A comprehensive study of young B stars in NGC 2264

I. Space photometry and asteroseismology^⋆

¹ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
e-mail: konstanze.zwintz@uibk.ac.at
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Graz University of Technology, Institute for Communication Networks and Satellite Communication, Inffeldgasse 12, 8010 Graz
⁴ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁵ University of Vienna, Institute for Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁶ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁸ The University of Texas at Austin, McDonald Observatory, 2515 Speedway, Stop C1402, Austin, TX 78712, USA

Received: 22 December 2016
Accepted: 18 March 2017

Abstract

Context. Space photometric time series of the most massive members of the young open cluster NGC 2264 allow us to study their different sources of variability down to the millimagnitude level and permit a search for slowly pulsating B (SPB)-type pulsation among objects that are only a few million years old.

Aims. Our goal is to conduct a homogeneous study of young B-type stars in the cluster NGC 2264 using photometric time series from space in combination with high-resolution spectroscopy and spectropolarimetry obtained from the ground. The latter will be presented in a separate follow-up article.

Methods. We performed frequency analyses for eleven B stars in the field of the young cluster NGC 2264 using photometric time series from the MOST, CoRoT, and Spitzer space telescopes and the routines Period04 and SigSpec. We employ the MESA stellar evolution code in combination with the oscillation code GYRE to identify the pulsation modes for two SPB stars that exhibit short period spacing series.

Results. From our analysis we identify four objects that show SPB pulsations, five stars that show rotational modulation of their light curves caused by spots, one star that is identified to be a binary, and one object in the field of the cluster that is found to be a non-member Be star. In two SPB stars we detect a number of regularly spaced pulsation modes that are compatible with being members of a g-mode period series.

Conclusions. Despite NGC 2264’s young age, our analysis illustrates that its B-type members have already arrived on the zero-age main sequence (ZAMS). Our asteroseismic analysis yields masses between 4 and 6 M_⊙  and ages between 1 and 6 million years, which agree well to the overall cluster age.