Precise Asteroseismic Ages for the Helmi Streams

TL;DR

This study precisely determines the ages of two Helmi stream red giants using detailed asteroseismic modeling of individual oscillation modes, confirming the stream's progenitor existed over 12 billion years ago and highlighting the importance of mode frequencies for accurate age estimates.

Contribution

First detailed asteroseismic age measurements of Helmi stream stars using individual mode frequencies, improving age accuracy and understanding of the stream's origin.

Findings

Ages of HD 175305 and HD 128279 are approximately 11.16 and 12.52 Gyr.

Precise ages support the hypothesis that the Helmi streams' progenitor is at least 12 Gyr old.

Using individual mode frequencies yields more accurate ages than global asteroseismic parameters.

Abstract

The Helmi streams are remnants of a dwarf galaxy that was accreted by the Milky Way and whose stars now form a distinct kinematic and chemical substructure in the Galactic halo. Precisely age-dating these typically faint stars of extragalactic origin has been notoriously difficult due to the limitations of using only spectroscopic data, interferometry, or coarse asteroseismic measurements. Using observations from NASA's Transiting Exoplanet Survey Satellite, we report the detailed asteroseismic modeling of two of the brightest red giants within the Helmi streams, HD 175305 and HD 128279. By modeling the individual oscillation mode frequencies and the spectroscopic properties of both stars, we determine their fundamental properties including mass, radius, and age ($\tau$). We report $\tau = 11.16 \pm 0.91$ Gyr for HD 175305 and $\tau = 12.52 \pm 1.05$ Gyr for HD 128279, consistent with previously inferred star-formation histories for the Helmi streams and the differential chemical abundances between the two stars. With precise ages for individual stream members, our results reinforce the hypothesis that the Helmi streams' progenitor must have existed at least 12 Gyr ago. Our results also highlight that the ages of metal-poor, $\alpha$-enhanced red giants can be severely underestimated when inferred using global asteroseismic parameters instead of individual mode frequencies.