Fundamental Questions in Astrophysics: Guidelines for Future UV Observatories PDF
Preview Fundamental Questions in Astrophysics: Guidelines for Future UV Observatories
FUNDAMENTAL QUESTIONS IN ASTROPHYSICS: GUIDELINES FOR FUTURE UV OBSERVATORIES Editedby: ANAI.GO´MEZDECASTROandWILLEMWAMSTEKER ReprintedfromAstrophysicsandSpaceScience Volume303,Nos.1–4,2006 LibraryofCongressCataloging-in-PublicationDataisavailable ISBN1-4020-4838-6(hardbook) ISBN1-4020-4839-4(eBook) ISBN978-1-4020-4838-6(hardbook) ISBN978-1-4020-4839-4(eBook) Published bySpringer, P.O.Box17,3300AADordrecht,TheNetherlands. Pictureleft:insertedastributetothelateWillemWamstekerwholikedthisimageverymuch Picturesright: Top:GalaxyhaloiftheUniversereionizedatredshift15or6,byKenjiBekki&MasashiChiba,TohukuUniversity,Japan Below:SonicPointmodelofKiloHertzQPOs,byM.Colleman,F.K.Lamb&D.Psaltis Below:Simulationsofaccretiondisks,byJ.F.Hawley,S.A.Balbus,J.M.Stone Below:SimulationsoftheinteractionoftheaccretiondiskandthemagnetizedstarinaTTauriSystem,byBrigittavon Rekowsky&AxelBranderger Bottom:Artistillustrationoftheevaporationofanexoplanetatmosphere,byEuropeanSpaceAgencyandAlfred Vidal-Madjar(Institutd’AstrophysiquedeParis) Printedonacid-freepaper AllRightsReserved (cid:1)c Springer2006 Nopartofthematerialprotectedbythiscopyrightnoticemaybereproducedorutilizedinanyformor byanymeans,electronicormechanical,includingphotocopying,recordingorbyanyinformationstorage andretrievalsystem,without writtenpermissionfromthecopyrightowner. PrintedintheNetherlands TABLEOFCONTENTS Foreword 1–2 M.A.BarstowandK.Werner/StructureandEvolutionofWhiteDwarfsandtheirInteractionwiththeLocal InterstellarMedium 3–16 Isabella Pagano, Thomas R. Ayres, Alessandro C. Lanzafame, Jeffrey L. Linsky, Benjam´ın Montesinos and MarcelloRodono` /KeyProblemsinCool-StarAstrophysics 17–31 Ana I. Go´mez de Castro, Alain Lecavelier, Miguel D’Avillez, Jeffrey L. Linsky and Jose´ Cernicharo / UV CapabilitiestoProbetheFormationofPlanetarySystems:FromtheISMtoPlanets 33–52 BorisT.Ga¨ansicke,DomitilladeMartino,ThomasR.Marsh,CaroleA.Haswell,ChristianKnigge,KnoxS. LongandStevenN.Shore/UltravioletStudiesofInteractingBinaries 53–68 WillemWamsteker,JasonX.Prochaska,LucianaBianchi,DieterReimers,NinoPanagia,AndrewC.Fabian, ClaesFransson,BorisM.Shustov,PatrickPetitjean,PhillippRichterandEduardoBattaner/TheNeed forUltraviolettoUnderstandtheChemicalEvolutionoftheUniverse,andCosmology 69–84 RosaM.Gonza´lezDelgado/StarburstsatSpaceUltravioletWavelengths 85–102 NoahBrosch,JohnDavies,MichelC.FestouandJean-ClaudeGe´rard/AViewtotheFuture:UltravioletStudies oftheSolarSystem 103–122 WolframKollatschnyandWangTing-Gui/ActiveGalaxiesintheUV 123–132 Ana I. Go´mez de Castro, Willem Wamsteker, Martin Barstow, Noah Brosch, Norbert Kappelmann, Wolfram Kollatschny, Domitilla de Martino, Isabella Pagano, Alain Lecavelier des E´tangs, David Ehenreich, DieterReimers,RosaGonza´lezDelgado,FranciscoNajarroandJeffLinsky/FundamentalProblemsin Astrophysics 133–145 NorbertKappelmannandJu¨rgenBarnstedt/GuidelinesforFutureUVObservatories 147–151 F.Najarro,A.HerreroandE.Verdugo/MassivestarsintheUV 153–170 AstrophysSpaceSci(2006)303:1–2 DOI10.1007/s10509-006-9175-z Foreword (cid:1)C SpringerScience+BusinessMediaB.V.2006 Modern astrophysics is a mature science that has evolved Thevolumecontainsaseriesofreviewarticlesthatanalyze from its early phase of discovery and classification to a thescientificrequirementsformodernUVinstrumentation. physics-oriented discipline focussed in finding answers to ThefirstarticlesummarizesthesciencecaseforUVastron- fundamentalproblemsrangingfromcosmologytotheorigin omy.After,severalarticlestargetingthemajorresearchfields anddiversityoflife-sustainablesystemsintheUniverse.For ofastrophysicsfromSolarSystemtocosmologicalresearch this very reason, progress of modern astrophysics requires areincluded.ThesearticlesanalyzewhyandwhichUVin- the access to the electromagnetic spectrum in the broadest strumentationisrequiredtomakeprogressineachfield.The energy range. The Ultraviolet is a fundamental energy do- book ends with a summary of the UV instrumentation de- mainsinceitisoneofthemostpowerfultooltostudyplas- manded by the community and a brief update on techno- mas at temperatures in the 3,000–300,000 K range as well logical requirements. All articles in this volume have gone as electronic transitions of the most abundant molecules in throughthepeer-reviewsystemofthejournal“Astrophysics theUniverse.Moreover,theUVradiationfieldisapowerful andSpaceScience.” astrochemicalandphotoionizingagent. ThisbookcontainsthethoughtsandworkoftheNetwork The impact of UV instruments in astronomical research for UV astrophysics (NUVA) and the UV community at- can be clearly traced through the considerable success of large.Bytheendof2002,agroupofEuropeanastronomers theInternationalUltravioletExplorer(IUE)observatoryand coming from a broad range of areas: from fundamental as- successorinstrumentssuchastheGHRSandSTISspectro- trophysicalresearchtoobservationalexpertiseintheoptical, graphson-boardtheHubbleSpaceTelescope(HST),orthe UVandX-rayranges,aswellasspaceinstrumentsdevelop- FUSE satellite operating in the far UV (90–120nm range). mentteams,joinedeffortstoevaluatetheneedtodevelopnew Ofparticularimportancehasbeenaccesstohighresolution UVinstrumentationforthecomingdecadeinordertoachieve R (cid:2) 40,000–100,000 spectra providing an ability to study some of the major scientific objectives of the astronomical the dynamics of hot plasma and separate multiple galactic, community and make full profit of the large astronomical stellarorinterstellarspectrallines.Furthermore,theGALEX facilitiesplannedforotherspectralranges;thisgroupsetthe satelliteisprovidingnewexcitingviewsofUVsources. seedfortheNetworkforUltraVioletAstrophysics(NUVA). Thisbookdescribesthefundamentalproblemsinmodern Atthattime,STISwasworkinginHSTandtheCosmicOri- astrophysics that cannot progress without easy and wide- gins Spectrograph (COS) was being built for replacement; spreadaccesstomodernUVinstrumentation.Threeamong HST was thought to last till 2010/12, FUSE was working themshouldbestressedbyitsrelevance: nominallyandGALEXwasclosetobelaunched.However, thebigspaceagencieshadnoplansfornewUVspectroscopic 1. Extrasolar planetary atmospheres and astrochemistry in missions unless the large optical/UV telescope included in thepresenceofstrongUVradiationfields. theNASAOriginsplantargetedtoenterdevelopmentphase 2. ChemicalevolutionoftheUniverseandthediffusebary- in2015–2020andtobelaunchedinthesecondquarterofthis oniccontent. century. Unfortunately, on Friday 16 January 2004 NASA 3. The physics of accretion and outflow: the astronomical informedthattheplannedshuttlemissiontoserviceandup- engines. grade HST (SM-4) including the substitution of STIS by Springer 2 AstrophysSpaceSci(2006)303:1–2 COS,hadbeencancelled.JustfewmonthslaterSTISfailed. theonlyUVspectroscopicfacilityavailableworld-wide.The NoaccesstohighresolutionUVspectroscopyhasbeenfea- project is driven by a broad international colaboration led sibleduringmostof2005sinceFUSEresumedobservations by Russia (ROSCOSMOS); there is a significant European in November 2005 after the failure of the third of the four participationintheproject.WillemWamstekerwasalsoco- onboardreactionwheelsinDecember2004.TheNUVAwas editor of “Astrophysics and Space Science” and deeply in- officiallyestablishedinJanuary2004.Akeyobjectiveofthe volvedintheeditionofthisvolume. NUVA is to run an exploratory analysis to define the sci- Marcello Rodono´ was director of the Obsservatorio As- entificrequirementsforfutureUVobservatories.Thisbook trofisicodiCataniaandaveryrelevantmemberofthe“cool contains the first outcome of this work and its publication stars”community.Hewaskeenoffmulti-wavelengthstudies hasbeenfundedbytheNUVAasapartofitsactivities.The andanactivepromotoroftheWSO/UVproject. NUVAiswithintheOpticalInfraredCo-ordinationNetwork MichelFestouwasdirectorofl’ObservatoiredeBesanc¸on for Astronomy (OPTICON) funded by the European Com- and a passionate researcher. His work on UV spectroscopy missions6thFrameworkprogrammeundercontractnumber of comets is reknowned world-wide. He actively colabo- RII3-Ct-2004-001566. ratedwiththeNUVAintheidentificationoftheUVfacilities In these two years, some respected colleagues and dear neededtomakeprogressonSolarSystemResearch. friendswhoactivelypromotedandenthusiasticallycolabo- Wewouldliketodedicatethisbooktotheirmemory. ratedinthisprojecthavepassedaway. Finally,wewouldliketothankourOPTICONcolleagues Willem Wamsteker was the director of ESA’s IUE Ob- andveryespeciallyJohnDavies(OPTICONProjectScien- servatoryuntilthemissionterminatedin1996.Afterhisdi- tist) and Gerry Gilmore (OPTICON P.I.) for their support. rection,theIUEdataarchivebecamethefirstfullyinternet WealsoacknowledgethesupportoftheUniversidadCom- drivenastronomicalarchive.Willemwasapromotorofthe plutense de Madrid which is hosting and maintaining the NUVA and a key initiator of the World Space Observatory NUVAsite(www.ucm.es/info/NUVA). UV(WSO/UV)project;a1.7mUVtelescopeequippedwith stateoftheartinstrumentationprovidingafactorof10im- Prof.AnaIne´sGo´mezdeCastro provementonthehighresolutionspectroscopiccapabilities NUVA,chair ofHST/STIS. WSO/UVwillbelaunchedin2010becoming Madrid,March1st,2006 Springer AstrophysSpaceSci(2006)303:3–16 DOI10.1007/s10509-006-2061-x ORIGINAL ARTICLE Structure and Evolution of White Dwarfs and their Interaction with the Local Interstellar Medium M.A.Barstow·K.Werner Received:11March2005/Accepted:11August2005 (cid:1)C SpringerScience+BusinessMediaB.V.2006 Abstract The development of far-UV astronomy has been lowerlimittotheageoftheUniverse.Recently,ithasbeen particularlyimportantforthestudyofhotwhitedwarfstars. suggestedthatcoolwhitedwarfsmayaccountforasubstan- Asignificantfractionoftheiremergentfluxappearsinthefar- tial fraction of the missing mass in the galactic halo (Op- UVandtracesofelementsheavierthanhydrogenorhelium penheimer et al., 2001). White dwarfs are also believed to are,ingeneral,onlydetectedinthiswavebandoratshorter playaroleintheproductionoftypeIasupernovae,through wavelengths that are also only accessible from space. Al- possiblestellarmergersormasstransfer.Althoughnocan- thoughwhitedwarfshavebeenstudiedinthefar-UVthrough- didate precursor system has yet been found, they are used outthepast∼25years,sincethelaunchofIUE,onlyafew as “standard candles” to measure the distances of the most tensofobjectshavebeenstudiedingreatdetailandamuch remoteobjectsintheUniverseandtoimplyanon-zerovalue largersampleisrequiredtogainadetailedunderstandingof for the cosmological constant. It is intriguing that such lo- theevolutionofhotwhitedwarfsandthephysicalprocesses cal, low luminosity objects as white dwarf stars play a key thatdeterminetheirappearance.Wereviewherethecurrent role in some of the most important cosmological questions knowledgeregardinghotwhitedwarfsandoutlinewhatwork of our day, concerning the nature and fate of the Universe. needstobecarriedoutbyfuturefar-UVobservatories. To understand and calibrate cosmologically important as- pectsofwhitedwarfs,suchastheircoolingages,massesand Keywords Ultravioletastronomy·Spacemissions·White compositions,werequireathoroughunderstandingofhow dwarfs theirphotosphericcompositionsevolve.Atmosphericmetal abundances affect cooling rates and bias the determination oftemperatureandsurfacegravity.Reliablemassescanonly 1. Introduction be derived from accurate effective temperature and surface gravitymeasurements.Importantly,metalsaredifficulttode- 1.1. Whitedwarfstarsandthelocalinterstellarmedium tect in cool white dwarfs but still play an important role in inastrophysics cooling.Therefore,abundancesmeasuredinhotterstarspro- videimportantdataastowhatspeciesmaybepresentandin ManywhitedwarfsareamongtheoldeststarsintheGalaxy. whatquantities. Their space and luminosity distributions help map out the InterstellargasisafundamentalcomponentoftheMilky history of star formation in the Galaxy and can, in princi- Way and other galaxies. The local interstellar medium ple,determinetheageofthediskbyprovidinganimportant (LISM)isparticularlyimportantbecauseitiscloseenough tousfordetailedexaminationofitsstructure.Indeed,itisthe onlypartoftheUniverse,apartfromthesolarsystem,thatwe M.A.Barstow((cid:1)) DepartmentofPhysicsandAstronomy,UniversityofLeicester, can study directly, from the penetration of neutral particles UniversityRoad,LeicesterLE17RH,UK throughtheheliopause.Studyofthecompositionofthelocal e-mail:[email protected] interstellargastellsusabouttheevolutionoftheUniverseand ourgalaxy.Whilemodifiedconsiderablysincetheformation K.Werner Universita¨tTu¨bingen,FRG,Tu¨bingen ofthesolarsystem,theprocessesthathaveshapedtheISM Springer 4 AstrophysSpaceSci(2006)303:3–16 must have also affected the solar system and possibly even theenergylevels.However,accordingtothePauliExclusion the evolution of life. For example, the local radiation field Principle, no two electrons can occupy the same quantum andgaspropertiesoftheLISMsettheboundaryconditions state,whichhasafinitevolumeinposition-momentumspace, fortheheliosphere.Therefore,itregulatesthepropertiesof sothereisalimitwherearepulsiveforcearisingfromthis,the theinterplanetarymediumandthecosmicrayfluxesthrough “electrondegeneracypressure”,resistsfurthercompression thesolarsystem(seeFrischetal.,2002),whichmayaffect ofthematerial.Fowler(1926)showedthatthispressurecould theterrestrialclimate(Muelleretal.,2003). supportastellarmassagainstgravitationalcollapseandpro- The relationship between white dwarfs and the LISM is posedthatthismightexplaintheexistenceofwhitedwarfs. intimate.Theprocessofproducingwhitedwarfsinthedisk Combiningthisinsightwiththeequationsofstellarstructure, substantially enriches the content of the local ISM (Mar- Chandrasekhar(1931,1935)determinedthemass–radiusre- gio, 2001) and white dwarfs contribute significantly to the lationforwhitedwarfsandthemaximummass(1.4M(cid:4))that total cosmic abundance of metals (Pagel, 2002). They also electrondegeneracypressurecouldsupportagainstgravity, provideasignificantfractionofthetotalfluxofionizingra- theChandrasekharlimit.AsthisNobelPrizewinningwork diationwithintheLISMandwillaffecttheionizationstate has been extended by subsequent developments, the basic oftheinterstellargas,eveniftheLISMisnotinionization ideas remain unchanged. However, importantly, they have equilibrium.Atthesametime,whitedwarfsmaybeaccret- alsohardlybeentestedbydirectobservation. ingmaterialfromthedensercloudsintheISM,whichwill Theoretical and observational study of stellar evolution modifytheirphotosphericabundances.Finally,whitedwarfs hasplacedwhitedwarfsasonepossibleendpointofthepro- areidealbackgroundsourcesfordirectstudyoftheLISM. cess. In general terms, all stars with masses below about 8 timesthatoftheSunwillpassthroughoneormoreredgiant 1.2. Physicalstructureandclassificationofwhite phases before losing most of their original mass to form a dwarfs planetarynebula.Theremnantobject,awhitedwarf,isthe core of the progenitor star. In the absence of any internal Astronomershaveknownabouttheexistenceofwhitedwarf sourceofenergy,thetemperatureofawhitedwarf,afterits starsfor150years,sincethediscoveryofacompaniontothe birth, is determined by how rapidly stored heat is radiated brightest star in the sky, Sirius. Studying the regular wave- intospace.Estimatesofwhitedwarfcoolingtimesindicate like proper motion of Sirius, Bessel revealed the presence thatitwilltakeseveralbillionyearsforthestarstofadetoin- ofahiddencompanion,withthepaireventuallyresolvedby visibility.Hence,whitedwarfsareamongtheoldestobjects Clarkin1862andtheorbitalperiodrevealedtobe50years. in the galaxy. Since, the galaxy is younger than the cool- Later,thefirstspectroscopicstudyofthesystembyAdams ingtimescales;thelowesttemperature(oldest)whitedwarfs revealedagreatenigma.WhilethetemperatureofSiriusB yieldalowerlimittoitsage. wasfoundtobehigherthanSiriusA,itwasapparently1000 The basis for understanding the nature of most stars is times less luminous. This result could only be explained if analysisoftheiropticalspectraandclassificationaccording Sirius B had a very small radius, 1/100 R(cid:4) and similar to to the characteristics revealed. A number of physical pro- thatoftheEarth.However,withaknownmassof∼1M(cid:4)for cessescanaltertheatmosphericcompositionofawhitedwarf Sirius B, the implied density of 1400Tonnesm−3 was well asitcools.AsnotedbySchatzmann(1958),thestronggrav- above that of any form of matter known to 19th and early itational field (logg ∼8 at the surface) causes rapid down- 20thcenturyphysicists.Withinanumberofyears,ahandful warddiffusionofelementsheavierthantheprincipalHorHe of stars similar to Sirius B were found, mostly in binaries. component.Hence,Schatzmannpredictedthatwhitedwarf Theterm“whitedwarf”wascoinedlater,onthebasisofthe atmospheres should be extremely pure. Consequently, the visualcolorandsmallsize,whencomparedtootherstars. spectrashouldbedevoidofmostelements,showingsigna- Theanswertotheriddleoftheirstructurestemsfromthe turesofonlyhydrogenand,possibly,helium.Whitedwarfs developmentofthequantumstatisticaltheoryoftheelectron arethusdividedintotwomaingroupsaccordingtowhether gasbyEnricoFermiandPaulDirac.Whenatomsinamate- or not their spectra are dominated by one or other of these rialaresufficientlyclosetogether,theirmostweaklybound elements. The hydrogen-rich stars are given the classifica- electronsmovefreelyaboutthevolumeandcanbeconsid- tionDA,whilethehelium-richwhitedwarfsaredesignated eredtobehavelikeagas.Almostalltheelectronsinthegas DOifHeIIfeaturesarepresent(hotterthanabout45,000K) will occupy the lowest available energy levels. Any states andDBifonlyHeIlinesarevisible.Smallnumbersofhy- withthesameenergyaresaidtobe“degenerate.”Hence,this brid stars exist, with both hydrogen and helium present. gasisknownasthe“degenerateelectrongas.”Undercondi- In these cases, two classification letters are used, with the tionsofextremepressuretheelectronsareforcedtooccupy first indicating the dominant species. For example, DAO spacemuchclosertothenucleioftheconstituentatomsthan white dwarfs are mostly hydrogen but exhibit weak HeII in normal matter, breaking down the quantised structure of features. Springer AstrophysSpaceSci(2006)303:3–16 5 Fig.1 Schematicdescriptionof theproductionofH-richand He-richbranchesofwhitedwarf evolution The above classification scheme applies only to white 1.3. Thestructureofthelocalinterstellarmedium dwarfs with temperatures above ∼10,000K, when the H and He energy levels are sufficiently populated above the TheSunisembeddedintheLocalInterstellarCloud(LIC),a ground state to produce detectable features at visual wave- warm(T ∼10,000K),low-density(n ∼0.1cm−3)region, lengths. At lower temperatures, although H and/or He may which is observed in projection toward most, but not all, bepresenttheseelementsarenolongerdirectlydetectable. nearbystars.ModelsoftheLIC(RedfieldandLinsky,2000) Cool white dwarfs are divided into three main groups. DC showtheSunlocatedjustinsidetheedgeoftheLICinthedi- whitedwarfshavecontinuous,completelyfeaturelessspec- rectionoftheGalacticCenterandtowardtheNorthGalactic tra;DZwhitedwarfsareHe-richandhaveonlymetallines Pole(NGP).TheLICissurroundedbyahot106Ksubstrate visible; DQ white dwarfs show carbon features. Figure 1 andappearstobepartoftheexpandingcloudcomplexrep- summarisesthecurrentthoughtsregardingtherelationships resentingtheLoopIradioshell,whichiseitherasupernova between the hot white dwarf groups along with the princi- remnant or a wind blown shell from the Sco-Cen Associa- pal mechanisms that provide evolutionary routes between tion(Fig.2).Indeed,evidenceforshocksintheLICwithin them. the past 2 Myr is supported by EUV observations of ion- Gravity is not necessarily the only influence on photo- ized interstellar HeII (Lyu and Bruhweiler 1996; Barstow sphericcomposition.Itcanbecounteredbyradiationpres- et al., 1997). Also, surrounding the LIC is the low den- sure which acts outward to support heavy elements in the sityLocalBubblewithdimensionsof>200pc.SoftX-ray atmosphere,aprocesstermed“radiativelevitation.”Another andOVIobservationsimplythisregionislargelyfilledwith mechanism that can mix elements that have settled out in ahot(T ∼106K),extremelylow-density(n ∼10−2cm−3) thestellaratmosphereisconvection.Iftheconvectivezone plasma.Frisch(1995)andBruhweiler(1996)discusspossi- reaches down to the base of the atmosphere then heavy el- bleoriginsofLoopIandtheLocalBubble,butweneedto ements can be dredged back up into the outer atmosphere. resolvetwokeyquestions.WhatisthegeometryoftheLo- Afurthercomplicationisthatmaterialcanalsobeaccreted cal Bubble? What are the distribution, physical conditions, fromtheinterstellarmedium. andkinematicsofcloudsintheLocalBubble?Theanswers Springer 6 AstrophysSpaceSci(2006)303:3–16 Fig.2 ViewoftheLISMwithin 400pcoftheSun.TheSunis locatedattheendofthedashed lineextendingfromthestarβ CMa.Thedashedlineis ∼200pcinlength.Theregionof theLocalVoidorLocalBubble isdenoted.Mostofthevolume oftheLISMiscomposedof verylow-densitygas.Theradio loopstructuresarefurther indicated.ThestarsofSco-Cen areinteriortoLoopI.Regions ofhighdustextinction,regions wheredensecloudsarefound, arealsomarked.From Bruhweiler(1996) tothesequestionsalsobeardirectlyonwhetherinterstellar quencecannotyetbereadilyexplained.Forexample,theratio accretionispossibleinspecificwhitedwarfs. of H-rich to He-rich progenitors (4:1) is lower than that of thewhitedwarfsintowhichtheyevolve(Napiwotzki,1999). 2. ThecomplexnatureoftheDAwhitedwarf Furthermore,agapintheHe-richsequence,between∼45000 population and30000K,separatingtheDOandDBwhitedwarfs(We- semaeletal.,1985;Liebertetal.,1986;DreizlerandWerner, 2.1. Compositionandstructureofthestellar 1996)impliesthatHe-richwhitedwarfsmusttemporarilybe photospheresfromUVspectroscopy seenasDAstarsduetosomephysicalprocess. It is well established that the hottest (T ∼50,000K) TheemergencefromtheAsymptoticGiantBranch(AGB)of white dwarfs possess significant abundances of elements the two H- and He-rich groups, is qualitatively understood heavier than He in their atmospheres. Typical ranges are toarisefromdifferencesinpreciseevolutionarypaths.The showninTableIforthemostimportantelements.Massfrac- He-rich group undergoes a late helium shell flash, which tionsaregiven,withrespecttoHforDAsandHeforDOs, causes convective mixing of the outer envelope. Hydrogen since they are essentially independent of nuclear burning, isingestedandburntordilutedbythisprocess(e.g.,Herwig iftheelementdoesnotparticipateintheprocess.Observed etal.,1999).ThemajorityofyoungwhitedwarfsareofDA abundancesmayvaryconsiderablywithtemperature,asdis- type, out numbering the He-rich objects by a ratio of ∼7:1 cussedlater.Hence,welisttypicalvaluesandcommenton (Flemingetal.,1986;Liebertetal.,2005).Themorerecent, the possible ranges in each white dwarf pathway. Studying deeper, Sloan Digital Sky Survey appears to yield an even howphotosphericabundancepatternschangeasthesestars highervalueof9:1fortheratioofDAstothetotalnumber cool can delineate the evolutionary pathways followed by ofDOandDBwhitedwarfs(Kleinmanetal.,2004).How- whitedwarfs.Forexample,abundanceenhancementsinC, ever, the complex relationships between these groups and N and O may be a sign of near exposure of the C O core a demonstrable temperature gap in the He-rich cooling se- duringtheAGBphaseorofalateHe-shellburningepisode. Springer
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