arXiv:1207.6957v1 [astro-ph.SR] 30 Jul 2012 eivdt etest hr h yaooriginates. dynamo the (tachocli where region site transition the the be , to elemen believed the fluid iner In their 1991). that to Zahn due however, e.g. zone radiative expected, adjacent is the into It penetrate e gradient one. temperature radiative adiabatic the the bound where border Their regi the fulfilled. energy at are is lie the (CZ) criterion - Schwarzschild zones envelope convective the where the description, pro of standard adiabatic an parts the as treated deeper In be scales can the elements time fluid in short by least transport on mixed at are - elements and chemical the which envelop convective in have (MS) main-sequence Low- Introduction 1. o hc h onadpue r olne bet modify and to convection radiat penetrative able to of close longer extent remains no that are stratification temperature plumes stratificat the flows downward temperature downward the adiabatic by which to - low close mixing a material establish and that - transport heat tive ueteetn fteP nteSn(etoiue l 1993; al. et (Berthomieu (2011) Sun al. the et Christensen-Dalsgaard in 1994). PC al. the et Monteiro of to extent the attempted the have of sure 1990; studies importance helioseismic the (Gough region, to behaviour tachocline Owing whi (periodic) 1994). Vorontsov spacings, oscillatory & frequency Roxburgh an characteristic show some frequ oscillations as then the well impacts as and profile cies speed v sound of is be the regime radiative change in a can abrupt to convective This a The from asteroseismology. stars. transport energy of MS means low-mass by in achieved convection penetrative of unknown largely still is and principles first from derived be uy2,2018 24, July srnm Astrophysics & Astronomy eertv ovcin(C orsod oe to corresponds (PC) convection Penetrative rca on hrfr st n bevtoa signature observational find to is therefore point crucial A eevd,21;acpe 2012 , accepted 2012; , Received 2 3 1 Conclusions. envelop convective outer the below convection penetrative zone. convection envelope the of Results. base the frequenc of oscillation properties the the and structure internal the derive Methods. Aims. oa xetoe itnerpeetn 6 representing distance a over extent an to neoeo h oa-ieoclao D525rcnl obs recently 52265 HD oscillator solar-like the of envelope h ne onayo h xr-iigrgo sfuda 0 at found is region extra-mixing the of boundary inner The e words. Key ntttd hsqed ens nvri´ eRne ,CNR 1, Rennes Universit´e de e-mail: Rennes, de Physique de Institut bevtied ai,LSA NSUR80,F915Meudon F-92195 8109, UMR CNRS LESIA, Paris, de Observatoire bevtied ai,GP,CR M 11 -29 Meudon, F-92195 8111, UMR CNRS GEPI, Paris, de Observatoire eama hrceiigteiwr rniinfo convect from transition inward the characterizing at aim We esi intr fevlp eertv convection: penetrative envelope of signature Seismic [email protected] h esi niaosceryrva htt etrepresen best to that reveal clearly indicators seismic The eivsiae h rgno n pcfi etr on nth in found feature specific one of origin the investigated We seoesooy-sas neir tr:oscillations stars: - interiors stars: - asteroseismology hs eut otiuet h ahciecharacterizati tachocline the to contribute results These aucitn.ylmj-HD52265-AAletter no. manuscript / roesoti tr cannot stars in overshoot or h oo trH 52265 HD CoRoT the . e et ftettlselrrdu,sgicnl larger significantly radius, stellar total the of cents per 0 .Lebreton Y. ffi in convec- cient L te othe to etter v.The ive. e htbs ac h bevtosadue esi indic seismic a used and observations the match best that ies o be- ion i (see tia .Tepntaiedsac setmtdt be to estimated is distance penetrative The e. e is ne) isible quals re yteCRTsatellite. CoRoT the by erved mea- cess. . aries have . ABSTRACT 800 ons 1 en- ch , es 2 ts s M 21 -54 ens France Rennes, F-35042 6251, UMR S ± n ..Goupil M.J. and 0 tr:idvda:H 52265 HD individual: stars: - . vra siae xeto 0 of extent neces estimated is overshoot an envelope over convective that found recently ) h frequencies The 4). rmtegon,te rmsaeb the by space from then ground, the from no-over a and formulation overshoot one. - t betw ballistic and intermediate - convective smooth, classical the be between must transition stratification the radiative that show also to egt.Tehg-ult oa aaadwd vial ran degrees available mode wide of and values data solar high-quality The height). pcrm n dnie 8rlal o-erepmdso d of solar-lik p-modes p-mode low-degree typical reliable grees 28 a star identified found G0V and high-metallicity They spectrum, a . 33719), an (HIP 52265 hosting HD star the of isos edn oa cuayi rqec measurements tenths frequency few photomet in accuracy high-precision an 2010) to al. leading missions, et (Koch Kepler and 2002) h eido h silto sepce ob ogr(Roxbu longer be to expected is st oscillation 1993). the the inside of deeper la period located a the is for discontinuity Moreover, below. the extent, one temp PC radiative adiabatic the the to from jump gradient increa larger ature an a causes with that grows PC t signal of expects extent oscillatory then the One of discontinuity. amplit amplitude the its the of and height star the to the on related inside depend T be discontinuity to therein). the found references of is and location component oscillatory the 2009, of Roxburgh period e.g. detecta (see conducted be been to stars have expected in ones CZ only a the of modes, base low-degree the using at PC of frequencies tions 004 v ordaieeeg rnpr ttebs fteconvecti the of base the at transport energy radiative to ive ni tr te hnteSun. the than other stars in on h bevdpoete fH 26,mdl utinclude must models 52265, HD of properties observed the t France France , E oa-ieoclain aebe dnie nmn tr fi stars many in identified been have oscillations Solar-like alte l 21)aaye the analysed (2011) al. et Ballot R ditor D525feunysetu.W oeldtesa to star the modelled We spectrum. frequency 52265 HD e ℓ where = 0 µ , 1 z hoeia tde ftee the of studies Theoretical Hz. R , = n order and 2 3 1 . 3 R ν n ⊙ ,ℓ steselrradius. stellar the is r nterne1500 range the in are ℓ n loe hitne-asar tal. et Christensen-Dalsgaard allowed nterne14 range the in hnwa sfudfrteSun. the for found is what than . 37 ∼ 0 H . 95 P CoRoT H ( H P hc corresponds which , P stepesr scale pressure the is CoRoT − ff silto spectrum oscillation trsniieto sensitive ator cso h oscilla- the on ects − 4(e hi Table their (see 24 2550

c Bgi tal. et (Baglin S 2018 ESO µ zwt a with Hz ve d to ude rand ar e a een shoot eof ge sary sing rger fa of rgh hat the ble er- rst he he ry e- 1 e Y. Lebreton and M.J. Goupil: Seismic signature of envelope penetrative convection: the CoRoT star HD 52265 frequency at maximum amplitude νmax = 2090 ± 20µHz. The er- ror on each frequencyis a few tenths of µHz. Such a high quality 0.045 data set enables to probe the interior structure of the star. Here HD52265 we report on one evidence for penetrative convection below the ξ = 0.0 0.040 ξ upper convective region of HD 52265 as indicated by its internal = 1.3 structure modelling. 0.035 ) n (

2. HD 52265: observations and modelling 10

/ 0.030 01

2.1. Global and seismic observational constraints rr 0.025 To model HD 52265 we adopted the effective temperature Teff = 6120 ± 110 K and metallicity [Fe/H] = 0.22 ± 0.05 dex that we derived from an average of 20 spectroscopic de- 0.020 terminations reported since 2001. We adopted the L = 2.053 ± 0.053L that we derived from the Hipparcos paral- 0.015 ⊙ 1700 1800 1900 2000 2100 2200 2300 2400 2500 lax (van Leeuwen 2007), Tycho magnitude and bolometric cor- νnl (µHz) rection calculated according to VandenBerg & Clem (2003). We related the ratio of heavy elements mass fraction Z to hydro- Fig. 1. Observed ratios rr01/10(n) (black squares) as a function of fre- gen mass fraction X to [Fe/H] through [Fe/H] = log(Z/X) − quency νn,0/n,1 for HD 52265 compared to the rr01/10(n) ratios in the models without PC (ξ = 0.0, magenta) and with PC (ξ = 1.3, red). log(Z/X)⊙ and adopt (Z/X)⊙ = 0.0244 from the Grevesse & Noels (1993) solar mixture. In the following we will focus on the ability of models to reproduce the values of the seismic indicators rr01(n) and rr10(n) (T, ρ, κ, σ are the temperature, density, opacity, and Boltzmann introduced by Roxburgh & Vorontsov (2003), which are defined constant, respectively). The free parameter ξ is of the order as of unity but has to be calibrated by comparing stellar models to observations. The seismic indicators rr01/10 that we consid- rr01(n) = dd01(n)/∆ν1(n) ; rr10(n) = dd10(n)/∆ν0(n), (1) ered here are sensitive to the change in the temperature deriva- tive hence to the transition from an unstable to a stable strat- where ification. The amplitude of the periodic signal is smaller for a 1 smoother transition and cannot be detected if the transition is dd (n) = (ν − 4ν + 6ν − 4ν + ν ) (2) 01 8 n−1,0 n−1,1 n,0 n,1 n+1,0 highly smooth. We therefore concentrated on determining the 1 adiabatic extent of the overshoot region. We imposed accord- dd10(n) = − (νn−1,1 − 4νn,0 + 6νn,l − 4νn+1,0 + νn+1,1), (3) ingly that the temperature gradient in the overshooting zone is 8 the adiabatic gradient. where ∆νℓ(n) = νn+1,ℓ − νn,ℓ are the standard large frequency Equilibrium models were calculated and adjusted to sat- separations (Tassoul 1980). Roxburgh & Vorontsov showed that isfy the constraints provided by the global parameters (L, Teff the ratios rr01 and rr10 (hereafter rr01/10) are sensitive to the and surface [Fe/H]) and the 28 reliable observed frequencies of sharp variation of the sound speed in the transition region be- Ballot et al. (2011). The model frequencies were calculated with tween the upper CZ and the radiative layers beneath (see also the LOSC adiabatic oscillation code (Scuflaire et al. 2008) for Ot´ıFloranes et al. 2005). Roxburgh (2009), using high-quality the whole range of observed mode orders and degrees, and the solar data, confirmed that these indicators exhibit an oscilla- observed and modelled seismic indicators were derived consis- tory behaviour as a function of the frequency. From the 28 re- tently. We corrected the model frequencies from the so-called liable individual frequencies determined by Ballot et al. (2011), near surface effects using the empirical correction proposed by we calculated the rr01/10(n) ratios. The variations of rr01(n) Kjeldsen et al. (2008). The frequency differences may be sen- (resp. rr10(n)) as a function of frequency νn,0 (resp. νn,1) are sitive to surface effects, however, we recall that Roxburgh & plotted in Fig 1. Ballot et al. already noticed a periodic varia- Vorontsov (2003) demonstrated that the ratios of small to large tion of the separations d01(n) = ν0,n−1 − 0.5(ν1,n + ν1,n−1) and separations are quite independent of the surface treatment and d10(n) = −ν1,n + 0.5(ν0,n + ν0,n+1) and suggested that it could be are therefore efficient probes of the interior. the signature of the CZ boundary. The periodic variation is even We used the Levenberg-Marquardt minimization method as clearer in the rr01/10(n) separation, and Section 3 is devoted to described in Miglio & Montalb´an (2005) to adjust the age and confirming the origin of this periodic signal and to characteriz- mass, the initial helium and metallicity, the mixing-length pa- ing it. rameter for convection, the convective core overshooting extent, and surface effects parameter so that the model of HD 52265 fits the observations best, within the error bars. The goodness 2.2. Internal structure models and their oscillations of fit was evaluated through minimization of the reduced χ2: 2 = − −1 · Nobs − 2 We modelled HD 52265 with the evolution code Cesam2k χ (Nobs 1) Pi=1 (xi,mod xi,obs)/σi,obs , where Nobs is (Morel & Lebreton 2008) and the input physics and param- the number of observational constraints considered, xi,mod and th eters described in Lebreton (2011). We considered the model xi,obs are the modelled and observed values of the i parameter, for penetrative convection below the convective envelope pro- respectively, and σi,obs is the error on xi,obs. We point out that posed by Zahn (1991). In this model the distance of fluid pen- the convective core overshooting extent derived from prelimi- etration into the radiative zone reads Lp = (ξ/χP)HP, where nary model calibrations of HD 52265 is low, lower than 0.10Hp. χP = (∂ log χ/∂ log P)ad is the adiabatic derivative with respect Because the properties of the central region do not affect the ef- to pressure P of the radiative conductivity χ = 16σT 3/(3ρκ) fect we study at the transition region, the models presented here

2 Y. Lebreton and M.J. Goupil: Seismic signature of envelope penetrative convection: the CoRoT star HD 52265

0.010 0.40 HD52265 base of convective envelope ξ 0.35 = 1.3 P

ln sinusoidal fit 0.30 d /

T 0.25 0.005 ln 0 . d

0.20 0 =

ξ 0.15 , 10 0.10 / 0.0 0.2 0.4 0.6 0.8 01 0.000 r/R⊙ rr 1.4 − ) 10 2 convective envelope /

− 1.2 01 s . HeI ionisation 1.0 rr cm −0.005 4 0.8 10 ( 0.6 τ d

/ 0.4 c d 0.2 −0.010 500 1000 1500 2000 2500 3000 3500 1700 1800 1900 2000 2100 2200 2300 2400 2500 τ (s) νnl (µHz)

Fig. 2. Top: Temperature gradient as a function of radius in a model with Fig. 3. Difference of the ratios rr01/10(n) as a function of frequency be- penetrative convection (continuous line) and a standard model (dashed tween the standard model (no PC, ξ = 0.0) and (i) the model with PC line) of HD 52265. Bottom: Sound speed gradient with respect to the with an extent of ξ = 1.3 (red continuous line), (ii) the observations R (black). The dashed red line is a sinusoidal fit of the red curve (see acoustic depth τ(r) = R dr/cs for the same models. r text). Note that to calculate the differences, models were splined on the observed frequency grid. A detailed comparison with observations is provided in Fig. 4. do not include core overshooting. The detailed properties of the models will be presented in a forthcoming paper. Table 1. Fit of the signal S ≡ ∆rr01/10(n) = rr01/10(n)S − rr01/10(n)ξ=0. 2 Case 1: ∆rr01/10(n) = (a/ν + b/ν ) cos(4πνT + 2φ)+offset. Case 2: 2 ∆rr01/10(n) = (a/ν) cos(4πνT + 2φ) + (b/ν ) sin(4πνT + 2ψ)+offset. 3. Extent of penetrative convection Model fit (Mξ=1.3) ; fit of raw observations (O) ; spline adjustment on observations (Os). The best-fit models yield total χ2-values in the range 2-3 what- ever the value of the PC and therefore cannot be distinguished signal ST a b φ ψ offset on that basis. The models have an age A ≃2 Gyr, a mass [s] [µHz] [104µHz2] [rad] [rad] 10−4 M ≃ 1.25 M⊙ and a radius R ≃ 1.3 R⊙ in quite reasonable agree- Case 1 ment with the values derived by Escobar et al. (2012) in their Mξ=1.3 2153 -8.29 0.75 -1.32 - -0.5 recent modelling of the star. We now examine the impact of PC O 2187 -34.1 5.76 -1.64 - -8.4 on the rr01/10(n) frequency ratios. Os 2204 -36.0 6.18 -1.86 - -7.6 We first compared models with ξ = 0.0 and 1.3. The varia- Case 2 tions of the ratios rr01/10(n) with frequency are shown in Fig.1. Mξ=1.3 2182 -9.03 1.03 -1.89 −1.28 -0.5 The amplitude of the oscillatory component is significantly O 1999 -48.9 9.14 1.24 2.09 -6.3 smaller for ξ = 0.0 than for ξ = 1.3. The impact of a nonva- Os 2040 -49.1 9.15 0.63 1.44 -6.0 nishing ξ on the structure of the model with ξ = 1.3 compared with a model without PC is illustrated in Fig.2. Differences ap- pear in the transition region below the convection zone while ev- To examine the ability of models to reproduce the observed rr01/10 oscillatory trend, the difference between the signals from erywhere else, the structures coincide. The top of Fig. 2 shows 2 the behaviour of the temperature gradient below the CZ in the the observations and models was measured with a specific χ quantity where the variables are xi = rr01/10(n). Considering model of HD 52265. Without PC the CZ is located at rSc, the 2 Schwarzschild radius. The temperature gradient drops from its models with increasing ξ, we found that the variation of χ shows a minimum between ξ = 1.2 and 1.3. The signals from adiabatic value 0.4 at the border of the convective envelope and 2 takes the value of the radiative gradient. In contrast, when PC models with ξ ≤ 1.1 or ξ ≥ 1.4 lead to significantly larger χ , excluding these models as acceptable. We therefore found a PC takes place below rSc as in model with ξ = 1.3, a discontinuity in the temperature gradient occurs at the bottom of the mixed extent of ξ ≃ 1.25 ± 0.10 for HD 52265. This translates into an region (including the PC extent), resulting in a nearby density overshoot distance of dov = 0.95 ± 0.08 Hp corresponding to discontinuity. This sharpens the variation of the adiabatic sound dov = 0.060 ± 0.004 R. 1/2 speed cs = (Γ1P/ρ) at the base of the convective envelope in models including PC (Γ is the first adiabatic index). The bottom 1 4. Seismic location of the base of the adiabatic of Fig. 2 shows the sound speed gradient as a function of the R temperature stratification τ = / acoustic depth (r) Rr dr cs. Rapid variations in the sound speed gradient can be seen, which correspond to the HeI ioniza- The structure of inner regions below the transition layer is un- tion (τ ∼ 620 s) and to the inner limit of the external CZ. At the affected by a change in ξ. It is therefore reasonable to assume base of the CZ, the variation of dcs/dτ for the model including that the long-term trend seen in Fig.1 for both signals with and PC behaves as a near discontinuity (τ ∼ 2780 s), whereas is re- without PC is the same and has its origin deeper in the star. This 2 2 mains continuous in the standard model where only d cs/dτ is enables us to remove it to isolate the oscillatory component aris- marginally discontinuous (τ ∼ 2450 s). ing from the base of the CZ of model with ξ = 1.3. We interpo-

3 Y. Lebreton and M.J. Goupil: Seismic signature of envelope penetrative convection: the CoRoT star HD 52265 lated the rr01/10(n) signals of the models with ξ = 0.0 and 1.3 on the same frequency grid and computed their differences. The 0.010 differences plotted in Fig.3 are clearly generated by including HD52265 PC in the model with ξ = 1.3. We obtain an almost pure sinu- HD52265 splined sinusoidal fit soid, the characteristics of which can be estimated and related to sinusoidal fit

0 0.005 . the properties of the region of penetrative convection. Slightly 0 =

ξ different expressions of the sinusoidal component arising from , 10 /

the CZ base that is expected to be visible in several seismic in- 01 dicators of distant stars have been established theoretically (see rr 0.000 − e.g. Roxburgh & Vorontsov 1994; Monteiro et al. 2000; Basu OBS , et al. 2004; Mazumdar 2005; Verner et al. 2006; Houdek & 10 /

Gough 2007;Roxburgh2009).Guided by them, we assumed that 01 rr − the model oscillation in Fig.3, which stems from the difference 0.005 ∆rr01/10(n) = rr01/10(n)ξ=1.3 − rr01/10(n)ξ=0.0, can take two nearly equivalent forms expressed in the caption of Table 1. Roxburgh −0.010 (2009) expressed the periodicity of the signal as 1/(2Tξ=1.3), 1700 1800 1900 2000 2100 2200 2300 2400 2500 which providesthe acoustic radius Tξ=1.3 = τt−τ(r) atthebaseof νnl (µHz) the CZ. τ(r) is the acoustic depth and τt ≡ τ(r = 0) = 1/(2h∆νi), where h∆νi is the mean asymptotic large frequency separation. Fig. 4. Difference of the ratios rr01/10(n) as a function of frequency be- The amplitude is a slowly varying function of ν with a term in tween the standard model (no PC, ξ = 0.0) and the raw observations ν−1 (resp. ν−2), arising from a discontinuity in the first (resp. sec- (black) or a spline fit of observations (continuous blue line). The dashed ond) derivative of the sound speed. We list in Table 1 the values lines are the fits of the raw and splined data (see Table 1). of Tξ=1.3 and other parameters of the fit of the sinusoidal signal of Fig.3 (χ2 = 1.6 10−4). The results weakly depend on the as- fit With the above results, we provided an additional clue that sumed form for ∆rr (n). We find T ≃ 2167 ± 15 s, which 01/10 ξ=1.3 the physical description of the turbulent convection must be im- is higher than values obtained by Monteiro et al. (2000) because proved in stars, particularly at interfaces with radiative regions. Monteiro et al. considered less overshoot and slightly smaller Future modelling of the dynamics in the region of the tachocline stellar . These T values can now be compared with ξ=1.3 will have to comply with our findings as well as with the results the theoretical value, T = τ − τ(r ), directly computed from ov t ov previously obtained for the Sun. the equilibrium model with ξ = 1.3, where rov is the radius at the base of the adiabatic region and τt = 4913 s for the model with ξ = 1.3. The fitted value Tξ=1.3 falls within ∼ 35s(∼ 2%) of the References resulting theoretical value Tov = 2132 s, the shift Tξ=1.3 − Tov being similar to that found for the Sun by Roxburgh (2009). Baglin, A., Auvergne, M., Barge, P., et al. 2002, in ESA Special Publication, Vol. The same procedure was applied to the observed signal, 485, Stellar Structure and Habitable Planet Finding, ed. B. Battrick, F. Favata, I. W. Roxburgh, & D. Galadi, 17–24 which is compared to the model with ξ = 0.0 and the sinusoidal Ballot, J., Gizon, L., Samadi, R., et al. 2011, A&A, 530, A97 2 −3 fits are plotted in Fig. 4. The results of the fits (χfit = 10 ) are Basu, S., Mazumdar, A., Antia, H. M., & Demarque, P. 2004, in ESA Special listed in Table 1. The period Tobs = 2100 ± 100 s agrees quite Publication, Vol. 559, SOHO 14 Helio- and Asteroseismology: Towards a well with that derived for model ξ = 1.3. We conclude that the Golden Future, ed. D. Danesy, 313 Berthomieu, G., Morel, P., Provost, J., & Zahn, J.-P. 1993, in Astronomical base of the adiabatically stratified region of HD 52265 is located Society of the Pacific Conference Series, Vol. 40, IAU Colloq. 137: Inside within 2% of the radius rov/R = 0.800 ± 0.004. the Stars, ed. W. W. Weiss & A. Baglin, 60–62 Christensen-Dalsgaard, J., Monteiro, M. J. P. F. G., Rempel, M., & Thompson, M. J. 2011, MNRAS, 414, 1158 5. Conclusions Escobar, M. E., Th´eado, S., Vauclair, S., et al. 2012, A&A, 543, A96 Gough, D. O. 1990, in Lecture Notes in Physics, Berlin Springer Verlag, We have detected an oscillatory signal with a significant am- Vol. 367, Progress of Seismology of the Sun and Stars, ed. Y. Osaki & plitude in the variation of the separations rr01/10(n) with fre- H. Shibahashi, 283 quency for the CoRoT star HD 52265. A comparison with the Grevesse, N. & Noels, A. 1993, in Origin and Evolution of the Elements, ed. same signal arising from appropriate stellar models shows that it N. Prantzos, E. Vangioni-Flam, & M. Casse, 15–25 Houdek, G. & Gough, D. O. 2007, MNRAS, 375, 861 cannot be reproduced unless penetrative convection is included Kjeldsen, H., Bedding, T. R., & Christensen-Dalsgaard, J. 2008, ApJ, 683, L175 at the base of the outer convective envelope. This is the first Koch, D., Borucki, W., Jenkins, J., et al. 2010, in 38th COSPAR Scientific time that such a feature is firmly detected in a star other than Assembly, Vol. 38, 2513 the Sun. A best fit of the signal provides a measure of the ex- Lebreton, Y. 2011, ArXiv e-prints arXiv:1108.6153 Mazumdar, A. 2005, A&A, 441, 1079 tent of the mixed region below the CZ, in terms of the proxy Miglio, A. & Montalb´an, J. 2005, A&A, 441, 615 dov = 0.95 ± 0.08 Hp. Monteiro, M. J. P. F. G., Christensen-Dalsgaard, J., & Thompson, M. J. 1994, The periodic signal for HD 52265 is more pronounced than A&A, 283, 247 for the Sun (Christensen-Dalsgaard et al. 2011), with a longer Monteiro, M. J. P. F. G., Christensen-Dalsgaard, J., & Thompson, M. J. 2000, period, and so is the measured extent of PC in terms of H and MNRAS, 316, 165 p Morel, P. & Lebreton, Y. 2008, Ap&SS, 316, 61 normalized radius. We point out that HD 52265 is similar to the Ot´ıFloranes, H., Christensen-Dalsgaard, J., & Thompson, M. J. 2005, MNRAS, Sun in all aspects except for the higher metallicity. Therefore, to 356, 671 understand the amplitude difference between the two stars, it is Roxburgh, I. W. 1993, in Astronomical Society of the Pacific Conference Series, important to investigate the impact of metallicity on the structure Vol. 42, GONG 1992. Seismic Investigation of the Sun and Stars, ed. T. M. Brown, 173 and dynamics of the tachocline. Progress is expected in a near Roxburgh, I. W. 2009, A&A, 493, 185 future when seismic missions will provide similar observations Roxburgh, I. W. & Vorontsov, S. V. 1994, MNRAS, 268, 880 for many stars spanning a wide metallicity range. Roxburgh, I. W. & Vorontsov, S. V. 2003, A&A, 411, 215

4 Y. Lebreton and M.J. Goupil: Seismic signature of envelope penetrative convection: the CoRoT star HD 52265

Scuflaire, R., Montalb´an, J., Th´eado, S., et al. 2008, Ap&SS, 316, 149 Tassoul, M. 1980, ApJS, 43, 469 van Leeuwen, F., ed. 2007, Astrophysics and Space Science Library, Vol. 350, Hipparcos, the New Reduction of the Raw Data VandenBerg, D. A. & Clem, J. L. 2003, AJ, 126, 778 Verner, G. A., Chaplin, W. J., & Elsworth, Y. 2006, ApJ, 638, 440 Zahn, J.-P. 1991, A&A, 252, 179

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