Dramatic Spectral and Photometric Changes of Pleione (28 Tau) between 2005 November and 2007 April

Abstract

Recent photometric and spectroscopic observations of Pleione are reported. Pleione showed brightness declines in both the $B$ and $V$ bands by $\sim$ 0$\rlap {.}{^{\rm m}}$25 between 2006 January and 2007 March. Significant decreases in the strengths and widths of the long-lived emission components in H$\alpha$ and H$\beta$ were observed during this period, while additional outer double-emission components appeared in these lines. Many metallic shell lines strengthened during the observation period, being accompanied by central quasi-emission components. We interpret these spectral changes in terms of a double disk: the progressive decline of a misaligned old disk in due course of precession and the formation and growth of a new disk in the star’s equatorial plane.

Properties and nature of Be stars

“…During the transition from a single-peaked to double-peaked emission, there is some time interval when the Hα profile has a characteristic wine-bottle shape. The occasional presence of additional absorption components has been already noted by Iliev et al (2007) or Tanaka et al (2007) and is typical of all recorded shell phases of BU Tau. Besides the occasional presence of one or more additional absorptions, extended red emission wings are seen on some Hα profiles.…”

“…This clearly corresponds to the gradual development of the hydrogen shell spectrum according to our spectra -see also Tanaka et al (2007). Emission has been slowly fading from JD 2453000 until now, when its peak intensity represents only about 30% of the intensity seen in our earliest spectra.…”

“…According to their interpretation, the old disk was precessing in space as suggested by Hirata (2007). Our spectra cover a much longer time interval, including the one studied by Tanaka et al (2007), and as Fig. 2 shows, the change of the Hα profile was smooth.…”

“…He further argues that also the change in the Hα profiles from a weak double emission with a strong central absorption core to a strong emission with a wine-bottle shape indicates that the disk was first seen more or less edge-on and later more face-on. Tanaka et al (2007) studied the spectra of BU Tau from Nov. 2005 until April 2007, which cover the period of a formation of the new shell phase. They argue that a new disk was formed in the equatorial plane of the B star while the old disk was decaying but still present.…”

“…It underwent several phase transitions between B, Be, and Be shell phases, accompanied by pronounced light variations; see, e.g. Gulliver (1977), Sharov & Lyuty (1976), Iliev et al (1988), Sharov & Lyutyj (1992), Hirata & Kogure (1976), Hirata & Kogure (1977), Hirata (1995), Doazan et al (1988), Iliev et al (2007), and Tanaka et al (2007).…”