The Be Star Newsletter, Volume 40 - July 2011
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Hα Spectroscopy and V Variations of Be Star 28 Tauri (Pleione)Ernst Pollmann
Building on the review article by Hassforther (2008) on the same subject, with this contribution I would like to show the correlation between V brightness and the Hα equivalent width (EW). As a member of the Pleiades cluster the spectroscopic binary Be star (spectral type B8e) 28 Tau is well known as a shell star with photometric and spectroscopic long-term variations and cyclic changes in its spectrum from a Be phase to a Be-shell phase (Fig. 1) since the 19th century. Since 1938, an alternation of Be-shell and Be phases has been reported with a 35–36 years cycle.
Figure 1. High-resolution grating spectrum of 28 Tau (2009/11/19) in its current Be-shell phase The observed changes of the spectral characteristics from a Be phase to a Be-shell phase (and back) with this period is, because the disk "for some reason" (probably perturbation by the companion star at periastron) is not in the equatorial plane but slanted to the equator and precesses around the central star. This is manifested as variations of the Hα line profile (Hummel, 1998). Currently the last Be-phase > Be-shell-phase observations (especially during the period November 2005 to April 2007) are interpreted as follows: every 35–36 years the spectroscopic companion star at periastron produces a mass loss from the primary star. As a result, a new disc is formed in the equatorial plane. During this process the old disc is already out of this equatorial plane, due to the precession caused by the companion. This process is then directly manifested in Hα emissivity. The historical V light curve in Fig. 2, from the database by S. Otero (private communication 2009), published V data by Tanaka et al. (2007) and other sources (see legend in this figure), and the Hα EW long-term monitoring by professional and amateur observations in Fig. 3 (sources see legend in this figure) made it possible to look for any correlation between V and EW. Because 74 simultaneous measurements of V and EW could be identified from all measurements, the correlation in Fig. 4 between Hα EW and overall brightness shows, to what extent the brightness increase of V is influenced by the brightness of the disc (determined by the Hα EW). The EW error bars in Fig. 4 correspond to the attainable accuracy of one observation in one night (±1 Å), the error bars in V corresponds to an assumed average measurement accuracy of all involved observers (± 0.03 mag).
Figure 2. Historic V light curve from various sources
Figure 3. Time behaviour of the Hα equivalent width from professional and amateur observations
Figure 4. Correlation of the Hα equivalent width versus V brightness from 74 simultaneous measurements If the variation in brightness are mainly caused by variations in the brightness of the disc, then at EW = 0 the (photospheric) V brightness (strictly taken only at Hα wavelength 6563 Å) would be approximately 5.4 mag. In other words, each increase above this value would therefore be attributable to Be star disc emissivity.
A large number of amateur spectra by J. Guarro, CH. Buil and B. Mauclaire are available to continue investigations in this area. They can be found in the BeSS database for Be stars of the observatory Paris Meudon
References
Hassforther, B. 2008, BAV-Rundbrief (ISSN 0405-5497) 1, 35
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