Calculating the Mass of Horizontal-Branch Stars with HIPPARCOS

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Horizontal-branch stars in globular clusters turned out to have a mass of ~=0.4 M_(solar), which followed from T_(eff) and log(g) (as derived from photometry, Balmer line spectroscopy plus Balmer profile models), the luminosity of these stars from UV + visual spectrophotometry, and the distance to the globular cluster. Such a mass value is in contradiction with values from the theory of evolution of stars. Hipparcos parallaxes to a few field-HB stars have been used to do the same analysis. The input data are T_(eff) and log(g) from the literature, the luminosity from IUE + visual spectrophotometry, and the Hipparcos distance. Here the mean of the masses is found to be M_(HB) = 0.38 +/- 0.07 M_(solar). The masses determined for globular cluster HB stars and for field HB stars agree with each other, but deviate from stellar evolution theory. The cause for the discrepancy is discussed. The most likely cause is problems with the determination of the gravity from the comparison with theoretical Balmer profiles. An unlikely increase of the distance to the globular cluster by 0.4 mag would bring the mass of its HB-stars up to 0.6 M_(solar). However, the mass of the field HB stars stays fixed at 0.38 M_(solar), based on the Hipparcos parallaxes.

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