Observations of the ultraviolet-bright star Y453 in the globular cluster M4 (NGC 6121)

• Autores: Dixon, William V.; Chayer, Pierre; Latour, Marilyn; Miller Bertolami, Marcelo Miguel; Benjamin, Robert A.
• Año de publicación: 2017
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Abundances; Atmospheres; Ultraviolet; Stars; NGC 6121 (estrella); Y453 (estrella)
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

We present a spectral analysis of the UV-bright star Y453 in M4. Model fits to the star´s optical spectrum yield Teff ∼ 56,000 K. Fits to the star´s FUV spectrum, obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope, reveal it to be considerably hotter, with Teff ∼ 72,000 K. We adopt Teff = 72,000 ± 2000 K and log g = 5.7 ± 0.2 as our best-fit parameters. Scaling the model spectrum to match the star´s optical and near-infrared magnitudes, we derive a mass M∗ = 0.53±0.24 M⊙ and luminosity = log L/L⊙ 2.84±0.05, consistent with the values expected of an evolved star in a globular cluster. Comparing the star with post-horizontal-branch evolutionary tracks, we conclude that it most likely evolved from the blue horizontal branch, departing the asymptotic giant branch before third dredge-up. It should thus exhibit the abundance pattern (O-poor and Na-rich) characteristic of the second-generation (SG) stars in M4. We derive the star´s photospheric abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are roughly 0.25 dex greater than those of the cluster´s SG stars, while the Si and S abundances match the cluster values. Abundances of the iron-peak elements (except for iron itself) are enhanced by 1-3 dex. Rather than revealing the star´s origin and evolution, this pattern reflects the combined effects of diffusive and mechanical processes in the stellar atmosphere.
Fil: Dixon, William V.. Space Telescope Science Institute; Estados Unidos
Fil: Chayer, Pierre. Space Telescope Science Institute; Estados Unidos
Fil: Latour, Marilyn. Friedrich-alexander University Erlangen-nuremberg; Estados Unidos
Fil: Miller Bertolami, Marcelo Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina
Fil: Benjamin, Robert A.. University Of Wisconsin Whitewater; Estados Unidos