Metallicity effect on stellar granulation detected from oscillating red giants in open clusters

¹ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
e-mail: enrico.corsaro@oact.inaf.it
² Instituto de Astrofísica de Canarias, 38205 Tenerife, Spain
³ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁴ Laboratoire AIM, CEA/DRF – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
⁵ Space Science Institute, 4750 Walnut street Suite#205, Boulder CO 80301, USA
⁶ Department of Astronomy, New Mexico State University, PO Box 30001, MSC 4500, Las Cruces, NM 88003-8001, USA
⁷ Apache Point Observatory, 2001 Apache Point Road, PO Box 59, Sunspot, NM 88349, USA
⁸ Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
⁹ Department of Astronomy, Ohio State University, 140 W 18th Ave, OH 43210, USA
¹⁰ Department of Physics and Astronomy, Vanderbilt University, 1807 Station B, Nashville, Tennessee 37235, USA
¹¹ School of Physics, University of New South Wales, NSW 2052, Australia
¹² Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
¹³ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁵ Unidad de Astronomía, Universidad de Antofagasta, Avenida Angamos 601, 1270300 Antofagasta, Chile

Received: 3 May 2017
Accepted: 19 July 2017

Abstract

Context. The effect of metallicity on the granulation activity in stars, and hence on the convective motions in general, is still poorly understood. Available spectroscopic parameters from the updated APOGEE-Kepler catalog, coupled with high-precision photometric observations from NASA’s Kepler mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds.

Aims. We aim to determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from [Fe/H] ≃ − 0.09 to 0.32. The parameters describing the granulation activity of these stars and their frequency of maximum oscillation power, ν_max, are studied while taking into account different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations.

Methods. We adopted the Bayesian code Diamonds for the analysis of the background signal in the Fourier spectra of the stars. We performed a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature.

Results. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass.

Conclusions. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods. The trend in amplitude is in qualitative agreement with predictions from existing 3D hydrodynamical simulations of stellar atmospheres from main sequence to red giant stars. We confirm that the granulation activity is not sensitive to changes in the stellar core and that it only depends on the atmospheric parameters of stars.