## 4. A mass outburstIf the onset of the emission is due to a sudden ejection of mass, the observed timescale provides an opportunity to make a simple estimate of the mass loss rate. In order to do this, we need some basic parameters such as the distance, mass and radius of HD 76534. Membership of the Vela-R2 association has been used to assign a distance d = 870 pc (Herbst, 1975). Using this, Hillenbrand et al. (1992) derived R = 7.5 and M = 11 for = 1. Although there is room for improvement on these values, they are sufficiently accurate for the present purpose. The volume emission measure is calculated as follows. From
The outer radius of the gas is dependent on the travel time which
we take as 10,000 s ( 2.8 hours, the timescale of
the variability) and the velocity law, v(r) = v However, the H line profile is symmetric and
double peaked. This suggests that the ionized region does not
originate in a spherically symmetric wind, because then some profile
asymmetry should be apparent. Applying instead a more conventional
geometry for the Be star case, where the matter is compressed into a
disk, the estimate for the mass loss rate becomes less trivial. Since
the de-projected velocities in the circumstellar disk would be larger
than observed (the In a flattened geometry, smaller mass loss rates are capable of matching the observed emission measure. The reasoning is as follows. For example, the volume occupied by a disk with an opening half angle of is about ten times less than that occupied by a sphere. In order to conserve mass, the electron density must be ten times larger. Since the resulting emission measure is proportional to the square of the density times the volume, one obtains an emission measure that is of order ten times larger for the same mass loss rate in the spherically symmetric case. Conversely, in order to obtain the same emission measure, a mass loss rate which is the square root of ten smaller than in the spherically symmetric case will reproduce the same EM. We thus obtain a few times 10 © European Southern Observatory (ESO) 1997 Online publication: July 8, 1998 |