Hd-160529 - A New Galactic Luminous Blue Variable

The galactic early-type A hypergiant HD 160529 has been photometrically monitored in the Stromgren system during the past 18 years. HD 160529 has become fainter in the optical range by 0.5 mag during the past eight years. In addition, pulsation-like variations with a quasi-period of 57 days and with a peak-to-peak amplitude of about 0.1 mag in b and y were found. The fading was not completely monotonous; a fall of at least 0.2 mag in b and y within about 40 days around JD2446500 is particularly remarkable. This jump is accompanied by a change in the temperature-sensitive c1 index of about -0.14 mag. HD 160529 has become visually fainter and hotter during the past eight years, thus becoming a new case of a galactic Luminous Blue Variable (LBV). Model calculations on the basis of the photometry and previously published as well as new spectroscopic observations have shown HD 160529 to be a close counterpart of R 110, a recently discovered LBV in the LMC. From this comparison an absolute magnitude of M(V) = -8.9 and a distance of 2.5 kpc is derived. The stellar parameters characterising the maximum state are estimated to be T(eff) almost-equal-to 8000 K, log g almost-equal-to 0.55, R almost-equal-to 330 R., from which the mass is estimated to be only M almost-equal-to 13 M.. From the H-alpha profile a lower limit of the mass-loss rate of HD 160529 was calculated, which is about 10 times higher than that of R 110; this could be a consequence of the higher metallicity of the galactic LBV. The low mass of HD 160529 makes this luminous blue star a good candidate for a post-RSG (red supergiant phase) object, providing some evidence that (as sometimes argued) low metallicity is not a necessary prerequisite for evolution of massive stars backward to higher temperatures.