HD 195636: A metal-poor rotator near the HB/AGB transition

The color temperature and Balmer jump inferred from UBV and Stromgren photometric indices, the low gravity and low metallicity derived from spectrum analysis, and weakness of all CH molecular lines combine to suggest that HD 195636 is in an evolutionary state near the transition between the horizontal branch and asymptotic giant branch. The projected equatorial rotational velocity of PID 195636 is V-e sin i=25 km/s, a value at least 2.5 times greater than that expected if known blue horizontal branch axial rotators in globular clusters conserve envelope angular momentum during horizontal branch evolution. Constancy of the radial velocity on time scales of 3, 10, and 90 days indicates that the axial rotation is not due to co-rotation in a short-period binary. Therefore, it seems most plausible that transfer of angular momentum from core to envelope occurred during HB evolution. The Balmer line profiles are peculiar. H alpha is abnormally shallow, as if the core were filled by emission, and higher members of the Balmer series are significantly broader than those of HD 200564, a slightly cooler giant star of similar metallicity. The space velocity of HD 195636 calculated for an assumed RHB/AGB luminosity, M-V=-0.5, is 470 km/s retrograde, a high but not extraordinary value. (C) 1997 American Astronomical Society.