TOI-2458 b: A mini-Neptune consistent with in situ hot Jupiter formation

Abstract

We report on the discovery and spectroscopic confirmation of TOI-2458 b, a transiting mini-Neptune around an F-type star leaving the main-sequence with a mass of $M_{\star }=1.05\pm 0.03$ M${}_{\odot }$, a radius of $R_{\star }=1.31\pm 0.03$ R${}_{\odot }$, an effective temperature of $T_{\mathrm{eff}}=6005\pm 50$ K, and a metallicity of $-0.10\pm 0.05$ dex. By combining TESS photometry with high-resolution spectra acquired with the HARPS spectrograph, we found that the transiting planet has an orbital period of $\sim$3.74 days, a mass of $M_p=13.31\pm 0.99$ M${}_{\oplus }$ and a radius of $R_p=2.83\pm 0.20$ R${}_{\oplus }$. The host star TOI-2458 shows a short activity cycle of $\sim$54 days revealed in the HARPS S-index and H$\hat{I}\pm$ times series. We took the opportunity to investigate other F stars showing activity cycle periods comparable to that of TOI-2458 and found that they have shorter rotation periods than would be expected based on the gyrochronology predictions. In addition, we determined TOI-2458's stellar inclination angle to be $i_{\star }=10.6_{-10.6}^{+13.3}$ degrees. We discuss that both phenomena (fast stellar rotation and planet orbit inclination) could be explained by in situ formation of a hot Jupiter interior to TOI-2458 b. It is plausible that this hot Jupiter was recently engulfed by the star. Analysis of HARPS spectra has identified the presence of another planet with a period of $P=16.55\pm 0.06$ days and a minimum mass of $M_p\sin i=10.22\pm 1.90$ M${}_{\oplus }$. Using dynamical stability analysis, we constrained the mass of this planet to the range $M_c\simeq (10,25)$ M${}_{\oplus }$.

25 pages, 20 figures, accepted for publication in Astronomy & Astrophysics