Probing habitable regions with SRG/eROSITA

TL;DR

This study leverages the SRG/eROSITA all‑sky X‑ray survey, cross‑matched with Gaia DR3, to build a 3750‑star main‑sequence sample and derive stellar $L_X$ and $L_{EUV}$, enabling estimation of $F_{XUV,HZ}$ for planets in the habitable zone. Using the empirical $L_{EUV}$–$L_X$ scaling, it constructs $L_{XUV}$ distributions and shows cooler, magnetically active stars exhibit higher $L_{XUV}/L_{bol}$, driving potentially hazardous irradiation in their HZs. The habitability assessment proceeds with Kop2014‑based HZ flux calculations and an energy‑limited atmospheric escape model, revealing $F_{XUV,HZ}$ spanning $10^0$–$10^5$ erg cm$^{-2}$ s$^{-1}$ and mass‑loss rates that scale strongly with incident flux; local and Galactic maps identify regions with elevated XUV environments. The results underscore the pivotal role of stellar high‑energy emission in exoplanet atmospheric evolution and habitability, and they highlight the need for direct XUV measurements to improve constraints on atmospheric escape across diverse stellar hosts.

Abstract

Stellar high-energy radiation is a key driver of atmospheric erosion and evolution in exoplanets, directly affecting their long-term habitability. We present a comprehensive study on stellar high-energy radiation and its impact on exoplanetary atmospheres, leveraging data from the \textit{SRG/eROSITA} all-sky survey. Our sample consists of 3750 main-sequence stars identified by cross-matching with \textit{Gaia} DR3. Utilizing X-ray spectral fits from the \textit{eROSITA} catalog, we computed X-ray ($L_X$) and combined extreme-ultraviolet (EUV) luminosities ($L_{\mathrm{EUV}}$), which we used to derive XUV fluxes at the habitable zone ($F_{\mathrm{XUV,HZ}}$). We find that the majority of stars in our sample are significantly more XUV-active than the Sun, with habitable zone fluxes ranging from $10^0$ to $10^5$ erg~cm$^{-2}$~s$^{-1}$. The ratio of $L_{\mathrm{XUV}}/L_{\mathrm{bol}}$ is found to be higher for cooler, magnetically active stars, highlighting their potentially hazardous nature for planetary atmospheres. Applying the energy-limited escape model, we computed atmospheric mass-loss rates for hypothetical earth-like planets located at the habitable zone of each star. We also present local maps for distances up to $500$~pc of the average XUV flux, revealing ``hazard zones'' where stellar radiation could significantly influence planetary atmospheric evolution. This work demonstrates the power of X-ray surveys in constraining the high-energy environments of exoplanets and underscores the critical role of stellar activity in planetary habitability.

Figures (11)

• Figure 1: Distribution of hardness ratios for the analyzed sample. The hardness ratio ($HR$) is defined as $HR = (H-S)/(H+S)$, where $H$ is the unabsorbed flux in the hard X-ray band ($0.5-1.0$ keV) and $S$ is the unabsorbed flux in the soft X-ray band ($0.2-0.5$ keV). The hardness ratio provides a proxy for the spectral shape, with higher values corresponding to harder spectra and lower values to softer emission.
• Figure 2: Color-magnitude diagram of the sources, using absolute magnitude ($M_G$) and color ($BP-RP$) values from Gaia DR3. The color scales with X-ray luminosity ($L_{X}$), showing the relationship between stellar evolutionary status and coronal activity.
• Figure 3: Histogram of the combined X-ray and extreme-UV luminosity ($L_{\mathrm{XUV}}$) for the main-sequence sample.
• Figure 4: Combined X-ray and extreme-UV luminosity ($L_{\mathrm{XUV}}$) as a function of Gaia$BP-RP$ color. Only sources with $\log_{10}(L_{\mathrm{XUV}})$ between $10^{27}$ and $10^{33}$ erg s$^{-1}$ are shown to improve visualization.
• Figure 5: Combined X-ray and extreme-UV luminosity ($L_{\mathrm{XUV}}$) as a function of Gaia absolute magnitude $M_G$.