Letter to the Editor

Evidence for a sharp structure variation inside a red-giant star

¹ Institut d'Astrophysique et de Géophysique de l'Université de Liège, 17 Allée du 6 Août, 4000 Liège, Belgium e-mail: a.miglio@ulg.ac.be
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ LESIA, UMR8109, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon, France
⁴ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
⁵ INAF – Osservatorio di Roma, via Frascati, 33 Monteporzio Catone, RM, Italy

Received: 21 July 2010
Accepted: 3 September 2010

Abstract

Context. The availability of precisely determined frequencies of radial and non-radial oscillation modes in red giants is finally paving the way for detailed studies of the internal structure of these stars.

Aims. We look for the seismic signature of regions of sharp structure variation in the internal structure of the CoRoT target HR 7349.

Methods. We analyse the frequency dependence of the large frequency separation and second frequency differences, as well as the behaviour of the large frequency separation obtained with the envelope auto-correlation function.

Results. We find evidence for a periodic component in the oscillation frequencies, i.e. the seismic signature of a sharp structure variation in HR 7349. In a comparison with stellar models we interpret this feature as caused by a local depression of the sound speed that occurs in the helium second-ionization region. Using solely seismic constraints this allows us to estimate the mass (M = 1.2_-0.4^+0.6 ) and radius (R = 12.2_-1.8^+2.1 ) of HR 7349, which agrees with the location of the star in an HR diagram.