An asteroseismic study of the O9V star HD 46202 from CoRoT space-based photometry

1.4 MB·English

by  M. Briquet

Astronomy&Astrophysicsmanuscriptno.15690 (cid:13)c ESO2011
January31,2011
An asteroseismic study of the O9V star HD46202 from CoRoT
⋆
space-based photometry
M.Briquet1,⋆⋆,C.Aerts1,2,A.Baglin3,M.F.Nieva4,P.Degroote1,N.Przybilla5,A.Noels6,F.Schiller5,M.
Vucˇkovic´1,R.Oreiro1,K.Smolders1,M.Auvergne3,F.Baudin7,C.Catala3,E.Michel3,andR.Samadi3
1 InstituutvoorSterrenkunde,K.U.Leuven,Celestijnenlaan200D,B-3001Leuven,Belgium
e-mail:[email protected]
2 DepartmentofAstrophysics,IMAPP,UniversityofNijmegen,POBox9010,6500GLNijmegen,TheNetherlands
3 LESIA,CNRSUMR8109,Universite´PierreetMarieCurie,Universite´DenisDiderot,ObservatoiredeParis,92195MeudonCedex,
1
France
1
4 MaxPlanckInstituteforAstrophysics,KarlSchwarzschildStr.1,GarchingbeiMu¨nchenD-85741,Germany
0
5 Dr.KarlRemeisObservatory&ECAP,UniversityofErlangen-Nuremberg,Sternwartstrasse7,D-96049Bamberg,Germany
2
6 Institutd’AstrophysiqueetdeGe´ophysique,UniversityofLie`ge,Baˆt.B5C,Alle´edu6Aouˆt17,B-4000Lie`ge,Belgium

n 7 Institutd’AstrophysiqueSpatiale(IAS),Baˆtiment121,F-91405,OrsayCedex,France
a
J Received;accepted

8
2 ABSTRACT

] The O9V star HD46202, which is a member of the young open cluster NGC 2244, was observed by the CoRoT satellite in
R October/November2008duringashortrunof34days.Fromtheveryhigh-precisionlightcurve,weclearlydetectβCep-likepulsation
S frequencieswithamplitudesof∼0.1mmagandbelow.Acomparisonwithstellarmodelswasperformedusingaχ2asameasurefor
thegoodness-of-fitbetweentheobservedandtheoreticallycomputedfrequencies.Thephysicalparametersofourbest-fittingmodels
.
h arecompatiblewiththeonesdeduced spectroscopically.Acoreovershootingparameterα = 0.10±0.05pressurescaleheightis
ov
p required.Noneoftheobservedfrequenciesaretheoreticallyexcitedwiththeinputphysicsusedinourstudy.Moretheoreticalworkis
- thusneededtoovercomethisshortcominginhowweunderstandtheexcitationmechanismofpulsationmodesinsuchamassivestar.
o
AsimilarexcitationproblemhasalsobeenencounteredforcertainpulsationmodesinβCepstarsrecentlymodelledasteroseismically.
r
t
s Keywords.Asteroseismology–Stars:oscillations–Stars:variables:early-type–Stars:individual:HD46202
a
[

2 1. Introduction cillatory behaviour. Radiation-driven winds have thus been ig-
v noredwhencomputinginstabilitystrips(e.g.,Pamyatnykh1999,
2 Asteroseismology is the domain of astrophysics that probes Miglioetal.2007ab).Ontheotherhand,theorypredictstheoc-
6 the internalstructure of stars by using their stellar oscillations. currenceofso-calledstrangemodeoscillationsinstarscloseto
1 Different classes of pulsating stars are known across the H-R
the Humphreys-Davidsonlimit. These are due to a convection
3 diagram, includingpulsating main-sequenceB-type stars (with
zoneintheouterstellarlayerswhereadensityinversionoccurs
1. masses between 3 and 20 M⊙), such as the so-called β Cep andtheradiationpressuredominates(e.g.,Glatzel&Kiriakidis
0 stars and the slowly pulsating B stars (e.g., Aerts et al. 2010). 1993, Papaloizou et al. 1997, Saio et al. 1998). Review papers
1 Currently, the part of the H-R diagram corresponding to even onthetopicsareavailablebyGlatzel(2009)andbySaio(2009).
1 hotter and more massive main-sequence stars has been poorly
Observationshave not established that there are strange modes
: exploredfor asteroseismic purposes,boththeoreticallyand ob-
v inOstars.
servationally.
Xi On the observational side, there are a few O-type stars for
Due to lack of firm observational detections, the theory of
whichspectroscopicandphotometricvariabilitylinkedtostellar
r stellar oscillations has hardly been applied to O star models.
a pulsation has been searched for and found from ground-based
On the one hand, one expects, for late O-type stars such as
data.
HD46202 (O9V) studied in this paper, that oscillation modes
Convincingevidenceofstellarpulsationhasbeendetectedin
excitedbytheκmechanismoccurwithperiodsofseveralhours,
ζ Oph (Kambeet al. 1997),HD93521(Howarth&Reid 1993,
just as for the well-established class of β Cep stars. The theo-
Rauwetal.2008),ξPersei,andλCephei(deJongetal.1999).
retical treatment for the lower mass main-sequence O stars is
Infact, itis difficultto discoverpulsationmodesin O-typetar-
the same as the one for the early-type B stars because stel-
getsbecausetheobservedvariabilitythatmightcomefrompul-
lar winds are not expected to play an importantrole in the os-
sationshasverylowamplitude(e.g.,Henrichs1999forasum-
mary).
Sendoffprintrequeststo:[email protected]
⋆ The CoRoT space mission was developed and is operated by the Inthiscontextclearerdetectionscanbeexpectedfromlong
FrenchspaceagencyCNES,withtheparticipationofESA’sRSSDand uninterruptedphotometrictimeseriesatmicro-magnitudepreci-
ScienceProgrammes,Austria,Belgium,Brazil,Germany,andSpain. sionobtainedfromspacesatellitessuchasCoRoT(Convection,
⋆⋆ PostdoctoralFellowoftheFundforScientificResearchofFlanders RotationandplanetaryTransits,Auvergneetal.2009).Aspart
(FWO),Belgium of the asteroseismology programme,several O-type stars were
1
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
observed by CoRoT during the second short run SRa02 point-
-0.0015
ing towards the anticentre of the Galaxy, e.g., the O-star bina-
ries HD46149 (Degroote et al. 2010) and HD47129 (Mahy et
-0.001
al. 2010). In this paper, we present a study of the single O9V
star HD46202, which is a member of the young open cluster
NGC2244intheRosetteNebula. -0.0005
e
d
u
nit 0
2. FrequencyanalysisonCoRoTphotometry g
a
M
The CoRoT data of HD46202 were taken during 34 days be- 0.0005
tween2008October8 and2008November12with anaverage
time sampling of 32 seconds. This involves a frequency reso-
0.001
lution of ∼0.03d−1. After removing all flagged measurements,
such as the ones suffering from hot pixels during the passage
0.0015
through the South Atlantic Anomaly, the light curve consists
0 1 2 3 4 5 6 7 8
of 81781 datapoints. As in most CoRoT light curves, a down-
Time (days)
ward trend considered to be of instrumental origin is visible.
Moreover, the light curve presents three jumps during the last Fig.1. Part of the detrended CoRoT light curve, converted to
five days of observation. We divided the light curve into four magnitude (mag = -2.5 log (flux), once the average has been
10
partsand,foreachoftheseselectedregions,weremovedatrend substracted).Thefirstdatapointcorrespondsto2454759HJD.
using a linear approximation. To visualise the variability, only
part of the detrended light curve is shown in Fig.1. We can
clearly see a variation with a period of the order of 2 days. 0.00012
Moreover,variationsonashortertimescalearealsoobserved.
The Scargle periodogram (Scargle 1982) of the detrended 0.0001
light curve is presented in Fig.2. Clear peaks are detected be-
tween0.5and5d−1.Thepeakat13.97d−1isnotintrinsictothe g) 8e-05
a
starbutcorrespondstotheorbitalfrequencyoftheCoRoTsatel- m
lite. To determinethe wholefrequencyspectrumof thestar we de ( 6e-05
usedthemethodologydescribedinDegrooteetal.(2009a),who u
analysednumerousB-typepulsatorsusinglightcurvesfromthe plit
m 4e-05
exoplanet data of the CoRoT satellite. This methodology uses A
theusualmethodofsuccessiveprewhitening,alongwithastatis-
ticalstop criterionbased onthe onefirstintroducedbyScargle 2e-05
(1982). The statistical stop criterion defined in Degroote et al.
(2009a)wasadoptedinsteadofasignal-to-noise(S/N)criterion 0
(Bregeretal.1993).Thereasonisthat,althoughasignalorigi- 0 2 4 6 8 10 12 14
natingfromnonsinusoidalprocesses(e.g.,rednoise)isalsocon- Frequency (d-1)
sideredto be significant,andthusprewhitenedusinga sum-of-
Fig.2.ScargleperiodogramofthedetrendedCoRoTlightcurve,
sinesmodel,isolatedpeaksatloweramplitudescanberecovered
convertedtomagnitude.
outsideoftheseregions.
Thefrequencieswiththehighestamplitudes,whichturnout
to be typical βCep-like pulsation modes, are listed in Table1.
Inaddition,allthefrequencieswithanamplitudelowerthanthe Wesearchedforrotationalfrequencysplittingswiththeau-
onecorrespondingtothesatellite’sorbitalfrequencyaregivenin tomatic methods described in Degroote et al. (2009a), but we
theAppendix.Intotal67frequenciesarefoundtobesignificant couldnotunambiguouslyorconfidentlyfindsuchmultiplets.For
byouradoptedstopcriterion.Forcomparisonpurposes,wealso thehighestamplitudemodes,thefrequencies f4, f5,and f6 (see
indicatethesignal-to-noise(S/N)ratio,whichiscomputedusing Table1)mightbelongtothesamemultipletandthefrequencies
a 6d−1 box-interval centred on the considered frequency. The f8, f10, and f11 might belong to another one. However, recent
errorestimateonthefrequencyiscomputedasinDegrooteetal. work on the β Cep star 12 Lacertaetaughtus to be cautiousin
(2009a),i.e.,usingtheformulaofMontgomery&O’Donoghue attributing frequenciesto multiplets without empirical identifi-
(1999),butalsotakingcorrelationeffectsintoaccountaccording cationofthemodedegree.Forthislattertarget,Dziembowski&
totheformalismbySchwarzenberg-Czerny(2003). Jerzykiewicz(1999)assumedthatthreeofthestrongestmodes,
Recent studies of CoRoT lightcurvesof B-type stars high- almostequidistant,belongtothesamemultipletintheirseismic
lightedvariationsintheamplitudeoffrequenciesandinthefre- modelling.Later,aphotometricmodeidentificationbyHandler
quencies themselves as a function of time (e.g., in the βCep etal.(2006)ruledoutthisassumptionbyprovingthatthethree
star HD180642, Degroote et al. 2009b; in the B0.5IVe star modesareactually associatedwith threedifferentvaluesofthe
HD49330, Huat et al. 2009). In order to check the stability degreeℓ.Itisobviousthatreliableempiricalmodeidentification
of the frequencies of HD46202 listed in Table1, we applied frommulticolourphotometry,and/orspectroscopy,isindispens-
a short time Fourier transform. Different window widths were ablefordefiniteconclusions.
used,leadingtotheconclusionthatthefrequenciesarestablein CoRoT observations also revealed solar-like oscillations in
time. There is no convincing proof of changing amplitudes ei- two massive stars: the β Cep star HD180642 (Belkacem et al.
ther.Thedetectedlowamplitudevariationsintimearetypicalof 2009)andtheO-typebinaryHD46149(Degrooteetal. 2010).
beatingeffectsforamultiperiodicpulsator(Fig.3). To search for the signature of such oscillations in HD46202,
2
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
Table1.Thehighest-amplitudefrequenciesderivedfromthede-
0.011
trendedCoRoTlightcurveaftersubsequentprewhitening,their
correspondingamplitude,andsignal-to-noiseratio. 0.01
0.009
ID f(d−1) f(µHz) Amplitude(mmag) S/Nratio
f 0.510 5.901 0.1079 9.3
1 0.008
f 4.856 56.204 0.0934 11.7 2
2 1/
f3 4.691 54.295 0.0798 10.7 S) 0.007
f 2.290 26.500 0.0708 8.5 V
4 T
f 2.643 30.586 0.0645 7.7 ( 0.006
5
f 3.004 34.769 0.0587 7.2
6
f 2.195 25.404 0.0460 5.7 0.005
7
f 1.462 16.919 0.0426 5.6
8 0.004
f 2.222 25.722 0.0339 4.8
9
f 0.804 9.305 0.0314 4.5
10 0.003
f 2.127 24.613 0.0307 4.3
11 -20 0 20 40 60 80
f∗ 13.973 161.728 0.0298 10.6
12 v (km s-1)
∗ thesatellite’sorbitalfrequency Fig.4. Temporal variance spectrum (TVS) computed from the
cross-correlated profiles computed by taking all elements into
account, except H. The line corresponds to the 99% signifi-
5 cancelevelforthevariabilityevaluatedfollowingtheapproach
0.14 ofFullertonetal.(1996).
4 0.12
First, variabilitywas searched in individualline profiles. Then,
) 0.10 to increase the S/N of the spectra and improve the detection
1
d(cid:0) 3 of the line-profile variations, we used cross-correlated profiles
cy ( 0.08 computedusing the least squaresdeconvolution(LSD) method
n
e (Donatietal.1997)andtakingallelementsintoaccountexcept
u
eq2 0.06 H. We also computed cross-correlated profiles associated with
fr onlyoneelementforallpossibleelements,suchashelium,sil-
0.04
icon, nitrogenand oxygen.To performour frequencyanalysis,
1 we used the software package FAMIAS1 (Frequency Analysis
0.02
andModeIdentificationforAsteroseismology,Zima2008).The
significanceofvariabilityinstellarspectracanbequantifiedby
0.00
5 10 15 20 25 30 35 a temporal variance spectrum (TVS, Fullerton et al.1996). We
time (HJD - 2454745 )
computedTVS diagramsfor individualline profiles and cross-
Fig.3.TheshorttimeFouriertransformofthedetrendedCoRoT correlatedprofiles(Rauwetal.2001).Oneofthemisillustrated
light curve, computed with a window width of 10 days. The inFig.4.Evenwithsuchstate-of-the-arttechniques,nospectro-
colourscaleisinmmagunits. scopicvariabilitycouldbedetected.
we performed an autocorrelation of the time series and peri- 4. Fundamentalparameters
odogram,bothfortheoriginaldetrendedlightcurveandforthe
In the next section, we compute stellar models for our target,
lightcurveprewhitenedwiththehighest-amplitudefrequencies.
whichmatchthepulsationfrequenciesdiscoveredintheCoRoT
Wecouldnotfindanystructuretypicalofsolar-likeoscillations
photometry.Thefundamentalparametersandchemicalcompo-
withamplitudesabove0.009mmag.
sition of HD46202 are also used as additional constraints. To
deduce the latter, we used a high-quality spectrum with ex-
3. Searchforvariabilityinspectra tended wavelength coverage, taken with the ESO/MPG 2.2m
telescopewithFEROS(Kauferetal.1999).Sixindividualspec-
Fromthephotometricamplitudesofthe pulsationmodesbeing tra,observedbetween2008April16–26of720sexposuretime
0.1 mmag or less, it is clear that these modes cannot be de- eachwereextractedfromtheESOarchiveandreducedwiththe
tectableinspectraofmoderateS/N.Nevertheless,thestudyofa MIDASpipeline.Coadditionofthesedatagaveaspectrumwith
timeseriesofspectrallinesremainsrelevantwhensearchingfor S/N ofmorethan500perresolutionelementintheopticalblue
otherkindsofvariability.First,wecancheckforthepresenceof region.
avariablestellarwind.Also,modesofveryhigh-degreeℓmight
Our quantitative spectral analysis follows the methodology
bevisibleinthespectrallines(e.g.,asintheβCepstarω1Sco,
described by Nieva & Przybilla (2007, 2008). In brief, non-
Telting & Schrijvers 1998), although they are not observed in
LTEline-formationcalculationswereperformedwiththecodes
theCoRoTlightcurve.Additionally,somespectrallinesassoci- Detail and Surface (Giddings 1981; Butler & Giddings 1985;
atedtoonlycertainelementsmightbevariablewithaperiodicity
both updated by K. Butler) on the basis of a prescribed LTE
linkedtothestellarrotationandmagneticfield. atmospheric structure (Atlas9, Kurucz 1993) and accounting
Consequently,we gathered44 high-resolutionspectra (R =
50000, S/N of ∼60) with the CORALIE e´chelle spectrograph 1 FAMIAShasbeendevelopedintheframeworkoftheFP6European
attachedto the 1.2mLeonardEulertelescope (LaSilla, Chile). CoordinationActionHELAS http://www.helas-eu.org/
3
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
Fig.5. Examples of line fits (red) for H, He,
and C to the high-quality observed spectrum
(black). The hydrogen and helium lines are
reproduced with a single synthetic spectrum,
whileforcarbonaslightlydifferentabundance
value isadopted per line, giving rise to a1-σ
uncertaintyof0.10dexduetoline-to-linescat-
ter.
Table 2. Atmosphericparameters,photometricpropertiesand fundamentalparametersof HD46202as derivedfromthe spectro-
scopicanalysis.
T (K) 34100±600 V 8.18±0.02 M/M 19.6±2.7
eff ⊙
logg(cgs) 4.17±0.07 B−V 0.17±0.02 R/R 6.5+0.9
⊙ −0.7
ξ(kms−1) 6±2 E(B−V) 0.49±0.03 logL/L 4.71±0.10
⊙
vsini(kms−1) 25±7 R 2.7±0.1 X(bymass) 0.715±0.033
V
ζ(kms−1) 15±7 M −3.73±0.25 Y(bymass) 0.271±0.033
V
M˙ (M yr−1) <10−7 B.C. −3.30±0.05 Z(bymass) 0.014±0.001
⊙
d (kpc) 1.3±0.2 M −7.03±0.26 τ (Myr) 2±1.5
spec bol evol
forthemostrecentmodelatomsofPrzybillaetal.(2008).The theseelements,implyingmassfractionsforhydrogenX,helium
methodisequivalenttofullnon-LTEmodelatmosphereanalyses Y,andthemetalsZ inHD46202asindicatedinTable2,impli-
forlateO-typestarswithweakwinds(Nieva&Przybilla2007). citlyassumingthatthepresentsurfaceabundancesarerepresen-
The parameters were determined via simultaneous fitting tativeforthepristinevalues.
of all observed H lines and ionisation equilibria of Hei/ii and To verify the results from our spectroscopic analysis, we
Ciii/iv. Our results from the spectral fitting are summarised in compared our non-LTE model flux with the observed spectral
Table 2 forthe effectivetemperatureT , surfacegravitylogg, energy distribution of HD46202 in Fig. 6. For this UV spec-
eff
microturbulenceξ,andtheprojectedrotationalvsiniandmacro- trophotometrywasextractedfromtheIUEarchive,andJohnson
turbulentvelocityζ. Examplesof ourfits tothe observedspec- magnitudeswereadoptedfromMorel&Magnenat(1978).The
trum are shownin Fig. 5 for these finalparameters.Additional latter were transformed into absolute fluxes using zeropoints
modelswere calculatedwith the non-LTEcodeFastwind (Puls fromBesseletal.(1998).Theobservedfluxwasdereddenedac-
etal.2005),whichaccountsfornon-LTEmetallineblanketing, cordingtothereddeninglawdescribedbyCardellietal.(1989),
massloss,andsphericalgeometry,foranindependenttestofthe with both the colour excess E(B−V) and the ratio of total-to-
parameter determination.Similar atmosphericparameterswere selective extinction R requiring determination. The data were
V
derived from observed H and Hei/ii line spectra2, confirming normalisedinV.Anexcellentmatchoftheorywithobservation
the match of our hydrostatic approach also with hydrodynam- isobtainedforthevaluesgiveninTable2.ThestrongLyαline
icalmodelsforlateO-typemainsequencestarswithweakstel- mostlyhasinterstellarorigins.
larwinds.TheFastwindcalculationsalsoallowedthemass-loss This allowed the dereddened apparent visual magnitude to
rate M˙ from Hα line-profile fits to be constrained. The upper bedeterminedandthespectroscopicdistanced tobederived
spec
limitfoundcanbeconsideredasinsignificantinourcomparison followingRamspecketal.(2001).Therequiredstellar mass M
withstellarevolutionarymodels. wasobtainedbycomparisonofthestar’spositioninthelogT –
eff
The helium, carbon, nitrogen, and magnesium abundances loggdiagramwithevolutionarytrackscomputedasdescribedin
derived with Detail/Surface coincide with the cosmic abun- Sect.5(alsotheevolutionaryageτ wasestimatedthisway).
evol
dancestandard(CAS)fromearlyB-typestarsinthesolarneigh- Thisin turn facilitated the absolute visualmagnitude MV to be
bourhoodasproposedbyPrzybillaetal.(2008).Fromanexten- determined,alongwiththebolometricmagnitudeM ,thestel-
bol
sion ofthatwork(Nieva& Przybilla,in preparation)we found lar luminosity L, and the stellar radius R, by using the bolo-
evidencethatabundanceanalysesofO,Si,andFeforlateO-type metriccorrectionB.C.fromourmodel.Thus,afullyconsistent
starsrequireextensionsofthemodelatoms,whichisbeyondthe spectroscopicsolutionforthefundamentalstellarparametersof
scope of the present work. We therefore adopt CAS values for HD46202wasestablished,assummarisedinTable2.
Inconsequence,HD46202isanunevolvedstarclosetothe
2 Acomprehensiveanalysisofthemetallinespectrumisnotfeasible zero-age main sequence, which shows no signature of mixing
withFastwindatpresent. withCN-processedmaterial(Przybillaetal.2010).Thederived
4
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
3.7
3.8
3.9
4
4.1
g
g
lo 4.2
4.3
4.4
4.5
4.6
4.58 4.57 4.56 4.55 4.54 4.53 4.52 4.51 4.5 4.49 4.48
log T
eff
Fig.6. Fit of the Detail non-LTE model flux for our finally Fig.7. The 1-σ and 3-σ error boxes represent the position of
adoptedparametersfromTable2(redline)totheobservedspec- HD46202,deducedfromspectroscopy,in the logT –logg di-
eff
tralenergydistributionof HD46202(black line, squares)from agram. The positions of all the asteroseismic models having a
theUV tothe near-IR.Theobservationshavebeendereddened χ2 ≤ 2 are also shown, for X = 0.682 (in red), X = 0.715 (in
andnormalisedinV. green), and X = 0.748 (in blue). We refer to the text for more
explanations.
5.2.Matchingthehighest-amplitudefrequencies
colourexcess,R , distance,andagearein excellentagreement
V
withtheNGC2244clusterpropertiesdeterminedbyHensberge Toeachmodelofourcomputedgridofstellarevolutionarymod-
etal.(2000). els, we assigned a χ2-value, which comparesthe observed fre-
quencies fobs tothetheoreticallycomputedones fth,asfollows:
i i
N (fobs− fth)2
5. Comparisonwithstellarmodels χ2 =X i i (1)
(σobs)2
i=1 i
We checked that state-of-the-art stellar models with standard
physics can account for the frequency spectrum observed by where σobs is the observational uncertainty. The lower the χ2-
i
CoRoT. When X and Z are fixed, we attempted to asteroseis- value,thebetterthematchbetweenobservedandpredictedfre-
micallyconstrainthemodelparametersofHD46202,whichare quencies. We selected the observed frequencies with only the
themass,thecentralhydrogenabundance(orage),andthecore highest amplitudes, as listed in Table1. The χ2-value is com-
convectiveovershootingparameter. puted, and the theoretical frequency value is the closest to the
observedone.Becauseofthelackofmodeidentification,nore-
strictiononthedegreeℓwasimposedonthetheoreticalfrequen-
5.1.Numericaltoolsandmodelinputphysics cies,whichcorrespondtozonalmodes(m=0).
In Fig.7 the positions of all the models having a χ2 ≤ 2,
Stellar models for non-rotating stars were computed with the without any a priori restriction on logT , but with logg ≥
evolutionary code CLE´S (Code Lie´geois d’E´volution Stellaire, 3.89, are compared to the error box repreeffsenting the position
Scuflaire et al. 2008a). We used the OPAL2001 equation of ofHD46202inthelogT –loggdiagram,asspectroscopically
eff
state (Rogers & Nayfonov 2002; Caughlan & Fowler 1988), deduced.
withnuclearreactionratesfromFormicolaetal. (2004) forthe Weconcludethatthepositionsofourbest-fittingmodelsare
14N(p,γ)15O cross-section. Convective transport is treated by compatiblewiththespectroscopicone.Indeed,thediscrepancy
using the classical mixing length theory of convection(Bo¨hm- found in log g can be considered as small in view of current
Vitense 1958). For the chemical composition, we used the so- uncertaintiesin the theory of stellar structure and stellar atmo-
lar mixture from Asplund et al. (2005). We used OP opacity spheres.Despitegreateffortshavebeenmadetoidentifyandre-
tables (Seaton 2005) computed for this mixture. These tables ducenumeroussystematicerrorsofatmosphericparametersre-
arecompletedatlogT < 4.1withthelow-temperaturetablesof latedtothedatareduction,spectralmodelling,andspectralanal-
Fergusonetal.(2005). ysis(seeNieva&Przybilla2010forasummary),theremaining
When X ∈ [0.682,0.748] and Z ∈ [0.013,0.015] were systematicscannotbeexcluded.
adopted (Przybilla et al. 2008), a fine grid of stellar evolution- Oneof themain factorsin thiscontextisthe unknowntrue
arymodelswascomputedforamassbetween16and28M in rotational velocity v of HD46202. The star shows an excep-
⊙ rot
steps of 0.1 M and a core convective overshootingparameter tionallylowvsiniforthemassivestarpopulationofNGC2244,
⊙
α between 0 and 0.5 pressure scale heights in steps of 0.05. where about 30% of the stars have vsini>250kms−1 (Huang
ov
For each main-sequence stellar model, we calculated the theo- & Gies2006), indicating a large fraction of truly fast rotators.
reticalfrequencyspectrumoflow-orderp-andg-modeswitha The measured vsini of HD46202 is only a lower limit, since
degreeoftheoscillationupto ℓ = 4 usingastandardadiabatic v mayreachmuchhighervaluesifthestarisseennearlypole-
rot
codefornon-rotatingstellarmodels(Scuflaireetal.2008b). on.Thespectroscopicallyderivedloggwouldbethepolarvalue
5
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
Table 3. Physical parameters of HD46202 as derived from theobservedfrequenciesistheoreticallyexcitedusingtheinput
the asteroseismic modelling, X ∈ [0.682,0.748] and Z ∈ physicsdescribedinSect.5.1.
[0.013,0.015]. Suchanexcitationproblemhasalreadybeenencounteredfor
othermassivestars.Forinstance,thetwoβCepstars12Lacertae
T (K) 34800±780 (B2III)andνEridani(B2III)presentobservedmodesthathave
eff
logg(cgs) 3.93±0.03 not yet been predicted by pulsation models, although differ-
M/M⊙ 24.1±0.8 entattempts to solve the discrepanciesbetween theoryand ob-
τ (Myr) 4.3±0.5
evol servations have been suggested in the literature. We refer to
α 0.10±0.05
ov Dziembowski&Pamyatnykh(2008),andreferencestherein,for
thelatestdiscussionofthematter.Anotherchoiceofopacityta-
ble (e.g. OPAL opacities) and/or metal mixture (e.g. Grevesse
& Noels 1993) does not reconcile the observed and excited
in that case, while the average logg on the surface of the rota- model frequencies for HD46202. Miglio et al. (2007ab) show
tionallydeformedstarcouldbesignificantlylower.Fastrotation that more modes are found to be excited in pulsating B-type
would require not only a re-interpretation of the spectroscopi- stars when using OP opacities with the Asplund et al. (2005)
callyderivedparametersbutalsoconsiderationofnon-standard mixture,asadoptedinourstudy,comparedtothecasewherethe
physicsinthestellarstructureandevolutionarymodels(seee.g. OPAL opacities are considered. To solve shortcomings in our
Ekstro¨met al.2008), with additionalconsequencesfor the pre- understandingofexcitationmechanismsinmassivestarsand,in
dicted frequencyspectrum andits excitation.We concludethat HD46202inparticular,isbeyondthescopeofthispaperbutde-
the potential nature of HD46202 as a fast rotator seen nearly servesto be addressedin depth.One explanationmightbe that
pole-on could resolve the remaining discrepancy between the currentopacitiesarestillunderestimatedintheregionwherethe
spectroscopicandasteroseismicsolutions.Thewaytocheckthis drivingofpulsationmodesoccurs.
wouldbetogatheraveryhigh-resolutiontimeseries(typically
R∼100000ofhighS/N(>300)spectroscopycoveringthebeat
pattern of the oscillations). In this way, a spectroscopic mode 6. Conclusions
identificationcanbeachievedwithcertaintyand,alongwithit,
The space white-light CoRoT photometryreveals the presence
anestimateoftheinclinationangleofthestar.
of β Cep-like pulsationsof very small amplitudes(∼0.1 mmag
We note that a similar shift in logg was also obtained for
and below) in the O9V star HD46202. By means of CoRoT
the βCep star θ Ophiuchi(Briquetetal. 2007). For thistarget,
data, variability associated to stellar pulsations, albeit of a dif-
the identification of a fundamentalradial mode could pinpoint
ferentnature,wasalsodetectedintheO-typebinaryHD46149
theseismic logg-valuewhile thespectroscopiclogg-valuewas
(Degrooteetal.2010)andinPlaskett’sstar(Mahyetal.2010).
foundtobeabout0.15dexhigher.
Thesediscoveriesunambiguouslyprovetherearestellaroscilla-
tionsinO-typestars,whileopeningthewayforpromisingaster-
5.3.Mostprobablemodelparameters oseismicmodellingofthiskindofmassivestar.
For HD46202, we found stellar models compatible with
In Fig.7, we distinguishtwo groupsof models, shown as open the observed asteroseismic (pulsation frequencies) and non-
squaresandasfilledcircles.Lookingdeeperintothetheoretical asteroseismic constraints (effective temperature and surface
frequency spectrum of these two groups, we conclude that the gravity). For the modelthat best explains the observedoscilla-
mostprobablemodelsare thoserepresentedas opensquaresin tions, the frequency f = 4.856d−1 is identified as the funda-
2
Fig.7.Ourargumentsarethefollowing. mental radial mode. Empirical mode identification is desirable
For the models shown as filled circles, the theoretical fre- inordertoconfirmthisconclusionandtoidentifythewavenum-
quencies matching the observed frequencies of Table1 corre- bers (ℓ,m) of other modes. This task is, however, challenging
spondtoℓ = 3orℓ = 4modesonly,exceptforthefrequency f6 becauseoftheverylowamplitudeofthemodes.
whichhasanℓ = 1.Itisveryunlikelythatnomodewithℓ = 0 Another challenge concerns the excitation of the modes.
and ℓ = 2 is observed among the highest amplitudes modes. Even using OP opacities with the Asplund et al. (2005) solar
Thereversescenario,i.e.withonlyℓ =0,1,2modesamongthe mixture,noneof the observedfrequenciesof HD46202is pre-
highestamplitudemodes,iscertainlymoreplausibleforreasons dicted to be excited by non-adiabatic computations, which ex-
ofcancellationeffectsforhigherdegreemodes. plain,atleastingeneral,theexcitationofthemodesintheless
Themodelsshownasopensquaresfit f2 = 4.856d−1 asthe massivepulsatingBstars.Miglioetal.(2007ab)extensivelyex-
fundamentalradialmode, f3 = 4.691d−1 as an (ℓ,n) = (2,−1) ploredtheimpactofthechoiceofopacitytablesandmetalmix-
mode and f6 = 3.004d−1 as an (ℓ,n) = (1,−1) mode. For the tures on the instability domains for stars with masses between
modes with lower frequency values, the theoretical frequency 2.5 and 18 M and for different metallicity values Z. Similar
⊙
spectrumisdenser,soanunambiguousidentificationismoredif- studies for higher masses, also including mass loss, are neces-
ficult.Ourbest-fittingmodelhasamassbetween23.3and24.9 saryforacomparisonwiththenewlydiscoveredoscillationsin
M⊙ andacoreconvectiveovershootingparameterbetween0.05 massiveO-typestarsfromspacedatawithunprecedentedqual-
and 0.15 pressure scale heights. The other physicalparameters ity. Our observational results presented here form a good ba-
aregiveninTable3. sisforsuchatheoreticalstudy.Moreover,adetailedexploration
of the physics in the outer layers leading to the opacity-driven
modes detected in the O9V star HD46202 and the modes of
5.4.Checkingtheexcitationofthemodes
stochastic nature detected in the O8.5V star HD46149will al-
For all models represented in Fig.7, we checked the excita- lowanin-depthevaluationofthevariousexcitationmechanisms
tion of the pulsation modes with the linear non-adiabatic code that seem to be at work in slowly rotating O-type stars in the
MADdevelopedbyDupretetal.(2002).Wefoundthatnoneof main-sequencephase.
6
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
Acknowledgements. WethankAndreaMiglioforprovidingushiscodetocom- Rauw,G.,Morrison,N.D.,Vreux,J.-M.,Gosset,E.,&Mulliss,C.L.2001,A&A,
putetheχ2function.Theresearchleadingtotheseresultshasreceivedfunding 366,585
fromtheEuropeanResearchCouncilundertheEuropeanCommunity’sSeventh Rauw,G.,DeBecker,M.,vanWinckel,H.,Aerts,C.,Eenens,P.,Lefever,K.,
Framework Programme (FP7/2007–2013)/ERC grant agreement n◦227224 Vandenbussche,B.,Linder,N.,Naze´,Y.,Gosset,E.2008,A&A,487,659
(PROSPERITY), from the Research Council of K.U.Leuven, from the Fund Rogers,F.J.,Nayfonov,A.2002,ApJ576,1064
forScientific ResearchofFlanders, andfromtheBelgian federalsciencepol- Saio,H.2009,CoAst,158,245
icyofficeBelspo.F.S.acknowledges financialsupportbythe“Studienstiftung Saio,H.,Baker,N.H.,Gautschy,A.1998,MNRAS,294,622
des deutschen Volkes”. We thank the GAPHE team of the department of Scargle,J.D.1982,ApJ,263,835
Astrophysics,GeophysicsandOceanographyoftheUniversityofLie`geforpro- Schwarzenberg-Czerny, A. 2003, in Astronomical Society of the Pacific
vidingustheircodetocomputethetemporalvariancespectrum. Conference Series, Vol. 292,Interplay ofPeriodic, Cyclic andStochastic
VariabilityinSelectedAreasoftheH-RDiagram,ed.C.Sterken,383
Scuflaire,R.,The´ado,S.,Montalba´n,J.,Miglio,A.,Bourge,P.-O.,Godart,M.,
Thoul,A.,Noels,A.2008a,Ap&SS,316,83
Scuflaire,R.,Montalba´n,J.,The´ado,S.,Bourge,P.-O.,Miglio,A.,Godart,M.,
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7
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
TableA.1. ThefrequenciesderivedfromthedetrendedCoRoTlightcurveaftersubsequentprewhitening,theircorrespondingerror,amplitude,
phase,andsignal-to-noiseratio.Thesignaliswrittenas 67 A sin[2π(f(t−t )+φ)]-0.00000169,wheret =2451545HJD.
Pi=1 i i 0 i 0
ID f(d−1) Error f(µHz) A(mag) φ(2πrad) S/N
f 0.50982 0.00023 5.90069 0.000108 0.174 9.28
1
f 4.85606 0.00025 56.20440 0.000093 0.087 11.68
2
f 4.69107 0.00029 54.29479 0.000080 -0.292 10.65
3
f 2.28961 0.00029 26.50012 0.000071 0.048 8.54
4
f 2.64259 0.00033 30.58553 0.000064 0.369 7.69
5
f 3.00402 0.00037 34.76875 0.000059 -0.421 7.23
6
f 2.19493 0.00048 25.40428 0.000046 -0.372 5.72
7
f 1.46182 0.00051 16.91921 0.000043 0.265 5.59
8
f 2.22240 0.00060 25.72222 0.000034 0.446 4.84
9
f 0.80396 0.00067 9.30509 0.000031 -0.353 4.46
10
f 2.12660 0.00071 24.61343 0.000031 0.256 4.30
11
f 13.97330 0.00076 161.72801 0.000030 -0.121 10.60
12
f 2.15488 0.00078 24.94074 0.000028 -0.072 3.95
13
f 2.35706 0.00077 27.28079 0.000028 -0.205 4.09
14
f 2.49231 0.00079 28.84618 0.000027 -0.115 4.09
15
f 0.54203 0.00082 6.27350 0.000026 -0.389 4.00
16
f 0.57365 0.00084 6.63947 0.000022 -0.016 4.03
17
f 0.32639 0.00086 3.77766 0.000031 0.392 3.98
18
f 0.40030 0.00096 4.63310 0.000027 -0.016 3.61
19
f 0.22654 0.00100 2.62199 0.000021 0.148 3.52
20
f 0.35462 0.00101 4.10440 0.000023 0.176 3.59
21
f 0.66703 0.00106 7.72025 0.000019 -0.434 3.46
22
f 3.08004 0.00110 35.64861 0.000018 0.168 3.39
23
f 0.85580 0.00115 9.90509 0.000016 0.409 3.31
24
f 4.38227 0.00128 50.72072 0.000016 0.199 3.02
25
f 1.48753 0.00128 17.21678 0.000017 0.077 3.06
26
f 2.00846 0.00129 23.24606 0.000018 -0.424 3.07
27
f 0.99298 0.00130 11.49282 0.000017 0.275 3.07
28
f 1.11162 0.00141 12.86597 0.000015 0.072 2.90
29
f 0.71597 0.00145 8.28669 0.000015 0.496 2.86
30
f 2.07063 0.00145 23.96562 0.000015 0.252 2.89
31
f 0.43954 0.00147 5.08727 0.000015 -0.155 2.88
32
f 1.67322 0.00149 19.36597 0.000014 -0.327 2.88
33
f 0.25381 0.00151 2.93762 0.000015 -0.472 2.90
34
f 3.02746 0.00151 35.04005 0.000016 0.275 2.93
35
f 1.91309 0.00159 22.14225 0.000012 -0.022 2.83
36
f 11.22560 0.00170 129.92593 0.000012 0.282 4.65
37
f 2.89645 0.00173 33.52373 0.000013 -0.272 2.63
38
f 4.55862 0.00174 52.76181 0.000012 -0.083 2.66
39
f 4.76013 0.00174 55.09410 0.000012 0.346 3.10
40
f 0.93383 0.00173 10.80822 0.000013 -0.053 2.74
41
f 1.42504 0.00173 16.49352 0.000013 0.278 2.77
42
f 3.38899 0.00174 39.22442 0.000012 0.381 2.82
43
f 0.14792 0.00177 1.71204 0.000012 -0.493 2.79
44
f 2.79809 0.00179 32.38530 0.000011 0.463 2.80
45
f 0.61406 0.00183 7.10718 0.000013 0.236 2.78
46
f 0.78215 0.00185 9.05266 0.000013 -0.360 2.79
47
f 1.76133 0.00189 20.38576 0.000011 0.025 2.79
48
f 1.98728 0.00191 23.00093 0.000013 0.138 2.76
49
f 1.58434 0.00195 18.33727 0.000011 0.015 2.77
50
f 0.95913 0.00200 11.10104 0.000010 -0.281 2.73
51
f 4.43451 0.00202 51.32535 0.000010 -0.318 2.83
52
f 2.32122 0.00203 26.86597 0.000010 -0.459 2.78
53
f 1.13398 0.00204 13.12477 0.000010 0.437 2.80
54
f 3.21544 0.00208 37.21574 0.000010 -0.190 2.78
55
f 8.77345 0.00217 101.54456 0.000009 0.458 3.38
56
f 1.72569 0.00219 19.97326 0.000010 -0.045 2.67
57
f 1.85355 0.00213 21.45313 0.000010 -0.492 2.80
58
f 0.29858 0.00216 3.45579 0.000010 0.410 2.77
59
8
Briquetetal.:AnasteroseismicstudyoftheO9VstarHD46202fromCoRoTspace-basedphotometry
TableA.1.continued.
ID f(d−1) Error f(µHz) A(mag) φ(2πrad) S/N
f 1.02122 0.00218 11.81968 0.000009 -0.250 2.80
60
f 1.20689 0.00216 13.96863 0.000011 0.370 2.86
61
f 3.91015 0.00219 45.25637 0.000009 0.381 2.74
62
f 5.91544 0.00222 68.46574 0.000009 -0.040 2.89
63
f 1.24273 0.00226 14.38345 0.000010 -0.160 2.85
64
f 1.95592 0.00224 22.63796 0.000009 0.479 2.91
65
f 1.31797 0.00228 15.25428 0.000009 -0.195 2.91
66
f 0.69020 0.00223 7.98843 0.000009 0.258 3.00
67
9