Constraints on the structure of the core of subgiants via mixed modes: the case of HD 49385

Abstract

Context. The solar-like pulsator HD 49385 was observed with the CoRoT satellite over a period of 137 days. The analysis of its oscillation spectrum yielded precise estimates of the mode frequencies over nine radial orders and distinguished some unusual characteristics, such as some modes outside the identified ridges in the échelle diagram and that the curvature of the = 1 ridge differs significantly from that of the = 0 ridge. Aims. We search for stellar models that can reproduce the peculiar features of the oscillation spectrum of HD 49385. After showing that they can be accounted for only by a low-frequency = 1 avoided crossing, we investigate the information provided by the mixed modes about the structure of the core of HD 49385. Methods. We propose a toy-model to study the case of avoided crossings with a strong coupling between the p-mode and g-mode cavities in order to establish the presence of mixed modes in the spectrum of HD 49385. We then show that traditional optimization techniques are ill-suited to stars with mixed modes in avoided crossing. We propose a new approach to the computation of grids of models that we apply to HD 49385. Results. The detection of mixed modes leads us to establish the post-main-sequence status of HD 49385. The mixed mode frequencies suggest that there is a strong coupling between the p-mode and g-mode cavities. As a result, we show that the amount of core overshooting in HD 49385 is either very small (0 < α ov < 0.05) or moderate (0.18 < α ov < 0.20). The mixing length parameter is found to be significantly lower than the solar one (α CGM = 0.55 ± 0.04 compared to the solar value α = 0.64). Finally, we show that the revised solar abundances of Asplund ensure closer agreement with the observations than the classical ones of Grevesse & Noels. At each step, we investigate the origin and meaning of these seismic diagnostics in terms of the physical structure of the star. Conclusions. The subgiant HD 49385 is the first star for which a thorough modeling has been attempted to reproduce all the properties of an avoided crossing. It has provided the opportunity to show that the study of the coupling between the cavities in these stars can provide valuable insight into open questions such as core overshooting, the efficiency of convection, and the abundances of heavy elements in stars.

Estimating stellar mean density through seismic inversions

“…This time, the results are far better and are within 2.5% of the mean densities from Deheuvels & Michel (2011) for the SOLA inversions and for the KBCD approach, and 3.5% for the scaling law based on the large frequency separation. Paradoxically, the pre-main-sequence models now give the results which are closest to Deheuvels & Michel (2011). Once more, the scaling law is somewhat different from the two other approaches, which is probably indicative of surface effects.…”

“…19. The horizontal triple-dot-dashed lines are the mean densities corresponding to the four families of solutions in Deheuvels & Michel (2011). In the upper panel, the results are particularly poor.…”

“…This failure is caused by differences in mode labelling between the pre-and post-main-sequence models owing to the presence of mixed modes in the latter. The inversion results were based on an identification which corresponds to some of the bestfitting models from Deheuvels & Michel (2011), who applied Newton's method to a grid of models specifically designed to handle mixed modes. Such an identification is ill-adapted for pre-main-sequence models and to a lesser extent for some of the post-main-sequence models because of the jumps it introduces in the n values.…”

“…Once more, the scaling law is somewhat different from the two other approaches, which is probably indicative of surface effects. In order to investigate the influence of mixed modes on SOLA mean density inversions, we used a low-overshoot model from Deheuvels & Michel (2011) with the Asplund et al (2005) mixture. Inversions were carried out for three different sets of modes:…”

Asteroseismic age determination for dwarfs and giants

“…This behaviour results in a deviation from the asymptotic behaviour of pure p-modes, and the magnitude of that deviation can provide strong constraints on the core properties (and thus age) of subgiant stars. Initial investigations have found correlations between this mixed-mode character and stellar mass (e.g., Benomar et al 2012;Deheuvels & Michel 2011), but more studies are needed to properly characterise the sensitivity of these type of pulsations to stellar structure and the systematics involved in determining stellar properties by using them as the observables to fit.…”