gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT

0.97 MB·English

by  K. Zwintz

Astronomy&Astrophysicsmanuscriptno.pmsgdor-aa2 c ESO2013
(cid:13)
December11,2013
γ Doradus pulsation in two pre-main sequence stars discovered by
CoRoT ⋆
K.Zwintz1,2⋆⋆,L.Fossati3,T.Ryabchikova4,A.Kaiser1,M.Gruberbauer5,T.G.Barnes6,A.Baglin7,andS.
Chaintreuil7
1 UniversityofVienna,InstituteofAstronomy,Tu¨rkenschanzstrasse17,A-1180Vienna,Austria
e-mail:[email protected]
2 InstituutvoorSterrenkunde,K.U.Leuven,Celestijnenlaan200D,B-3001Leuven,Belgium
3 Argelander-Institutfu¨rAstronomiederUniversita¨tBonn,AufdemHu¨gel71,53121Bonn,Germany
3 4 InstituteofAstronomy,RussianAcademyofSciences,Pyatnitskaya48,119017Moscow,Russia
1 5 DepartmentofAstronomyandPhysics,St.MarysUniversity,Halifax,NSB3H3C3,Canada
0 6 TheUniversityofTexasatAustin,McDonaldObservatory,82Mt.LockeRd.,McDonaldObservatory,Texas79734,USA
2 7 LESIA,ObservatoiredeParis-Meudon,5placeJulesJanssen,92195Meudon,France

n Received/Accepted
a
J ABSTRACT

6
Context.Pulsationsinpre-mainsequencestarshavebeendiscoveredseveraltimeswithinthelastyears.Butnearlyallofthesepul-

] satorsareofδScuti-type.γDoradus-typepulsationinyoungstarshasbeenpredictedbytheory,butlackobservationalevidence.
R Aims.Wepresenttheinvestigationofvariabilitycausedbyrotationand(γDoradus-type)pulsationintwopre-mainsequencemem-
S bersoftheyoungopenclusterNGC2264usinghigh-precisiontimeseriesphotometryfromtheCoRoTsatelliteanddedicatedhigh-
. resolutionspectroscopy.
h Methods.The variability found using the CoRoT data was combined with the fundamental parameters and chemical abundances
p derivedfromhigh-resolutionspectroscopy,obtainedattheMcDonaldObservatory,todiscussthepresenceofpulsationandrotation
- inthetwo NGC2264 cluster members. Timeseries photometry of NGC2264VAS20 and NGC2264VAS87 wasobtained by the
o
CoRoTsatelliteduringthededicatedshortrunSRa01inMarch2008.NGC2264VAS87wasre-observedbyCoRoTduringtheshort
r
t runSRa05inDecember2011andJanuary2012.FrequencyanalysiswasconductedusingPeriod04andSigSpec.Thespectralanalysis
s
wasperformedusingequivalentwidthsandspectralsynthesis.
a
[ Results.The frequency analysis yielded ten and fourteen intrinsic frequencies for NGC2264VAS20 and NGC2264VAS87, re-
spectively,intherangefrom0to1.5d 1whichareattributedtobecausedbyacombinationofrotationandpulsation.Theeffective
−
1 temperatureswerederivedtobe6380 150KforNGC2264VAS20and6220 150KforNGC2264VAS87.Membershipofthetwo
± ±
v starstotheclusterisconfirmedindependentlyusingX-rayfluxes,radialvelocitymeasurementsandpropermotionsavailableinthe
1 literature.ThederivedLithiumabundancesoflog n(Li)=3.34and3.54forNGC2264VAS20andNGC2264VAS87,respectively,
9 areinagreementwiththeLithiumabundanceforotherstarsinNGC2264ofsimilarT reportedintheliterature.
eff
9 Conclusions.WeconcludethatthetwoobjectsaremembersofNGC2264andthereforeareintheirpre-mainsequenceevolutionary
0 stage.Theirvariabilityisattributedtobecausedbyrotationandg-modepulsationratherthanrotationonly.Assumingthatpartof
. theirvariabilityiscausedbypulsation,thesetwostarsmightbethefirstpre-mainsequenceγDoraduscandidates.
1
0 Keywords.stars:pre-mainsequence-stars:oscillations-stars:individual:NGC2264VAS20,NGC2264VAS87-techniques:
3 photometric-techniques:spectroscopic
1
:
v
i 1. Introduction fromgravitationalcontraction,havebeenreportedseveraltimes
X
withinthelastyears(e.g.,Ripepietal.2006,Zwintzetal.2009),
r Pre-andpost-mainsequenceevolutionarytracksintersectclose but the majority of these pulsators are of δ Scuti -type. PMS
a to the zero-age main sequence (ZAMS) and cross the instabil-
δ Scuti stars pulsate in p-modesand have spectral types from
ity regionsof δ Scuti and γ Doradus stars. Stars of both evo-
AtoearlyF;theirpulsationperiodsrangefrom 20minutesto
lutionary stages are therefore expected to show both types of ∼
6.5hours.However,recentlyaPMSδScuti-γDoradus hybrid
pulsations. δ Scuti and γ Doradus pulsators are well estab-
star,CoID0102699796,wasfoundusingdataobtainedwiththe
lishedamongthehydrogencoreburning(post-)mainsequence
CoRoTsatellite(Ripepietal.2011).
stars (e.g., Rodriguez et al. 2000, Handler 1999). Pulsations in
The classical (post-) main sequence γ Doradus stars have
pre-mainsequence(PMS)stars,whichgaintheirenergymainly
spectraltypesfromlateAtoFandareknowntopulsateinhigh
orderg-modeswithperiodsrangingfromapproximately0.3toa
Sendoffprintrequeststo:K.Zwintz,
e-mail:[email protected] fewdays(e.g.,Kayeetal.1999).Themodulationoftheradiative
⋆ The CoRoT space mission was developed and is operated by the fluxatthebaseoftheconvectiveenvelopehasbeensuggestedto
French space agency CNES, with participation of ESA’s RSSD and betheexcitationmechanismforγDoradus stars(e.g.Guziket
ScienceProgrammes,Austria,Belgium,Brazil,Germany,andSpain. al,2000,Dupretetal.2005).Similarpropertiesareexpectedfor
⋆⋆ Pegasus Marie Curie post-doctoral fellow of the Research γ Doradus stars in their PMS evolutionary phase. Bouabid et
Foundation-Flanders al.(2011)determinedthetheoreticalseismicpropertiesofPMS
1
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
γ Doradus pulsators, but no confirmedmembersof this group (CoID0223979759),named VAS20 and VAS87 hereafter, and
havebeenreportedyet. aresubjectofthiswork.
The search for γ Doradus pulsation in PMS stars is com- VAS20 (RA = 06:40:05.66,DE = +09:35:49.3)has
2000 2000
plicated by the likely presence of rotational modulationdue to a V magnitude of 11.248mag (Sung et al. 1997) and a photo-
spotsinsuchyoungobjects.CandidatesforPMSγDoradus pul- metricallydeterminedspectraltypeF5(Young1978).AnX-ray
sationarelocatedinthesameregionoftheHertzsprungRussell fluxwasdetectedbyROSAT/HRIobservations(Flaccomioetal.
(HR) diagram as the hotter members of the group of T Tauri 2000)favoringthestar’syouth.Additionally,Fu˝re´szetal.(2006)
stars. ThelightcurvesofTTauristarsarewell-knowntoshow listedthestarasaclustermemberaccordingtoitsradialveloc-
regular and irregular variations caused by spots on the surface ity.Themembershipprobabilityisgivenas94%byVasilevskis
andtheinteractionofthestarswiththeircircumstellarenviron- etal.(1965).King(1998)reportsaneffectivetemperature(T )
eff
ment(e.g.,Alencaretal.2010).γDoradus typepulsationoccurs of 6219 140K using B V and V I photometry of Sung
± − −
on the same time scales as variabilitycaused by spots and sur- et al. (1997) and calibrations of Bessel (1979), and a Lithium
faceinhomogeneities,i.e.,withperiodsontheorderofuptofew abundance log n(Li) = 3.56 derived from low-resolution spec-
days.Itisthereforehardtounambiguouslydistinguishbetween troscopy.
the influence of rotation and pulsation in such stars (see, e.g., VAS87(RA =06:40:56.95,DE =+09:48:40.7)hasa
2000 2000
Uytterhoevenetal.2010). V magnitudeof12.323mag(Sungetal.1997)andaphotomet-
As γ Doradus stars have periods on the order of up to a ricallydeterminedspectraltypeF9(Young1978).Themember-
few days,theyare noteasy to observefromground.Only with ship probability is given as 96% by Vasilevskis et al. (1965).
dedicatedmulti-sitecampaignsitispossibletoresolvethesefre- VAS87 is also an X-ray source detected by the CHANDRA
quencies,bothspectroscopicallyandphotometrically.Thehigh- satellite (Ramirez et al. 2004). Thereforethe star is verylikely
precisionphotometryobtainedwiththeCoRoTsatelliteallowed to beyounganda memberofNGC2264.Inthe studybyKing
forthefirsttimetostudythesestarsinmoredetail. (1998)aT =6135 140Kandalog n(Li)=3.65aregiven.
eff
±
InthisstudywepresenttheanalysisofCoRoTlightcurves
andhigh-resolutionspectroscopyfortwo PMSmembersof the
youngopenclusterNGC2264. 3. TheCoRoTlightcurves
3.1.Observationsanddatareduction
2. NGC2264:VAS20andVAS87
TheCoRoTsatellite(Baglin2006)waslaunchedon2006,Dec
TheyoungopenclusterNGC2264(α =6h41m,δ =+9 27th,fromBaikonuraboardaSoyuzrocketintoapolar,inertial
2000 2000 ◦
53)hasbeenstudiedfrequentlyinallwavelengthregimesinthe circular orbit at an altitude of 896km. CoRoT carries a 27-cm
′
past.ItislocatedintheMonocerosOB1associationabout30pc telescopeandcanobservestarsinside twoconesof 10◦ radius,
abovethegalacticplaneandhasadiameterof 39arcminutes. oneatRA = 06 : 50andtheotheratRA = 18 : 50.Thefieldof
Sungetal.(1997)reportaclusterdistanceof75∼9 83pcwhich viewofthetelescopeisalmostcircularwithadiameterof3.8◦.
correspondstoadistancemodulusof9.40 0.25m±ag. EachCCDisasquareof1.3◦.Thefilterbandwidthrangesfrom
The age of NGC2264 can only be det±ermined with a rela- 370to1000nm.
tivelylargeerrorasitsmainsequenceconsistsonlyofmassive CoRoT observed NGC2264 includingVAS20 and VAS87
O and B stars and stars of later spectral types are still in their forthe first time for23.4daysin March2008duringthe Short
pre-mainsequencephase.Additionally,notallclustermembers RunSRa01withintheframeworkoftheAdditionalProgramme
might have formed at the same time and star formation is re- (Weiss 2006). A second short run, SRa05, on the cluster
ported to still be going on in the cluster (e.g., Flaccomio et al. NGC2264 was conducted in December 2011 / January 2012
1999). Therefore different values for NGC2264’s age are re- duringwhichonlyVAS87wasre-observedwithatimebaseof
portedintheliterature;theyliebetween3and10millionyears about39days.
(e.g.,Sungetal.2004,Sagaretal.1986).Ifstarsarestillbeing Forbothobservingruns,thecompleteclusterwasplacedin
formedinNGC2264,someofitsmembersmightevenbeonly one Exofield CCD and data were taken for all stars in the ac-
0.15millionyearsold(Sungetal.1997). cessiblemagnituderange,i.e.,from10to16maginR.The100
NGC 2264 is not significantly affected by reddening and brighteststarsinthefieldofNGC2264weretheprimarytargets
differential reddening is negligible as well. The mean value of tosearchforstellarpulsationsamongPMSclustermembers.
E(B V)=0.071 0.033mag reported by Sung et al. (1997) The reduced N2 data for both stars and both runs were ex-
− ±
is in good agreementwith values found by other authors (e.g., tractedfromtheCoRoTdataarchive.TheCoRoTdatareduction
Walker1956,Parketal.2000). pipeline(Auvergneetal.2009)flagsthosedatapointsthatwere
The first ever discovered PMS pulsators, the two PMS obtainedduringpassagesofthesatelliteovertheSouthAtlantic
δ Scuti stars V588Mon and V589Mon (Breger et al. 1972), Anomaly (SAA) and replaces them with linearly interpolated
aremembersofNGC2264.Sincethenseveralstudieshavebeen values. We did not use these ‘SAA-flagged’ data points in our
already devoted to the search for and analysis of its pulsat- analysis. The light curves of VAS20 and VAS87 obtained in
ing members, e.g., using data from the MOST space telescope 2008 (see Figures 1 and top left panel in Figure 2) consist of
(Zwintzetal.2009). 2704and2600datapoints,respectively,withasamplingtimeof
Usingthehigh-precisiontimeseriesphotometryofallacces- 512sec.The2011/12datasetofVAS87hasbeenobservedwith
sible stars in the field of NGC2264 obtained with the CoRoT a sampling time of 32 sec and therefore comprises 90073 data
satellite, we conducted a homogeneous survey for pulsating points.Wecheckedforthepresenceofcontaminantstarsinthe
candidates in the magnitude range between 10 and 16mag point-spreadfunctionandmaskusedfortheobservationsforall
in R. Cluster members showing variability on time scales be- threelightcurvesandfoundnocontaminatingsources.
tween about 0.3 to a few days have been the prime candidates For VAS20 chromatic light curves in red, green and blue
to search for PMS γ Doradus pulsation. Two of these are wereobtainedbyCoRoT,whileforVAS87dataweretakenonly
NGC2264VAS20 (CoID0500007018) and NGC2264VAS87 in white light. The chromatic light curves were used to check
2
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
thevariabilityofVAS20.Thevariationsarepresentinallthree
-15.680
colorswith the expecteddifferentphotometricamplitudes.The
-15.678
toppanelsofFigures1and2showthelightcurvesinwhitelight
g
forthetwostarswherethefluxesobtainedinthethreecolorsfor ma -15.676
VAS20weresummedtoincreasethesignal-to-noise. st. -15.674
n
i
-15.672
VAS20wasalsoobservedbytheMOST(Walkeretal.2003)
satellite in 2006(Zwintzetal. 2009) andVAS87wasincluded 2990 2995 3000 3005 3010
intheMOSTobservationsofNGC2264in2011/12.Asthetwo HJD - 2445545.0 [d]
frequency [ Hz]
starsare ratherfainttargetsforMOST, the lowamplitudevari- 0 5 10 15 2µ0 25 30
abilityiscompletelyhiddeninthenoiseandtherespectivelight
curvescannotbeusedforanyfurtherinvestigation. 2.5 F1
original
g] 2.0
a
m
m 1.5
3.2.ComparisonoftheCoRoTdataofSRa01andSRa05 ude [ 1.0 F3 F2
The light curves of VAS87 (Figure 2) observed in 2008 (i.e., mplit 0.5
SRa01)and2011/12(i.e.,SRa05)showadramaticdifferencein a
theamplitudesoftheobservedvariability.Adetailedcompara- 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
tivestudyofthecharacteristicsoftheNGC2264observationsby
frequency [c/d]
CoRoTobtainedinbothyearsshowedthatseveralinstrumental 0 5 10 15 20 25 30
effectsare present.Theseinstrumentaleffects,as describedbe- 0.8
original - F1F2F3
low,makeitverydifficultforustointerpretamplitudechanges F4
betweenthetwoobservingyears.Forthatreasonwedonotcon- ag] 0.6 F5
sideramplitudevariationsinthispaper. m F7
m
DifferentinstrumentalsettingswereusedforCoRoTin2008 de [ 0.4 F9 F8 F6
u
and 2011/12 which might affect the measured amplitudes: For plit 0.2 F10
m
the observations in 2008, the CCD E1 was used, but as it is a
notoperativeany more, the observationsin 2011/12were con- 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
ducted using the second Exoplanet CCD, E2. For the short
frequency [c/d]
run SRa01 the best possible observing season for NGC2264
was chosen (i.e., March 2008) minimizing the influence of
scattered light. Due to the coordinated simultaneous observa- Fig.1. CoRoT lightcurve(top panel) forVAS20, originalam-
tionsofCoRoT withthesatellites Spitzer(Werneretal. 2004), plitude spectrum (grey) from 0 to 3d−1 (bottom X axis), i.e.,
Chandra(Weisskopfetal.2002)andMOST(Walkeretal.2003) from 0 to 34.72µHz (top X axis), with the first three intrinsic
in 2011/12 not the optimum observing period was taken (i.e., frequencies identified (black), residual amplitude spectra after
December2011andJanuary2012).Thereforedifferentorienta- prewhiteningthethreehighestamplitudefrequencies(wherethe
tionsoftheCoRoTsatelliteinthetworunshadtobechosenand remaining 7 intrinsic frequencies are again marked in black).
differentphotometrictemplateshadtobeused.Thedifferentori- Notethedifferenty-axesscales.
entationof the cluster on the CCD couldhave some impacton
thepropertiesoftheobservations,e.g.,theCCDtemperaturein
the2011/12observationswashigherthanin2008.Additionally, 3.3.FrequencyAnalysis
theagingoftheCCDresultedinanincreasedrateofhotpixels
For the frequency analyses, we used the software package
andahigherdarkcurrentlevel.
Period04(Lenz&Breger2005)thatcombinesFourierandleast-
squaresalgorithms.Frequencieswerethenprewhitenedandcon-
ManyoftheseeffectsarecorrectedbytheCoRoTpipe-line
sideredtobesignificantiftheiramplitudesexceededfourtimes
inafirstapproximation,anddonotaffectthetime-domainprop-
the local noise level in the amplitude spectrum (Breger et al.
erties. But the situation is more difficult concerningthe ampli-
1993, Kuschnig et al. 1997). We verified the analysis using
tudesofindividualmodes.Forinstance,aperturephotometrycan
theSigSpecsoftware(Reegen2007).SigSpeccomputessignif-
beaffectedbythebackgroundlevelsofthestellarfieldandofthe
icancelevelsforamplitudespectraoftimeserieswitharbitrary
CCD.Anuncertaintyontheestimationofthebackgroundlevel
time sampling.Theprobabilitydensityfunctionofa givenam-
of 10% leads to an uncertainty on the amplitude of the same
plitudelevelissolvedanalyticallyandthesolutionincludesde-
order.Theestimateofthebackgroundusedbythestandardcor-
pendencesonthefrequencyandphaseofthesignal.
rection in the CoRoT pipe-line dependson many factors, such
In2008,VAS20andVAS87wereobservedwithasampling
astheproximityofthebackgroundwindows,theorientationof
time of 512s, correspondingto a Nyquist frequencyof 84d 1;
the CCD, and the shape of the photometrictemplate. So it can −
hence,thefrequencyanalysiswasconductedintherangefrom0
varyfromonetargettoanother.Adetailedstudyoftheseeffects
to84d 1.TheRayleighfrequencyresolution1/T correspondsto
iscurrentlyinpreparation(Baglinetal.2012). −
0.04d 1, where T is the lengthof the observations(23.4days).
−
As the above described effects influence the ultra-precise In thecase oftwo frequenciespresentwithin1/T,Kallingeret
CoRoTdatasignificantly,therelativeamplitudesforVAS87ob- al.(2008)haveshownusingnumericalsimulationsthattheup-
tained from the SRa01 data are different to those derived from perlimitforthefrequencyerroris1/(4 T)(seealsotheirFigure
·
theSRa05data.Thisiswhyinthispaperwedonotdiscussam- 3)iftheSigSpecsignificance(sig)ishigherthan16.Inourcase
plitudevariations. this correspondsto a resolution of 0.01d 1. For sig < 16, the
−
3
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Fig.2.CoRoTlightcurves(toppanels)forVAS87fromSRa01(2008data,leftpanel)andSRa05(2011/12data,rightpanel;light
curvewith32secsamplingingreyandbinnedto512secinblack)tothesameX-axesscales,originalamplitudespectra(grey)from0
to2d 1(bottomXaxis)i.e.,from0to23.15µHz(topXaxis),withthesixcommonintrinsicfrequenciesidentifiedinblack(middle
−
panels), residual amplitudespectra after prewhiteningthe six frequencies(bottom panels) where the additionaleight frequencies
foundinthe2011/12dataaremarkedinblack(bottomrightpanel).NotethedifferentY-axesscalesinthetopandmiddlepanels.
frequencyerroris1/(T √sig),i.e.,thevaluesgiveninparen- found in the 2011 data set. From these 14 frequenciesin total,
∗
theses in Tables 1 and 2. In our case only F10 in VAS20 has seven could be explained as linear combinations (again within
sig < 16. three times the frequencyuncertainties) which are identified in
In2011,VAS87wasobservedwithasamplingtimeof32s, the last column of Table 2 and seven are clearly independent
correspondingtoaNyquistfrequencyof1218d 1.Thecomplete from each other. Due to the instrumentaldifferencesin the ob-
−
runlastedforabout39days,butduetoinstrumentaleffects,the servationsfrom2008and2011(seeSection3.2),therespective
first few days had to be omitted from the analysis. The total amplitudesofthecommonfrequenciescannotbeusedforanas-
length of the data set (shown in Figure 2) used in this inves- trophysicalinterpretation.ForVAS87webelievethatF5isthe
tigation comprises 35 days, which correspondsto a Rayleigh frequencywhichindicatestherotationalperiod(seeSection5).
frequencyresolutio∼nof0.028d−1.ApplyingtheKallingeretal. ApplyingthecriteriabyKallingeretal.(2008),allfrequen-
(2008) criterion, we get an upper limit of the frequency error ciesforVAS20andVAS87areresolvedandarethereforecon-
of 0.007d−1, if the SigSpec significanceis higherthan 16.The sideredintrinsictothestars.
frequencyerrorsforsig < 16aregiveninTable2.
UsingtheCoRoTlightcurvefrom2008,VAS20showsten
intrinsic frequencies in the range from 0 to 1.35d 1 (Figure 1
−
andTable1).Linearcombinationsbetweenthefrequencieshave 4. Spectroscopicanalysis
beensoughtwithinthreetimesthefrequencyuncertainty.Three
Fundamentalparametersandabundancesofbothstarswerede-
frequencies, i.e., F4, F9 and F10, might be explained as linear
termined from spectra obtained on 2010, December 15th and
combinations,theothersevenareclearlyindependentfromeach
16th with the Robert G. Tull Coude´ Spectrograph (TS) on the
other. We believe that the frequencycorrespondingto the rota-
2.7-mtelescopeofMcDonaldObservatory.Intheadoptedcon-
tionalperiodofVAS20isF1(seeSection5).
figuration the cross-dispersed e´chelle spectrograph has a re-
ForVAS87thedatasetsfrom2008and2011wereanalyzed
solving power of 60000 and delivered spectra of VAS20 and
independentlyandtheresultswerethencomparedtoeachother.
VAS87withsignal-to-noiseratios(S/N)perpixel,calculatedat
Asthedatasetobtainedin2011issignificantlylonger,thenoise
levelintheamplitudespectrumisdecreasedandadditionalfre- 5000Å, of 100 and 65, respectively. Each spectrum covered
∼
quencies could be identified. Six frequencies are common to the wavelength range from 3633–10849Å with gaps between
both data sets (see Table 2), eight additional frequencies were thee´chelleordersatwavelengthslargerthan5880Å.
4
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Table 1. Results of the frequency analysis of VAS20: frequencies and periods, amplitudes, signal-to-noise values and SigSpec
significancesaregivensortedbytheprewhiteningsequence.Therespectivelast-digiterrorsofthefrequenciescomputedaccording
toKallingeretal.(2008)aregiveninparentheses.Possiblelinearcombinationsarelistedinthelastcolumn.
No. freq freq period amp S/N sig combi
# [d 1] [µHz] [d] [mmag]
−
F1 0.672(2) 7.78(2) 1.488 2.726 14.8 428.9
F2 0.226(3) 2.61(4) 4.427 0.818 8.6 164.6
F3 0.117(4) 1.35(5) 8.566 0.638 7.4 118.6
F4 0.347(4) 4.02(4) 2.879 0.572 8.9 136.3 3F3
F5 0.160(5) 1.85(6) 6.257 0.380 6.5 72.8
F6 0.760(6) 8.80(7) 1.315 0.299 4.7 47.4
F7 0.397(8) 4.60(9) 2.518 0.217 4.3 27.2
F8 0.723(9) 8.37(10) 1.383 0.320 7.5 22.3
F9 0.506(9) 5.86(11) 1.977 0.182 4.5 20.3 F1-F5
F10 1.350(13) 15.62(15) 0.741 0.105 3.6 11.5 2F1
Table2.ResultsofthefrequencyanalysisofVAS87usingdatafrom2008and2011:frequenciesandperiods,amplitudes,signal-
to-noisevaluesandSigSpecsignificancesforbothyearsaregivensortedfirstbytheprewhiteningsequenceofthe2008data and
thenofthe2011/12data.Therespectivelast-digiterrorsofthefrequenciescomputedaccordingtoKallingeretal.(2008)aregiven
inparentheses.Inthelastcolumnpossiblelinearcombinationsarelisted.
No. freq08 freq08 amp08 S/N-08 sig08 freq11 freq11 amp11 S/N-11 sig11 period combi
# [d 1] [µHz] [mmag] [d 1] [µHz] [mmag] [d]
− −
F1 0.519(3) 6.01(3) 1.081 9.6 219.3 0.511(2) 5.92(2) 0.451 5.1 323.8 1.925
F2 0.309(3) 3.58(4) 0.410 10.6 167.6 0.291(3) 3.36(3) 0.769 13.8 100.1 3.236
F3 0.236(4) 2.73(5) 0.641 9.0 114.1 0.251(1) 2.90(1) 1.996 19.9 748.1 4.235
F4 0.554(7) 6.41(9) 0.250 6.1 31.9 0.540(2) 6.25(2) 0.404 6.2 243.6 1.806 F2+F3
F5 0.274(9) 3.17(10) 0.447 9.6 23.1 0.2620(3) 3.033(4) 4.098 23.6 7980.9 3.655 F1-F3
F6 0.811(9) 9.38(55) 0.174 4.1 19.7 0.809(3) 9.36(4) 1.943 4.06 73.0 1.233
F7 0.110(1) 1.27(1) 0.804 7.1 916.5 9.091
F8 0.145(2) 1.68(2) 0.501 5.3 307.8 6.897
F9 0.079(2) 0.92(2) 0.560 7.1 365.3 12.658 2F2-2F3
F10 0.184(2) 2.13(2) 0.526 7.1 235.3 5.435
F11 0.359(2) 4.15(2) 0.362 5.9 211.1 2.786 F3+F7
F12 0.782(3) 9.05(3) 0.225 3.8 95.6 1.279 3F1-3F3
F13 0.399(4) 4.62(4) 0.243 4.7 65.9 2.506 F1-F7
F14 0.645(4) 7.47(5) 0.186 4.1 54.3 1.550 3F3-F7
BiasandFlatFieldframeswereobtainedatthebeginningof lines.ForVAS87weobtainedT =6220 150K,logg=3.8 0.2
eff
each night,while severalTh-Arcomparisonlamp spectra were and υ =1.7 0.5kms 1, while for ±VAS20 we obta±ined
mic −
obtainedeachnightforwavelengthcalibrationpurposes.There- T =6380 150±K,logg=4.0 0.2andυ =1.8 0.5kms 1.The
eff mic −
± ± ±
ductionwasperformedusingtheImageReductionandAnalysis excitation and ionisation equilibria obtained for VAS20 and
Facility1 (IRAF). Thespectra were normalizedby fitting a low VAS87 are shown in Figures3 and 4. For both stars, the mea-
orderpolynomialtocarefullyselectedcontinuumpoints. sured equivalent widths span between about 20 and 250mÅ.
To compute model atmospheres of the target stars we em- GiventhefactthattheuncertaintyonT isonlystatisticaland
eff
ployedtheLLmodelsstellarmodelatmospherecodebyShulyak doesnottakeintoaccountthepossiblepresenceofvarioussys-
etal(2004),whilewecalculatedsyntheticspectrawithSYNTH3 tematic effects, we increased the uncertainty to 200K. By im-
(Kochukhov 2007). Our main source of atomic parameters for posing the ionisation equilibrium, we took into account NLTE
spectral lines is the VALD database (Kupka et al. 1999). The effectsforFe,asgivenbyMashonkinaetal.(2011).
LTE abundance analysis was based on equivalent widths, ana-
Itisextremelyimportanttochecktheparametersobtainedin
lyzedwitha modifiedversion(Tsymbal1996)oftheWIDTH9
thiswaywithotherindicatorsasthismethodmightbestrongly
code(Kurucz1993).
affectedbysystematics(seeforexampleRyabchikovaetal.2009
We determined the microturbulence velocity (υ ) impos-
mic andFossatietal.2011).Forthisreasonweusedhydrogenlines
ing the equilibrium between line abundance and equivalent
and lines with extended wings to check T and logg, respec-
width for Fei lines, logg imposing the ionisation equilibrium eff
tively.Note thatat thistemperaurethe hydrogenlineshave the
for Fe, and T imposing the excitation equilibrium for Fei
eff advantageofnotbeingverysensitivetogravityvariations.
1 IRAF (http://iraf.noao.edu) is distributed by the National Optical In PMS stars various spectral lines, particularly the hydro-
Astronomy Observatory, which is operated by the Association of genlines,couldshowemissionasyoungstarsmightstillbesur-
Universities for Research in Astronomy (AURA) under cooperative rounded by the remnants of their birth clouds. If the hydrogen
agreementwiththeNationalScienceFoundation. lineswouldbepartiallyorcompletelyfilled,thedeterminationof
5
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Within the uncertainties, the hydrogen lines confirmed the
effective temperatures we derived for both stars with the exci-
tation equilibrium.We confirmed the derived logg values with
the analysis of the Mgi line profile at λ5172Å (for more de-
tails on the adopted procedure see Fossati et al. 2011), which,
in this temperature regime, is particularly sensitive to gravity
variations. Our fundamental parameters provide the best over-
alldescriptionoftheavailableobservables:hydrogenandmetal-
lic line profiles. Our analysis shows that the two stars are very
similarandthisisdemonstratedalsoinFigure5whichshowsa
comparisonbetweenthespectraofthetwostars.
Fig.3.Toppanel:correlationbetweenlineabundanceandexci-
tationpotentialforFeiasafunctionofT forVAS20.Middle
eff
panel:Fei(asterisks)andFeii(diamonds)abundanceasafunc-
tionofloggforVAS20.Asmallhorizontalshiftwasappliedto
the points for visualisation purposes. Bottom panel: excitation
equilibrium for both Fei (asterisks) and Feii (diamonds) lines
for VAS20. The correlationsare shown respectivelyby a solid
andadashedline.
Fig.5. Comparison between the observed spectra of VAS20
(dashed line) and VAS87 (solid line) in the region around the
Mgibtriplet(toppanel)andHγline(bottompanel).Bothpanels
present for comparison a synthetic spectrum (dash-dottedline)
calculatedwithT =6300Kandlogg=3.9.Tobetterappreciate
eff
the similarities between the two stars, we convolved the spec-
trumofVAS87tothesameυsiniofVAS20.
By fitting synthetic spectra to several weakly blended
lines we measured a υsini of 42 2kms 1 for VAS20, and
−
18 1kms 1for VAS87. Given the±relatively large υsini, we
−
±
also performedthe analysis of VAS20 using spectral synthesis
(formoredetailsseeFossatietal.2007),obtainingresultscon-
sistent with the equivalent width results. Given the T of the
Fig.4.SameasFigure3,butforVAS87. eff
two stars we expectthe presence of some macroturbulenceve-
locity(υ ).ForVAS20,υsini issolargethatthelineprofiles
mac
do not require any υ , while for VAS87, to be able to fit si-
mac
multaneouslythecoreandthewingsofstronglines,weneeda
theeffectivetemperature(Teff)fromtheselineswouldbeimpos- υmac ofabout12kms−1.Thisvalueisslightlylargerthanwhat
sible.ForbothVAS20andVAS87thereisnosignofemission isexpectedformainsequencestarsofthisT (Valenti&Fisher
eff
andifanyitwouldaffectjustthelinecoreswhichcannotanyway 2005),whichmightbeconnectedtothefactthatthisstarisinits
betakenintoaccountduetoNLTEeffects.Wethereforeusedthe PMSphase.ForVAS20we werenotableto measureυ be-
mac
HγlinetoestimateTeff.HαwasnotcoveredandHβisaffected causeoftheratherlargeυsini whichpreventedusfromseeing
byadefectintheimagingsystemofthespectrograph.We nor- theeffectsofυ broadeningonthelinewings.Inthisrespect,
mac
malized the Hγ line continuum using the artificial flat-fielding the υsini we obtained for VAS20 is likely to contain also the
techniquedescribedinBarklemetal.(2002),whichhasproven hiddeninformationonυ ,whichcannotbedirectlymeasured.
mac
to be successful with TS data (e.g., Fossati et al. 2011). To re- Foreachstarwedeterminedtheabundancesofabout15el-
duce systematic uncertainties in the temperature determination ements, which are given in Table 3. Notice that the spectrum
due to the hydrogen lines normalization, we obtained low res- of VAS87 is badly affected by spikes, making both the con-
olution (R 10000)spectra, coveringHα (6470–6710Å), with tinuum normalisation and the equivalent width measurements
∼
the1.8-mtelescopeoftheDominionAstrophysicalObservatory rather uncertain, in particular in the blue spectral region. This
(Canada). Low resolution spectra allow a better control of the isresponsiblefortheslightlylargerabundanceuncertaintiesfor
normalization. VAS87, compared to that of VAS20. For both stars, we ob-
6
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Table3.LTEatmosphericabundancesinprogramstarswiththe
error estimates based on the internal scattering from the num-
ber of analysed lines, n. For comparison purpose, the last col-
umngivestheabundancesofthesolaratmospherecalculatedby
Asplundetal.(2009).
Ion VAS20 VAS87 Sun
log(N/N ) n log(N/N ) n log(N/N )
tot tot tot
Lii 8.55 0.16 1 8.60 0.10 1 10.99
Nai −5.83±0.08 2 − 5.±68: 2 −5.87
Mgi −4.63±0.03 4 4−.55 0.12 4 −4.44
Sii −4.58±0.09 7 −4.87±0.14 5 −4.53
Siii − 4.±49: 1 − ± −4.53
Cai 5−.64 0.09 16 5.71 0.14 10 −5.70
Scii −8.97±0.09 6 − 9.±16: 1 −8.89
Tii −7.00±0.08 6 7−.08 0.15 3 −7.09
Tiii −7.03±0.06 8 −7.14±0.13 6 −7.09
Cri −6.37±0.17 15 −6.39±0.10 6 −6.40
Crii −6.44±0.14 8 −6.06±0.08 5 −6.40 Fig.6. Comparison between the observed spectra (black solid
Mni −6.62±0.15 4 −6.44±0.14 5 −6.61 line) of VAS20 (upperpanel) and VAS87 (lower panel) in the
Fei −4.55±0.10 127 −4.63±0.15 45 −4.54 regionofLilineatλ 6707Å andsyntheticspectra(reddashed
Feii −4.52±0.08 17 −4.58±0.20 14 −4.54 line)calculatedwith∼thebestfittingLiabundance.Notethedif-
Coi − ± − 7.±13: 1 −7.05
Nii 5.77 0.10 16 5−.83 0.08 8 −5.82 ferenty-axesscales.
Cui − 8.±03: 1 − 7.±68: 1 −7.85
Yii 9−.94 0.08 2 9−.66 0.03 2 −9.83
Baii −9.29±0.04 3 −9.51±0.05 2 −9.86
− ± − ± −
tained abundance values comparable to the solar ones, except
forBaandLiwhichareoverabundant,inareasonableagreement
withintheerrorstotheoverallclustermetallicityof-0.15(Lynga
1987).ForLiwederivedanabundanceoflog n(Li)=3.49 0.16
±
forVAS20andlog n(Li)=3.44 0.10forVAS87(seeFigure6).
±
The difference in the uncertainties of the Li abundance for the
two stars reflects the difference in the projected rotational ve-
locity (υsini). We obtained the Li abundance from the lines
at λ 6707Å adopting hyperfine structure from Smith et al.
(1998∼)andthemeteoritic/terrestrialisotopicratioLi6/Li7=0.08
(Rosman&Taylor1998).ThegivenerrorsontheLiabundance
werecalculatedtakingintoaccounttheuncertaintiesonthefun-
damentalparameters(seeFossatietal.2009). Fig.7. Comparison between LLmodels theoretical fluxes (full
line), calculated with the fundamental parameters derived for
VAS20, with Johnson (crosses), 2MASS (diamonds), Spitzer
4.1.SpectralenergydistributionandHertzsprung-Russell
(triangles),andWISE(squares)photometryconvertedtophysi-
diagram
calunits.
Figure 7 and 8 show the fits of the synthetic fluxes, calcu-
lated with the fundamentalparameters derived for VAS20 and
VAS87,respectively,totheobservedJohnson(Sungetal.1997),
2MASS (Zacharias et al. 2005), Spitzer (IRAC - Sung et al. of reddening across the cluster reported in the literature (e.g.,
2009) and WISE (Cutri et al. 2012) photometry, converted to Sungetal.1997).Figure8showsalsoasteepincreaseinfluxin
physical units. For this we used the calibrations provided by theWISEphotometryofthetworedchannels,butthisfeatureis
Bessel et al. (1998), van der Bliek et al. (1996), Sung et al. notpresentintheSpitzerphotometry.Itisthereforepossiblethat
(2009),andWrightetal.(2010),respectively.Adoptingtheclus- theWISEphotometryhasbeencontaminatedbythebackground
terdistanceandreddeninggivenbySungetal.(1997),weesti- cloudwhichwouldberatherbrightatinfraredwavelengths.
matedthestellarradiusofVAS20tobe3.5 0.5R/R ,whilefor Usingthespectroscopictemperatures,weputthetwostarsin
VAS87 we derived a radius of 2.2 0.5R/R±. This c⊙omparison theHR-diagram.Wecalculatedtheluminosityofeachstarpho-
alsoconfirmsthetemperatureweob±tainedfo⊙rthetwostars. tometrically, adopting the magnitudes in the V band, a cluster
Along the line of sight of VAS87 there is a dense cloudof distanceof759 83pcandareddeningofE(B V)=0.071 0.033
± − ±
material which we believe being behind the star, as it would (Sungetal.1997).WeusedthebolometriccorrectionbyBalona
otherwise be clearly visible in the spectral energy distribution, (1994). For VAS20 and VAS87 we obtained logL/L =1.235
which we can fit with the same reddeningapplied for VAS20. and0.805,respectively.Givenall the uncertaintiesand⊙the fact
This is also supported by the presence of only small amounts thatweadoptedabolometriccorrectionvalidformain-sequence
7
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Doradus stars (see Bouabid et al. 2011, their Figure 8). The
cool border of the predicted theoretical (PMS) γ Doradus in-
stability strip lies at about 6700K, but such a high T would
eff
notallow to fit bothhydrogenandmetallic linesfor bothstars.
Thereforethetwostarslieslightlyoutsidethepredicted,theoret-
icalPMSγDoradusinstabilitystrip(Bouabidetal.2011).This
isnottoosurprisingassimilardiscrepanciesarefoundinother
groups of (post-)main sequence pulsators. In particular several
oftheγDoradus starsfoundwithdatafromtheKeplersatellite
(Kochetal.2010)arelocatedattemperaturessimilartothatof
VAS20andVAS87(Balonaetal.2012).
Additionally, the theoretical seismic properties of PMS γ
Doradushavebeeninvestigatedwithoutaconfirmedmemberof
thisgroupofpulsators.Acomparisonoftheobservationstothe
PMS γ Doradus theory might yield new constraints on the lo-
cationof the respectiveinstability region.Moretheoreticaland
observational work will be performed in the future to investi-
gatethisindetail.AsthePMSγDoradus instabilitystripcoin-
Fig.8.SameasFigure7,butforVAS87.
cideswellwiththeinstabilitystripofthe(post-)mainsequence
γ Doradus stars, in Figure 9 we show the location of VAS20
and VAS87 in the HR-diagram with respect to the “classical”
stars,we decidedto applya conservativeuncertaintyonthelu-
γDoradus instabilitystripbyDupretetal.(2004).
minosityof0.1dex.
On the basis of the evolutionary tracks we derived masses
and radii for the two stars. For VAS20 we obtained a mass
of 2.17 0.15M , a radius of 3.5 0.5R and a surface grav-
ity of 3±.69 0.1⊙1, while for VAS±87 we⊙ obtained a mass of
±
1.55 0.15M , a radius of 2.1 0.5R and a surface gravity of
3.98±0.11.T⊙hestellarradii,der±ivedfr⊙omtheevolutionarytracks
±
areinremarkablegoodagreementwithwhatweobtainedfrom
thefitofthespectralenergydistribution.Withinone-sigma,the
spectroscopicloggvaluesarealso inagreementwith thosede-
rivedfromtheevolutionarytracks.
VAS20andVAS87aretwostarsbornfromthesamemolec-
ularcloudthataremembersofNGC2264andthathavenearly
thesameT andloggvalues,butslightlydifferentmasses,radii
eff
andluminosities(seeTable4foranoverviewofthefundamental
parameters)which place them at differentlocationsin the HR-
diagram.Atpresentwecanonlyspeculateaboutthereasonfor
thisdiscrepancy.
One possible explanation might be connected to the pres-
ence ofsequentialstar formationin NGC2264andthatits star
forming activity is still going on (e.g., Flaccomio et al. 1999).
Fig.9.PositionsofVAS20(asterisk)andVAS87(diamond)in
The ages of stars in the cluster lie between 0.8 and 8 million
the HR-diagram in comparison with evolutionary tracks (bot-
years where intermediate mass stars and very low mass stars
tompanel)andisochrones(toppanel).InbothpanelstheZAMS
were formedfirst andand massive stars downto aboutone so-
is indicated by a dotted line. Evolutionary tracks for PMS
lar mass were formed later on (Sung et al. 1997). To assess if
stars with masses of 2.25M/M , 2.00M/M , 1.75M/M , and
an age spread in NGC2264would be a possible reasonfor the
1.50M/M are shown respectiv⊙ely by a so⊙lid, dashed,⊙dash-
slightdifferencesinthefundamentalparametersofthetwostars,
dotted an⊙d dash-triple dotted line. Isochrones with ages of
we included the isochronesin Figure 9 and obtainedan age of
10Myr, 8Myr, 6Myr, and 2Myr are shown respectively by a
4 1Myr for VAS20 and 10 2Myr for VAS87. In the present
solid, dashed,dash-dottedanddash-triple-dottedline. The bor- ± ±
case, the higher mass star (VAS20) would have to be formed
ders of the γ Doradus instability strip (Dupretet al. 2004) are
later than the lower mass star (VAS87). This would fit to the
markedwiththicksolidlines.
theoryofstarformationinNGC2264(Sungetal.1997)asde-
scribedabove.
Figure 9 shows the position of the two stars in the HR- Another possible explanationfor the differencesin the two
diagram, in comparison to PMS evolutionary tracks from stars’ fundamental parameters could be that VAS20 is part of
Guenther (2012, priv. communication)using the YREC evolu- a binary system. We have assessed that the CoRoT photome-
tion code (Demarque et al. 2008) with physics as described in try is not affected by a contaminating source and we do not
Guentheretal.(2009)andisochronesbyDemarqueetal.(2008). find any evidence for binarity in our spectroscopic measure-
For both evolutionary tracks and isochrones we adopted solar mentsandinthespectralenergydistribution.Theloggvalueof
metallicity.Also includedin Figure 9is the instability strip for VAS87 derivedfrom our spectroscopic data agrees well to the
γDoradus stars(Dupretetal.2004). corresponding value determined from the evolutionary tracks.
The derived T values are relatively low compared to the But for VAS20 there is a difference of about 0.3 dex between
eff
theoretical computations of the instability strip for PMS γ thetwovalues.Wethereforehypothesizethattheluminosityof
8
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
VAS20isoverestimatedwhenextractedfromspectroscopybe- Thelowtemperaturesofthe“comparison”starsensurethattheir
causeVAS20mightbepartofanunresolvedbinarysystem. variabilityislikelytobeproducedbyrotationalmodulation,as
nopulsation(otherthansolar-likeoscillationswhichareinadif-
ferentfrequencyrange,i.e.,largerthantheNyquistfrequencyof
Table4.FundamentalparametersforVAS20andVAS87. 84d 1) can be present, and that their level of activity is equal
−
or higher than that of VAS20 and VAS87. A typical example
isgivenin Figure10:thelightcurve(toppanel)andamplitude
VAS20 VAS87
spectrum(bottompanel)ofCoID0223980447observedin2008
T [K] 6380 150 6220 150
eff ± ± isshown.CoID0223980447hasaspectraltypeofK6.Thefre-
logg 4.0 0.2 3.8 0.2
υ [kms 1] 1.8±0.5 1.7±0.5 quencyanalysisshowstwoindependantfrequencies(F1andF2
υmsiicni [km−s 1] 42± 2 18± 1 in Figure 10) and a third frequency which is 1/2F1 indicating
−
M[M ] 2.17 ±0.15 1.55 ±0.15 the presence of non-linear effects. From the frequency analy-
logL[⊙L ] 1.2±0.1 0.8±0.1 sesofthespottedstars,wecanconfirmthefindingsbyAlencar
R[R ] ⊙ 3.5±0.5 2.2±0.5 etal. (2010) andwe infer thatrotationalmodulationcan be re-
⊙ ± ± sponsibleforamaximumofthreefrequenciesinthe23-daylong
CoRoTdatasetsfrom2008.
4.2.Comparisonwithphotometriccolors
Young stars are often still surroundedby the remnants of their -10.2
birth clouds which affects the observed photometric colors.
Thereforetheapplicationofphotometriccalibrations(developed
-10.0
formainsequencestars)toPMSobjectswilloftenleadtoerro-
g
a
neousvalues. m
In the present case, we used the available Stro¨mgren pho- inst. -9.8
tometry(Strometal.1971,Nerietal.1993)andthephotometric
calibration by Moon & Dworetsky (1985) to test this hypothe-
sis. For VAS20 we get T =5800K, logg=3.6 and a metallic-
eff -9.6
ity [Fe/H] = -0.76 and T =6400K, logg=4.6 and a metallic-
eff
ity[Fe/H]=-0.61forVAS87.AlthoughtheseT valuesmight
eff 2990 2995 3000 3005 3010
be used as a first estimate, the logg values of 3.6 and 4.6 are,
HJD - 2445545.0 [d]
respectively, too low and too high for these stars and the quite
frequency [ Hz]
low[Fe/H]valuesdonotmatchtheoverallclustermetallicityof µ
0 5 10 15 20
-0.15(Lynga1987).Thisillustratesthattheresultsofphotomet-
riccalibrationsforthedeterminationofthefundamentalparam- 0.10 F1
etersofPMSstarscannotbetrustedandthatonlyspectroscopic
measurementscangivereliablevalues. 0.08
g] 0.06
a
5. Rotation&Pulsation m
mp [ 0.04 1/2*F1
As star spots andγ Doradus pulsationcause variabilityon the a F2
sametimescales,i.e.,inthelowfrequencydomain,weheredis-
0.02
cussthenatureoftheobservedvariationsinVAS20andVAS87
whichweattributetoacombinationofrotationandpulsation.
0.00
Surfaceinhomogeneitiescancausecomplexvariabilityfea- 0.0 0.5 1.0 1.5 2.0
frequency [c/d]
tures in the light curves and amplitude spectra due to multiple
spotsonthesurface,differentialrotationormigratingstarspots
(Strassmeier 2009). Nevertheless, frequencies originating from Fig.10. The “comparison” T Tauri star CoID0223980447:
rotationalmodulationaremostlyaccompaniedbynon-linearef- CoRoT light curve (top panel) from SRa01 (2008) and origi-
fectswhichcanbeidentifiedintheamplitudespectraasharmon- nalamplitudespectrum(bottompanel)from0to2d 1 (bottom
−
icsandlinearcombinations. X axis), i.e.,from0 to 23.15µHz (topX axis), wherethethree
PotentialPMSγDoradus pulsatorsarelocatedinthesame significantfrequenciesareidentifiedinblack.
regionoftheHR-diagramastheTTauristarswhichareknown
toshowregular,semi-regularandirregularvariabilities.Forthe
classical T Tauri stars in NGC2264, Alencar et al. (2010) re- UsingdatafromtheKeplersatellite,Balonaetal.(2012)find
porta mean rotationalperiod between 3 and 4 days using data threetypesofstarswithmultiplepeaksinthefrequencydomain
obtainedduringthesameCoRoTrunin2008. of γ Doradus pulsators: stars with one or two dominantpeaks
Based on this information, we performed additional fre- with asymmetric light curves (their ASYM group), stars with
quency analyses of cluster members with known variability two close frequencieswith symmetric light curves (their SYM
causedbyspots(i.e.,theclassicalTTauristars)whichhavespec- group)andstarsshowingmultiplelowfrequencies(theirMULT
tral types from K0 to M0. These “comparison” stars were ob- group).WhilestarsintheMULTgroupareverylikelyγDoradus
servedwithCoRoTinthesamerunin2008,i.e.,havethesame pulsators,theremightbeamixofspottedstarsandpulsatorsin
datasetlengths,butaremuchcoolerthanVAS20andVAS87. theirsampleoftheASYMandSYMgroups(Balonaetal.2012).
9
K.Zwintzetal.:PMSγDoradus candidatesinNGC2264
Fromtheiranalysestheauthorsalsoconcludethatthepulsation findthatrotationalmodulationcanberesponsibleforthreefre-
periodsoftheirstarsareclosetotherotationalperiods. quenciesatmaximum,butcannotexplainthepresenceoftenand
Thelightcurvesofourtwostarslookrathersymmetric,but fourteenintrinsicfrequenciesidentifiedinVAS20andVAS87.
oncethepeakwiththehighestamplitude(i.e.,F1forVAS20and The probability that VAS20 and VAS87 are members of
F5forVAS87;seeFigures1and2)issubtracted,multiplepeaks NGC2264isveryhighduetotheirreportedX-rayfluxes,radial
atlowamplitudesremainwhichstronglyindicatesthepresence velocitymeasurementsandpropermotions.Theradialvelocity
ofpulsation. (υr)oftheclusterNGC2264wasderivedfrommeasurementsof
Usingaυsinivalueof42kms−1andastellarradiusof3.5R 13starsandisgivenas17.68±2.26kms−1byKharchenkoetal.
forVAS20,themaximumrotationperiodcorrespondsto 4.2d⊙. (2005). From ourspectra we deriveυr valuesof 17.7 1.0 and
We identifyF1 at 0.672d−1, i.e., a periodof about1.5 d∼ays, to 19.3±0.9kms−1 for VAS20 and VAS87, respective±ly, which
be likely caused by rotation because it is the highest and most areinagreementwiththeclusteraverage.
dominantamplitudepeakinouranalysiswiththeotherfrequen- Ourspectroscopicanalysesaddanothercrucialpieceof in-
cies lying at significantly lower amplitudes (i.e., smaller than formation:the relatively high Li abundance we measured is in
1mmag).Withtheseparameters,theprojectedinclinationangle excellentagreementwiththeoneobtainedbySestito&Randich
oftherotationalaxisiscomputedtobe23degrees. (2005)forotherstarsofsimilarTeff inthiscluster.TheLiabun-
For VAS87 a υsini value of 18kms 1with a stellar radius dancesreportedby King (1998) are higherthan the valuesthat
−
of 2.2R would correspond to a maximum rotation period of we derivedbutagreewithintheuncertaintiesof0.2dex.These
6.1d. ⊙In our frequency analysis, the by far most dominant systematicdifferencesmightoccurbecauseweusedahigherTeff
∼and highest amplitude peak in the 2011/12 data set is F5 at andtookintoaccountthefullstructureoftheline.Additionally,
0.262d 1correspondingto a periodof 3.816d. In the 2008data ourmeasurementsoftheequivalentwidthsaresmallerby20mÅ
−
the highest amplitude peak is F1 at 0.519d 1which is about comparedtoKing(1998).
−
twice F5. As the surfaces of young stellar objects are likely As the variability observed in VAS20 and VAS87 is very
not homogeneous, we assume that in 2008 the surface inho- likelycausedbya combinationofrotationalmodulationandg-
mogeneities were not that prominent than in 2011 and that modepulsationandasbothtypesofvariationsoccuronthesame
pulsational and rotational activities occurred on similar levels timescales,itiscurrentlyhardtounambiguouslydistinguishbe-
in 2008. In this context, we still have to remember that the tweenthem.ButthemultiplefrequenciesdiscoveredinVAS20
CoRoTphotometrymightbeaffectedbyinstrumentaleffectsto andVAS87provideafirststrongindicationforanobservational
apresentlyunknownextentandthatnotallthelargeamplitude detection of γ Doradus pulsation in PMS stars which was the-
differencesbetweenthedatasetsfrom2008and2011/12might oretically predicted (Bouabid et al. 2011). These first observa-
beinterpretedastrophysically.WethereforeconcludethatF5at tionalcandidatesforPMSγDoradus starswillbeanimportant
0.262d 1(valuefromthe2011dataset)correspondingtoaperiod inputfortheoryandhelptocontinuethedevelopmentofastero-
−
of3.816dislikelytobecausedbyrotation.Thecorresponding seismicmodelsforPMSstars.
projectedinclinationangleoftherotationaxisisthen36degrees
forVAS87. Acknowledgements. KZ receives a Pegasus Marie Curie Post-doctoral
Fellowship ofthe Research Foundation –Flanders (FWO)and acknowledges
We conclude that rotational modulation may explain only
supportfromanAPARTfellowshipoftheAustrianAcademyofSciences.We
few of the detectedfrequenciesin VAS20 and VAS87, butthe wish tothank the referee foruseful comments that allowed us to clarify cer-
clear multi-periodicityof both stars shows that rotation cannot tain parts of this paper. We thank Oleg Khochukov and Denis Shulyak for
be the only cause of their variability and that indeed g-modes their support. We are grateful to Ettore Flaccomio for his extensive knowl-
edge about NGC2264 and the respective literature. Wethank Michel Breger
havebeenobserved.
forfruitfuldiscussionsinpreparationofthepaper.Spectroscopicdatawereob-
tained withthe 2.7-mtelescope atMcDonald Observatory, Texas, USandat
the Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics,
6. Conclusions NationalResearchCouncilofCanada.
VAS20andVAS87wereobservedforabout23.4dayswiththe
CoRoT satellite during the short run SRa01. VAS87 was re- References
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T valuesof6380Kand6220Kagree,withintheuncertainties, Balona,L.A.,Guzik,J.A.,Uytterhoeven,K.,etal.2012,MNRAS,415,3531
eff
Barklem,P.S.,Stempels,H.C.,AllendePrieto,C.,etal.2002,A&A,385,951
withthoseofKing(1998).
Bessel,M.S.1979,PASP,91,589
Usingourmeasuredυsinivaluesof42kms−1 and18kms−1 Bessel,M.S.,Castelli,F.&Plez,B.1998,A&A,333,231
combinedwith the derived stellar radii of 3.5R and 2.2R for Breger,M.1972,ApJ,171,539
VAS20andVAS87,respectively,weidentifyF1⊙at0.672d⊙1for Breger,M.1993,A&A,271,482
−
VAS20 and F5 at 0262d 1 for VAS87 to be likely caused by Bouabid,M.-P.,Montalba´n,J.,Miglio,A.,etal.2001,A&A,531,145
− Cutri,R.M.,etal.2012,WISEAll-SkyDataRelease
rotation,whiletheotherfrequenciesareinterpretedasg-modes
Demarque,P.,Guenther, D.B.,Li,L.H.,Maxumdar,A.,Straka, W.S.2008,
ofγDoradus pulsation. AstrophysicsandSpaceScience,316,31
Inthiscontext,wealsocomparedVAS20andVAS87tosev- Dupret,M.-A.,Grigahce`ne,A.,Garrido,R.,Gabriel,M.,&Scuflaire,R.2004,
A&A,414,L17
eraladditionalvariablestarsinNGC2264inthespectralrange
Dupret,M.,Grigahce´ne,A.,Garrido,R.,Gabriel,M.,Scuflaire,R.2005,A&A,
fromK0 to M0.Atsuchcooltemperaturesvariabilitycan only
435,927
becausedbyspotsonthestellarsurface.Fromthisanalysiswe Flaccomio,E.,Micela,G.,Sciortina,S.,etal.1999,A&A,345,521
10