g.Matsuura et al.2005).This has consequences for dust formation:amorphous dust may form easily around low metallicity carbon stars,while minerals such SiC and MgS do not,ginvig a mineral-poor or ’soft’dust.Sloan et al.(2006)and Zijlstra et al.(2006),using Spitzer spectra,indeed ?nd that the SiC and MgS features are weaker in the Magellanic Clouds.Oxygen-rich stars can only form dust from metal-dependent minerals,and their dust formation e?ciency is expected to be strongly suppressed at low metallicity.Mass loss and dust formation at low metallicity is therefore expected to be dominated by carbon stars.

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