Non-radial, non-adiabatic solar-like oscillations in RGB and HB stars

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by  M. Grosjean

Non-radial, non-adiabatic solar-like oscillations in RGB and
HB stars
M.Grosjean1,3,a,M.A.Dupret1,K.Belkacem2,J.Montalban1,A.Noels1,andR.Samadi2
1 Institutd’AstrophysiqueGe´ophysiqueetOceanographiedel’Univertiste´deLie`ge,Belgium
2 LESIA,ObservatoiredeParis-Meudon,France
3 3 Doctorant,BoursierF.R.I.A.
1
0
2
Abstract. CoRoTandKeplerobservationsofredgiantsrevealrichspectraofnon-radial
n
a solar-likeoscillationsallowingtoprobetheirinternalstructure.Wecomparethetheoret-
J ical spectrum of two red giants in the same region of the HR diagram but in different
evolutionaryphases.Wepresenthereourfirstresultsonthe inertia,lifetimesandampli-
5
1 tudesoftheoscillationsanddiscussthedifferencesbetweenthetwostars.

]
R 1 Introduction
S
. The two modelsconsideredin this study are a red giantbranch (RGB) anda helium burningmodel
h
p (HB).BothwerecomputedwiththecodeATON[9].Theirmassis1.4M⊙ andtheirradiusis11.9R⊙.
- MLT is used to described the convection with αMLT = 1.9 and the initial chemical composition is
o X =0.7andZ =0.02.Tocomputethemodelifetimes,weusethenon-adiabaticpulsationcodeMAD
r [2] with a non-local time-dependant treatment of the convection [4]. The amplitudes are computed
t
s usingastochasticexcitationmodel[8]andsolarparameterstodescribetheturbulenceintheenvelope.
a
[

1
2 Results
v
9
4
2
3
. 100 100
1 l =0
0 10-1 10-1 ll ==12
:13 Inertia 1100--32 Inertia 1100--32
v
i 10-4 l = 0 10-4
X l = 1
10-5 l = 2 10-5
r
a 25 30 35 40 45 25 30 35 40 45
Frequency (µHz) Frequency(µHz)
Fig.1.Inertiaofl=0,1and2modesfortheRGBmodel(left)andtheHBmodel(right).Thankstotheinertia
wecandistinguishtwotypeofnon-radialmodes.Themodeswithlowvalueofinertiaaretrappedintheenvelope
(p-typebehavior).Asintheadiabaticcasethel=2modesarebettertrappedintheenvelopethanthel=1[5].We
alsonoteahigherdensityofmodesandabettertrappingintheRGBmodel(seealso[6]).
a e-mail:[email protected]
EPJWebofConferences
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l = 0
3 l = 1 2
l = 2 1000
Lifetime (days) 10092 Lifetimes(days) 1006246 l =0
8 4
7 l =1
6 2 l =2
25 30 35 40 45 25 30 35 40 45
Frequency (µHz) Frequency(µHz)
Fig. 2. Lifetimes of l=0, 1 and 2 modes for the RGB model (left) and the HB model (right). The oscillatory
behaviorofthelifetimeintheHBmodelcomesfromtheoscillationsoftheinertia.Itisnolongerpresentinthe
RGBmodelduetoahighradiativedampingforallmodesnottrappedintheenvelope.
1.6 1.4 l =0
l =1
1.4 1.2 l =2
Amplitudes (m/s) 11000.....20864 lll === 012 Amplitudes(m/s) 1000....0864
0.2 0.2
0.0 0.0
25 30 35 40 45 25 30 35 40 45
Frequency (µHz) Frequency(µHz)
Fig.3.Amplitudesofl=0,1and2modesfortheRGB(left)andtheHB(right)models.Becauseoflargerradiative
damping(leftpanels)andlargerinertia(rightpanels)ofl=1modestrappedintheenvelope,theiramplitudesare
smallerthanthecorrespondingl=2andl=0modes.Thel=1tol=0amplitudesratiosaresmallerfortheRGBdue
totheimportantradiativedamping.Thisresultsisnotincompatiblewithcurrentobservations[7].
3 Conclusions
Asalreadyfoundbyadiabaticcomputations[5],thespectrumdensityislargerintheRGBmodel.Lifetimesand
amplitudesindicatethatmoreg-dominatedmixed-modesshouldmorebevisibleintheHBthanintheRGBmodel.
ThisisduetoabettertrappingefficiencyintheRGBmodel(see[5])whichresultsfromitslargerdensitycontrast.
Computationsoflifetimesshowtheimportanceoftakingintoaccounttheradiativedampingtopredictvisibilities
of mixed-modes. Further development and comparisons with observations will allow us to better characterise
convectionanditsinteractionwithoscillationsintheoutermostlayersofredgiants.
References
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