Improved Constraints for the XUV Luminosity Evolution of Trappist-1

TL;DR

This study refines the understanding of TRAPPIST-1's XUV luminosity evolution, indicating a shorter saturation period and confirming the star's planets likely received significant XUV energy over time.

Contribution

It provides new observational constraints and a revised model for TRAPPIST-1's XUV luminosity evolution, especially the saturation duration and decay rate.

Findings

Saturation phase lasted approximately 3.14 Gyr.

XUV luminosity decayed exponentially after saturation.

Planets received an estimated 10^{30}-10^{32} erg of XUV energy.

Abstract

We re-examine the XUV luminosity evolution of TRAPPIST-1 utilizing new observational constraints (XUV and bolometric luminosity) from multi-epoch X-ray/UV photometry. Following the formalism presented in Fleming et al. (2020), we infer that TRAPPIST-1 maintained a saturated XUV luminosity, relative to the bolometric luminosity, of $\log_{10}$(L$_{\rm XUV}$/L$_{\rm bol}$) $= -3.03_{-0.23}^{+0.25}$ at early times for a period of $t_{sat} = 3.14_{-1.46}^{+2.22}$ Gyr. After the saturation phase, we find L$_{\rm XUV}$ decayed over time by an exponential rate of $\beta_{\rm XUV} = -1.17_{-0.28}^{+0.27}$. Compared to our inferred age of the system, ${\rm age} = 7.96_{-1.87}^{+1.78}$ Gyr, our result for $t_{sat}$ suggests that there is only a $\sim4\%$~chance that TRAPPIST-1 still remains in the saturated phase today, which is significantly lower than the previous estimate of 40\%. Despite this reduction in $t_{sat}$, our results remain consistent in the conclusion that the TRAPPIST-1 planets likely received an extreme amount XUV energy -- an estimated integrated XUV energy of $\sim10^{30}-10^{32}$ erg over the star's lifetime.