The evolutionary status of the blue halo stars

This paper presents a new approach to the observational study of the advanced evolution of Population II stars. The discussion is based on the use of the (log g, log Th )-diagram as an H-R diagram. In particular, advantage is taken of our ability to make direct comparisons, in the (log g, log Te )-plane, between observations of individual field and cluster stars and the predictions of interior theory. The distribution of the high-latitude faint blue stars over T6ff is first studied on the basis of their distribution in the [(U - B), (B - V)]-diagram. It is shown that the principal stellar sequence of the latter distribution is not uniform but displays two gaps, at colors corresponding to log Teff 4.11 (gap 1) and log Teff 4.33 (gap 2). Because globular-cluster blue-horizontal- branch sequences show significant structure at similar values of Teff it is suggested that the gaps are a horizontal-branch phenomenon. The narrow-band spectrophotometric techniques used previously to study blue-horizontal- branch (BHB) stars in globular clusters are applied to a sample of high-velocity and high4atitude faint blue stars. The atmospheric parameters g and Teff, together with the interstellar reddening, are derived for individual program stars by comparing the observed narrow-band colors with the emergent fluxes computed for a grid of approximate line-blanketed model atmospheres. An investigation of the (log g, log Th )-distribution of globular cluster and field blue stars, together with the properties of gaps 1 and 2, leads to a luminosity-independent (log g, log Teff)classification for blue halo stars. This classification, which is based on g and Teff values taken from the literature and from the present study, encompasses blue halo stars in the temperature range 4.48 > log Teff > 3.86. Five subgroups, designated HL, A, BC, D, and wd are isolated. Comparison of the observed stellar distribution in the (log g, log Te )-diagram with existing evolutionary tracks shows that current theoretical concepts of the advanced evolution of Population II stars can explain the majority of blue halo stars. The group D stars (most of which would be classified spectroscopically as subdwarf B) are difficult to fit into the evolutionary scheme. The comparison between observation and theory leads also to a tentative explanation for the gaps observed in the field star two-color diagram. This explanation depends on the sensitivity of the evolutionary behavior of low-mass (0.50 o < 0.60 e) horizontal-branch stars to the amount of matter in their hydrogen-rich envelopes. If this hypothesis is correct it indicates that the observed gap positions, once properly calibrated, can be used to study the physical and chemical properties of the blue halo stars. Subject headings: faint blue stars - globular clusters - horizontal-branch stars - stellar evolution