Reassessing the Relationship Between Stellar X-ray Luminosity and Age with eROSITA Data Release 1

TL;DR

The study re-evaluates the stellar X-ray luminosity–age relation using eROSITA DR1 data cross-matched with asteroseismic ages, expanding the sample with four new asteroseismic stars and incorporating X-ray measurements for six additional targets. It models the relation in log space as $ \\log L_{x,n} = m \\log t + b $ using Differential Evolution MCMC while incorporating a jitter term to account for astrophysical variability, and it also tests the impact of upper limits and a possible mass dependence. The results show a shallower age slope, $m \approx -1.37 \pm 0.47$, when variability is accounted for, consistent with the younger-star regime and suggesting that X-ray luminosity is not a reliable standalone age indicator for older stars. Upper limits do not significantly alter the conclusions, and there is tentative evidence for mass dependence, though the interpretation remains inconclusive. Overall, X-ray luminosity remains useful as part of a multi-method age-dating approach rather than as a sole diagnostic for stellar ages, with implications for calibrating ages in exoplanet studies.

Abstract

Accurate stellar dating provides crucial information about the formation and development of planetary systems. Existing age-dating techniques are limited in terms of both the spectral type and age range they can accurately probe, and many are unreliable for stars older than 1 Gyr. Recent studies have suggested that a star's X-ray luminosity correlates strongly with stellar age and shows a steep fall-off at ages older than 1 Gyr. In this work, we present X-ray luminosity relationship values from eROSITA for four previously unassessed stars. Additionally, we reassess the X-ray luminosity/age relationship present in 24 main-sequence stars older than a gigayear. We confirm that a correlation does appear to exist between stellar age and X-ray luminosity at ages older than 1 Gyr. However, we measure a shallower slope with age than previous research for older stars, similar to what was found for younger stars. We also find evidence for significant astrophysical variability in a star's X-ray luminosity, which will likely limit the precision with which X-ray measurements can yield age estimates. We also find weak evidence for mass dependence in the X-ray luminosity/age relationship. These results suggest that although X-ray luminosity correlates with stellar age, it may not serve as a reliable standalone age indicator and is better used as part of a broader suite of age-dating methods.

Figures (2)

• Figure 1: Relationship between X-ray luminosity and stellar age. We include both archival measurements from booth2017improved and also incorporate newly measured targets with asteroseismic ages and X-ray luminosity from eROSITA. The points are color-coded by stellar type. The uncertainties are shown with a dark, smaller uncertainty reflecting the measurement uncertainties and the transparent larger uncertainties reflecting the addition of the jitter term. The dark and light blue shaded regions indicate the $1\sigma$ and $2\sigma$ uncertainties in the age-activity relationship, and the dark blue line indicates our best-fit model. Compared to the booth2017improved model fit (shown by the dashed black line), we find a significantly shallower slope.
• Figure 2: The X-ray luminosity/age relationship incorporating mass dependence. (a) Measurements of and best-fit relationship between X-ray luminosity as a function of stellar age, after subtracting the best-fit mass dependence. (b) Measurements of and best-fit relationship between X-ray luminosity as a function of stellar mass. As in Figure \ref{['fig:alltargets']}, the dark and light blue shaded regions indicate the $1\sigma$ and $2\sigma$ uncertainties in the age/mass/X-ray luminosity. We see tentative ($\approx 3\sigma$) evidence for a mass dependence in the X-ray luminosity/age relationship.