LMC origin of the hyper-velocity star HE 0437-5439. Beyond the supermassive black hole paradigm

0.54 MB·English

by  N. Przybilla

Astronomy&Astrophysicsmanuscriptno.hvs (cid:13)c ESO2008
February4,2008
L   E
− ⋆
LMC origin of the hyper-velocity star HE0437 5439
Beyond the supermassive black hole paradigm
8 N.Przybilla1,M.F.Nieva1,U.Heber1,M.Firnstein1,K.Butler2,R.Napiwotzki3,andH.Edelmann1
0
0
2 1 Dr.Remeis-SternwarteBamberg,Sternwartstr.7,D-96049Bamberg,Germany
2 Universita¨tssternwarteMu¨nchen,Scheinerstr.1,D-81679Mu¨nchen,Germany
n 3 CentreforAstrophysicsResearch,UniversityofHertfordshire,CollegeLane,HatfieldAL109AB,UK
a
J Received...;accepted...

9
2 ABSTRACT

] Context.Hyper-velocitystarsmovesofastthatonlyasupermassiveblackhole(SMBH)seemstobecapabletoacceleratethem.Hence
h theGalacticcentre(GC)istheironlysuggestedplaceoforigin.Edelmannetal.(2005)foundtheearlyB-typestarHE0437−5439to
p betooshort-livedtohavereacheditscurrentpositionintheGalactichaloifejectedfromtheGC,exceptifbeingabluestragglerstar.
- ItsproximitytotheLargeMagellanicCloud(LMC)suggestedanoriginfromthisgalaxy.
o
Aims.ThechemicalsignaturesofstarsattheGCaresignificantlydifferentfromthoseintheLMC.Hence,anaccuratemeasurement
r
t oftheabundancepatternofHE0437−5439willyieldanewtightconstraintontheplaceofbirthofthishyper-velocitystar.
s Methods.High-resolutionspectraobtainedwithUVESontheVLTareanalysedusingstate-of-the-artnon-LTEmodellingtechniques.
a
Results.WemeasuredabundancesofindividualelementstoveryhighaccuracyinHE0437−5439aswellasintworeferencestars,
[
fromtheLMCandthesolarneighbourhood, respectively.Theabundancepatternisnotconsistentatallwiththatobservedinstars

1 neartheGC,rulingouranoriginfromtheGC.However,thereisahighdegreeofconsistencywiththeLMCabundancepattern.Our
v abundanceresultscannotruleoutanoriginintheoutskirtsoftheGalacticdisk.However,wefindthelifetimeofHE0437−5439to
6 bemorethanthreetimesshorterthanthetimeofflighttotheedgeofthedisk,renderingaGalacticoriginunlikely.
5 Conclusions.OnlyoneSMBHisknowntobepresentinGalaxyandnoneintheLMC.HencetheexclusionofanGCoriginchallenges
4 theSMBHparadigm.Weconcludethattheremustbeothermechanism(s)toacceleratestarstohyper-velocityspeedthantheSMBH.
4 Wedrawattentiontodynamicalejectionfromdensemassiveclusters,thathasrecentlybeenproposedbyGvaramadzeetal.(2008).
.
1 Keywords.Galaxy:halo–MagellanicClouds–stars:abundances,distances,early-type,individual(HE0437−5439)
0
8
0
1. Introduction cationintheGalactichalo.However,itstraveltime(100Myrs)
:
v was found to be much longer than its main sequence lifetime
i StarsmovingfasterthantheGalacticescapevelocitywerefirst (≈25–35Myrs),renderingaGCoriginunlikely,thoughpossible
X predictedtoexistbyHills(1988).Thefirstsuchhyper-velocity
if HE0437−5439 would be a blue straggler star. Alternatively
r stars (HVSs) were discovered serendipitously only recently
thestarcouldhaveoriginatedfromtheLargeMagellanicCloud
a
(Brownetal. 2005; Hirschetal. 2005; Edelmannetal. 2005,
(LMC)asitismuchclosertothisgalaxy(18kpc)thantotheGC.
E05:HE0437−5439at723kms−1 heliocentricradialvelocity).
An accurate determination of its elemental abundance pat-
Asystematicsearchforsuchobjectshasresultedinthediscov-
tern should allow to distinguish between an origin in the
eryofsevenadditionalHVSuptonow(seeBrownetal.2007).
LMC or in the GC because their chemical compositions dif-
Hills(1988)predictedthatthetidaldisruptionofabinaryby
fer distinctively. HE0437−5439 is by far the brightest HVS
asupermassiveblackhole(SMBH)couldleadtotheejectionof
known(V=16.36,Bonanos&Lo´pez-Morales2008)andthere-
starswithvelocitiesexceedingtheescapevelocityofourGalaxy.
foresuitedbestforadetailedanalysis.Henceweobtainedhigh-
TheGalacticcentre(GC)isthesuspectedplaceoforiginofthe
resolution,highS/NspectraofHE0437−5439attheESOVLT
HVSsasithostsaSMBH.
withUVESandperformedaquantitativespectralanalysisusing
Both, the moderate rotational velocity (vsini=54kms−1)
state-of-the-artnon-LTEmodellingtechniques.
andthechemicalcomposition(consistentwithsolarabundances
withinafactorofafewfromaLTEanalysisofaspectrumwith
S/N≈5)wereconsideredevidenceforamainsequencenatureof 2. Observationsandquantitativenon-LTEanalysis
HE0437−5439(E05).Thisputthestaratadistanceof≈60kpc.
Nine spectra of HE0437−5439with a total integrationtime of
Numericalkinematicalexperimentswerecarriedouttotrace
the trajectory of HE0437−5439from the GC to its present lo- ∼4hwereobtainedonJanuary12,2006,coveringtherangefrom
3750 to 4950Å and 5700 to 9400Å at a resolving power of
Sendoffprintrequeststo:[email protected] R≈35000. The data reduction followed procedures described
⋆ BasedonobservationscollectedattheEuropeanSouthernObser- by Koesteretal. (2001). We took a spectrum of the DC white
vatory,Paranal,Chile(DDTproposal276.B-5024). dwarf WD0123−262, such that a reliable continuum normali-
2 Przybillaetal.:LMCoriginofthehyper-velocitystarHE0437−5439
Fig.1. Comparison of spectrum synthesis for HE0437−5439 (full line) with observation (grey). The main spectral features are
identified.WeakinterstellarCaH&KlinesareduetotheGalacticforeground,indicativeforonlyasmallamountofreddening.
sation in the blue couldbe achieved,a regioncontainingmany Table1.Stellarparametersoftheprogrammestars&elemental
broadBalmerlines.Moreover,themethodremovesfringesinthe abundancesε(X)=log(X/H)+12(numberoflinesanalysedin
redreliably,improvingconsiderablyonthestandardUVESdata parentheses)
reductionpipeline.ApeakS/Nof≈80perresolutionelementis
HE0437−5439 NGC2004-D15 HR3468
measuredforthecoaddedspectrumintheblue.
T (K) 23000±1000 21500±1000 22900±400
Two comparison stars with similar atmospheric pa- eff
logg(cgs) 3.95±0.10 3.70±0.10 3.60±0.05
rameters are considered: the LMC object NGC2004-D15 ξ(km/s) 1±1 1±1 5±1
(Kornetal. 2002, K02) and HR3468 in the solar neighbour- vsini(km/s) 55±2 48±3 11±2
hood(Nieva&Przybilla2008,NP08).Thesamedatareduction M/M⊙ 9.1±0.8 9.5±0.8 11.6±0.5
proceduresasusedforHE0437−5439areappliedtotheUVES Age(Myr) 18±3 23±1 14.5±0.5
spectrumofNGC2004-D15,obtainedfromtheESOarchive.A ε(C) 8.13±0.12 (3) 8.05±0.10 (1) 8.36±0.10(17)
comparisonwiththeLMCstarallowsforaconsistencytestwith ε(C) ... ... 8.47±0.04 (2)
the LMCbaseline abundancesdeterminedfrommain sequence ε(N) 7.58±0.04 (7) 7.15±0.06 (4) 8.10±0.08(54)
ε(O) 8.70±0.10 (1) ... 8.82±0.02 (4)
B-typestarsinpreviousstudies(e.g.K02;Trundleetal.2007).
ε(O) 8.71±0.07(40) 8.52±0.09(31) 8.80±0.09(45)
The quantitativeanalysis of the 3 stars was carried outfol-
ε(Mg) 7.40±0.10 (1) 7.25±0.10 (1) 7.67±0.03 (2)
lowingthemethodologydiscussedbyNieva&Przybilla(2007)
ε(Si) 7.20±0.10 (3) 7.37±0.15 (3) 7.47±0.09 (7)
and NP08. In brief, non-LTE line-formation calculations were ε(Si) 7.26±0.05 (4) 7.30±0.12 (3) 7.43±0.06 (5)
performed on the basis of line-blanketed model atmospheres, ε(Si) ... ... 7.52±0.05 (2)
using updatedversionsof D and S (Giddings1981; ε(Fe) 7.47±0.10 (3) 7.20±0.13 (3) 7.40±0.11(37)
Butler&Giddings 1985). State-of-the-art model atoms were
employed–H:Przybilla&Butler(2004);He:Przybilla(2005);
C/:Nieva&Przybilla(2006),NP08;N:Przybilla&Butler
(2001);O:Przybillaetal.(2000);Mg:Przybillaetal.(2001); lines.Microturbulenceξwasderivedinthestandardwaybyde-
Fe as adopted by Moreletal. (2007); an updated model manding that the line equivalent widths within an ion be inde-
for O, originally by Becker&Butler (1988). Our extensions pendentof abundance.Elemental abundanceswere determined
and improvements of the original Si// model atom from fromχ2-minimisationoffitstoindividualmetallineprofiles.
Becker&Butler(1990)werecrucialforouranalysisasthision- Basic stellar parameters and metal abundances for the pro-
ization equilibrium is our primary temperature indicator. Tests grammestarsarelistedinTable1.Massesandevolutionarylife-
withhigh-S/NspectraofGalacticstarsshowedthatthemodels timeswere determinedby comparisonwith evolutionarytracks
allow theobservedspectraforthese elementstobe reproduced from Schalleretal. (1992) and Schaereretal. (1993), for solar
withhighaccuracyandthereforeatmosphericparametersandel- andLMCmetallicity,respectively.Notethattheactualmetallic-
ementalabundancestobedeterminedwithsmalluncertainties. ity of HE0437−5439 is Z≈0.013, which gives a significantly
The atmospheric parameters (T , logg) were determined lower age (18Myr) than found by E05 (25–35Myr). The cor-
eff
frommetalionizationequilibria(mainlySi//,butalsoO/ responding distance to HE0437−5439 is 61±9kpc, in perfect
and C/, where available) and the Stark-broadened Balmer agreementwithE05.
Przybillaetal.:LMCoriginofthehyper-velocitystarHE0437−5439 3
starsintheLMC.Nitrogenisknowntohavealowpristineabun-
danceintheLMC(e.g.K02).WethusconfirmNGC2004-D15
tobeessentiallyunaffectedbymixingwithCN-cycledmaterial.
AbundancesintheGalacticstarHR3468arenear-solar,except
for enriched N. The abundances in HE0437−5439 tend to be
intermediate,withasolarN/Cratio.
3. ConstraintsontheoriginofHE0437−5439from
itschemicalsignature
WeshallnowcomparetheabundancepatternofHE0437−5439
tothechemicalsignaturesofthetwosuggestedplacesoforigin,
theGalacticcentreontheonehandandtheLMContheother.
Fig.2.Metalabundancesinthethreeprogrammestars(Table1),
The Galactic centre. A comparison of the abundance pattern
relative to the solar standard (Grevesse&Sauval 1998): of HE0437−5439 to that of a sample of stars near the GC
HE0437−5439(blacksymbols),NGC2004-D15(redsymbols)
(Cunhaetal. 2007; Najarroetal. 2008) is depicted in Fig. 3
andHR3468(opensymbols).Ionicspeciesareencodedaccord-
(upperpanel),foraveragevaluesoverallionsofanelement.The
ing to the legend. Solar and half-solar values are indicated by
sum of carbonand nitrogenis consideredto removesignatures
dottedanddashedlines,respectively.
of mixing with nuclear-processed matter from the individual
abundances (as catalysts, their total number is conserved in
the CN-cycle). The α-elements oxygen, magnesium, silicon,
The uncertainties in the atmospheric parameters were con-
and C+N are super-solar and enhanced relative to iron in the
strained by the quality of the simultaneous fits to all diagnos-
GC sample. In HE0437−5439,however,they are subsolar and
ticindicators:allhydrogenandheliumlines,andmultiplemetal
depleted with respect to iron. This rules out HE0437−5439to
ionizationequilibria.Suchagoodmatchhasbeenreportednever
beofGCorigin.
before in B-star analyses. The helium abundance in the three
stars was found to be consistent with the solar value. Standard LMC. AcomparisonoftheabundancepatterninHE0437−5439
deviationsσformetalabundanceswerecalculatedfromthein- with the LMC reference star is made in the middle panel of
dividuallineabundancesinanion.Thederiveduncertaintiesare Fig. 3. Both patterns are very similar as all error bars overlap.
extremelylowforB-starsanalyses.Weadoptedσ=0.10dexfor ThereisatendencyfortheabundancesofHE0437−5439tobe
ionswithonlyoneobservedline.Inaddition,systematicerrors slightlyhigherthaninNGC2004-D15(from0.04dexforC+N
needtobeconsideredbecauseofuncertaintiesinthestellarpa- to 0.27dex for Fe, with an exceptionin Si). These abundances
rameters,atomicdataandthequalityofthespectra.Infact,the are fully consistent with today’s knowledge of the abundance
precisionof the analysisis limitedmostly bythenoise levelof scatter within the LMC (e.g. Luck&Lambert 1992; Hilletal.
thespectra.Alaboriousproceduretominimisesystematicshas 1995),consistentwithanoriginofHE0437−5439inthisgalaxy.
been developedby NP08, which allows us to estimate the sys-
Galactic disk. We are reluctant to accept the LMC origin be-
tematicuncertaintiesinelementalabundancestobe0.10dexfor
fore having discussed other options for a Galactic disk origin.
HR3468 and 0.15dex for the other two stars. This significant
From Fig. 3 (lower panel) it can be seen that HE0437−5439
improvementwithrespecttopreviousBstaranalysescanbeob-
and the solar neighbourhood star HR3468 have similar abun-
tained only when major sources of systematic errors are elim-
dances to within error limits, when compared on an element-
inated (NP08). The resulting synthetic spectrum is compared
to-elementbasis. However,there is a tendencyforall elements
with observation of HE0437−5439 in Fig. 1, for many strate-
except iron to be less abundantin HE0437−5439,by 0.09dex
gicspectralregions.Thematchbetweentheoryandobservation
(O)to0.27dex(Mg).Suchanabundancepatternwouldrestrict
isexcellentwithintheS/Nlimitations.Thefitsobtainedforthe
the place of birth of HE0437−5439to outside the solar circle
othertwostarsareofsimilarquality.
because of the Galactic abundance gradient. Consequently, we
Abundancepatternsforthethreeprogrammestarsarevisu-
cannot rule out an origin in the outskirts of the Galactic disk
alisedinFig.2,relativetothesolarstandard(Grevesse&Sauval
fromthechemicalsignaturealone.
1998)1.Notethesmallline-to-line-scatterandtheagreementof
abundancesfordifferentionsoftheelements,simultaneouslyfor
allionizationequilibria.Thiscanbeachievedonlywhenthestel- 4. Kinematicsrevisited
lar parameters are constrained well and highly reliable model
Edelmannetal. (2005) suggested the LMC as place of origin
atomsareemployedinthenon-LTEcalculations.Metalioniza-
solelybecauseofthediscrepancybetweenevolutionarylifetime
tionequilibriashowamuchhighersensitivitytoparametervari-
and time of flight. We have recalculated the kinematics on the
ationsthantheH/Helinesalone.Thisexplainspartoftheparam-
basisofthesignificantlyreducedageofHE0437−5439,follow-
eteroffsetwithregardtotheLTEanalysisofHE0437−5439by
ingtheprocedureofE05.Suchan estimatehasalso toaccount
E05.WederivedasomewhatlowerT andloggforNGC2004-
eff for the large space motionof the LMC (e.g.Piateketal. 2008)
D15thanK02,thoughwithinthemutualuncertainties.
of about 500pc/Myr in direction east-north-east. The total dis-
ElementalabundancesinNGC2004-D15areabouthalfso-
tance to travel by HE0437−543 after ejection ∼18Myr ago is
lar,exceptfornitrogen,whichisingeneralagreementwiththe
therefore ∼19+7kpc. Hence, the total ejection velocity should
baseline metallicity derived in previous studies of early-type −3
amount to ∼1000kms−1, if originating in the kinematic centre
1 Similarconclusionsaredrawninthefollowingiftherevisedsolar oftheLMC.AccountingfortheextensionoftheLMC,ejection
abundancesofGrevesseetal.(2007)areused. velocitiesmayvaryby∼100kms−1.
4 Przybillaetal.:LMCoriginofthehyper-velocitystarHE0437−5439
Because no SMBH is known to exist in the LMC,
the SMBH slingshot scenario or other ejection models (e.g.
Baumgardtetal. 2006) invokinga SMBH are ruled out.Hence
theremustbe an additionalphysicalmechanismcapableof ac-
celeratingamassivestartoaspacevelocityof1000kms−1.
Two such alternative scenarios have been proposed. ) A
close encounter of a binary with an intermediate-mass black
hole(IMBH) moremassive than103M⊙ hasbeenproposedas
a viable ejection mechanism by Gualandris&PortegiesZwart
(2007), who suggest the populous dense clusters NGC2004
or NGC2100 in the LMC could harbour such an IMBH.
) Dynamical ejection by interaction of massive binaries in
the cores of dense clusters is plausible (Gvaramadzeetal.
2008). This process is more likely to occur in the LMC than
in the Galaxy because the LMC hosts a significant number
of sufficiently massive and dense clusters. From the list of
Mackey&Gilmore (2003), eight young clusters can be iden-
Fig.3. Comparison of metal abundances in HE0437−5439 tified to be of the right age to qualify as candidate place of
(blackdots) with a sample of starsclose to the GC(blue trian- birthofHE0437−3954.PrimarycandidatesareNGC2100and
gles),upperpanel,theLMCreferencestarNGC2004-D15(red NGC2004,followedbyNGC1850andNGC1847.
diamonds),middlepanel,andthesolarneighbourhoodreference In consequence, hyper-velocity stars are not such simple
starHR3468(circles),lowerpanel.Errorbarsaccountforstatis- probes for the shape of the Galactic halo as suggested e.g. by
ticandsystematicuncertainties. Gnedinetal.(2005).Thefundamentalassumptionoftheexclu-
siveoriginofHVSsintheGChastobedropped,asacceleration
mayoccurinclustersthroughouttheGalacticdiskaswell.
Flight times fromthe outerGalactic disk to the currentpo-
Acknowledgements. We thank the Director General of ESO for making this
sition of HE0437−5439 are depicted in Fig. 4 of E05, who
studypossiblebygrantingdirector’sdiscretionarytimeandthestaffoftheESO
suggested that the age discrepancy could be overcome if the Paranalobservatoryfortheirvaluablesupport.M.F.N.andM.F.gratefullyac-
HVSwerea bluestragglerformedbya mergeroflessmassive knowledge financial supportbytheDeutsche Forschungsgemeinschaft (grants
stars. Our revised mass would require a merger of two ∼5M⊙ HE1356/44-1andPR685/3-1).
stars to form a blue straggler with the observedparametersfor
HE0437−5439. Their individual life times are about the same References
asthetimeofflight,renderinganoriginintheGalaxyveryun-
likely. Baumgardt,H.,Gualandris,A.,&PortegiesZwart,S.2006,MNRAS,372,174
Becker,S.R.,&Butler,K.1988,A&A,201,232
Becker,S.R.,&Butler,K.1990,A&A,235,326
Bonanos,A.Z.,&Lo´pez-Morales,M.2008,ApJ,submittedarXiv:0712.1825
5. Conclusion Brown,W.R.,Geller,M.J.,Kenyon,S.J.,&Kurtz,M.J.2005,ApJ,622,L33
Brown,W.R.,Geller,M.J.,Kenyon,S.J.,etal.2007,ApJ,671,1708
We haveperformeda quantitativespectralanalysisof theHVS
Butler, K.&Giddings, J.R.1985, in Newsletter ofAnalysis ofAstronomical
HE0437−3954andreferenceobjectsintheLMCandthesolar Spectra,No.9(Univ.London)
neighbourhood.Useofstate-of-the-artnon-LTEmodellingtech- Cunha,K.,Sellgren,K.,Smith,V.V.,etal.2007,ApJ,669,1011
niquesallowedpreciseelementalabundancestobeobtained. Edelmann,H.,Napiwotzki,R.,Heber,U.,etal.2005,ApJ,634,L181(E05)
Giddings,J.R.1981,Ph.D.Thesis,UniversityofLondon
From the results of the spectroscopic analysis we can rule
Gnedin,O.Y.,Gould,A.Miralda-Escude´,J.,Zentner,A.R.2005,ApJ,634,344
outa GC origin for HE0437−3954because its abundancepat- Grevesse,N.,&Sauval,A.J.1998,SpaceSci.Rev.,85,161
ternindicatesneitherhighmetallicitynorα-enrichment.Onthe Grevesse,N.,Asplund,M.,&Sauval,A.J.2007,SpaceSci.Rev.,130,105
other hand a high degree of consistency with the LMC pattern Gualandris,A.,&PortegiesZwart,S.2007,MNRAS,376,L29
Gvaramadze, V. V., Gualandris, A., & Portegies Zwart, S. 2008, preprint
is found when accounting for an abundance scatter within the
astro-ph/0702735
LMC2.Theobservedabundancepatterncouldalsobeconsistent Hill,V.,Andrievsky,S.,&Spite,M.1995,A&A,293,347
withanoriginfromtheoutskirtsoftheGalacticdisk,inthecase Hills,J.G.1988,Nature,331,687
HE0437−3954 being a blue straggler. However, a comparison Hirsch,H.A.,Heber,U.,O’Toole,S.J.,&Bresolin,F.,2005,A&A,444,L61
Koester,D.,Napiwotzki,R.,Christlieb,N.,etal.2001,A&A,378,556
ofthetime-of-flightwiththelifetimeofthebluestragglerprac-
Korn,A.J.,Keller,S.C.,Kaufer,A.,etal.2002,A&A,385,143
ticallyrulesoutthisscenario.Hence,HE0437−3954isunlikely
Luck,R.E.,&Lambert,D.L.1992,ApJS,79,303
ofGalacticorigin. Mackey,A.D.,&Gilmore,G.F.2003,MNRAS,338,85
Edelmannetal. (2005) suggested that HE0437−3954 was Morel,T.,Butler,K.,Aerts,C.,Neiner,C.,&Briquet,M.2007,A&A,457,651
ejectedfromtheLMC.Ourspectroscopicanalysislendsstrong Najarro,F.,Figer,D.F.,Hillier,D.J.,etal.2008,ApJ,accepted
Nieva,M.F.,&Przybilla,N.2006,ApJ,639,L39
supporttothisscenario.Theabundancepatternisconsistentwith
Nieva,M.F.,&Przybilla,N.2007,A&A,467,295
that of our LMC reference star. An ejection velocity of about Nieva,M.F.,&Przybilla,N.2008,A&A,inpressarXiv:0711.3783(NP08)
1000kms−1 is required for the star to have reached its posi- Piatek,S.,Pryor,C.,&Olszewski,E.W.2008,AJ,acceptedarXiv:0712.1764
tionfromthe LMC. Accuratepropermotionmeasurementsare Przybilla,N.2005,A&A,443,293
Przybilla,N.,&Butler,K.2001,A&A,379,955
neededtofinallyconfirmtheLMCorigin.
Przybilla,N.,&Butler,K.2004,ApJ,609,1181
Przybilla,N.,Butler,K.,Becker,S.R.,etal.2000,A&A,359,1085
2 Morecomprehensiveinvestigationsthanpresentlyavailable,using Przybilla,N.,Butler,K.,Becker,S.R.,&Kudritzki,R.P.2001,A&A,369,1009
improvedanalysistechniquesonlargersamplesofyoungstarscovering Schaller,G.,Schaerer,D.,Meynet,G.,&Maeder,A.1992,A&AS,96,269
theentireregionoftheLMCareneededtodrawdefiniteconclusionson Schaerer,D.,Meynet,G.,Maeder,A.,&Schaller,G.1993,A&AS,98,523
thedegreeofchemical(in)homogeneityinthatgalaxy. Trundle,C.,Dufton,P.L.,Hunter,I.,etal.2007,A&A,471,625