Nitrogen-rich field stars are not necessarily old

Leitinger, Ellen

doi:10.5281/zenodo.17366800

Nitrogen-rich field stars are not necessarily old

Leitinger, Ellen

Publisher: Zenodo
DOI: 10.5281/zenodo.17366800
Repository: zenodo.org
Language: en

Abstract

Globular cluster (GC) stars exhibit distinct chemical abundance patterns, which can classify them into multiple stellar populations. When red giant stars with these chemical signatures are discovered in the Galactic field, rather than within GCs, they are assumed to have once belonged to GCs. We have expanded the search for such chemically peculiar stars in the Kepler field, targeting metal-poor stars located in the halo, thin and thick disk, combining asteroseismic properties with chemical abundances and kinematics, in order to investigate their potential origins. Combining data from APOGEE DR17, Gaia DR3, and Kepler, we have identified a sample containing: primordial stars - with chemical signatures typical of field stars, and enriched stars - with strong nitrogen enrichment and corresponding carbon and oxygen depletion. The asteroseismic ages of stars in our sample indicate that most of the enriched stars are too young to have originated from globular clusters. This has prompted a search for alternative explanations of their origins, such as enrichment through a binary companion, which could mimic the enrichment signature observed in GCs.

Full text

Nitrogen-rich field stars are not necessarily old Ellie Leitinger Most Galactic globular clusters contain: Primordial stars with light element abundances similar to field stars of the same metallicity Enriched stars enriched in N, Na, Al and depleted in C, O, Mg, in comparison to primordial stars [1] with A. Miglio, J. Montalban, D. Massari, A. Bragaglia, W.E. van Rossem, K. Brogaard, A. Mazzi & J.S.Thomsen Discovering enriched stars in the Milky Way field - rather than in globular clusters – leads to the assumption they were either 1) ejected from globular clusters, or 2) once belonged to a now dissolved cluster The estimated fraction of Milky Way field stars originating from globular clusters varies between 11% [2] to 70% [3] Using APOGEE DR17 abundances, we selected a sample of metal-poor primordial and enriched red giant stars from the halo, thin and thick disk contained in the Kepler field, in order to investigate their potential origins PARAM [4],[5],[6] Estimations on mass & age by fitting Δ𝜈, 𝜈𝑚𝑎𝑥, evolutionary state, [Fe/H] and Teff With uncertainties on stellar ages of around 20% References: [1] see e.g.Bastian & Lardo 2018, [2] Hank+20, [3] Belokurov+23, [4] Willet+25 (submitted), [5] Miglio+21, [6] Rodrigues+17, [7] Montalban+21 AIMS [7] Estimations using individual-mode frequencies, which are more sensitive to the internal structure of the star With uncertainties on stellar ages of around 10% From Kepler asteroseismology, we can determine masses and ages through Bayesian stellar parameter estimation tools: The typical age range of Galactic globular clusters is 8.9 to 14.5 Gyr In our sample: the primordial stars have ages consistent with stars of the thick disk, but most of the enriched stars are too young and massive to have originated from Milky Way globular clusters Mass transfer between binary red giant stars and asymptotic branch stars (AGB) could reproduce the same enrichment signatures observed in enriched globular cluster stars, which could explain these young stellar ages We confirm at least one enriched star (KIC 10796857) is a spectroscopic binary, with another (KIC 8350894) showing enrichments in the elements Ba and La - indicative of potential mass transfer with an AGB companion The same method is currently being applied to K2 and TESS stars, in order to expand the sample and investigate further signatures associated with binary mass transfer Background image credit: Gaia DPAC