The EXOTIME project: signals in the O–C diagrams of the rapidly pulsating subdwarfs DW Lyn, V1636 Ori, QQ Vir, and V541 Hya^★,★★

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: mackebrandt@mps.mpg.de
² Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ INAF – Osservatorio Astrofisico di Torino, strada dell’Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Korea Astronomy and Space Science Institute, Daejeon 34055, South Korea
⁵ Department of Physics and Astronomy, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
⁶ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁷ German Aerospace Center (DLR), Remote Sensing Technology Institute, Münchener Str. 20, 82234 Weßling, Germany
⁸ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, University of Tübingen, 72076 Tübingen, Germany
⁹ University of Delaware, Department of Physics and Astronomy Newark, DE 19716, USA
¹⁰ Delaware Asteroseismic Research Center, Mt. Cuba Observatory, Greenville, DE 19807, USA
¹¹ Embry-Riddle Aeronautical University, Department of Physical Science and SARA, Daytona Beach, FL 32114, USA
¹² Florida Institute of Technology, Department of Physics & Space Sciences, Melbourne, FL 32901, USA
¹³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁴ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁵ Institute of Theoretical Physics and Astronomy, Vilnius University, Gostauto 12, Vilnius 01108, Lithuania
¹⁶ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly-Thege M. út 15-17, 1121 Budapest, Hungary
¹⁷ MTA CSFK Lendület Near-Field Cosmology Research Group, Konkoly Observatory, Budapest, Hungary
¹⁸ Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65897, USA

Received: 22 November 2019
Accepted: 12 March 2020

Abstract

Aims. We aim to investigate variations in the arrival time of coherent stellar pulsations due to the light-travel time effect to test for the presence of sub-stellar companions. Those companions are the key to one possible formation scenario of apparently single sub-dwarf B stars.

Methods. We made use of an extensive set of ground-based observations of the four large amplitude p-mode pulsators DW Lyn, V1636 Ori, QQ Vir, and V541 Hya. Observations of the TESS space telescope are available on two of the targets. The timing method compares the phase of sinusoidal fits to the full multi-epoch light curves with phases from the fit of a number of subsets of the original time series.

Results. Observations of the TESS mission do not sample the pulsations well enough to be useful due to the (currently) fixed two-minute cadence. From the ground-based observations, we infer evolutionary parameters from the arrival times. The residual signals show many statistically significant periodic signals, but no clear evidence for changes in arrival time induced by sub-stellar companions. The signals can be explained partly by mode beating effects. We derive upper limits on companion masses set by the observational campaign.