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

Felix Mackebrandt ,L Lanteri,S Schuh,D Kilkenny,S Benatti,M Paparó,R Silvotti,L Molnár,R Claudi,A Bonanno ,Ronny Lutz ,T Ôtani ,E M Green,R H Østensen,T D Oswalt,T Nagel,J L Provencal,R Janulis,S.-L Kim,A Frasca

doi:10.1051/0004-6361/201937172

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.

Highlights

Subdwarf B stars are sub-luminous stars with a mass of about 0.5 M located at the blue end of the horizontal branch, Photometric data of Fig. 1, results in Figs. 8, 10, 12, and 14, and figures in the appendix are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/cat/J/A+A/638/A108Based on observations obtained at the 0.9 m SARA-KP telescope, which is operated by the Southeastern Association for Research in Astronomy.which is the so-called extreme horizontal branch (EHB, Heber 1986)
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
We investigated variations in the arrival times of their dominant stellar pulsation modes to draw conclusions about secular period drifts and possible sub-stellar companions

Summary

Introduction

Subdwarf B stars (sdBs) are sub-luminous stars with a mass of about 0.5 M located at the blue end of the horizontal branch, Photometric data of Fig. 1, results in Figs. 8, 10, 12, and 14, and figures in the appendix are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/cat/J/A+A/638/A108. In order to detect sub-stellar companions orbiting rapidly pulsating sdB stars, the EXOTIME observational programme (EXOplanet search with the TIming MEthod) has been taking long-term data since 1999. The detection method demands the observation of a target for a total time base at least as long as one orbit of a potential companion, which can span several years This requires coordinated campaigns with observatories using ~1–4 m telescopes. Similar to the single-frequency analysis, the observational epochs are fitted individually using the sum of sinusoidal functions to yield the observed phase information O.

DW Lyn

V1636 Ori

Findings

V541 Hya