Identifying and characterising the population of hot sub-luminous stars with multi-colour MeerLICHT data

Abstract

Context. Colour–magnitude diagrams reveal a population of blue (hot) sub-luminous objects with respect to the main sequence. These hot sub-luminous stars are the result of evolutionary processes that require stars to expel their obscuring, hydrogen-rich envelopes to reveal the hot helium core. As such, these objects offer a direct window into the hearts of stars that are otherwise inaccessible to direct observation. Aims. MeerLICHT is a wide-field optical telescope that collects multi-band photometric data in six band filters (u, g, r, i, z, and q), whose primary goals are to study transient phenomena, gravitational wave counterparts, and variable stars. We showcase MeerLICHT’s capabilities of detecting faint hot subdwarfs and identifying the dominant frequency in the photometric variability of these compact hot stars, in comparison to their Gaia DR3 data. We hunt for oscillations, which will be an essential ingredient for accurately probing stellar interiors in future asteroseismology. Methods. Comparative MeerLICHT and Gaia colour–magnitude diagrams are presented as a way to select hot subdwarfs from our sample. A dedicated frequency determination technique is developed and applied to the selected candidates to determine their dominant variability using time-series data from MeerLICHT and Gaia DR3. We explore the power of both datasets in determining the dominant frequency. Results. Using the g − i colour, MeerLICHT offers a colour–magnitude diagram that is comparable in quality to that of Gaia DR3. The former, however, is more sensitive to fainter objects. The MeerLICHT colour–colour diagrams allow for the study of different stellar populations. The frequency analysis of MeerLICHT and Gaia DR3 data demonstrates the superiority of our MeerLICHT multi-colour photometry in estimating the dominant frequency compared to the sparse Gaia DR3 data. Conclusions. MeerLICHT’s multi-band photometry leads to the discovery of high-frequency faint subdwarfs. Continued observations tuned to asteroseismology will allow for mode identification using the method of amplitude ratios. Our MeerLICHT results are a proof-of-concept of the capacity of the BlackGEM instrument currently in the commissioning stage at ESO’s La Silla Observatory in Chile.