First stellar abundances in the dwarf irregular galaxy Sextans A

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A (= DDO 75) from high-resolution spectra obtained with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Kueyen telescope (UT2) of the ESO Very Large Telescope (VLT). Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron-group abundance was determined from these three stars to be [[( Fe II, Cr II)/ H]] = - 0.99 +/- 0.04 +/- 0.06.(2) This is the first determination of the present-day iron-group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar alpha-element abundance was determined from the alpha-element magnesium as [[alpha(Mg I)/ H]] = - 1.09 +/- 0.02 +/- 0.19. This is in excellent agreement with the nebular alpha-element abundances as determined from oxygen in the H II regions. These results are consistent from star to star, with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ ("on the spot'') enrichment. The alpha/Fe abundance ratio is [[alpha(Mg I)/ Fe II, Cr II]] = -0.11 +/- 0.02 +/- 0.10, which is slightly lower but consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies, NGC 6822 and WLM. The results of near-solar [alpha/Fe] ratios in dwarf galaxies is in stark contrast with the high [alpha/Fe] results from metal-poor stars in the Galaxy ( which plateau at values near + 0.4 dex) and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [alpha/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.