Observations of the new luminous blue variable R 110 of the Large Magellanic Cloud during an F star-phase.

Abstract

The LMC star R 110 has increased its visual brightness by about 0.5 mag in the last ten years. During its brightest observed phase in January and February 1989, we obtained UBVRI and JHK photometry, low dispersion spectroscopy with IUE, and high dispersion spectroscopy in the optical with CASPEC and in the satellite UV with IUE. During the brightening, the measured B - V and U - B colours have become considerably redder. From these photometric data R 110 can be classified as a new Luminous Blue Variable. In the coarse of the brightening and reddening, the spectrum has evolved from a B-type P Cyg spectrum to an almost normal F0-supergiant spectrum. With M_bol_ = -8.9, however, it is now the most luminous early-F supergiant of the LMC. The absorption lines have an extended blue wing which is interpreted as due to a shell expanding with a mean velocity of about 50 km s^-1^. We derive the following parameters for the present bright phase of R 110: T_eff_ = 7600 K, log g = 0.45, R_*_ = 310 R_sun_ and M = 10 M_sun_. The comparatively low mass indicates that R 110 is in its post-red supergiant evolutionary phase. From the Hα emission we estimate a mass-loss rate of about 10^-6^ M_sun_ yr^-1^ during the outburst and a similar value of a few times 10^-6^ M_sun_ yr^-1^ before the brightening. Surprisingly, and in contrast to other LBV's, the star shows no evidence for an enhanced mass-loss rate during this small outburst. The brightening and the gross spectroscopic and photometric properties during outburst described to the formation of a bloated cool and opaque quasi-static pseudo-photosphere.