Astron. Astrophys. 318, 198-203 (1997)

4. A mass outburst

If 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 = 7.83 mag (Thé et al. 1985), and EW(H ) = -9 Å, we find F(H ) = 1.5 10^-11 erg cm^-2 s^-1. Taking the H recombination coefficient for a (T = 10⁴ K, = 10⁹ cm^-3) gas (Hummer & Storey, 1987), we obtain the emission measure EM (= dV) = 1.7 10⁵⁷ cm^-3. Taking the extinction at the R band into account, the value becomes twice as large.

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) = v₀ + ( - v₀)(1 - R /r) , where v₀ is the initial velocity, the velocity at infinity, and R the radius of the star. Adopting a value of the terminal velocity of 1000 km s^-1, v₀ = 10 km s^-1 and = 0.7 (Friend & Abbott 1986), the distance travelled is 0.33 R . At this radius, the velocity happens to be 380 km s^-1, a similar figure to the observed line broadening of 370 km s^-1. So, if the line width were entirely due to kinematic broadening, the adopted velocity law parameters combine to give a consistent picture. Assuming spherical symmetry and the above parameters, we find a mass loss rate of 10^-8 yr^-1 is able to match the observed emission measure. Other combinations of the above parameters have been explored and we find that the choice is not critical in terms of the order of magnitude deduced for the mass loss rate.

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 v i of HD 76534 is quite low, suggesting a near pole-on orientation), we have no value for the outflow velocity. Furthermore, we can not uniquely disentangle the contribution of the different kinematical and non-kinematical factors governing the line-broadening. In view of such problems, we can do no more than adapt the result from the spherically symmetric case, to provide an order of magnitude estimate for a disk mass loss rate.

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^-9 yr^-1, which is comparable to the 3.8 10^-9 yr^-1 Hanuschik et al. (1993) determine for µ Cen.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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