Abundances of neutron-capture elements in the Hot Extreme-Helium Stars V1920 Cygni and HD 124448

0.13 MB·English

by  Gajendra Pandey

DRAFTVERSIONJANUARY1,2004
PreprinttypesetusingLATEXstyleemulateapjv.11/12/01
ABUNDANCESOFNEUTRON-CAPTUREELEMENTSINTHEHOTEXTREME-HELIUMSTARSV1920
CYGNIANDHD1244481
GAJENDRAPANDEY2,3,DAVIDL.LAMBERT2,N.KAMESWARARAO3,C.SIMONJEFFERY4
DraftversionJanuary1,2004
ABSTRACT
AnalysisofHST STISultravioletspectraoftwohotextremeheliumstars(EHes):V1920CygandHD124448
providethefirstmeasurementsofabundancesofneutron-captureelementsforEHes. Althoughthetwostarshave
similar abundancesforelementsupthroughtheiron-group,theydifferstrikinglyintheirabundancesofheavier
elements: V1920Cyg is enriched by a factor of 30 in light neutron-captureelements (Y/Fe, Zr/Fe) relative to
4 HD124448.Thesedifferencesinabundancesofneutron-captureelementsamongEHesmirrorsthatexhibitedby
0 theRCrBstars, andisevidencesupportingtheviewthatthereisanevolutionaryconnectionbetweenthesetwo
0
groupsofhydrogen-deficientstars. Also,theabundancesofYandZrinV1920Cygprovideevidencethatatleast
2
oneEHestarwentthroughas-processsynthesisepisodeinitsearlierevolution.

n
Subjectheadings:stars: abundances–stars: chemicallypeculiar–stars: evolution
a
J

4 1. INTRODUCTION dance analyses for a pair of hot EHes of similar temperature
1 and gravityand with almost identicalcompositionsexceptfor
Extremeheliumstars(EHes)arecarbon-richB-andA-type
abundancesofYandZr.
supergiants in which surface hydrogen is merely a trace ele-
1
ment(Jefferyetal. 1987). RCrBstarsaresimilarlyhydrogen-
v 2. OBSERVATIONS
3 poor carbon-rich F- and G-type supergiants characterised by
6 steep and irregular declines in visual brightness (Asplund et Ultraviolet: V1920Cyg (aka HD225642 and LS II +33 5)
2 al. 2000). Understanding the origins of these rare luminous andHD124448wereobserved(programID:GO9417)on2002
1 H-deficient stars remains a challenge. Detailed analyses of October 18 and 2003 July 21, respectively, with the HST’s
0 thestar’schemicalcompositionsholdcluestotheirorigins. A STISNear-UV/MAMA,using the E230Mgratingand 0.′′2 ×
4 significantlacunapresentlyexists: theabundancesofneutron- 0.′′06 aperture, which provides a resolving power (λ/∆λ) of
0 captureelementsinEHesareunknown. Here,weprovideand 30,000. Two spectra for each star were obtained: V1920Cyg
/
h commentonthefirstestimatesoftheseabundancesforapairof (DataSetsO6MB06010andO6MB06020withexposuretimes
p EHes. 1844 s and 2945 s, respectively) and HD124448 (Data Sets
- Two scenarios are contenders to account for the EHes and O6MB02010andO6MB02020withexposuretimes1977sand
o
the RCrB stars: the merger of a He white dwarf with a C-O 3054 s, respectively). The spectrum covered the wavelength
r
t white dwarf (Webbink1984), and a final shellflash in a post- rangefrom1840Åto2670Å.Sincetheabsorptionprofilesare
s
a AGB star onthe whitedwarfcoolingtrack(Ibenet al. 1983). broadforV1920Cyg(projectedrotationalvelocityvsini∼40
: A final shell flash appears most likely responsible for the re- km s- 1; Jeffery et al. 1998) and HD124448 (vsini ∼ 20 km
v markablestarsFGSge(Langer,Kraft&Anderson1974;Gon- s- 1;Schönberner&Wolf1974),thecoaddedspectrafromtwo
i
X zalezetal. 1998)andV4334Sgr(Sakurai’sobject- Duerbeck exposureswere rebinned to a lower resolution to improve the
r & Benetti 1996; Asplund et al. 1997) with RCrB-like light signal-to-noise(S/N) ratio, which is about100at 2500Å.The
a curves, high overabundancesof the neutron-captureelements, resultingspectrahavearesolvingpowerof7500(V1920Cyg)
andprobableorcertainH-deficiency.Whitedwarfmergersap- and15000(HD124448).
peartoaccountforthecompositionsofEHesand(probably)the Optical: A high-resolution optical spectrum of V1920Cyg
RCrBs (Pandeyet al. 2001; Saio & Jeffery2002; Asplundet wasobtainedon1996July25attheW.J.McDonaldObserva-
al. 2000). Thes-processabundancesinEHesarecrucialclues tory’s 2.7-m telescope with the coudé cross-dispersed echelle
because, in the merger model, enrichment of neutron-capture spectrograph(Tulletal. 1995)ataresolvingpowerof60,000.
elementsisnotexpectedunlesssomes-processingoccursdur- Theobservingprocedure,thedetector,andthewavelengthcov-
ingthemerger(Pandeyetal. 2001). erageareasdescribedinPandeyetal. (2001).
For hot EHes (T >14000 K), the lighter neutron-capture
eff
elements(Sr,Y, Zr)andtheheavierelements(Baandthelan- 3. ABUNDANCE ANALYSIS
thanides) are undetectable in optical spectra because ioniza-
Model atmospheres from the code STERNE and synthetic
tion equilibrium ensures that the dominant ion is X2+, whose
spectra computedwith the Belfast LTE code SPECTRUM are
strongest lines are principally in the ultraviolet. Fortunately,
combined in the analysis (Jeffery, Woolf, & Pollacco 2001).
the EHes have appreciable ultraviolet flux and are observable
InputparametersforSTERNEincludingthecompositionwere
athigh-spectralresolutionwiththeHubbleSpaceTelescope′s
takenfrompreviousabundanceanalyses. Theseparametersare
SpaceTelescopeImagingSpectrograph.Here,wereportabun-
theeffectivetemperatureT =16180±500K,thesurfacegrav-
eff
1BasedonobservationsobtainedwiththeNASA/ESAHubbleSpaceTelescope,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Inc.
(AURA)underNASAcontractNAS5-26555
2DepartmentofAstronomy;UniversityofTexas;Austin,TX78712-1083;[email protected]
3IndianInstituteofAstrophysics;Bangalore,560034India;[email protected],[email protected]
4ArmaghObservatory;CollegeHill,Armagh,BT619DG,UK;[email protected]
1
2 Abundancesofneutron-captureelementsinhotextreme-HeliumStars
itylogg=2.00±0.25(cgsunits),themicroturbulentvelocityξ respectively. Theestimatedabundanceuncertaintyisfollowed
= 15±5km s- 1, andthe abundanceratio C/He = 1% by num- in brackets by the number of useful lines. The uncertainty is
ber of atoms for V1920Cyg (Jeffery et al. 1998), and T = the combined uncertainty from the estimated errors in the at-
eff
15500±800K, log g = 2.10±0.20(cgs units), ξ = 10 km s- 1, mosphericparametersor,iflarger,theline-to-linescatterinthe
and C/He = 1% for HD124448 (Schönberner & Wolf 1974; abundances.Notethatfortheultravioletspectra,thesamelines
Heber1983). Derivedabundancesare sufficientlyclose to the weregenerallyusedforbothstarsand,hence,theabundancera-
inputvaluesthatiterationisunnecessary.Thecontinuousopac- tiosbetweenthestarsareindependentoftheadoptedgf-values.
ityisdominatedbyphotoionizationofneutralheliumandelec- Severalconsistencycheckswereappliedtoouranalyses,es-
tronscatteringwithelectronssuppliedbyhelium. Inthissitua- peciallyto themorecompleteanalysisofV1920Cyg. Ioniza-
tion, the HeI line strengths are insensitive to the abundances tionequilibriumforFeII/FeIIIissatisfiedforbothstars,andfor
of the trace elements, and to Teff, but are sensitive to grav- SII/SIIIforV1920Cyg.Excitationequilibriumisfoundforthe
ity because the strong lines have Stark-broadenedwings. The FeIII opticallinesforV1920Cyg; theultravioletlinesusedin
adopted models satisfactorily reproduce the optical and ultra- our analysesdo not offera range in their lower excitation po-
violetHeI lineprofiles(V1920Cyg). Thederivedabundances tential. Whenan elementprovideslinesin the opticalandthe
(Table1)fromspectumsynthesesaregivenaslogǫ(X)andnor- ultraviolet spectra of V1920Cyg, the abundances are in good
malizedwithrespecttototalmasswherelogΣµ ǫ(X)=12.15 agreement. Our results for V1920Cyg are in good agreement
X
withµastheatomicweight. withthosebyJefferyetal. fromalowerresolutionopticalspec-
trumoflimitedbandpass.TheagreementforHD124448isless
TABLE 1 goodbetweenourultraviolet-basedabundancesandthosefrom
Chemi
al Abundan
es in two hot EHes aphotographicopticalspectrum(Heber1983).Ourlimitonthe
H abundance for V1920Cyg from the absence of the Hα line
V1920Cyg is more than 2 dex less than the abundanceofferedby Jeffery
etal. fromtheHβ line. Heber(1983)puttheHabundanceof
Spe
ies Z Solara Opti
al UV HD124448 HD124448atlogǫ(H)<7.5.
Hi 1 12.00 <6.2(H(cid:11)) (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1) 4. NEUTRON-CAPTURE ELEMENTS
Hei 2 10.98 11.5(4) 11.5(1) (cid:1)(cid:1)(cid:1)
OuropticalandultravioletspectraofhotEHeswerescanned
Cii 6 8.46 9.6(cid:6)0.2(8) 9.7(cid:6)0.1(3) 9.4(cid:6)0.1(1)
forlinesofYIII, ZrIII,andofthedoubly-ionizedlanthanides.
Nii 7 7.90 8.6(cid:6)0.3(7) (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1)
InspectionoftheultravioletspectrashowedthatthetwoEHes
Oii 8 8.76 9.6(cid:6)0.2(2) (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1)
differdramaticallyintheabundancesofYandZr. Thispointis
Mgii 12 7.62 7.7(cid:6)0.2(1) 7.8(cid:6)0.2(1) 7.7(cid:6)0.2(1)
highlightedbyFigure1. InFigure1a,theZrIIIlineat2656.47
Siii 14 7.61 (cid:1)(cid:1)(cid:1) 7.1(cid:6)0.3(1) 6.9(cid:6)0.3(1)
Sii 16 7.26 7.3(cid:6)0.2(5) (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1) Å is comparable in strength to the MgII lines at 2660.8 Å in
Siii 7.1(cid:6)0.1(3) (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1) V1920Cyg(lowerspectrum)butinHD124448theZrIIIlineis
Mnii 25 5.58 (cid:1)(cid:1)(cid:1) 4.6(cid:6)0.3(1) 4.7(cid:6)0.3(1) extremelyweak.Figure1bshowsasimilar,albeitlessdramatic
Mniii (cid:1)(cid:1)(cid:1) 5.0(cid:6)0.2(4) 4.8(cid:6)0.2(4) comparisononaccountofblends,fortheYIIIlineat2367.2Å.
Feii 26 7.54 (cid:1)(cid:1)(cid:1) 6.8(cid:6)0.3(10) 7.0(cid:6)0.3(10) These differences are not attributable to differences in stellar
Feiii 7.0(cid:6)0.3(4) 6.8(cid:6)0.1(4) 7.2(cid:6)0.1(4) parameters. Thiscomparisoneliminatesthepossibilitythatthe
Coii 27 4.98 (cid:1)(cid:1)(cid:1) 4.6(cid:6)0.3(1) 4.6(cid:6)0.3(1) lineswhichweattributetoYIIIandZrIIIaremerelyunidenti-
Niii 28 6.29 (cid:1)(cid:1)(cid:1) 5.6(cid:6)0.3(2) 5.6(cid:6)0.3(3) fiedlinesofironorothermoreabundantspecies.
Znii 30 4.70 (cid:1)(cid:1)(cid:1) 4.6(cid:6)0.3(1) 4.2(cid:6)0.3(1) The measured wavelengths of YIII and ZrIII lines were
Srii 38 2.99 <4.3 (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1) taken from Epstein & Reader (1975) and Khan, Chaghtal &
Yiii 39 2.28 (cid:1)(cid:1)(cid:1) 3.1(cid:6)0.3(3) 1.8(cid:6)0.3(1) Rahimullah (1981), respectively. The YIII resonance lines at
Zriii 40 2.67 (cid:1)(cid:1)(cid:1) 3.6(cid:6)0.2(8) 2.6(cid:6)0.2(3)
2414.60,2367.23,and 2327.31Åare detected in the spectrum
Laiii 57 1.25 (cid:1)(cid:1)(cid:1) <2.2 (cid:1)(cid:1)(cid:1)
of V1920Cyg with the 2367.23Å line seen as a contributor
Ceiii 58 1.68 (cid:1)(cid:1)(cid:1) <2.0 <1.5
to a blended line in the spectrum of HD124448 (Figure 1).
Ndiii 60 1.54 <1.8 (cid:1)(cid:1)(cid:1) (cid:1)(cid:1)(cid:1)
LinesofZrIIIat2664.27,2656.47,2643.82,2620.56,2593.70,
a 2102.26,1921.94,and1863.98ÅinthespectrumofV1920Cyg
Re
ommendedsolarsystemabundan
esfromTable2ofLodders
(20T0h3e).adoptedgf-valuesweretakenfromtheNISTdatabase5 were used to set the abundance. Three of the ZrIII lines at
2664.27, 2656.47, and 2643.82Åwere detectable in the spec-
for H, He, Mg, Si, S, MnII, Co, Ni, Zn, and Sr, Wiese, Fuhr trumofHD124448.
& Deters (1996) for C, N, and O, and Kurucz’s database6 for
Figure 1 showsour synthesesof the regionaround2656.47
MnIII andFeIII. ForZrIII, threesetsoftheoreticalgf-values Å. A small change in the assumed Fe abundance between
are in good agreement: Redfors (1991), Reader & Acquista
V1920Cyg (logǫ(Fe) = 6.8) and HD124448 (logǫ(Fe) = 7.1)
(1997),andCharro,López-Ayuso&Martín(1999). Weadopt
isrecognized.
Redfors(1991)gf-valueswiththesuggestedcorrectionbySik-
An upper limit to the Sr abundance for V1920 Cyg is es-
strömetal. (1999)forYIIIandZrIII;theestimateduncertainty
timated from the non-detection of the resonance line of SrII
in the gf-values is reportedto be within 10%. The gf-values
4215.52Å.
forLaIII, CeIII, andNdIII arefromDREAM database7, Bié- An unsuccessful search was conducted for the doubly-
mont,Quinet&Ryabchikova(2002),andZhangetal. (2002),
ionized lanthanides. We mention here only those lanthanides
5http://physics.nist.gov/cgi-bin/AtData/lines_form
6http://kurucz.harvard.edu
7ftp://mail.umh.ac.be/pub/ftp_astro/dream/LaIII
Pandey,G.etal. 3
1.2 1.2
1 1
0.8 0.8
0.6 0.6
2656 2658 2660 2662 2364 2365 2366 2367 2368
FIG. 1.— TheobservedspectraofV1920CygandHD124448arerepresentedbyfilledcirclesandcrosses,respectively. Panel-ashowstheregionincludingthe
ZrIIIlineat2656.5Å.SyntheticspectraforfourdifferentZrabundancesareshownforV1920Cygwithlogǫ(Zr)=3.6providingasatisfactoryfittotheobserved
line.Theabundancelogǫ(Zr)=2.6providesafittothesharperlineintheHD124448spectrum.Panel-bshowstheregionincludingtheYIIIlineat2367.2Åwhich
isblendedwithaNiIIline.Thesyntheticspectrumwithlogǫ(Y)=3.1providesafittoV1920Cyg’sspectrum,andlogǫ(Y)=1.8toHD124448’sspectrum.Ineach
Panel,principallinesareidentified.
providing significant upper limits to the abundances. A star-to-star variation in the abundancesof the neutron-capture
LaIII 2379.37Å resonance line provides the upper limit for elements.For‘majority’RCrBs(Lambert&Rao1994),[Y/Fe]
V1920Cyg. An upper limit to the Ce abundance is obtained and [Zr/Fe] ranges from +0.3 to about +1.6 but with smaller
bycomparingthesyntheticandobservedprofileofthelowex- valuesfor[Ba/Fe], [La/Fe], andpresumablyotherlanthanides
citationCeIII2603.59Åline.Thetwostrongestresonancelines (Asplund et al. 2000). This range and the non-solar ratio
of Y and Zr to Ba and lanthanides was confirmed by Rao &
ofNdIIIat5193.06Åandat5294.10Åsettheupperlimittothe
Lambert (2003) from a differential analysis of the newly dis-
NdabundanceinV1920Cyg.
covered RCrB star V2552 Oph and RCrB. Among ‘minor-
ity’RCrBs (i.e., veryFe-poorstars), somewhatmoreextreme
5. DISCUSSION
values are known with a similar contrast between light and
Our abundance analysis of optical and ultraviolet spectra neutron-captureelements. ThecoolpeculiarRCrBUAqrhas
of V1920Cyg confirms the results obtained by Jeffery et al. very extreme neutron-capture element enrichments: [Y/Fe] =
(1998) for elements from C to the iron-group. We extend the +3.3and[Ba/Fe]=+2.1(Vanture,Zucker&Wallerstein1999;
earlier analysis to Mn, Co, Ni, and Zn, and, in particular, to alsoBond,Luck&Newman1979).LimiteddataforcoolEHes
the neutron-capture elements Y and Zr. We similarly con- suggest Sr, Y, or Zr abundances within the RCrB range, say
firm and extend the previous analyses of HD124448 (Schön- [X/Fe]of0.0to+0.9(Pandeyetal. 2001).
berner&Wolf1974;Heber1983). Wehaveprovidedthefirst, Our STIS spectra demonstrate that the star-to-star variation
abundances of neutron-captureelements for hot EHes. These in [Y/Fe] and [Zr/Fe] among hot EHes is at least as great as
neutron-captureelementabundancessuggestthatatleastinthe among the ‘majority’ RCrB stars: V1920Cyg with [Y/Fe] ≃
EHestarV1920Cyg,s-processnucleosynthesisdidoccurinits [Zr/Fe] ≃ 1.6 is at one end of the range and HD124448with
earlierevolution. [Y/Fe]≃[Zr/Fe]≃+0.1isattheotherend. Unfortunately,the
An interesting similarity is suggested by these abundances upperlimitssetonabundancesoflanthanidesdonotpermitus
andthoseoftheRCrBs. AfeatureoftheRCrBstarsisthelarge to check that EHes follow RCrBs in showing smaller enrich-
4 Abundancesofneutron-captureelementsinhotextreme-HeliumStars
ments of these heavier elements. The upper limits set for Ce duringthemerger(Pandeyetal. 2001).ThesurfaceoftheC-O
andNdinV1920Cyg([Ce/Fe]≤+1.1and[Nd/Fe]≤+1.0)hint white dwarf, the former core of an AGB star, will be rich in
atabehaviorsimilartotheRCrBs. s-processproductsbutlittle of this C-rich materialis required
As noted in the Introduction, two scenarios are potential at the surfaceof the star after accretionof He-rich materialto
sources of H-deficientluminousstars. A final-flash in a post- accountfor the observedabundancesof the lightelements. A
AGBstars,andamergerofaHewithaC-Owhitedwarf. The thin residual He-shellaroundthe C-O white dwarf core could
C and O abundancesof the hot EHes are consistent with pre- contributes-processproducts.TheHewhitedwarfanditspos-
dictions of white dwarf mergersbutnot the currentfinal-flash sibleH-richskinarenotexpectedtoberichinneutron-capture
models (Pandey et al. 2001; Saio & Jeffery 2002). Saio & elements. One supposes that accretion of the He white dwarf
Jeffery(2002)alsoshowthataEHestarformedbyaccretionof maybeaccompaniedbyanepisodeofnucleosynthesisinclud-
He-richmaterialbyaC-Owhitedwarfhasthepulsationalprop- ing release of neutronsthrough13C(α,n)16O with 13C created
ertiesofrealEHestars. Additionally,amergerbetteraccounts byH-burning.If,asseemsplausible,thestrengthoftheneutron
forlargelinewidthsseeninspectraofEHes;theaccretingstar sourceandefficiencyofneutroncapturesvariesfrommergerto
is spun up by the accreted gas from the (former) orbiting He merger, the range in the neutron-captureelement abundances,
white dwarf. Yet, the neutron-captureelement abundances of asseen here forthe EHe pair V1920Cyg andHD124448and
theEHesare,perhaps,morereadilyexplainedbythefinal-flash knownamong RCrBs, results. Aspects of the compositionof
scenario,assuggestedbythefinal-flashcandidatesFGSgeand the minority RCrBs suggest nucleosynthesismay accompany
V4334Sgrwithneutron-captureelementoverabundances. The theaccretionofHe-richmaterialbytheC-Owhitedwarf. The-
rangeinrelativeoverabundancesofYandZrtoBaandthelan- oretical studies of the merger scenario are to be sought with
thanidesvariesgreatlybetweenthepair: V4334Sgrhasahigh carefulexaminationoftheattendantnucleosynthesis.
relativeoverabundance(Asplundetal. 1997),butFGSgehasa WethanktherefereeGlennWahlgrenforanincisivereport,
lowrelativeoverabundance(Gonzalezetal. 1998). CarlosAllendePrietoforreadingandcommentingonadraftof
Enrichmentof neutron-captureelementsis notexpectedfor thispaper. WeacknowledgesupportfromtheSpaceTelescope
theEHesunlesssynthesisbyneutronsviathes-processoccurs ScienceInstitutethroughgrantGO-09417.
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