First detection of photospheric depletion in the LMC

0.15 MB·English

by  M. Reyniers

Astronomy&Astrophysicsmanuscriptno.6878 (cid:13)c ESO2008
February5,2008
LE
⋆
First detection of photospheric depletion in the LMC
M.Reyniers⋆⋆ andH.VanWinckel
InstituutvoorSterrenkunde,DepartementNatuurkundeenSterrenkunde,K.U.Leuven,Celestijnenlaan200D,3001Leuven,Belgium
e-mail:[email protected]
ReceivedDecember5,2006;acceptedDecember31,2006
7
0
ABSTRACT
0
2
Context.RecentphotosphericabundancestudiesofgalacticfieldRVTauristarsshowthatdepletionofrefractoryelementsisrather

n commonintheseevolvedobjects.
a Aims.Theprocessthatcreatesthischemicalanomalyisnotunderstoodwell,butitprobablyrequiresthepresenceofgravitationally
J bounddustinabinarysystem.Wetestforthepresenceofdepletioninextra-galacticobjects.

1 Methods.Adetailedphotosphericabundancestudyonthebasisofhigh-qualityUVESspectrawasperformedontheRVTauristar
1 intheLMC:MACHO82.8405.15.Abundanceswerederivedusingacriticallycompiledlinelistwithaccuratelog(gf)valuesandthe
latestKuruczmodelatmospheres.
1 Results.With[Fe/H]=−2.6incombinationwith[Zn/Fe]=+2.3and[S/Ti]=+2.5,MACHO82.8405.15displaysastrongdepletion
v abundancepattern.TheeffectofthedepletioniscomparabletothestrongestdepletionsseeninfieldGalacticRVTauristars.
7 Conclusions.ThechemicalanalysisofMACHO82.8405.15provesthatthedepletionprocessalsooccursintheextragalacticmembers
2 oftheRVTauripulsationclass.OurprogramstarisamemberofalargersampleofLMCRVTauriobjects.Thissampleisunique,
3 sincethedistancesofthemembersarewell-constrained.Furtherstudiesofthissamplearethereforeexpectedtogaindeeperinsight
1 intothepoorlyunderstooddepletionphenomenonandoftheevolutionarystatusofRVTauristarsingeneral.
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Keywords.Stars:AGBandpost-AGB–Stars:abundances–Stars:individual:MACHO82.8405.15–MagellanicClouds
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/
h 1. Introduction Theextensivechemicalstudiesof the photospheresof field
p
RV Tauri stars showed, however, that none of the RV Tauri
-
o It is now almost one century ago that Seares&Haynes (1908) stars shows evidence of post 3rd dredge-up chemical enrich-
r noted how RV Tauri, the prototype of the RV Tauri variables, ments, such as enhanced carbon or enhanced s-process abun-
st showsa lightcurvewith subsequentdeepandshallowminima. dances (except for maybe V453Oph, see Derooetal. 2005).
a Thisobservation,togetherwiththe requiredlimitsonthespec- Instead, the abundance studies (see e.g. Giridharetal. 2000,
v: traltype (F to K), is still used as the definingcharacteristic for 2005;Maasetal.2005,andreferencestherein)revealthatsevere
i theRVTaurivariables.Prestonetal.(1963)definedthreespec- abundance anomalies are observed in mainly RVB stars, with
X troscopicclasses:ThestarsoftypeRVAhavespectraltypeG-K, underabundancesofelementswithahighcondensationtemper-
r and show strong absorptionlines butnormalCN or CH bands. ature. The origin of these depletion patterns probablyinvolves
a
TheRVBstarsaregenerallysomewhathotter(spectraltypeF), chemicalfractionationdue to dust formationin the circumstel-
weak-linedobjectsthatshowenhancedCNandCHbands.RVC lar environment,followed by a decoupling of the gas and dust
stars are also weak-lined stars, but they show normal bands of withareaccretionofthecleanedgasonthestellarphotosphere.
CHandCN. It was realised that the spectroscopic classes of Prestonetal.
The exact evolutionary stage of RV Tauri stars is still not could be attributed to a different degree of depletion, with the
veryclear,buttheyareprobablyinthepost-AGBstageofevo- RVB stars the most heavily depleted. The whole process prob-
lution,asfirstarguedbyJura(1986).ManyoftheRVTauristars ably requires a stable circumstellar environment, like a stable
show an infrared excess, which is caused by the circumstellar disk (Watersetal. 1992). Since a stable disk is only likely to
dustthatwas likelyformedby heavymass lossduringthe pre- formin abinarysystem,itisbelievedthatalldepletedRV Tau
cedingAGBphase.TheMACHOexperimentinthe90srevealed starsareprobablybinaries(VanWinckel2003;DeRuyteretal.
the first extragalactic RV Tauri stars in the Large Magellanic 2006). Note that depletion does not seem to occur in globular
Cloud. Alcocketal. (1998) showed the light curvesof abouta clusterRVTauristars,orinfieldRVTauristarsoflowmetallic-
dozen RV Tauri stars in the LMC, with “formal periods” (the ity ([Fe/H]<−1). Recently, we (Reyniersetal. 2006) reported
timebetweentwosuccessiveminima)between40and120days, ontheintrinsicmetal-poorRVTauristarMACHO47.2496.8in
definedasinglePLrelationforthesestars,andstrengthenedthe theLMC,whichisuniquefordisplayingaverystrongs-process
post-AGBinterpretationofRVTauristarsbytheirluminosities. enrichment. This corroboratesthe post-AGB nature of this ob-
ject,butaddstothechemicaldiversityobservedinthispulsation
class.
Sendoffprintrequeststo:M.Reyniers
⋆ based on observations collected at the European Southern PreliminaryworkbyLloydEvans&Pollard(2004)showed,
Observatory,Chile(programme074.D-0619(A)) on the basis of low-resolution spectra, that at least two RV
⋆⋆ PostdoctoralfellowoftheFundforScientificResearch,Flanders Tauri stars found through the MACHO experiment are RVB
2 M.ReyniersandH.VanWinckel:FirstdetectionofphotosphericdepletionintheLMC
Table 1. Log of the high-resolution VLT-UVES observations,
with small spectral gaps between 5757Å and 5833Å and be-
tween8521Åand8660Åduetothespatialgapbetweenthetwo
UVESCCDs.
date UT exp.time wavelength S/N
start (sec) interval(Å)
2005-02-08 01:43 3600 4780−6808 130
3758−4983 110
2005-02-09 02:37 7200
(cid:26)6705−10084 120
stars,andhencepotentiallydepleted.Inthisletter,wefocuson
the brightestofboth objects:MACHO82.8405.15(m =14.3).
V
Alcocketal. (1998) found a period of 93.1days for this ob-
jectanda meanabsolutevisualmagnitudeofM =−4.52.The
V
amplitude of the pulsation is significant, with a peak-to-peak
of 0.89 in the MACHO V magnitude. Here, we present a de-
tailed abundance analysis of MACHO82.8405.15, based on
high-resolution,highsignal-to-noiseVLT-UVESspectra.
2. Observationsandanalysis
Spectra were taken with UVES on the VLT-UT2 (Kueyen) in
visitor mode by one of us (HVW). Details of the observations,
togetherwithsomeindicativesignal-to-noise(S/N)ratios,canbe
foundin Table 1. The spectra were taken just beforeminimum
light.Thereductionwasperformedinthededicated“UVEScon-
text” of the ESO MIDAS software packet, including all stan-
dard reduction steps. Optimal extraction was used, except for
Fig.1. The VLT+UVES spectrum of MACHO82.8405.15, to-
the spectrum taken in the first night, since this was taken with
gether with the spectrum of ACHer, around the 4810Å Zn
theslicerandoptimalextractionisnotimplementedforthistype
line(upperpanel)andthe6155Åoxygentriplet(lowerpanel).
ofdata.AsamplespectrumcanbefoundinFig.1.
ACHerisadepletedfieldRVTauristarthatissomewhatcooler
Themethodofanalysisissimilartothemethodusedinour
(T =5500K) than MACHO82.8405.15.It is extensively dis-
previous publications on similar stars (e.g. Derooetal. 2005). eff
cussed in VanWinckeletal. (1998). In the upperpanel, the Zn
First, a complete line identification was performed, using the
lineisremarkablystrong,whilelinesofotherironpeakelements
line lists of The´venin (1989, 1990). Then, lines with a clean
arealmostabsent.Inthelowerpanel,theoxygentripletaround
profile and an equivalent width smaller than 150mÅ were se-
6155Å is clearly visible, indicating the hotter temperature for
lected for abundance calculation purposes. The log(gf) values
MACHO82.8405.15.Notethatthespectruminthelowerpanel
of the selected lines were taken from the line list with accu-
hasquitealotoftelluricemission.Correctionforthisemission
rate oscillator strengths described in VanWinckel&Reyniers
is difficult, since this partof the spectrum is observedwith the
(2000).Lineswithoutanaccuratelog(gf)valuewerediscarded.
imageslicer,hencenoskywindowisavailable.
Forthe abundancecalculation,we usedthe latestKuruczmod-
els(http://kurucz.harvard.edu),incombinationwiththeLTE
line analysis program MOOG by C. Sneden (April 2002 ver-
sion). Since initially only 24 Fe and 3 Fe lines were found as a result of the lower electron density and of the weaker UV
withourusuallinelist,11Feand4Felineswereaddedfrom blanketing.
The´venin(1989, 1990), althoughthese papersare generallyre- We had to follow an alternative scheme to deduce the at-
gardedasalessaccuratesourceforoscillatorstrengths. mosphericparameters.First, we estimated the gravity fromthe
In our search for the atmospheric parameters of luminositythatisgiveninAlcocketal.(1998).Thisestimateis
MACHO82.8405.15, we realised that the classical “spec- of course dependent on the adopted effective temperature, the
troscopic” method of studying the iron lines failed. The usual initial mass, and the reddening. However, for all temperatures
techniqueofdemandingexcitationequilibriumfortheFelines compatiblewithanF-starspectrum,alowgravityoflogg≤1.0
resultedinaneffectivetemperatureofT =7000K,butsucha isfound.Afurtherdecreaseintheparameterspacewasobtained
eff
higheffectivetemperaturegivesunrealisticsupersolarzinc(Zn) throughfittingtheHγandHδBalmerlinesusingKuruczBalmer
abundances. Moreover, at T =7000K, ionisation equilibrium profiles. The Hα and Hβ lines were not used for this purpose
eff
can only be attained with a high gravity of logg=2.5. The since they show strong emission features. The wings and the
reasontheclassicalmethodfailsisnotclear,butisprobablyre- coresofthetwolineswerefittedsimultaneously,withmoreem-
latedtothedynamicalcharacterofthephotosphere(seeSect.3) phasisonagoodfitforthewings.Wecouldconstraintheparam-
and/orto the well-knownnon-LTEeffectof UV overionisation eterspace furthertomodelswithT ≤6000K bythisBalmer-
eff
in the atmospheres of late-type stars (e.g. Shchukinaetal. profilefitting.Fromacomparisonofphotometrytakenwiththe
2005). This effect is expected to be greater in metal-poor stars Swiss Euler telescope at La Silla (see Reyniersetal. 2006, for
M.ReyniersandH.VanWinckel:FirstdetectionofphotosphericdepletionintheLMC 3
Fig.3. The spectral energy distribution (SED) of
Fig.2.ThefinalHγandHδBalmerfits. MACHO82.8405.15. The dereddened photometry is shown
with triangles, while the Kurucz model is overplotted with a
solid line. The photometry includes: U, B, V magnitudes in
more details on these observations) with synthetic colours, we
the Geneva system; I in the Cousins system; I, J, and K from
inferredthatthereddeningiscertainlysmall,withanupperlimit
DENIS; and J, H, and K from 2MASS. The optical photom-
ofE(B-V)=0.2.Ifwecombinethisinformationwiththegravity
etry shown here is taken at a more luminous phase than the
fromtheluminosityandtheBalmerfits,wecouldconcludethat
UVES spectroscopy on which the atmospheric parameters are
theonlyconsistenttemperatureisT =6000K.Thefinalchoice
eff based. The SED fits, however, were only used to constrain the
betweenthetwoconsistentgravitiesatthistemperature,0.5and
reddeningtowardsMACHO82.8405.15.
1.0,wasmadeonthebasisoftheconsistencyintheabundances,
resultinginlogg=0.5.TheHγandHδBalmerfitswiththefinal
parametersareshowninFig.2;thespectralenergydistribution
isshowninFig.3.Notethatthereisaphasedifferenceof0.24
betweentheUVESspectraonwhichtheBalmerfittingwasper-
formedand the photometry,implying a high uncertainty in the
derivedparameters(atleast±250KinT and±1.0inlogg).We
eff
wanttostress,however,thatintheparameterdetermination,the
photometrywasusedsolelyto constrainthereddeningtowards
MACHO82.8405.15.
3. Radialvelocitiesofindividuallines
Beforeturningtotheactualabundances,theradialvelocitiesof
theindividualspectrallinesneedtobenoted.Duringouranaly-
sis, we noticedthatsomelinesweremorevelocity-shiftedthan
others. Moreover, a clear correlation exists between the radial
velocity and the excitation potential of a line, with lower ve-
locities for lines with a high excitation potential. This is illus-
trated in Fig. 4. It is indicative of an optical depth effect due
Fig.4.Theclearcorrelationbetweentheheliocentricradialve-
to a differential velocity field in the line-formingregion of the
locitiesofindividuallinesandtheirlowerexcitationpotentials.
dynamical photosphere of this pulsating star. In field RVTauri
stars,thisshock-wavepassagecanleadtoline-splittingatsome
pulsational phases (e.g. Gilletetal. 1990). Very similar veloc-
the number of lines used; the third one is the mean equivalent
ity effects were also detected in the small-amplitude pulsating
width;thefourthcolumngivestheabsoluteabundancesderived
post-AGBstarHD56126(Le`breetal.1996;Barthe`setal.2000; logǫ=log(N(el)/N(H))+12;σ ,thefifthcolumn,istheline-to-
ltl
Fokinetal.2001),andwereagainattributedtothepassageofa
linescatter;thesixthcolumngivestheabundancerelativetothe
shock wave. Note that the UVES spectra were taken just be- sun [el/H]. For the referencesof the solar abundances(seventh
foreminimumlight,i.e.thephaseatwhichanewshockwaveis column)neededtocalculatethe[el/H]values:seeReyniersetal.
expectedtodevelop(Gilletetal.1989).Theadoptionofplane-
(2006). The dust condensation temperatures (last column) are
parallel,LTEKuruczmodelscanthusonlybeafirstapproxima-
takenfromLodders(2003).
tionoftherealphotosphere.
The abundances are also graphically presented in Fig. 5,
where the abundance relative to solar [el/H] is plotted against
condensationtemperature.Theclearanti-correlationasseenon
4. Resultsanddiscussion
Fig. 5 is undoubtfully recognised as a depletion pattern: the
The final abundances can be found in Table 2. The first col- slightlysubsolarabundancesofZnandSareexpectedforafield
umnofthistablegivesthe actualion;thesecondcolumngives LMC-star, while ironis depletedby morethan a factorof 300.
4 M.ReyniersandH.VanWinckel:FirstdetectionofphotosphericdepletionintheLMC
Table2.AbundanceresultsforMACHO82.8405.15.
MACHO82.8405.15
T =6000K
eff
logg=0.5(cgs)
ξ =3.5kms−1
t
[M/H]=−2.5
ion N W logǫ σ [el/H] sun T
λ ltl cond
C 13 63 8.00 0.13 −0.57 8.57 40
N 9 46 8.04 0.16 0.05 7.99 123
O 3 26 8.53 0.10 −0.33 8.86
[O] 2 102 9.16 0.08 0.30 8.86
Na 4 52 5.80 0.14 −0.53 6.33 953
Mg 2 38 5.29 0.01 −2.25 7.54 1327
S 3 37 6.99 0.02 −0.34 7.33 655
Ca 3 25 4.24 0.15 −2.12 6.36 1505
Sc 1 17 0.12 −3.05 3.17 1647
Ti 4 22 2.18 0.26 −2.84 5.02 1573 Fig.5. The anti-correlation between the abundance of an el-
Cr 2 49 3.26 0.04 −2.41 5.67 1291 ement ([el/H]) and its condensation temperature. This indi-
Cr 3 39 3.40 0.07 −2.27 5.67 1291
cates that a very efficient depletion process has taken place
Mn 3 28 3.60 0.13 −1.79 5.39 1150
Fe 35 30 5.08 0.25 −2.43 7.51 1328 in MACHO82.8405.15.The errorbar is the line-to-line scatter
Fe 7 52 4.87 0.19 −2.64 7.51 1328 listedinTable2.
Zn 4 87 4.26 0.06 −0.34 4.60 723
Y 1 24 −0.26 −2.50 2.24 1647
Ba 3 105 0.13 0.15 −2.00 2.13 1447 DeRuyteretal. 2006) that binarity plays a key role in the cre-
ationofastabledustycircumstellarenvironmentthatisprobably
neededfortheprocesstooccur.High-resolutioninfrareddataof
MACHO82.8405.15wouldyieldinvaluableinformationonthe
TheCNOelementshavelowcondensationtemperaturesandare physicalconditionsandmineralogyofthecircumstellarenviron-
typicallyexpectednottobedepleted.Thelowcarbonabundance ment.
of [C/H]=−0.6 makes clear that MACHO82.8405.15 has not Our first extragalactic detection of a depleted photosphere
experienced a 3rd dredge-up. One remarkable result is the de- in a luminous star shows that the depletion process may be
tectionoftwoforbidden[O]linesat6300.23Åand6363.88Å, rathercommonelsewhere.MACHO82.8405.15isamemberof
bothinabsorption.Theabundancesderivedfromtheselinesare a larger sample of luminous LMC RV Tauri objects. Our re-
muchhigherthantheabundancesderivedfromtheOtripletat cent analysis of one of the other objects (MACHO47.2496.8,
6155Å (see Table 2). The [O] lines are known notto be very Reyniersetal.2006)shows,however,averydifferentchemical
sensitivetonon-LTEeffects,sothislargeabundancedifference resultwith a verystrongs-process-enhancedphotospherein an
isnotexpected.Thetwo oxygenabundancescanbe broughtin intrinsically metal-poor environment of [Fe/H]=−1.4. Clearly
agreement if one adopts a slightly lower temperature (5750K) the LMC RVTauri stars are also very diverse chemically. A
combined with a very low gravity of logg=−0.4. Such a low global study of these objects will enable us to better constrain
gravity is, however, not compatible with the luminosity. The boththedepletionprocessitself,aswellasthetrueevolutionary
abundances of the s-process elements Y and Ba show an ex- statusofRVTauristars.
pectedbehaviourandareprobablynotintrinsicallyenhanced.
Acknowledgements. Itis apleasure tothank Clio Gielen forthe construction
oftheSED,SophieSaesenfortheEulerobservations, theanonymousreferee
forthequickandconstructivereport,andtheGenevastaffforobservationtime
5. Conclusion
ontheEulertelescope. MRacknowledges financialsupportfromtheFundfor
ScientificResearch-Flanders(Belgium).
In this letter, we have found evidence that the photospheric
depletion phenomenon, which is frequently observed in field
RV Tauri stars, also occurs in the extragalactic members of
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