Title: Gaia Excess Magnitude Uncertainties as a Proxy for Stellar Age

Abstract: Young stars are known to be more active, leading to a long-studied correlation between stellar age and photometric variability. Although individual, similarly aged stars seem to largely vary in photometric amplitude, the age-variability relation is effective over a large group of stars. To quantify this relationship, we approximate variability using excess photometric uncertainty in Gaia photometry (Var_G, Var_BP, and Var_RP) and calibrate using associations with well-constrained ages and membership lists. We find these metrics demonstrate a Skumanich-like correlation, scaling approximately as t^−0.4, and can predict group ages within 10-20% for associations younger than ≃2.5 Gyr. However, the resulting age uncertainties tend to be larger, primarily due to finite group sizes. Our index proves most effective for the youngest populations (<100 Myr), where uncertainties are similar to or better than uncertainties derived from more established methods, such as from a color-magnitude diagram. Given Gaia’s observation of photometry for over 1.5 billion stars, the index is widely accessible and quick to compute. Further, the index can be used at intermediate ages (200-500 Myr) where there are limited pre- or post-main-sequence stars and other methods exhibit higher uncertainties. As a demonstration of further applications, we show how the index can be used to find new associations and test whether a group of co-moving stars constitutes a genuine co-eval population. We apply our methods to newly identified groups from data mining Gaia astrometry (known as the Theia groups) within 350pc and find ≳90% are inconsistent with randomly drawn field stars and ≃80% have variability ages consistent with CMD ages. This suggests a vast majority of the Theia groups are likely real populations, contradicting earlier studies of these groups.

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