Evolved stars with inconsistent age estimates: Abundance outliers or mass transfer products?

TL;DR

This study interrogates the reliability of age estimates by comparing asteroseismic ages to abundance-derived ages for 8,803 red giants from APOKASC-3 and SDSS-V MWM. Using a simple kNN regression in the space of [Mg/H], [Fe/Mg], and log(g), the authors obtain abundance ages that generally reproduce asteroseismic ages within about two gigayears, but identify 377 outliers—Mass Accretor Candidates and Mass Donor Candidates—likely arising from binary mass transfer. Through control-sample comparisons and KS tests across diverse diagnostics (RV, RUWE, rotation, surface abundances, UV excess, and orbital invariants), the work finds that most outliers are better explained as age anomalies rather than abundance anomalies, with strong signatures in C/N and Na/Mg for the MAC and MDC groups. The study highlights a fundamental limit on the reliability of asteroseismic ages for stars with binary histories and calls for targeted follow-up observations to confirm mass-transfer scenarios and refine age-dating for stellar populations.

Abstract

In the Milky Way disk there is a strong trend linking stellar age to surface element abundances. Here we explore this relationship with a dataset of 8,803 red-giant and red-clump stars with both asteroseismic data from NASA Kepler Mission and surface abundances from the SDSS-V MWM. We find, with a k-nearest-neighbors approach, that the [Mg/H] and [Fe/Mg] abundance ratios predict asteroseismic ages to an accuracy of about 2 Gyr for the majority of stars. That said, there are substantial outlier stars whose surface abundances do not match their asteroseismic ages. Because asteroseismic ages for these stars are fundamentally based on density or mass, these outliers are mass-transfer candidates. Stars whose surface abundances predict a younger age (higher mass) than what's seen in the asteroseismology are mass accretor candidates (MAC); stars whose abundances predict an older age (lower mass) than the asteroseismology age are mass donor candidates (MDC). We create precise control samples, matched according to (1) surface abundances and (2) asteroseismic ages, for both the MAC and MDC stars; we use these to find slight differences in rotational velocity, [C/N], and [Na/Mg] between the mass-transfer candidates and their abundance neighbors. We find no drastic differences in kinematics, orbital invariants, UV excess, or other stellar abundances between outliers and their abundance neighbors. We deliver 377 mass-transfer candidates for follow-up observations. This project implicitly suggests a fundamental limit on the reliability of asteroseismic ages, and supports existing evidence that age-abundance outliers are products of binary mass transfer.

Figures (10)

• Figure 1: Left: [Mg/H] vs. [Fe/Mg] for our sample, colored by asteroseismic age. Younger stars (darker colored) tend to have higher [Fe/Mg], while older stars (lighter colored) tend to have lower [Fe/Mg]. Right: Same as left panel, but now colored by our abundance ages.
• Figure 2: Comparison of asteroseismic ages and abundance ages for all stars in our sample. Left: asteroseismic ages vs. abundance ages in Gyr. Most stars fall near the 1:1 line indicating strong agreement between abundance and asteroseismic ages. Two distinct outlier populations are highlighted: Mass Accretor Candidates (MAC; dark purple crosses), which appear anomalously younger in abundance ages than in asteroseismic ages, and Mass Donor Candidates (MDC; orange stars), which appear older in abundance ages than in asteroseismic ages. The MAC and MDC populations are stars whose surface abundances are inconsistent with their inferred asteroseismic masses, and are selected using both linear and logarithmic residual thresholds (see Section \ref{['outlier_section']}). Right: Same as left, but with logarithmic ages.
• Figure 3: Distributions of residuals between the linear (left) and logarithmic (right) asteroseismic and abundance ages for all stars in our sample. calculated in Equations \ref{['eq:delta_age_lin']} and \ref{['eq:delta_age_log']}. We show the respective linear and logarithmic thresholds used to classify MAC (purple) and MDC (orange) outliers.
• Figure 4: Stellar [Mg/H] vs. [Fe/Mg] for the full stellar sample (grey dots), with MAC (dark purple crosses) and MDC (orange stars) highlighted in the left and right panels, respectively. The MAC and MDC identification is described in Section \ref{['outlier_section']}. These age abundance outliers span a broad range of surface abundance values, with MDC concentrated at higher [Fe/Mg] and MAC spanning both low- and high-[Fe/Mg].
• Figure 5: Top row: distributions of stellar parameters for the MAC (dark purple), MAC abundance neighbors (pink) and MAC age neighbors (yellow). Stellar parameters include [Mg/H] (first column), [Fe/Mg] (second column), asteroseismic age (third column) and $\log(g)$ (fourth column). In each panel we quote the KS statistics between the MAC parameter distribution and that of the abundance (KS$_{\rm abd}$) and age (KS$_{\rm age}$) neighbors. Bottom row: same as top but for the MDC (orange), MDC abundance neighbors (pink), and MDC age neighbors (yellow).