Estimation of the XUV radiation onto close planets and their evaporation

Abstract: Context: The current distribution of planet mass vs. incident stellar X-ray flux supports the idea that photoevaporation of the atmosphere may take place in close-in planets. Integrated effects have to be accounted for. A proper calculation of the mass loss rate due to photoevaporation requires to estimate the total irradiation from the whole XUV range. Aims: The purpose of this paper is to extend the analysis of the photoevaporation in planetary atmospheres from the accessible X-rays to the mostly unobserved EUV range by using the coronal models of stars to calculate the EUV contribution to the stellar spectra. The mass evolution of planets can be traced assuming that thermal losses dominate the mass loss of their atmospheres. Methods: We determine coronal models for 82 stars with exoplanets that have X-ray observations available. Then a synthetic spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination of the EUV stellar flux, calibrated with real EUV data, allows us to calculate the accumulated effects of the XUV irradiation on the planet atmosphere with time, as well as the mass evolution for planets with known density. Results: We calibrate for the first time a relation of the EUV luminosity with stellar age valid for late-type stars. In a sample of 109 exoplanets, few planets with masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense photoevaporation takes place in a short period of time, as previously found in X-rays. The scenario is also consistent with the observed distribution of planet masses with density. The accumulated effects of photoevaporation over time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr. Conclusions: Coronal radiation produces rapid photoevaporation of the atmospheres of planets close to young late-type stars. More complex models are needed to explain fully the observations.

• The paper quantifies XUV radiation's role in driving atmospheric photoevaporation using synthetic coronal models from 82 X-ray observed stars.
• It establishes a correlation between EUV luminosity and stellar age, highlighting the evolving impact of XUV emissions in late-type stars.
• Analysis of 109 exoplanets, including HD 209458b's loss of ~0.2 MJ, demonstrates significant mass loss in younger, close-orbiting planets.

Estimation of XUV Radiation Impact on Planetary Outcomes

The paper "Estimation of the XUV radiation onto close planets and their evaporation" elaborates on the crucial role of extreme ultraviolet (XUV) radiation from stars on the atmospheric dynamics of exoplanets situated in close proximity. Utilizing coronal models of stars to extend the analysis from X-rays to the largely unobserved EUV range, the study undertakes a detailed assessment of planetary mass loss through photoevaporation, offering significant insights into both stellar and planetary evolution.

The primary focus lies in calculating the total irradiation impact of XUV radiation across a significant spectrum (1–912 Å) on the atmospheres of planets, specifically for cases where thermal escapes dominate. With data from 82 stars with existing X-ray observations, synthetic spectrum models were created to estimate the full range of XUV effects. The calibration of these models provides a basis for understanding the age relation of EUV luminosity across late-type stars, which is pivotal in tracing the evolutionary trajectory of planetary mass.

Major Findings and Numerical Results:

• A correlation between the EUV luminosity and the stellar age, valid for late-type stars, was established, offering a new perspective on the evolutionary dynamics of stellar emissions.
• The study analyzed a sample of 109 exoplanets, revealing that a significant fraction of planets with masses higher than about 1.5 M$_{\rm J}$ receive substantial XUV flux. This supports the notion of rapid photoevaporation occurring primarily in younger stars.
• HD 209458b was calculated to have potentially lost about 0.2 M$_{\rm J}$ since achieving an age of 20 million years (Myr) due to accumulated XUV radiation.

Theoretical and Practical Implications:

The implications of this research are manifold. Theoretically, it underscores the impact of stellar radiation in shaping planetary atmospheric mass loss, contributing to the evolving distribution of planetary masses. Practically, the results indicate that stars' coronal activities should be considered integral to models predicting the habitability and lifecycle of exoplanets.

The paper suggests that as younger stars exhibit higher XUV emissions, they subsequently have a more substantial impact on the atmospheric evolution of their planetary systems, particularly affecting planets that reside in closer orbits. Future observational missions are directed to further refine these models by incorporating more complex atmospheric phenomena and considering additional variables such as planetary composition and the effect of planetary magnetic fields.

Future Directions:

The study paves the way for more comprehensive models of planet-star interactions and reinforces the necessity for expansive datasets and high-fidelity measurements in the EUV range. Such research endeavors will be crucial in accurately modeling and predicting the atmospheric stability and life-bearing potential of exoplanets, shifting the paradigm of exoplanet science to a more holistically integrated approach.

In conclusion, the paper significantly enhances our understanding of the photoevaporative processes induced by stellar XUV radiation, warranting further exploration into associated non-thermal losses and the long-term implications of such atmospheric erosion.