The blue horizontal-branch stars of omega Centauri

Abstract

view Abstract Citations (85) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The blue-horizontal branch stars of omega Cen. Newell, E. B. ; Rodgers, A. W. ; Searle, L. Abstract We have obtained photoelectric UBV photometry for twenty-six blue horizontal-branch stars in the halo globular cluster c~, Centauri (NGC 5139). For twenty of these stars 11j3 profiles have been determined from low-dispersion image-tube spectra. Atmospheric parameters are derived on the basis of the observed colors and line widths. The results, together with those obtained in a previously published investigation of NGC 6397, are used to define relations between effective temperature and intrinsic color for blue hori- zontal-branch stars. For nine stars hotter than O~ == 0.55, we derive a mean mass-luminosity ratio (Y1J~/L) = -2.02 ± 0.16. This value is in good agreement with that obtained for the blue horizontal-branch stars of NGC 6397 by Newell, Rodgers, and Searle and with the independent estimates obtained theoretically by Christy and by Iben and Faulkner. A distance modulus is estimated for w Ceñ by fitting its C-M diagram to that of M92. The result, (in - ~ - 14 0 ± 0.3 mag, is based on a modulus of 14 5 mag for M92 and leads to a mean luminos- ity (log L/Lo) = 1.71 ± 0.12. The estimates of (~Y1/L) and (log L/Lo) lead to a mean mass of (9J~) = 0.53 ± 0.20 9)~o. The blue horizontal-branch stars appear to occupy a wedge shaped domain in the theoretical H-R diagram. I. INTRODUCTION In two previous papers (Searle and Rodgers 1966; Newell, Rodgers, and Searle 1969; hereinafter Paper I and II, respectively) we reported observations of the blue horizontal- branch (BHB) stars in the globular cluster NGC 6397. These observations were designed to locate the BHB stars in the theoretical llertzsprung-Russell diagram and to determine their masses. In this paper we report similar observations of the BHB stars in the cluster w Cen. As in Paper II we have obtained U13 V photometry and low-dispersion spectra of the BHB stars, but narrow-band continuum observations proved impractical for these fifteenth-magnitude stars, and we have used instead the empirical relations between atmospheric parameters and broad-band colors that were derived for NGC 6397 in Paper II. From observations of this kind we can determine effective temperatures and surface gravities of the observed stars, and hence their mass-to-luminosity ratio. The observa- tions reported here and in Paper II show that, within the errors of observation, this ratio is constant along the horizontal branch and has the same value in both clusters. To determine the masses of the BHB stars, we need an independent determination of their luminosities. In Paper II, main-sequence fitting yielded <log L/Lo) = 1 ~64 ± 0.1 and <9J1)BHB = 0.55 ± 0.15 9J1o for the BHB stars in NGC 6397. These results are in excellent agreement with Christy's (1966a, b) conclusion that the metal-poor RR Lyrae stars have log L/Lo ~-~` 1.66 and masses in the range 0.45-0.60 9)~o. In the absence of photoelectric photometry for main-sequence stars in w Cen, we have been forced to use indirect arguments to determine the luminosity of the BHB stars. Nevertheless, the observational study reported here supports the conclusions of Paper II Publication: The Astrophysical Journal Pub Date: November 1969 DOI: 10.1086/150230 Bibcode: 1969ApJ...158..699N full text sources ADS | data products SIMBAD (1) GCPD (1)