2022MNRAS.516.2801G

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.