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

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_ odot $, a radius of $R_ odot $, an effective temperature of $T_ eff and a metallicity of $-0.10 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 oplus $ and a radius of $R_p=2.83 oplus $. The host star TOI-2458 shows a short activity cycle of sim 54\,days revealed in the HARPS S-index and Halpha 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_ $\,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 and a minimum mass of $M_p i=10.22 oplus $. Using dynamical stability analysis, we constrained the mass of this planet to the range $M_ c oplus