A transit survey to search for planets around hot subdwarfs: I. methods and performance tests on light curves from Kepler, K2, TESS, and CHEOPS

Abstract

Context. Hot subdwarfs experienced strong mass loss on the Red Giant Branch (RGB) and are now hot and small He-burning objects. Such stars constitute excellent opportunities to address the question of the evolution of exoplanetary systems directly after the RGB phase of evolution. Aims. In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes (Kepler, K2, TESS, and CHEOPS), with our custom-made pipeline SHERLOCK, in order to determine the occurrence rate of planets around these stars, as a function of orbital period and planetary radius. We also aim to determine whether planets previously engulfed in the envelope of their red giant host star can survive, even partially, as a planetary remnant. Methods. In this first paper, we perform injection-and-recovery tests of synthetic transits for a selection of representative Kepler, K2 and TESS light curves, to determine which transiting bodies, in terms of object radius and orbital period, we will be able to detect with our tools. We also provide such estimates for CHEOPS data, which we analyze with the pycheops package. Results. Transiting objects with a radius À 1.0 R C can be detected in most of Kepler, K2 and CHEOPS targets for the shortest orbital periods (1 d and below), reaching values as small as "0.3 R C in the best cases. Reaching sub-Earth-sized bodies is achieved only for the brightest TESS targets, and the ones observed during a significant number of sectors. We also give a series of representative results for farther and bigger planets, for which the performances strongly depend on the target magnitude, the length and the quality of the data. Conclusions. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The Kepler, K2 and CHEOPS data will allow us to search for planetary remnants, i.e. very close and small (possibly disintegrating) objects, which would have partly survived the engulfment in their red giant host.

Analysis of Early Science observations with the CHaracterising ExOPlanets Satellite (CHEOPS) using pycheops

“…CHEOPS also has the flexibility to schedule observations to coincide with the transits and eclipses of known exoplanets, or to search for suspected transiting exoplanets in multi-planets systems (Bonfanti et al 2021). pycheops has already been used for the analysis of CHEOPS data in several studies (Bonfanti et al 2021;Lendl et al 2020;Benz et al 2021;Leleu et al 2021a;Borsato et al 2021;Van Grootel et al 2021). CHEOPS observations are on-going so we can look forward to the publication of many exciting results from the partnership of this unique instrument and the pycheops software.…”

“…These "filler" observations ensure that CHEOPS continues to collect useful science observations during short intervals between time-critical observations of transits and eclipses. The filler programmes within the GTO are being used to study the variability of low mass stars on short time scales, and to search for remnants of planetary systems around hot subdwarf stars (Van Grootel et al 2021).…”

Physical Properties of 29 sdB+dM Eclipsing Binaries in Zwicky Transient Facility

“…Our idea is to assume that the primary components of sdB+dM are all canonical sdB star with a mass of 0.47 ± 0.03 M , a radius of 0.175 ± 0.025 R , and temperature about 30,000 K (Van Grootel et al 2021).…”

“…About 40% of the sdB stars have been found to be single. Although it has been argued that all sdB stars were evolved from binary stars (Pelisoli et al 2020), but several facts suggest that the merger channel of two He WDs can not satisfactorily explain the single sdB stars (Burdge et al 2020;Van Grootel et al 2021). First, the number of low-mass white dwarf binaries is small (Ratzloff et al 2019b).…”

Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection

“…The CHaracterising ExOPlanet Satellite (CHEOPS; Benz et al 2021) was launched in December 2019, and has been acquiring high-precision optical photometry since going into full scientific operation in April 2020. Unlike the observing strategies of Kepler and TESS, CHEOPS is a mission designed primarily to observe individual targets already known to host transiting planets, with the first results being presented in literature in recent months (Bonfanti et al 2021;Borsato et al 2021;Delrez et al 2021;Leleu et al 2021;Morris et al 2021b;Swayne et al 2021;Szabó et al 2021;Van Grootel et al 2021). With photometric precision and a wide optical bandpass 1 both comparable to that of Kepler, this strategy allows CHEOPS to extend photometric measurements of Ψ and occultation depth to the most amenable targets across most of the night sky.…”