X-ray and UV Observations of the Young Sun HIP~67522: Evidence of Lyman-alpha Absorption Within the Planetary System

TL;DR

HIP 67522, a 17 Myr solar analogue in Sco-Cen with two transiting planets, is used to probe the young Sun’s XUV environment and its impact on planetary atmospheres. We present time-tagged HST/STIS UV spectra spanning FUV/NUV/optical, compare the star’s SED to the modern Sun, and find that the X-ray (1–100 Å) and EUV (100–900 Å) contributions at $1\ \,\mathrm{AU}$ are $329$ and $672$ erg cm$^{-2}$ s$^{-1}$, respectively. Intriguingly, attempts to recover the intrinsic Ly-alpha emission fail due to a substantial neutral hydrogen population within the system extending to $|\Delta v|>500\ \mathrm{km\ s^{-1}}$, implying potential absorption from exoplanet atmospheric outflow or a stellar component and motivating UV transit spectroscopy for confirmation. The study derives a scaling relation between FUV Ly-alpha and EUV as a function of stellar age and shows that the high XUV conditions at $17$ Myr, together with HIP 67522b’s low density, align with models predicting strong early heating and atmospheric escape for close-in planets. This supports the view that young, active solar-type stars can drive substantial mass loss and shape the early evolution of their planetary systems.

Abstract

We present ultraviolet (UV) spectroscopy of the 17 Myr, G0V star, HIP 67522. The UV spectrum is characterized by strong chromospheric and transition region emission lines. There was moderate spectral variability during the observations consisting of 15% stochastic fluctuation and two small flares releasing $E_{UV} \simeq 2-4\times10^{32}$ ergs in each event. We compare the broadband spectral energy distribution (SED; 4.7 A - 13.0~$μ$m) of the star first presented in Thao et al. (2024) to the solar SED and show that X-ray/UV (XUV) flux density at 1 AU is $10^{2}-10^{5}$ stronger (from 1000 A down to 5 A) in the young star compared to the present-day Sun. Attempts to reconstruct the intrinsic Ly-alpha emission of the star failed to return physically realistic results. The discrepancy appears to arise from a population of neutral hydrogen within the system itself, extending to $> \pm500$ km/s. The absorption could be due to outflow from exoplanet atmospheric loss or from a stellar component; such a picture would require high spectral resolution observations and/or UV transit spectroscopy to confirm. Finally, we examine the evolution of the XUV emission from solar-type stars from ages of 17 Myr to 9.4 Gyr and derive a scaling relation between FUV Ly-alpha and EUV emission as a function of stellar age. X-ray (1--100 A) and EUV (100--900 A) contributions to high energy emission are 329 and 672 ergs/cm^2/s at 1 AU, respectively, suggesting that both may contribute to exoplanet heating at this epoch. The XUV emission levels at 17 Myr combined with the low density of the planet HIP67522b are consistent with models that predict that solar type stars born with high rotation and activity levels will drive substantial heating and escape on close-in, gaseous planets.