Red-giant seismic properties analyzed with CoRoT

Abstract

Context. The CoRoT 5-month long observation runs provide us with the opportunity to analyze a large variety of red-giant stars and derive their fundamental parameters from their asteroseismic properties. Aims. We perform an analysis of more than 4600 CoRoT light curves to extract as much information as possible. We take into account the characteristics of both the star sample and the method to ensure that our asteroseismic results are as unbiased as possible. We also study and compare the properties of red giants in two opposite regions of the Galaxy. Methods. We analyze the time series using the envelope autocorrelation function to extract precise asteroseismic parameters with reliable error bars. We examine first the mean wide frequency separation of solar-like oscillations and the frequency of the maximum seismic amplitude, then the parameters of the excess power envelope. With the additional information of the effective temperature, we derive the stellar mass and radius. Results. We identify more than 1800 red giants among the 4600 light curves and obtain accurate distributions of the stellar parameters for about 930 targets. We are able to reliably measure the mass and radius of several hundred red giants. We derive precise information about the stellar population distribution and the red clump. By comparing the stars observed in two different fields, we find that the stellar asteroseismic properties are globally similar, but that the characteristics are different for red-clump stars. Conclusions. This study demonstrates the efficiency of statistical asteroseismology: validating scaling relations allows us to infer fundamental stellar parameters, derive precise information about red-giant evolution and interior structure, analyze and compare stellar populations from different fields.

The underlying physical meaning of theνmax − νcrelation

“…This agrees with the results obtained by several authors for main-sequence stars (e.g., Bedding & Kjeldsen 2003), and red giants (e.g. Mosser et al 2010). The problem is now to assess the physical background underlying this relation.…”

“…Kjeldsen & Bedding 1995;Chaplin et al 2008Chaplin et al , 2009Stello et al 2009a) using ground-based data. More recently, the spacemissions CoRoT and Kepler confirmed those results by providing accurate and homogeneous measurements for a large sample of stars from red giants to main-sequence stars (e.g., Mosser et al 2010). Scaling relations are essential to study a large set of stars (e.g., Kallinger et al 2009;Stello et al 2009b) for which, in general, little is known, to provide a first order estimate for mass and radius (e.g., Basu et al 2010;Mosser et al 2010), or to probe the populations of red giants (Miglio et al 2009).…”

Adiabatic Solar-Like Oscillations in Red Giant Stars

“…observational works based on CoRoT and Kepler satellite data Huber et al, 2010;Mosser et al, 2010;Carrier et al, 2010). In the domain of validity of the asymptotic approximation for pressure modes, the spectrum of p modes can be described by (e.g.…”

“…On the other hand, the sample of red giants observed by CoRoT during the first two runs is dominated by red clump stars (Miglio et al, 2009;Mosser et al, 2010) and the analysis of their oscillation properties by Mosser et al (2011b) showed that all the modes are arranged in almost vertical lines corresponding to different radial orders, and that a large dispersion is found for dipole modes at ∆ ν ∼ 4 µHz, that is the value of the large separation corresponding to the red-clump luminosity. The comparison between these observational results and the theoretical predictions for the same stellar population (Montalbán et al, 2010b) is noteworthy.…”

An Introduction to Data Analysis in Asteroseismology

“…We start by partitioning the power spectrum into overlapping windows of variable width, w. The width w depends on the central frequency of the window, ν central , used as a proxy for ν max . We make use of the fact that the width of the p-mode bump approximately scales with ν max (e.g., Stello et al, 2007;Mosser et al, 2010), and so w is defined as w = (ν central /ν max, )w .…”