X-Ray and UV Observations of the Young Sun HIP 67522: Evidence of Ly <i>α</i> Absorption within the Planetary System

Abstract

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 ≃ 2–4 × 10 32 erg in each event. We compare the broadband spectral energy distribution (SED; 4.7 Å–13.0 μ m) of the star first presented in P. C. Thao et al. to the solar SED and show that the X-ray and UV (XUV) flux density at 1 au is 10 2 –10 5 stronger (from 1000 Å down to 5 Å) in the young star compared to the present-day Sun. Attempts to reconstruct the intrinsic Ly α 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 &gt;±500 km s −1 . 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–9.4 Gyr and derive a scaling relation between far-UV Ly α and extreme-UV (EUV) emission as a function of stellar age. X-ray (1–100 Å) and EUV (100–911 Å) contributions to high-energy emission are 329 and 672 erg cm −2 s −1 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 HIP 67522 b 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.

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