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First optical light from the supernova remnant G 17.4-2.3

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a r X i v :a s t r o -p h /0201482v 1 29 J a n 2002Astronomy &Astrophysics manuscript no.(will be inserted by hand later)First optical light from the supernova remnant G 17.4−2.3P.Boumis 1,F.Mavromatakis 1,and E.V.Paleologou 21University of Crete,Physics Department,P.O.Box 2208,71003Heraklion,Crete,Greece2Foundation for Research and Technology-Hellas,P.O.Box 1527,71110Heraklion,Crete,GreeceReceived 16November 2001/Accepted 29January 2002Abstract.Deep optical CCD images of the supernova remnant G 17.4–2.3were obtained and faint emission has been discovered.The images,taken in the emission lines of H α+[N ii ],[S ii ]and [O iii ],reveal filamentary structures in the east,south–east area,while diffuse emission in the south and central regions of the remnant is also present.The radio emission in the same area is found to be well correlated with the brightest optical filament.The flux calibrated images suggest that the optical filamentary emission originates from shock-heated gas ([S ii ]/H α>0.4),while the diffuse emission seems to originate from an H ii region ([S ii ]/H α<0.3).Furthermore,deep long–slit spectra were taken at the bright [O iii ]filament and clearly show that the emission originates from shock heated gas.The [O iii ]flux suggests shock velocities into the interstellar “clouds”greater than 100km s −1,while the [S ii ]λ6716/6731ratio indicates electron densities ∼240cm −3.Finally,the H αemission has been measured to be between 7to 20×10−17erg s −1cm −2arcsec −2.Key words.ISM:general –ISM:supernova remnants –ISM:individual objects:G 17.4-2.31.IntroductionThe Galactic supernova remnants (SNRs)have been iden-tified by both radio (non-thermal synchrotron emission)and optical (optical emission lines)surveys.New searches in both wavebands continue to identify galactic SNRs (Fesen &Hurford 1995,Fesen et al.1997;Green 2001and references therein;Mavromatakis et al.2000,2001,2002)but since the last few years,observations in X-rays have also detected new SNRs (e.g.Seward et al.1995).The ratio of [S ii ]/H αhas become the standard discriminator used in optical SNR observations because the photoionized nebulae (like H ii regions and planetary nebulae)usually exhibit ratios of about 0.1-0.3,while collisionally ionized nebulae (like known Galactic SNRs)show ratios typically greater than 0.4(Smith et al.1993).Fesen et al.(1985)suggested that a division at [S ii ]/H α<0.5does not pro-vide clear evidence to distinguish SNRs from photoionized regions and additional observations of the strong forbid-den oxygen lines ([O i ],[O ii ]and [O iii ])are needed to give a complete diagnostic.Furthermore,theoretical shock models ,generally predicted [S ii ]/H αratios of 0.5to 1.0for SNRs (Raymond 1979,Shull &McKee 1979).G 17.4−2.3is not a well known SNR,and was first de-tected by Reich et al.(1988)in their Effelsberg 2.7–GHz survey,while its radio image was published by Reich et al.(1990).It is classified as a circular supernova remnant with2P.Boumis et al.:First optical light from the supernova remnant G17.4−2.3mations concerning the observations and data reduction, while the results of our imaging and spectral observations are given in Sect.3and4,respectively.In the last section (Sect.5)we discuss the physical properties of G17.4–2.3.2.Observations2.1.ImageryThe observations presented here were performed with the 0.3m Schmidt-Cassegrain(f/3.2)telescope at Skinakas Observatory in Crete,Greece in August20and21,2001. The1024×1024(with19×19µm2pixels)Tektronix CCD camera was used resulting in a scale of4′′.1pixel−1and a field of view of70′×70′.Two exposures in[O iii]and[S ii]of2400s each were taken during the observations,while one exposure of1800 s and one of2400s were obtained with the Hα+[N ii]filter. Note that thefinal images in eachfilter are the average of the individual frames.The image reduction(bias subtraction,flat-field cor-rection)was carried out using the standard IRAF and MIDAS and their negative gray–scale representation using the STARLINK Kappa and Figaro packages.The astrome-try information was calculated for each image individually using stars from the Hubble Space Telescope(HST)Guide Star Catalogue(Lasker et al.1999).The spectrophotomet-ric standard stars HR5501,HR7596,HR7950,and HR8634 (Hamuy et al.1992;1994)were used for absoluteflux cali-bration.All coordinates quoted in this paper refer to epoch 2000.2.2.SpectroscopyLow dispersion long–slit spectra were obtained with the 1.3m telescope at Skinakas Observatory in2001August 21.The1300line mm−1grating was used in conjunction with a2000×800SITe CCD(15×15µm2pixels)which resulted in a scale of1˚A pixel−1and covers the range of4750˚A–6815˚A.The slit width is7.′′7and it was oriented in the south–north direction,while the slit length is7.′9.The coordinates of the slit centre areα=18h31m30s andδ=−14◦51′33′′and two spectra of3600s each were obtained.The spectrophotometric standard stars HR5501, HR7596,HR9087,HR718,and HR7950were observed in order to calibrate the spectra of G17.4–2.3.3.Imaging of G17.4–2.33.1.The Hα+[N ii]and[S ii]emission line images The major characteristic revealed from our Hα+[N ii] and[S ii]images(Fig.1and2,respectively)seems to be the low surface brightness of G17.4–2.3.The weak diffuse emission is present in the south,south–east and central areas of the remnant,while no emission is detected in the north–west part.The most interesting region lies in the east part of the remnant,where afilamentary structure ex-ists,which is very well correlated with the radio emission.Table 1.Typically measuredfluxes in the area of the brightestfilamentHα+[N ii]116.0111.2108.099.7[O iii]13.0 4.722.1–P.Boumis et al.:First optical light from the supernova remnant G17.4−2.33Fig.2.The[S ii]image of the area around G17.4–2.3, which has been smoothed to suppress the residuals from the imperfect continuum subtraction.Shadings run lin-early from0to20×10−17erg s−1cm−2arcsec−2.3.2.The[O iii]5007˚A imageIn contrast to the previous results,the image of the medium ionization[O iii]line showsclearlyfilamentary nature of the observed emission(Fig.3).This brightfil-ament extends for∼24′in the east,south–east,while no significant emission was found in other areas of the rem-nant.We do not detect[O iii]emission where the diffuse Hα+[N ii]and[S ii]emission is detected.Table1lists also typical[O iii]fluxes measured in different parts of thefilament.The latter,matches very well with the radio maps of G17.4–2.3at1400MHz and4850MHz,suggest-ing their association(Fig.4).The correlation shows that thefilament is located close to the outer edge of the radio contours but the low resolution of the radio images does not help us to determine the actual position of thefila-ment with respect to the shock front.The geometry of the [O iii]filament allows us to define a∼26′circle in diameter (dashed circle in Fig.3)for the remnant,with its center at α≃18h30m23.3s,δ≃−14◦45′25′′.Note that the optical angular size is in very good agreement with the value of ∼24′given in Green’s catalogue(Green2001).However, a larger angular diameter cannot be excluded since X–ray emission has not been detected so far and the radio shell is incomplete.4.The long–slit spectra from G17.4–2.3The low resolution spectra were taken on the relatively bright opticalfilament in the east part of the remnant (its exact position is given in Sect.2.2).In Table2,we present the relative linefluxes taken from two different Fig.3.G17.4–2.3imaged with the medium ionization line of[O iii]5007˚A.The image has been smoothed to suppress the residuals from the imperfect continuum sub-traction and the shadings run linearly from0to30×10−17 erg s−1cm−2arcsec−2.The projection of a∼26′diameter sphere that matches the location and orientation of the filament is also shown.apertures(Ia and Ib,Fig.5)along the slit.In particu-lar,apertures Ia and Ib have an offset of9′′.5and118′′.5 north of the slit centre,respectively.The apertures Ia and Ib were selected because they are free offield stars in an otherwise crowdedfield.In addition,these apertures in-clude sufficient line emission,especially in the blue part of the spectrum,to allow an accurate determination of the observed lines.The background extraction aperture was taken towards the north end of the slit.We utilized theflux calibrated images to identify the nature of this background emission used in the spectra.The[S ii]/Hαratio of∼0.2measured in the images suggests an H ii ori-gin for the background aperture emission.Assuming that its strength does not change appreciably at the Ia and Ib positions we performed the background subtraction to identify the SNR emission.The signal to noise ratios pre-sented in Table2do not include calibration errors,which are less than10%.Both extracted apertures show clearly that the observed optical emission originates shock heated gas,since the[S ii]/Hα>0.7.The[S ii]λ6716/6731ra-tio of1.2–1.4indicates low electron densities(Osterbrock 1989).The very strong[O iii]emission detected in aperture Ia suggests a shock velocity greater than100km s−1(Cox &Raymond1985),while the sulfur lines ratio indicate an electron density∼240cm−3(Osterbrock1989).However, taking into account the statistical errors on the sulfur lines,electron densities up to400cm−3are compatible with our measurements.The shock velocity implies by the4P.Boumis et al.:First optical light from the supernova remnant G17.4−2.3Fig.4.The correlation between the[O iii]emission and the radio emission at1400MHz(dash line)and4850MHz (solidline)is shown in thisfigure.The1400MHz(Condon et al.1998)and the4850MHz(Condon et al.1994)ra-dio contours scale linearly from1.1×10−3Jy/beam to0.1 Jy/beam and3.5×10−2Jy/beam to0.3Jy/beam,respec-tively.LDN379is also a strong radio source although not detected in[O iii].Note that in the north–east area of the filament wefind the well–defined radio shell of the SNR G17.8−2.6,which is also not detected in the optical.Ib spectrum could be less but still around100km s−1, while the electron density is even lower than in Ia(less than120cm−3).5.DiscussionThe supernova remnant G17.4–2.3shows up as an incom-plete shell in the radio bound without any X–ray emission detected so far.The low ionization images generally show diffuse emission in the south and south–east areas of the remnant.In contrast,afilamentary structure has been dis-covered in the medium ionization[O iii]line in the east, south–east region which is very well correlated with the radio emission at1400and4850MHz and could define the remnant’s outer edge.This correlation indicates that the observed emission is associated to G17.4–2.3.Both the calibrated images and the long–slit spectra suggest that the detected emission results from shock heated gas since the[S ii]/Hαratio exceeds the empirical SNR criterion value of0.4–0.5.Note that the H ii region found in the low ionization images shows a[S ii]/Hαratio of∼0.2.The easternfilament lies very close to this H ii region.The morphological differences between the low and medium ionization lines provide evidence for significant inhomo-geneities and density variations in the ambient medium.Fig.5.The spectra of apertures Ia(top)and Ib(bottom). Hester et al.(1987)suggested that the presence of such inhomogeneities and density variations would mainly af-fect the recombination zone where the low ionization lines are produced and it could also explain the[O iii]/Hαratio variations seen in the long–slit spectra.An interstellar extinction c at positions Ia and Ib(see Table2),of0.62(±0.33)and1.10(±0.17)or an A V of 1.27(±0.67)and2.26(±0.35)were measured,respec-tively.We have also determined the electron density mea-suring the density sensitive line ratio of[S ii]λ6716/6731. The densities we measure are below400cm−3.Assuming that the temperature is close to104K,it is possible to estimate basic SNR parameters.The remnant under in-vestigation is one of the least studied remnants and thus, the current stage of its evolution is unknown.The possibil-ities that the remnant is still in the adiabatic phase or in the radiative phase of its evolution cannot be excluded and will be examined in the following.The preshock cloud den-sity n c can be measured by using the relationship(Dopita 1979)n[SII]≃45n c V2s cm−3,(1)P.Boumis et al.:First optical light from the supernova remnant G17.4−2.35Table2.Relative linefluxes.Line(˚A)F a,b F a,b4959[OIII]70(9)4(2)6548[N ii]39(13)21(16)6584[N ii]142(44)75(57)6731[S ii]38(13)30(25)Absolute Hαflux d7.919[S ii]/Hα0.82(17)0.71(36)a Uncorrected for interstellar extinctionb Listedfluxes are a signal to noise weighted average of the individualfluxesc Numbers in parentheses represent the signal to noise ratio of the quotedfluxesd In units of10−17erg s−1cm−2arcsec−2Allfluxes normalized to F(Hα)=100where n[SII]is the electron density derived from the sulfur line ratio and V s is the shock velocity into the clouds in units of100km s−1.Furthermore,the blast wave energy can be expressed in terms of the cloud parameters by using the equation given by McKee&Cowie(1975)E51=2×10−5β−1n c V2s r s3erg.(2) The factorβis approximately equal to1at the blast wave shock,E51is the explosion energy in units of1051 erg and r s the radius of the remnant in pc.By using the upper limit on the electron density of400cm−3,which was derived from our spectra,we obtain from Eq.(1)that n c V2s<8.9.Then Eq.(2)becomes E51<0.08D31kpc, where D1kpc the distance to the remnant in units of1 kpc.An estimated value of N H∼5×1021cm−2is given by Dickey&Lockman(1990)for the column density in the direction of G17.4–2.3.Considering that(Ryter et al. 1975)N H=6.8(±1.6)×1021E B−V cm−2,(3) where A V=3.1×E B−V(Kaler1976),we obtain an N H of3×1021cm−2and5×1021cm−2for the two c values calculated from our spectra,respectively.A column den-sity of2+1−2×1021cm−2was measured towards an X–ray binary system,located1.2◦east of G17.4–2.3at a distance of∼3.1kpc(Rib´o et al.1999).Since,there are no other measurements of the interstellar density n0,values of0.1 and1.0will be examined.Following the result of Eq.(2) and assuming the typical value of1for the supernova ex-plosion energy(E51),wefind that the remnant may lie at distances greater than2.3kpc.Then,the lower interstellar density of∼0.1cm−3suggests that the column density is greater than7×1020cm−2,while for n0≈1cm−3it be-comes greater than7×1021cm−bining the previous results and assuming that the column density is found in the range of2−5×1021cm−2,then the lower interstellar density seems to be more probable.Using the results of Cioffiet al.(1988)and the above range of parameters for n0of0.1and1cm−3,the pressure driven snowplow(PDS)radii are calculated as38and14 pc,respectively.The radii,when compared with the esti-mated SNR radius of17D2.3kpc pc may suggest that the PDS phase has not begun yet.The derived velocities of the main shock front at the beginning of this stage are ∼300–400km s−1.Their comparison with the estimated shock velocities into the interstellar clouds result in a den-sity contrast of∼10between the ambient medium and the interstellar“clouds”.However,since neither the distance nor the interstellar medium density are accurately known, we cannot confidently determine the current stage of evo-lution of G17.4–2.3.6.ConclusionsThe faint supernova remnant G17.4–2.3was observed for thefirst time in major optical emission lines.The im-ages showfilamentary and diffuse emission structures.The bright[O iii]filament is very well correlated with the rem-nant’s radio emission at1400and4850MHz suggesting their association.Theflux calibrated images and the long–slit spectra indicate that the emission arises from shock heated gas.The observed opticalfilamentary structure provides some evidence for significant inhomogeneities in the ambient medium,implying that the main blast wave propagates into an inhomogeneous medium. Acknowledgements.The authors would like to thank the referee for his com-ments and suggestions which helped to clarify,and en-hance,the scope of this paper.Skinakas Observatory is a collaborative project of the University of Crete,the Foundation for Research and Technology-Hellas and the Max-Planck-Institut f¨u r Extraterrestrische Physik.This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service,provided by the NASA/Goddard Space Flight Center.ReferencesCase G.L.,and Bhattacharya D.1998,ApJ504,761CioffiD.F.,McKee C.F.and Bertschinger E.,1988,ApJ,334, 252Condon J.J.,Broderick J.J.,Seielstad G.A.,Douglas K.and Gregory P.C.,1994,AJ,107,1829Condon J.J.,Cotton W.D.,Greisen E.W.,Yin Q.F.,Perley R.A.,Taylor G.B.and Broderick J.J.,1998,AJ,115, 1693Cox D.P.and Raymond J.C.,1985,ApJ,298,651Dickey J.M.and Lockman F.J.,1990,ARAA,28,2156P.Boumis et al.:First optical light from the supernova remnant G17.4−2.3 Dopita M.A.,1979,ApJS,40,455Fesen R.A.,Blair W.P.and Kirshner R.P.,1985,ApJ,292,29Fesen R.A.and Hurford A.P.,1995,AJ,110,747Fesen R.A.,Winkler P.F.,Rathore Y.,Downes R.A.,WallaceD.and Tweedy R.W.,1997,AJ,113,767Green D.A.,Frail D.A.,Goss W.M.and Otrupcek R.,1997,AJ,114,2058Green D.A.,2001,A Catalog of Galactic Supernov a Remnants(2001version),Mullard Radio Astronomy ObservatoryCambridgeHester J.J.,1987,ApJ,314,187Hilton J.,White G.J.,Cronin N.J.and Rainey R.,1986,A&A,154,274Hamuy M.,Walker A.R.,SuntzeffN.B.,Gigoux P.,HeathcoteS.R.&Phillips M.M.,1992,PASP,104,533Hamuy M.,SuntzeffN.B.,Heathcote S.R.,Walker A.R.,Gigoux P.&Phillips M.M.,1994,PASP,106,566Kaler J.B.,1976,ApJS,31,517Lasker B.M.,Russel J.N.&Jenkner H.,1999,in the HSTGuide Star Catalog,version1.1-ACT,The Association ofUniversities for Research in Astronomy,IncLynds B.T.,1965,ApJS,12,163Mavromatakis,F.,Papamastorakis,J.,Paleologou,E.V.andVentura J,2000,A&A353,371Mavromatakis,F.,Papamastorakis,J.,Ventura,J.,Becker,W.,Paleologou,E.V.and Schaudel,2001,A&A370,265Mavromatakis,F.,Boumis P.and Paleologou, E.V.,2002,A&A,acceptedMcKee C.F.and Cowie L.L.,1975,ApJ,195,715Osterbrock D.E.,1989,Astrophysics of gaseous nebulae,eds.W.H.Freeman&CompanyRaymond J.C.,1979,ApJS,39,1Reich W.,F¨u rst E.,Reich P.and Junkes N.,1988,in IAUColloq.101,Supernova Remnants and the InterstellarMedium,eds.Roger R.S.and Landecker T.L.(CambridgeUniv.Press),293Reich W.,F¨u rst E.,Reich P.and Reif K.,1990,A&AS,85,633Rib´o M.,Reig P.,Mart´i and Paredes J.M.1999,A&A,347,518Ryter C.,Cesarsky C.J.and Audouze J.,1975,ApJ,198,103Seward F.D.,Dame T.M.,Fesen R.A.and Aschenbach B.,1995,ApJ,449,681Shull J.M.and McKee C.F.,1979,ApJ,227,131Smith R.C.,Kirshner R.P.,Blair W.P.,Long K.S.andWinkler P.F.,1993,ApJ,407,564。

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EXERCISE THIRTEENCPassage 1The first navigational lights in the New World were probably lanterns hung at harbor entrances. The first lighthouse was put up by the Massachusetts Bay Colony In L7~6 on Little Brewster Island at the entrance to Boston Harbor. Paid for and maintained by light dues" levied on ships, the original beacon was blown up in 1776. By then there were only a dozen or so true lighthouses in the colonies. Little over a century later, there were 700 lighthouses.The first eight erected on the West Coast in the 1850 5 featured the same basic New England design: a Cape Cod dwelling with the tower rising from the center or standing close by. In New England and elsewhere. though. lighthouses reflected a variety of architectural styles. Since most stations in the Northeast were built on rocky eminences, enormous towers were not the rule. Some were made of stone and brick, others of wood or metal. Some stood on pilings or stilts: some were fastened to rock with iron rods. Farther south. from Maryland through the Florida Keys. the coast was low and sandy. It was often necessary to build tall towers there-massive structures like the majestic Cape Hatteras, North Carolina. lighthouse. which was lit in 1870. At 190 feet. it is the tallest brick lighthouse in the country.Not withstanding differences in appearance and construction, most American lighthouses shared several features: a light. living quarters. and sometimes a bell (or. later. a foghorn). They also had something else in common: a keeper and. usually. the keeper's family. The keeper' 5 essential task was trimming the lantern 'Nick in order to maintain a steady bright flame. The earliest keepers came from every walk of life-they were seamen. Farmers, mechanics, rough mill hands-and appointments were often handed out by local customs commissioners as political plums. After the administration of lighthouses was taken over in 1852 by the United States Lighthouse 803rd. an agency of the Treasury Department. the keeper corps gradually became highly professional.1. What is the best title for the passage.(A) The Lighthouse on Little Brewster Island(B) T h Life of a Lighthouse Keeper(C) E arly Lighthouses in the United States(D) The Modern Profession of Lighthouse Keeping2. Why does the author mention the Massachusetts Bay Colony?(A) It was the headquarters of the United States Lighthouse Board.(B) Many of the tallest lighthouses were built there.(C) The first lantern wicks were developed there.(D) The first lighthouse in North America was built there.3. It can be inferred from the passage that light-houses in the Northeast did not needhigh towers because(A)ships there had high masts(B)coastal waters were safe(C) the coast was straight and unobstructed(D) the lighthouse were built on high places4. According to the passage. where can the tallest brick lighthouse in the UnitedStates be found?(A)Little Brewster Island(B)The Florida Keys(C)Cape Hatteras(D)Cape Cod5. In line 19, to which of the following does the word "They" refer?(A) Lighthouses(B) Differences(C) Quarters(D) Features6. It can be inferred from the passage that the Treasury Department, after assuming control ofthe lighthouses, improved which of the following?(A) The training of the lighthouse keepers(B) The sturdiness of the lighthouses(C)The visibility of the lights(D)The locations of the lighthouses7. Where in the passage does the author tell how lighthouses in the Northeast were fastened to the surrounding rock?(A) Lines 3-4(B) Line 12(C) Lines 14-15(D) Line 19Passage 2Homing pigeons are placed in a training program from about the time they are twenty - eight days of age. They are taught to enter the loft through a trap and to exercise above and around the loft, and gradually they are taken away for short distances in wicker baskets and released. They are then expected to find their way home in the shortest possible time.In their training flights or in actual races, the birds are taken to prearranged distant points and released to find their way back to their own lofts. Once the birds are liberated, their owners, who are standing by at the home lofts, anxiously watch the sky for the return of their entries. Since time is of the essence, the speed with which the birds can be induced to enter the loft trap may make the difference between gaining a win or a second place.The head of a homing pigeon is comparatively small, but its brain is one quarter larger than that of the ordinary pigeon. The homing pigeon is very intelligent and will persevere to the point of stubbornness some have been known to fly a hundred miles off course to avoid a storm.Some homing pigeon experts claim that this bird is gifted with a form of built - in radar that helps it find its own loft after hours of flight, for hidden under the head feathers are two very sensitive ears, while the sharp, prominent eyes can see great distances in daytime.Why do homing pigeons fly home? They are not unique in this inherent skill: it is found in most migratory birds, in bees, ants, toads, and even turtles, which have been known to travel hundreds of miles to return to their homes. But in the animal world. the homing pigeon alone can be trusted with its freedom and trained to carry out the missions that people demand.1. What is the purpose of the passage?(A) To convince the reader to buy a homing pigeon(B) To inform the reader about homing p1-qeons and their training(C) To protect homing pigeons against the threat of extinction(D) To encourage the owners of homing pigeons to set the birds free2. According to the passage, what happens to homing pigeons when they are about a month old?(A) They are kept in a trap.(B) They enter their first race.(C) They begin a training program.(D) They get their wings clipped and marked.3. In line 8, when the author states that the owners "anxiously watch the sky" there is the implication that the owners(A)want their pigeon to win the race(B)are sending radar signals to their pigeons(C)do not know whether the race began on time(D)do not trust the rules set down by the judges4. According to the passage, what is the difference between a homing pigeon and an ordinary one?(A) The span of the wings(B) The shape of the eyes(C) The texture of the feathers(D) The size of the brain5. The author mentions all of the following at tributes that enable a homing pigeon to return home EXCEPT(A)instinct(B)air sacs(C)sensitive ears(D)good eyes6.In line 16, the pronoun "it" refers to which of the following?(A) Radar(B) Bird(C) Loft(D) Form7. Why does the author mention bees, ants, toads, and turtles in the last pragraph?(A) To describe some unusual kinds of pets(B) To measure distances traveled by various animals(C) To compare their home - finding abilities with those of homing pigeons(D) To interest the reader in learning about other animalsPassage 3Central Park, emerging from a period of abuse and neglect, remains one of the mostpopular attractions in New York City, with half a million out - of - towners among the more than 3 million people who visit the park yearly. About 15 million individual visits are made each year.Summer is the season for softball, concerts, and Shakespeare; fall is stunning; winter is wonderful for sledding, skating, and skiing; and springtime is the loveliest of all. It was all planned that way.About 130 years ago Frederic Law Olmsted and his collaborator Calvert Vaux submitted their landscaping plan for a rectangular parcel two miles north of the town' s center. Thebarren swampy tract, home for squatters and a bone - boiling works that made glue, wasreported as 'a pestilential spot where miasmic odors taint every breath of air." It took 16 years for workers with pickaxes and shovels to move 5 million cubic feet of earth and rock, and to plant half a million trees and shrubs, making a tribute to nature-a romantic nineteenth -century perception of nature.What exists today is essentially Olmsted and Vaux's plan. with more trees, buildings, and asphalt. Landscape architects still speak reverently of Olmsted's genius and foresight, and the sensitive visitor can see the effects he sought.1. With what subject is the passage mainly concerned?(A) The lives of Olmsted and Vaux(B) N ew York City's tourist industry(C) E xamples of nineteenth - century art in New York City(D) The development of Central Park2. According to the passage. which is the prettiest time of year in Central Park?(A) Winter(B) S pring(C) S ummer(D) Fall3. It can be inferred that the rectangular parcel mentioned in line 9 is(A)the site of Central Park(B)a gift presented to New York(C)a skyscraper in New York(A) the proposed design for Central Park4. According to the passage. before Olmsted and Vaux began their work, the area now occupied by Central Park was(A)a romantic place(B)an infertile, marshy space(C)a green and hilly park(D) a baseball field5.It can be inferred from the passage that today' 5 landscape architects praise Olmstedfor his(A) enthusiasm for sport(B) skill at designing factories(C) concern for New York's homeless people(D) foresight in anticipating New York'surbanization6. Where in the passage does the author mention unpleasant smells?(A) Lines 1-3(B) Lines 5-7(C) Lines 10 - 12(D) Lines 15-16Passage 4The difference between a liquid and a gas is obvious under the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fills it to the level of a free surface. A gas forms no free surface but tends to diffusethroughout the space available; it must therefore be kept in a closed container or held by a gravitation field, as in the case of a planet's atmosphere. The distinction was a prominentfeature of early theories describing the phases of matter. In the nineteenth century, for example.one theory maintained that a liquid could be "dissolved" in a vapor without losing its identity.and another theory held that the two phases are made up of different kinds of molecules: liquidons and gasons. The theories now prevailing take a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both flow readily. They are fluids.The fundamental similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. Suppose a closed container partially filled witha liquid is heated. The liquid expands, or in other words becomes less dense; some of itevaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and pressure at which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be distinguished; there is a single, undifferentiated fluid phase of uniform density.1. Which of the following would be the most appropriate title for the passage?(A)The Properties of Gases and Liquids(B)High Temperature Zones on the Earth(C)The Beginnings of Modern Physics(D)New Containers for Fluids2. According to the passage, the difference between a liquid and a gas under normal conditions on Earth is that the liquid(A) is affected by changes in pressure(B) has a permanent structure(C) forms a free surface(D) is considerably more common3. It can be inferred from the passage that the gases of the Earth' 5 atmosphere are contained by(A)a closed surface(B)the gravity of the planet(C)the field of space(D) its critical point4. According to the passage, in the nineteenth century some scientists viewed liquidons and gasons as(A)fluids(B)dissolving particles(C)heavy molecules(D) different types of molecules5. According to the passage, what happens when the temperature is increased in aclosed container holding a liquid?(A) The liquid and gas phases become more similar.(B) The liquid and the gas become less dense.(C) The container expands.(D)The liquid evaporates out of the container.6. According to the passage, which of the following is the best definition of the critical point?(A) When the temperature and the pressure are raised(B) W hen the densities of the two phases are equal(C) W hen the pressure and temperature are combined(D) When the container explodesPassage 5Lucinda Childs's spare and orderly dances have both mystified and mesmerized audiences for more than a decade. Like other so - called "postmodern" choreographers. Childs sees dance as pure form Her dances are mathematical explorations of geometric shapes, and her dancers are expressionless, genderless instruments who etch intricate patterns on the floor in precisely timed. repetitive sequences of relatively simple steps. The development of Childs's career, from its beginning in the now legendary Judson Dance Theater. paralleled the development of minimalist art, although the choreographer herself has taken issue with those critics who describe her work as minimalist. In her view, each of her dances is simply "an intense experience of intense looking and listening." in addition to performing with her troupe, the Lucinda Childs Dance Company. Childs has appeared in the avant - garde opera Einstein on the Beach, in two off - Broadway plays, and in the films Jeonne d' Iman by Marie Jimenez and 21:12 Piano Bor.As a little girl, Childs had dreamed of becom~n9 an actress. She appeared regularly in student productions throughout her school years, and when she was about eleven she began to take drama lessons. it was at the suggestion of her acting coach that the youngster, who was, by her own admission. "clumsy, shapeless, and on the heavy side." enrolled in a dancing class Among her early teachers were Hanya Holm. the dancer and choreographer who introduced theWigman system of modern dance instruction to the United States, and Helen Tamiris, the Broadway choreographer. Pleased with her pupil's progress. Ms. Tamiris eventually asked the girl to perform onstage. After that exhilarating experience, Lucinda Childs "wasn't sure [she] even wanted to be an actress anymore.1. What is the passage mainly about?(A)Minimalist art(B)Mathematical forms(C)A choreographer(D) Broadway plays2. The word "its" in line 6 refers to(A) career(B)development(C)steps(D)the Judson Dance Theater3. The work of Lucinda Childs has been compared to which of the following?(A) Avant - garde opera(B) The Wigman system(C)Realistic drama(D) Minimalist art4.In which artistic field did Childs first study(A)Painting(B)Dance(C)Drama（D）Film。

A Supernova Riddle

A Supernova Riddle

a r X i v :0712.4035v 1 [a s t r o -p h ] 24 D e c 2007A Supernova RiddleDouglas C.Leonard 11The author is in the Department of Astronomy,San Diego State University,5500Campanile Drive,San Diego,CA 92182-1221,USA.E-mail:leonard@ Analysis of the polarization of light from supernovae can reveal the shape and distribution of matter ejected from exploding stars.In this “Perspectives”commentary (published:2007,Science ,315,193),we review the young field of Type Ia supernova spectropolarimetry and critically evaluate the recent work of Wang et al.(2007,Science ,315,212),in which a suggestive trend is found in data from 17Type Ia events.Roughly once per second in the observable universe,a star explodes and announces its death with an optical display that for weeks rivals the brilliance of its parent galaxy.These supernova events are classified into several types,but among the most interesting are those called type Ia supernovae (SNe Ia).Astronomers’love affair with these beacons began in earnest abouta decade ago when two groups put them to work as distance indicators and precisely mapped the expansion history of the universe well into the regime that gravity was expected to have imprinted its decelerating signature.Instead,the data revealed a universe presently accelerating in its expansion rate,a finding heralded by Science as the “Scientific Breakthrough of the Year”in 1998(1),and one that has since survived intense scrutiny and complementary experimental checks.Yet for all the fanfare and empirical success,it must be acknowledged that we are fundamentally ignorant:We do not know how these stars explode.On page 212of this issue,Wang et al.(2)identify a suggestive trend in an impressive set of SN Ia data that may point theway towards a deeper understanding of these enigmatic cosmic blasts.Despite an embarrassing dearth of direct observational evidence,thefirst part of the story of SNe Ia is largely considered settled.Each future SN Ia begins as a carbon-oxygen white dwarf –the compact corpse of a low-mass star like our Sun after its nuclear-burning life is over–accreting matter through some mechanism(massflow from the envelope of a close companion star seems most likely)until a critical central density is achieved and a thermonuclear runaway is triggered.There is general agreement that,once initiated,the burning front progresses through the star for a time as a subsonic deflagration.But at this point in the story,harmony ends and pitched battles begin,with some favoring an enduring deflagration front and others insisting on a transition to a supersonic detonation.The most recent“delayed detonation”models appear to better match observed SNe Ia:The events produced in these simulations are bright enough(a perennial problem for deflagration models)and have the proper ejecta composition and stratification(3).The mechanism that triggers the deflagration-detonation transition remains a mystery,however,and so the pure de-flagration model still retains its share of adherents.In any event,a complete comparison of the observable distinctions predicted by the two scenarios still awaits full,three-dimensional radi-ation transport simulations carried out at high enough resolution to resolve physical processes at very small scales.Into this fray,Wang et al.now step,armed with an upstart and poten-tially powerful observational tool:The ability to study the geometry of the supernova ejecta by analyzing the polarization properties of the light coming from the star shortly after explosion.Are supernovae round?Simple to pose,this question belies a menacing observational chal-lenge,given that all extragalactic supernovae remain point-like in the night sky throughout the critical early phases of their evolution.Fortunately,geometric information is encoded in the polarization properties of supernova light.The essential idea is that photons become polarized when they scatter off of free electrons,and hot,young supernova atmospheres contain an abun-dance of free electrons.Indeed,if we could view such an atmosphere as an extended source, rather than as an unresolvable point of light,we would expect to measure changes in both the direction and strength of the polarization as a function of position in the atmosphere.For a spherical,unresolved source,the directional polarization components cancel exactly and yield zero net polarization.Any deviation from perfect symmetry or roundness of the source in theplane of the sky,however,gives rise to a net polarization(see thefigure).There are two basic causes of supernova polarization.One is asphericity of the electron-scattering atmosphere;because electron scattering is independent of wavelength,it generally produces a uniform increase in the overall polarization level across the spectrum.In the other mechanism,asymmetry in the distribution of material(“clumpy ejecta”)above the electron-scattering photosphere unevenly screens the underlying light.Unlike global asphericity,this polarization mechanism is strongly dependent on wavelength,because only those spectral re-gions corresponding to line transitions of the chemical elements that make up optically thick clumps will be polarized.From spectropolarimetry gathered on seven events,previous work in this youngfield has found SNe Ia to have low overall polarizations but occasionally strong line polarization features (4—7).The emerging picture is thus one of a globally spherical photosphere with clumpy(or otherwise asymmetrically distributed)ejecta overlying it.How can such studies shed light on the Type Iaflame-propagation mystery?The latest models indicate that pure deflagrations leave behind lumpier ejecta than delayed detonations do(3,8).Spotting trends in SNe Ia data has a long tradition of bearing rich fruit.In1936,Walter Baade pointed out that the substantial homogeneity and extraordinary brightness of these objects could make them powerful cosmological tools.By the early1990s,however,it became clear that the dispersion in peak intrinsic luminosity(by more than a factor of ten),complicated their use as“standard candles”.Thefix came in1993,when Phillips(9)quantified a trend first noticed by Pskovskii(10)that intrinsically bright SNe Ia rise and decline in brightness more slowly than dim ones do.Various versions of the“light curve-width”relation have since provided the edifice upon which the entire SN Ia cosmology enterprise has been built,and served as touchstones for theoretical models of the explosions.It is just such a trend that Wang et al.now identify in spectropolarimetry of17SNe Ia: Bright events show systematically weaker line polarization than dim ones do.This trend is consistent with the idea that different SNe Ia make the transition from deflagration to detonation at different times.The sooner it happens,the brighter the supernova and the more completely scoured the ejecta will be of the clumps left behind by the deflagration front.The agreement between model predictions and observations strengthens the case for a detonation phase.Will all debate now end on the subject?It is doubtful.Critics will point out that the trend identified by Wang et al.specifically excludes all spectroscopically“peculiar”SNe Ia,which may comprise upwards of30%of the total population(11).Fundamental advances often come from consideration of the differences seen in a sample,rather than from the similarities alone. And some are likely to withhold any judgment until full three-dimensional models capable of resolving the clumps and quantitatively tracking the resulting polarization become available. Simply put,too many mysteries still surround SNe Ia for anyone to grow complacent.An important clue appears to have been wrested from nature,but we are not ready to resolve the riddle of SNe Ia just yet.References and Notes1.J.Glanz,Science,282,2156(1998).2.L.Wang,D.Baade,F.Patat,Science,315,212(2007);published online30November2006(10.1126/science.1121656).3.V.N.Gamezo,A.M.Khokhlov,E.S.Oran,Astrophys.J.,632,337(2005).4.D.C.Leonard et al.,Astrophys.J.,632,450(2005).5.L.Wang et al.,Astrophys.J.,591,1110(2003).6.D.A.Howell,P.H¨oflich,L.Wang,J.C.Wheeler,Astrophys.J.,556,302(2001).7.L.Wang et al.,Astrophys.J.,653,490(2006).8.M.Reinecke,W.Hillebrandt,J.C.Niemeyer,Astron.Astrophys.,391,1167(2002).9.M.M.Phillips,Astrophys.J.,413,105(1993).10.Y.P.Pskovskii,Soviet Astron.,21,675(1977).11.W.Li et al.,Astrophys.J.,546,734(2001).Clumpy: P > 0% Array netProducing supernova polarization.A spherical,unresolved supernova atmosphere produces zero net polarization(left),whereas a non-spherical atmosphere does not(center).Clumps of material that unevenly block the photosphere’s light can also produce a net supernova polariza-tion(right),and it is this mechanism that is thought to be responsible for the majority of theobserved polarization of SNe Ia.。

夜视眼镜发明的作文

夜视眼镜发明的作文

夜视眼镜发明的作文英文回答：Night vision goggles have revolutionized the way we see in the dark. These innovative devices use advanced technology to enhance our vision in low-light conditions, allowing us to navigate and observe our surroundings with ease. With night vision goggles, we can see clearly in the darkness, as if it were daytime. This remarkable invention has had a significant impact on various fields, from military operations to wildlife observation.One of the key advantages of night vision goggles is their ability to amplify the available light. They use image intensification technology to capture and enhance the faint light that is present in the environment. This technology works by collecting the small amount of light that is available, such as moonlight or starlight, and then amplifying it to produce a brighter image. As a result, even in pitch-black darkness, night vision goggles canprovide a clear and detailed view of our surroundings.In addition to amplifying available light, night vision goggles also have the ability to detect infrared radiation. This is particularly useful in situations where there is no ambient light available. Infrared radiation is emitted by objects and living beings as heat, and it is invisible to the naked eye. However, night vision goggles can detectthis radiation and convert it into a visible image. This allows us to see objects and living beings that would otherwise be invisible to us in the dark.Night vision goggles have a wide range of applications. In the military, they are used by soldiers to navigate and conduct operations in low-light conditions. They provide a tactical advantage by allowing soldiers to see their enemies before they can be seen themselves. Similarly, law enforcement agencies use night vision goggles to enhance their surveillance and reconnaissance capabilities. They can track criminals or suspects in the dark, increasing the chances of successful operations.Beyond the military and law enforcement, night vision goggles are also used in wildlife observation and research. Scientists and nature enthusiasts can observe nocturnal animals without disturbing their natural behavior. This has been particularly valuable in studying elusive and endangered species. Night vision goggles have also found applications in the field of hunting and outdoor activities, enabling hunters and outdoor enthusiasts to navigate and locate their targets in low-light conditions.In conclusion, the invention of night vision goggleshas had a profound impact on our ability to see in the dark. Through the use of advanced technology, these devicesamplify available light and detect infrared radiation, allowing us to see clearly in low-light conditions. From military operations to wildlife observation, night vision goggles have become an indispensable tool in various fields. Their ability to enhance our vision in the dark has opened up new possibilities and improved our understanding of the world around us.中文回答：夜视眼镜的发明彻底改变了我们在黑暗中的视野。

神奇的手电筒作文六百字

神奇的手电筒作文六百字

神奇的手电筒作文六百字英文回答：In the realm of extraordinary inventions, where imagination soars and ingenuity knows no bounds, there exists a marvel that defies the laws of conventional illumination – a magical flashlight. This extraordinary device, transcending the limitations of ordinary torches, possesses an inexplicable ability to illuminate even the darkest of nights with its ethereal glow.With a sleek and enigmatic exterior, the magical flashlight resembles a beacon of hope in the vast expanse of darkness. Its smooth contours and polished finish exude an air of sophistication, hinting at the exceptional power it wields. At its core lies a pulsating orb, radiating an intense luminescence that defies explanation.The magical flashlight's transformative capabilities are truly astonishing. When activated, a surge of etherealenergy courses through its circuitry, casting forth a brilliant beam of light that illuminates its surroundings with an otherworldly radiance. The beam, unlike any other, possesses an extraordinary range, piercing through dense shadows and obstacles with effortless ease. It can illuminate distant objects with pristine clarity, revealing details that would otherwise remain obscured.Beyond its unparalleled illumination, the magical flashlight possesses a unique and enigmatic property. It has the remarkable ability to project illusions into the surrounding environment. By manipulating the flow of light, the flashlight can create lifelike projections thatinteract with the physical world in astonishing ways. These projections can take on any form, from ethereal creatures to intricate landscapes, adding a touch of wonder to the mundane.However, the magical flashlight's true power lies not solely in its ability to illuminate and project illusions. It is in its transformative effect on those who wield it. When held in hand, the flashlight instils a sense of aweand wonder, inspiring courage in the face of adversity. It becomes a symbol of hope, reminding its bearer that even in the darkest of times, there is always light to guide the way.中文回答：在非凡的发明领域，想象力翱翔，独创性不受限制，那里存在着一个奇迹，它无视常规照明定律——一个神奇的手电筒。