The GAPS Programme at the TNG: LXX. TOI-5734b: A hot sub-Neptune orbiting a relatively young K dwarf with an Earth-like density

Abstract

Increasing interest in young exoplanets is leading to a growing effort to understand the formation and evolutionary processes responsible for their different architectures. One interesting target is TOI-5734, a relatively young K3-K4 dwarf star ($500_{-150}^{+300}$ Myr) showing a transiting candidate in photometric observations followed up with high-resolution spectroscopic data. Using Transiting Exoplanet Survey Satellite (TESS) photometry and High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) radial-velocity (RV) data, we aim to validate the presence of the companion TOI-5734b, measure its planetary mass, size, and its orbital parameters after having characterised its host star. We then aim to study its possible planetary composition and atmospheric evolution. By simultaneously modelling photometry and high-cadence RVs, we measured the radius, mass, and density of TOI-5734b precisely. In particular, we employed Gaussian processes (GPs) with a flexible kernel to discriminate between the stellar activity of the young host and planetary signals. We confirmed the planetary nature of TOI-5734b and measured its orbital period ($P_{\rm b}\sim6.18$ d), radius ($R_{\rm b} = 2.10^{+0.12}_{-0.12}$ $R_\oplus$), and mass ($M_{\rm b}=9.1^{+2.6}_{-2.6}$ $M_\oplus$). By measuring its density ($ρ_{\rm b}=0.98_{-0.30}^{+0.36}$ $ρ_\oplus$), we infer that TOI-5734b is close to having a rocky composition and an almost completely depleted primary envelope. Our results point toward the possibility of considering the target for atmospheric studies with present and future ground- and space-based facilities.

Figures (20)

• Figure 1: TESS target pixel file of Sector 20 of TOI-5734, which is marked with '1'. The other sources, extracted from the Gaia DR3 catalogue, have numbered circles of sizes proportional to the $G$-mag difference with our target. The colour bar shows the electron counts for each pixel. The orange squares represent the pixels used to construct the aperture photometry by the TESS pipeline. Grey arrows indicate the direction of the proper motions for all the plotted sources. The dashed green circle represents the reference three-pixel aperture to select the targets relevant for the dilution factor estimate.
• Figure 2: PATHOS light curves of TOI-5734 obtained using TESS data in Sectors 20, 40, and 60, with arrows pointing to the mid-transit times of the planet. The plot below shows the overall GLS and its envelope. The dashed red line represents the signal due to the stellar rotation period peaking at 11.12 d.
• Figure 3: GLS power spectrum for TOI-5734 based on HARPS-N RVs, stellar activity indicators (BIS span, $\log R^\prime_{\rm HK}$, FWHM, contrast), and their residuals after removing the long-term trend. The horizontal lines in the GLS periodograms indicate the FAP levels at 10$\%$, 1$\%$, and 0.1$\%$ power. The vertical green and red lines, respectively, highlight the positions of the planetary orbital period and stellar rotation period.
• Figure 4: TLS power spectrum of light-curve data of TOI-5734. The transiting signal is confirmed by the peak at $P_{\rm b}$=6.18414 d with an SDE value of 79, which is strongly above the thresholds (horizontal dashed lines) corresponding to the TLS false positive rates of 10$\%$, 1$\%$, and 0.1$\%$.
• Figure 5: Top: TESS phase-folded transits after subtracting the GP-modelled activity, along with the transit model shown with a black line and the residuals below it. Bottom: HARPS-N phased curve of RV data after subtracting the GP-modelled activity, with the planetary RV model (black line) and the residuals plotted below. The RV error bars are plotted without the jitter, the value of which is reported in Table \ref{['tab:prior_and_param_other']}.