skip to main content

Title: A helium-carbon correlation on the extreme horizontal branch in ω Centauri

Taking advantage of a recent FORS2/VLT spectroscopic sample of extreme horizontal branch (EHB) stars in ω Cen, we isolate 38 spectra well suited for detailed atmospheric studies and determine their fundamental parameters (T {sub eff}, log g, and log N(He)/N(H)) using NLTE, metal line-blanketed models. We find that our targets can be divided into three groups: 6 stars are hot (T {sub eff} ≳ 45,000 K) H-rich subdwarf O stars, 7 stars are typical H-rich sdB stars (T {sub eff} ≲ 35,000 K), and the remaining 25 targets at intermediate effective temperatures are He-rich (log N(He)/N(H) ≳ –1.0) subdwarfs. Surprisingly, and quite interestingly, these He-rich hot subdwarfs in ω Cen cluster in a narrow temperature range (∼35,000 K to ∼40,000 K). We additionally measure the atmospheric carbon abundance and find a most interesting positive correlation between the carbon and helium atmospheric abundances. This correlation certainly bears the signature of diffusion processes—most likely gravitational settling impeded by stellar winds or internal turbulence—but also constrains possible formation scenarios proposed for EHB stars in ω Cen. For the He-rich objects in particular, the clear link between helium and carbon enhancement points toward a late hot flasher evolutionary history.
Authors:
; ;  [1] ;  [2] ;  [3] ;  [4]
  1. Département de Physique, Université de Montréal, Succ. Centre-Ville, C.P. 6128, Montréal, QC H3C 3J7 (Canada)
  2. ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München (Germany)
  3. Istituto Nazionale de Astrofisica, Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio Catone (Italy)
  4. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
Publication Date:
OSTI Identifier:
22370259
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 795; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERES; CARBON; CORRELATIONS; DIFFUSION; ELEMENT ABUNDANCE; GALAXIES; HELIUM; STAR CLUSTERS; STARS; STELLAR WINDS; TURBULENCE