Hα Outbursts of Mu-Centauri - a Clue to the BE Phenomenon

Details Hα Outbursts of Mu-Centauri - a Clue to the BE Phenomenon Hα Outbursts of Mu-Centauri - a Clue to the BE Phenomenon

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274..356h&link_type=abstract

Astronomy and Astrophysics, Vol. 274, NO. 2/JUL(II), P. 356, 1993

Astronomy and Astrophysics

Astrophysics

54

Line Profiles, Stars: Be, Stars: Circumstellar Matter, Individual Stars: Μ Cen, Accretion Disk

Scientific paper

We present a photoelectric record of circumstellar Hα emission in μ Cen (B2 IV-Ve, υ sin i 155 km s-1), a mildly active bright southern Be star. The observations cover a period of about 200 d in 1987. During that time μ Cen showed four major outbursts, characterized by an increase of emission intensity from almost 0 to 2-4 Å equivalent width within 2-5 d. After such outbursts, emission steadily fades away with a typical time-scale of 20-80 d. This fading process is sometimes interrupted by minor outbursts. V/R ratios can be quite asymmetric during outbursts, but quickly become symmetric upon and after maximum emission. For part of the observed period photometric data are available as well. There is some evidence that the star is met in a state of elevated visual brightness during phases of Hα emission activity. We determine the radius of the circumstellar disk which reaches a maximum value of 4-6 stellar radii during emission maximum, and is smaller before and after maximum.
We interpret our observations in terms of simple ejection models. The Hα outbursts are likely to be caused by ejection of matter from a region somewhat above the stellar surface, preferentially in the equatorial plane. A certain fraction of the ejected matter is expected to collisionally obtain circularized orbits - with excess angular - momentum acquired by forwardly directed ejection - and to form thereby a circumstellar Keplerian disk (decretion disk). The further evolution of the disk is likely to be dominated by viscosity forcing most of the disk material to spiral back onto the star within ˜100 days, in rough agreement with the observed fading timescale.
The ejection process could be due to magnetic activity, i.e. magnetic flares or loops. We believe that μ Cen represents a prototype of Be star activity which - when enhanced in efficiency by a factor of 2 or 3 - can produce quasi-stationary envelopes as observed in many Be stars.

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