Detection of a large sample of γ Doradus stars from Kepler space photometry and high-resolution ground-based spectroscopy^⋆,⋆⋆

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: andrew@ster.kuleuven.be
² Department of Astrophysics, IMAPP, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Tavrian National University, Department of Astronomy, Simferopol, Ukraine
⁴ Isaac Newton Group of Telescopes, Apartado de Correos 321, 387 00 Santa Cruz de la Palma, Canary Islands, Spain
⁵ Royal Observatory of Belgium, 3 Avenue Circulaire, 1180 Brussel, Belgium

Received: 21 December 2012
Accepted: 24 May 2013

Abstract

Context. The launches of the MOST, CoRoT, and Kepler missions opened up a new era in asteroseismology, the study of stellar interiors via interpretation of pulsation patterns observed at the surfaces of large groups of stars. These space missions deliver a huge amount of high-quality photometric data suitable to study numerous pulsating stars.

Aims. Our ultimate goal is a detection and analysis of an extended sample of γ Dor-type pulsating stars with the aim to search for observational evidence of non-uniform period spacings and rotational splittings of gravity modes in main-sequence stars typically twice as massive as the Sun. This kind of diagnostic can be used to deduce the internal rotation law and to estimate the amount of rotational mixing in the near core regions.

Methods. We applied an automated supervised photometric classification method to select a sample of 69 Gamma Doradus (γ Dor) candidate stars. We used an advanced method to extract the Kepler light curves from the pixel data information using custom masks. For 36 of the stars, we obtained high-resolution spectroscopy with the HERMES spectrograph installed at the Mercator telescope. The spectroscopic data are analysed to determine the fundamental parameters like T_eff, log g, vsini, and [M/H].

Results. We find that all stars for which spectroscopic estimates of T_eff and log g are available fall into the region of the HR diagram, where the γ Dor and δ Sct instability strips overlap. The stars cluster in a 700 K window in effective temperature; log g measurements suggest luminosity class IV-V, i.e. sub-giant or main-sequence stars. From the Kepler photometry, we identify 45 γ Dor-type pulsators, 14 γ Dor/δ Sct hybrids, and 10 stars, which are classified as “possibly γ Dor/δ Sct hybrid pulsators”. We find a clear correlation between the spectroscopically derived vsini and the frequencies of independent pulsation modes.

Conclusions. We have shown that our photometric classification based on the light curve morphology and colour information is very robust. The results of spectroscopic classification perfectly agree with the photometric classification. We show that the detected correlation between vsini and frequencies has nothing to do with rotational modulation of the stars but is related to their stellar pulsations. Our sample and frequency determinations offer a good starting point for seismic modelling of slow to moderately rotating γ Dor stars.