Table Of ContentFUNDAMENTAL QUESTIONS IN ASTROPHYSICS:
GUIDELINES FOR FUTURE UV OBSERVATORIES
Editedby:
ANAI.GO´MEZDECASTROandWILLEMWAMSTEKER
ReprintedfromAstrophysicsandSpaceScience
Volume303,Nos.1–4,2006
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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
