For Be star observers:
A rare event is imminent: the Be star 28 Tau (Pleione) will soon appear in its Halpha spectrum, almost completely devoid of central shell absorption lines. This phenomenon is linked to the precession of the circumstellar disk, caused be tidal torque of the (unseen) companion star, which alters the disk inclination from a nearly edge-on to a nearly pole-on orientation by approximately i ~ 30°.
This change is reflected in variations of the Halpha equivalent width (EW) and the central depression (CD) features. The process recurs on a timescale of approximately 30 years.
I attached a pdf file, that shows something more related to that event.
Ernst Pollmann
Zusammenstellung.pdf (727.6 KB)
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@ernstpollmann Hi Ernst, thanks for sharing! This is an exciting event.
I looked through the PDF document; those rapid oscillations during the shell phase are fascinating:
You wrote that these oscillations happen when the secondary star causes the disk around the primary to tilt. Do you know how much tilt occurs during a single orbit? I am not sure whether to expect a fraction of a degree or tens of degrees.
P.S. Would you like for me to move this thread to the Spectroscopy forum? I suspect that a greater number of spectroscopic observers will see it over there.
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Hi TiDye Astronomer,
thank you for your feedback. As far as I know, the magnitude of the disk inclination during each periastron passage is not well established. I suspect that among professional researchers, this periastron-related inclination — which is clearly manifested in the depth of the central absorption — has received little attention so far.
Perhaps this will change in the future, once this effect becomes more widely known through our published amateur studies.
Ernst