Detection of additional absorption in the post-outburst spectra of MU Centauri

Abstract

A large number of photographic (92) and CCD (27) spectra of the pole-on Be star, Mu Cen, were obtained between February 1987 and May 1990. From the results of the above-mentioned spectra a weak outburst was detected in March 1987 which was probably the fourth after the major outburst of this star in early February 1987. Recently, two major outbursts of this star were detected between June 1989 and May 1990. From the comparison of the active (outburst and post-outburst period) and quiescent phase spectra, it has been found that the absorption depths and the equivalent widths of the ionized lines (Si II, S II, N II, Fe II, and Fe III) were stronger during the active phase than the quiescent phase. The existence of matter at polar latitudes or the increase of the photospheric temperature due to the nonradial pulsations have been suggested to explain the observed results. Also, the decrease in equivalent widths of H-alpha absorption line was detected, on the timescales of months, before the major outburst of April 1990. This may be used as an indicator for the development of an outburst, and it has been suggested that the conditions for an outburst are slowly built up, and that the nonradial pulsation probably plays a role in the development of the outburst condition.