Constraints on the mass and on the atmospheric composition and evolution of the low-density young planet DS Tucanae A b

Abstract

Context. Observations of young close-in exoplanets are providing initial indications for the characteristics of the population and clues to the early stages of their evolution. Transiting planets at young ages are also key benchmarks for our understanding of planetary evolution via the verification of atmospheric escape models. Aims. We performed radial velocity (RV) monitoring of the 40 Myr old star DS Tuc A with HARPS at the ESO-3.6 m to determine the planetary mass of its 8.14-day planet, which was first revealed by the NASA TESS satellite. We also observed two planetary transits with HARPS and ESPRESSO at ESO-VLT to measure the Rossiter-McLaughlin (RM) effect and characterise the planetary atmosphere. We measured the high-energy emission of the host with XMM-Newton observations to investigate models for atmospheric evaporation. Methods. We employed a Gaussian Processes (GP) regression to model the high level of the stellar activity, which is more than 40 times larger than the expected RV planetary signal. GPs were also used to correct the stellar contribution to the RV signal of the RM effect. We extracted the transmission spectrum of DS Tuc A b from the ESPRESSO data and searched for atmospheric elements and molecules either by single-line retrieval and by performing cross-correlation with a set of theoretical templates. Through a set of simulations, we evaluated different scenarios for the atmospheric photo-evaporation of the planet induced by the strong XUV stellar irradiation. Results. While the stellar activity prevented us from obtaining a clear detection of the planetary signal from the RVs, we set a robust mass upper limit of 14.4 M⊕ for DS Tuc A b. We also confirm that the planetary system is almost (but not perfectly) aligned. The strong level of stellar activity hampers the detection of any atmospheric compounds, which is in line with other studies presented in the literature. The expected evolution of DS Tuc A b from our grid of models indicates that the planetary radius after the photo-evaporation phase will be 1.8–2.0 R⊕, falling within the Fulton gap. Conclusions. The comparison of the available parameters of known young transiting planets with the distribution of their mature counterpart confirms that the former are characterised by a low density, with DS Tuc A b being one of the less dense. A clear determination of their distribution is still affected by the lack of a robust mass measurement, particularly for planets younger than ~100 Myr.

Short term variability of DS Tuc A observed with TESS

“…In this section, we present our analysis aiming to remove the long-medium components of stellar activity to automatically identify the energetic events at a short timescale. We analyse DS Tuc A, a relatively young (age around 40 Myr, Kraus et al 2014;Bell et al 2015;Crundall et al 2019;Benatti et al 2021) G6V star belonging to the physical binary system DS Tuc, with a light curve characterized by long-term photometric variations due to a spot cycle (Benatti et al 2021). DS Tuc A hosts an inflated Hot Jupiter (Benatti et al 2021).…”

“…We analyse DS Tuc A, a relatively young (age around 40 Myr, Kraus et al 2014;Bell et al 2015;Crundall et al 2019;Benatti et al 2021) G6V star belonging to the physical binary system DS Tuc, with a light curve characterized by long-term photometric variations due to a spot cycle (Benatti et al 2021). DS Tuc A hosts an inflated Hot Jupiter (Benatti et al 2021). The target is of particular interest since low-density atmospheres are very sensitive to high energy radiation-induced evaporation and photochemistry.…”

“…Planetary transits are quite difficult to model correctly and a bad model could result in false flares in the analysis. In order to avoid spurious effects due to transit, before proceeding with the analysis we remove from all the light curves used in this work the bins of the light curves corresponding to the transits of all the known planets for both systems (Benatti et al 2021;Martioli et al 2021). We consider for the transit time duration the value defined in literature.…”

New constraints on the future evaporation of the young exoplanets in the V1298 Tau system

“…For each initial value of M p , we calculated the core radius, R c , through the analytic relation proposed in Benatti et al (2021) and valid for planets with Earth-like composition. We then computed the radius of the atmospheric envelope as R env = R p − R c , and the initial atmospheric mass fraction, f atm = M env /M p , by inverting the relation given in Lopez & Fortney (2014).…”

“…Proceeding similarly as in Georgieva et al (2021) and in Benatti et al (2021), we evaluated the mass-loss rate of the planetary atmosphere using the hydrodynamicbased approximation developed by Kubyshkina et al (2018a) for planetary masses lower than 40 M ⊕ , or the energy-limited approximation (Erkaev et al 2007) for higher values. Then we computed the change in atmospheric mass fraction, planetary mass, and radius, from the present age to 5 Gyr, in steps of 1 Myr, assuming a fixed circular planetary orbit (Appendix A).…”