Asteroseismology of red giants from the first four months of Kepler data: Global oscillation parameters for 800 stars

TL;DR

This study analyzes solar-like oscillations in approximately 800 red giants observed by Kepler, revealing new insights into their oscillation properties, evolutionary states, and amplitude relations, with implications for stellar modeling.

Contribution

It provides the first comprehensive analysis of oscillation parameters for a large sample of red giants from Kepler data, including new relations and population evidence.

Findings

Different Delta nu - nu_max relation for red giants compared to main-sequence stars

Evidence for a secondary clump population with M > 2 M_sun

First amplitude-nu_max relation for Kepler red giants

Abstract

We have studied solar-like oscillations in ~800 red-giant stars using Kepler long-cadence photometry. The sample includes stars ranging in evolution from the lower part of the red-giant branch to the Helium main sequence. We investigate the relation between the large frequency separation (Delta nu) and the frequency of maximum power (nu_max) and show that it is different for red giants than for main-sequence stars, which is consistent with evolutionary models and scaling relations. The distributions of nu_max and Delta nu are in qualitative agreement with a simple stellar population model of the Kepler field, including the first evidence for a secondary clump population characterized by M ~> 2 M_sun and nu_max ~ 40-110 muHz. We measured the small frequency separations delta nu_02 and delta nu_01 in over 400 stars and delta nu_03 in over 40. We present C-D diagrams for l=1, 2 and 3 and show that the frequency separation ratios delta nu_02/Delta nu and delta nu_01/Delta nu have opposite trends as a function of Delta nu. The data show a narrowing of the l=1 ridge towards lower nu_max, in agreement with models predicting more efficient mode trapping in stars with higher luminosity. We investigate the offset epsilon in the asymptotic relation and find a clear correlation with Delta nu, demonstrating that it is related to fundamental stellar parameters. Finally, we present the first amplitude-nu_max relation for Kepler red giants. We observe a lack of low-amplitude stars for nu_max ~> 110 muHz and find that, for a given nu_max between 40-110 muHz, stars with lower Delta nu (and consequently higher mass) tend to show lower amplitudes than stars with higher Delta nu.