Photometric follow-up of the 20 Myr old multi-planet host star V1298 Tau with CHEOPS and ground-based telescopes

Abstract

Context. The 20 Myr old star V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. To date, the characterisation of its architecture and planets’ fundamental properties has been very challenging. Aims. The determination of the orbital ephemeris of the outermost planet V1298 Tau e remains an open question. Only two transits have been detected so far by Kepler/K2 and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow for better constraints to be set on the orbital period and would also help in determining an accurate radius for V1298 Tau e because the previous transits showed different depths. Methods. We observed V1298 Tau with the CHaracterising ExOPlanet Satellite (CHEOPS) to search for a third transit of planet e within observing windows selected to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed Kepler/K2 and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods. Results. We report the CHEOPS detection of a transit-like signal that could be attributed to V1298 Tau e. If so, that result would imply that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to ~45 days. Results from the ground-based follow-up marginally support this possibility. We found that i) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by Kepler/K2 and TESS; and ii) the transit observed by TESS is >30% deeper than that of Kepler/K2 and CHEOPS, and it is also deeper than the measurement previously reported in the literature, according to our reanalysis. Conclusions. If the new transit detected by CHEOPS is found to be due to V1298 Tau e, this would imply that the planet experiences TTVs of a few hours, as deduced from three transits, as well as orbital precession, which would explain the longer duration of the transit compared to the Kepler/K2 and TESS signals. Another and a priori less likely possibility is that the newly detected transit belongs to a fifth planet with a longer orbital period than that of V1298 Tau e. Planning further photometric follow-up to search for additional transits is indeed necessary to solve the conundrum, as well as to pin down the radius of V1298 Tau e.

The GAPS programme at TNG

“…None of the solutions presented above results in a resonant coupling between planets b and e. Preliminary results from ongoing observations of the ESA mission CHEOPS exclude the solutions at 44 and 46 days and are instead compatible with the one at about 45 days (Damasso et al 2023), which we adopted as the nominal period of V1298 Tau e in this work. (1)…”

“…The recent observational campaigns aimed at characterizing the planets around V1298 Tau (David et al 2019a,b;Suárez Mascareño et al 2022;Feinstein et al 2022;Damasso et al 2023) reveal that this young planetary system is highly peculiar. Its current architecture is quite compact, yet its planets do not appear to be globally locked in resonance Tejada Arevalo et al ( 2022), and their orbital characteristics argue for the system having crossed phases of dynamical instability (Sect.…”

“…However, the period ratio between planet b and d is too far from a 2:1 resonant ratio, effectively excluding the possibility that the system is currently in a resonant chain (Tejada Arevalo et al 2022). Furthermore, recent data indicate an orbital period of planet e (Feinstein et al 2022;Damasso et al 2023) that is not in resonance with that of planet b.…”