R. M. Humphreys, M. Koppleman, A. Helton, T. J. Jones, R. D. Gehrz (Univ. of Minnesota), R. M. Wagner (Univ. of Arizona)
Variable A is one of the highly luminous but unstable stars that define the upper luminosity limit in the HRD for evolved stars. At its maximum light in 1950 Var A was one of the visually brightest stars in M33 with an F-type spectrum and colors. It then rapidly declined by more than 3 magnitudes and became red. Optical and near-infrared photometry from 1977 - 1986 showed that Var A had gotten even fainter, was still red, and its spectrum from 1985 showed strong TiO bands. It had a strong infrared excess and was as bright at 10 microns as at its visual maximum in 1950 with a luminosity of 5 x 105 Lsun and a high mass loss rate ( 2 x 10-4 Msun yr-1). Its late-type spectrum was produced in a cool pseudo-photosphere or optically thick wind. Thus, Var A appeared to be a more extreme, more luminous version of rho Cas, well known for its "shell episodes" during which it fades and its normal F supergiant spectrum shows TiO bands. While in rho Cas these events last 1 - 2 years at most, in Var A, the outburst or eruption phase lasted at least 35 years and probably 40 - 45 years! A recent spectrum shows that the star's wind has returned to a warmer apparent temperature; however, it has remained faint. Its present faintness in the optical is presumably due to obscuration by circumstellar dust, although it is not red. The fading of its near-IR flux very likely corresponds to the changes in its wind possibly accompanied by the dissipation of warm dust near the star. Var A thus presents us with the opportunity to observe the formation and destruction of dust accompanying a major eruptive episode.
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.