Precise Asteroseismic Ages for Red Giants in the Helmi Streams
Lindsay, Christopher; Hon, Marc; Ong, Joel; García, Rafael; PALAKKATHARAPPIL, DINIL BOSE; Li, Tanda; Ruiz-Lara, Tomás; Helmi, Amina
- Publisher
- Zenodo
- Language
- en
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 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 (𝜏). We report 𝜏 = 11.2 ± 1.3 Gyr for HD 175305 and 𝜏 = 12.4 ± 0.8 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, α-enhanced red giants can be severely underestimated when inferred using global asteroseismic parameters instead of individual mode frequencies.
Full text
Precise Asteroseismic Ages for Red Giants in the Helmi Streams With: Marc Hon (MIT), Joel Ong (U. Hawaiʻi), Rafael García (U. Paris-Saclay), Dinil Palakkatharappil (U. Paris-Saclay), Jie Yu (ANU), Tanda Li (Beijing Normal U.), Tomás Ruiz-Lara (U. Granada), and Amina Helmi (U. Groningen) Christopher Lindsay. Yale Astronomy Session 3: Galactoarcheology. Monday, July 7th, 2025 TESS/Spitzer/NASA/JPL 1 I Will be on the Job Market this Fall!
2 The Milky Way grows through mergers with smaller galaxies Credit: ESA Kruijssen+ 2020
Red Giant Stars in the Helmi Streams are “Debris” from Accretion 3 Spitzer/NASA/JPL Helmi+ 1999 (HIPPARCOS data) Dodd+ 2023 (GAIA data)
4 When did this merger happen? What are the properties of the progenitor galaxy? Can asteroseismology inform models for how heavy element enhancement operates in dwarf galaxies? Answers to these questions require detailed studies of stars in the Halo.
5 With TESS, we can probe the interiors of stars of stars across the entire sky using asteroseismology. This allows for precise age determinations for giant stars in different kinematic components of our galaxy 5
6 Global Asteroseismology is Vital for Galactic Archeology A. Miglio+ 2021 (Kepler + APOGEE data) Compare the global oscillation properties of red giants and Temperatures, Luminosities, and Abundances from spectra with grids of stellar models to obtain ages to σ~25% uncertainty. These ages are used to study the formation and evolution of the Galactic Halo and Thick Disk. + Kepler, K2, and TESS have permitted scaling-relation based age determinations for thousands of stars in the Galactic Thin Disk, Thick Disk, and Halo. See Also: S. Grunblatt+ (2021), C.C. Borre+ (2022), C. Marasco+ (2025), and A. Theodoridis+ (2025)
Going Further: Detailed Asteroseismic Modeling with Individual Mode Frequencies •Previous work comparing results from individual frequency modeling with scaling relation estimates indicates that scaling relation results diverge for low-metallicity stars (e.g. D. Huber+ 2024, J. Larsen+ 2025). •Using individual frequencies enables more precise age determinations (σage ~ 10%) compared with scaling relation derived ages (σage ~ 25%), though uncertainties in modelling choices also play a role (Li & Joyce 2025). 7
8 We identify the oscillation mode frequencies of HD 175305 and HD 128279, two Halo stars in the Helmi Streams
9 Creating Stellar Models to Fit the Observations HD 175305 The modeling procedure uses the differential evolution algorithm to explore the space of initial mass, helium abundance, metallicity, and mixing length.
16 Future Applications of Detailed Metal-poor Asteroseismology: •Calibrate other age measurements techniques (isochrone fitting, scaling-relation based ages, gyrochronology, chemical clocks +) for more precise galactic archeology •Roederer+ 2010 measured Thorium and Europium in HD 175305 and 3 other Helmi streams stars and found cosmochronometric ages of 3.1 ± 7.9 Gyr. •Study the timescales of heavy element enrichment in dwarf galaxies •How fast are stars enhanced in r-process elements? • Determine the origin of the "young" α-rich stars in the Galaxy
17 Takeaways: •We conducted a detailed asteroseismic analysis of two halo star members of the Helmi streams. •The best fit asteroseismic ages and other parameters determined for HD 175305 and HD 128279 are consistent with previous isochrone-based studies of the star formation histories of the Helmi streams. •The old age of HD 128279 (12.52 Gyr) places a lower bound on when stars began to form in the progenitor of the Helmi streams. •Grid based modeling of metal poor stars using only global asteroseismic parameters tends to overestimate stellar masses, thereby underestimating stellar ages. To study metal poor stars in the halo, individual mode frequencies are necessary until low-metallicity corrections to the νmax scaling relation are determined. Thank you for your attention! I Will be on the Job Market this Fall!
Thank you for your attention! 18 I Will be on the Job Market this Fall!
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