Spectroscopy of the Millennium Outburst and Recent Variability of the Yellow Hypergiant Rho.pdf

a r X i v : a s t r o - p h / 0 3 1 2 0 7 4 v 1 3 D e c 2 0 0 3 Stars as Suns: Activity, Evolution and Planets To appear in ASP Conference Series, Vol. xxx, 218, IAU Symposium No. 219, 21-25 July 2003, Sydney, Australia Spectroscopy of the Millennium Outburst and Recent Variability of the Yellow Hypergiant Rho Cassiopeiae A. Lobel1, R. P. Stefanik, G. Torres, R. J. Davis Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge 02138 MA, USA I. Ilyin Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland A. E. Rosenbush Main Astronomical Observatory of the National Academy of Sciences, Ukraine Abstract. In the summer and fall of 2000 the yellow hypergiant ρ Cassiopeiae dimmed by more than a visual magnitude, while its effective temperature decreased from ～7000 K to below 4000 K over ～200 d. We observed the highest mass-loss rate of ～5% of the solar mass per year in a single stellar eruption so far (Lobel et al. 2003, ApJ, 583, 923). It is the third outburst of ρ Cas on record in the last century. During the outburst the enigmatic cool luminous hypergiant changed its spectral type form early F- to early M-type. The outburst produced an out- ward propagating circumstellar shock wave, resulting in a tremendous cooling of the entire atmosphere. The optical spectrum became com- parable to that of the red supergiant Betelgeuse, and revealed strongly blue-shifted molecular absorption bands of titanium-oxide (TiO). We de- termine from the newly formed TiO bands a gas mass-loss rate of M??5.4 ×10?2 M⊙ yr ?1, which is of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable η Carinae. During the pulsation cycles that followed the millennium outburst ρ Cas brightened up after mid 2002, and started to dim in early March 2003. Over the past two years since the outburst event we observe a very prominent inverse P Cygni profile in Balmer Hα. Strong Hα emission has not before been observed in the hyper