Local stars formed at z>10: a sample extracted from the SDSS

0.23 MB·English

by  Luca Sbordone

Local stars formed at z>10: a sample extracted from
the SDSS
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J LucaSbordone
7 Max-PlanckInstitutfürAstrophysik-Garching,Germany
1 GEPI,ObservatoiredeParis,CNRS,UniversitéParisDiderot,France

] Piercarlo Bonifacio
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GEPI,ObservatoiredeParis,CNRS,UniversitéParisDiderot,France
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INAF-OsservatorioAstronomicodiTrieste,Italy
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ElisabettaCaffau
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- ZAH–LandessternwarteHeidelberg,Germany,
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GEPI,ObservatoiredeParis,CNRS,UniversitéParisDiderot,France
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Hans-GünterLudwig
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ZAH–LandessternwarteHeidelberg,Germany,

2 GEPI,ObservatoiredeParis,CNRS,UniversitéParisDiderot,France
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As the Universe emerged from its initial hot and dense phase, its chemical composition was
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2 extremelysimple,beinglimitedtostableHandHeisotopes,andtracesofLi. Thefirststarsthat
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formedhadsuch initialcomposition. However,theyquicklybegantoproducea wholearrayof
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heavier nuclei, polluting the interstellar medium. While none among these first stars has been
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0 detectedtodate, anincreasingsampleexistsoftheirdirectdescendants,starswith afractionof
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heavyelementsoftheorderof1/1000ofthesolarvalue,orless. Inmostcases,suchstarsshould
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haveformedatredshiftofabout10orbeyond,andtheirchemicalcompositioncanprovidecrucial
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constraintstothenatureoftheveryfirststars. Extremelymetalpoor(EMP)starsareexceedingly
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a rare. We used the low resolution spectra obtained by the Sloan Digital Sky Survey (SDSS) to
search for EMP candidates: results of VLT-UVES high resolution follow-up for 16 of them is
presentedhere. A newly developedautomatic abundanceanalysisand parameterdetermination
code,MyGIsFOS,hasbeenemployedtoanalyzethedetailedchemicalabundancesofsuchstars.
11thSymposiumonNucleiintheCosmos
19-23July2010
Heidelberg,Germany.
∗
Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Localstarsformedatz>10: asampleextractedfromtheSDSS LucaSbordone
1. Target selectionandlow-resolutionmetallicityestimate
Candidate EMP stars were searched among stars with colors compatible with Halo the F-
G turn-ff population, and having been observed spectroscopically by SDSS (up to data release
6). SDSS low-resolution (R=2000) spectra were analyzed by means of an automatic abundance-
estimation code, comparing a number of metallic features against agrid of synthetic spectra. The
stellar effectivetemperature wasestimatedbymeansofatheoretical calibration oftheT /(g-z)
eff
relationship. Thegravitywaskeptfixedatlogg=4inthiscandidate-selection phase. SeeLudwig
etal.(2008)foramoredetailed description ofthecandidate selection phase.
2. Automated abundance analysiswithMyGIsFOS
The candidate EMP stars selected were then observed by means of UVES@VLTat a typical
resolution of about R=30,000, S/N∼40-50 per pixel. Three among them appeared to be carbon-
enhancedmetal-poorstarsandtheirfullanalysisispresentedbyBeharaetal.(2010). Sixteenmore
objects, notshowinganomalous CNOabundances, arepresented inthiscontribution.
Anew,fullyautomaticparameterestimationandabundanceanalysiscodehasbeendeveloped
andapplied tothefullchemical analysis ofthesestars. TheMyGIsFOScodeworksbyreplicating
astandard manualabundance analysis whereT isprovided fromoutside (e.g. fromphotometry
eff
calibration), gravity, microturbulence and [Fe/H] are derived by measuring selected Fe I and Fe
II features, and, after the atmosphere parameters are established, a full chemical analysis can be
performed by measuring lines of all the other significant elements. The user selects a number of
spectral regions for pseudo-continuum estimation, as well as all the needed metallic lines. Each
line is then fitted against a grid of pre-calculated synthetic spectra varying in T , logg, micro-
eff
turbulence, [Fe/H],and[a /Fe],bya c 2 minimization scheme. Stellarparametersareiteratedfrom
first-guess valuestothefinalones. AnexampleofMyGIsFOSoutput isshowninFig. 1.
ParameterestimationandfullchemicalanalysiswithMyGIsFOStakesbeween15and30sec-
onds per star on a mainstream PC. The code is currently available on collaboration basis, and is
foreseentobereleasedtothecommunityafterdebugging, performanceassessment, anddocumen-
tationarecompleted.
3. Abundance analysisresults
Comparedtothesampleanalyzedinthelargeprogram“FirstStars”(Bonifacioetal.,2009,and
referencestherein)thenewsampleappearsremarkablymoremetalpoor,assessingtheeffectiveness
of using SDSSfortarget selection (see Fig. 2). Theselection strategy proved highly effective: all
theselected targetswereconfirmedasEMPbythehighresolution analysis.
In fig. 3 we present [Cr/Fe] and [Ni/Fe] plotted against [Fe/H] for the new sample along
withthe“FirstStars”sample. Intheleftpanelthethree[CrII/Fe]measurements appear discrepant
withrespectofthe[CrI/Fe]measurements forthesamestars,underlining theimportance ofNLTE
effects for Crin EMPstars (Bonifacio et al., 2009; Bergemann & Cescutti, 2010) which were not
accounted forinthepresentanalysis.
Ofparticular interest is the evidence ofa sizable population ofNi-rich stars at extremely low
metallicities: 8 stars out of 16 show [Ni/Fe]>0.2. While the current analysis is preliminary, it
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Localstarsformedatz>10: asampleextractedfromtheSDSS LucaSbordone
Figure 1: An exampleof a set of MyGIsFOSfinalfits overFe I (blueboxes), Si I (greenbox), andSc II
(redbox)features. Darkgreyboxesoutlinepseudo-continuumregions. Thelightgrey,shadedboxisaFeI
featurewhichhasbeenrejectedasbad.
Figure2: ComparisonofthemetallicitydistributionofthesampleofEMPstarsextractedfromSDSS(left
panel)withtheonepresentedintheESOlargeprogram“FirstStars”(rightpanel)
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Localstarsformedatz>10: asampleextractedfromtheSDSS LucaSbordone
Figure3:Intheleftpanel,[CrI/Fe]isplottedagainst[Fe/H]fortheSDSSsample(filleddots)andthe“First
Stars” sample (crosses). The three red filled dots correspond to Cr II measurements in three stars of the
SDSSsample.Intherightpanel,[Ni/Fe]isplottedagainst[Fe/H],withthesamesymbols.
would appear that this result is indeed robust. A final analysis, including temperatures based on
3D-hydrodynamical Ha profiles,iscurrently underway.
References
Behara,N.T.,Bonifacio, P.,Ludwig,H.-G.,Sbordone,L.,GonzálezHernández,J.I.,&Caffau,E.
2010,A&A,513,A72
Bergemann, M.,&Cescutti,G.2010, arXiv:1006.0243
Bonifacio, P.,etal.2009,A&A,501,519
Ludwig,H.-G.,Bonifacio,P.,Caffau,E.,Behara,N.T.,GonzálezHernández, J.I.,&Sbordone,L.
2008,PhysicaScriptaVolumeT,133,014037
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