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ABSTRACT Standard stellar evolution theory poorly predicts the surface abundances of chemical species in low-mass, red giant branch (RGB) stars. Observations show an enhancement of p–p chain and CNO cycle products in red giant envelopes, which suggests the existence of non-canonical mixing that brings interior burning products to the surface of these stars. The 12C/13C ratio is a highly sensitive abundance metric used to probe this mixing. We investigate extra RGB mixing by examining: (1) how 12C/13C is altered along the RGB, and (2) how 12C/13C changes for stars of varying age and mass. Our sample consists of 43 red giants, spread over 15 open clusters from the Sloan Digital Sky Survey’s APOGEE DR17, that have reliable 12C/13C ratios derived from their APOGEE spectra. We vetted these 12C/13C ratios and compared them as a function of evolution and age/mass to the standard mixing model of stellar evolution, and to a model that includes prescriptions for RGB thermohaline mixing and stellar rotation. We find that the observations deviate from standard mixing models, implying the need for extra mixing. Additionally, some of the abundance patterns depart from the thermohaline model, and it is unclear whether these differences are due to incomplete observations, issues inherent to the model, our assumption of the cause of extra mixing, or any combination of these factors. Nevertheless, the surface abundances across our age/mass range clearly deviate from the standard model, agreeing with the notion of a universal mechanism for RGB extra mixing in low-mass stars.
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Evidence of deep mixing in IRS 7, a cool massive supergiant member of the Galactic nuclear star cluster
ABSTRACT The centre of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the nuclear star cluster (NSC), IRS 7, are presented for 19F, 12C, 13C, 14N, 16O, 17O, and Fe from a local thermodynamic equilibrium analysis based on spherical modelling and radiative transfer with a 25-M⊙ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both 12C (∼–0.8 dex) and 16O (∼–0.4 dex), while exhibiting an extremely enhanced 14N abundance (∼+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The 19F abundance is also heavily depleted by ∼1 dex relative to the baseline fluorine of the NSC, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abundances of the minor isotopes 13C and 17O are also derived, with ratios of 12C/13C ∼ 5.3 and 16O/17O ∼ 525. The derived abundances for IRS 7, in conjunction with previous abundance results for massive stars in the NSC, are compared with rotating and non-rotating models of massive stars and it is found that the IRS 7 abundances overall follow the behaviour predicted by stellar models. The depleted fluorine abundance in IRS 7 illustrates, for the first time, the potential of using the 19F abundance as a mixing probe in luminous red giants.
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- Award ID(s):
- 1927130
- PAR ID:
- 10464900
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 516
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 2801 to 2811
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/stac2393
