Astron. Astrophys. 317, L17-L20 (1997)

5. The Cas shell episode: a small scale LBV eruption?

In their review article Humphreys and Davidson (1994) give some examples to illustrate the wide variety of LBV light curves. Of particular interest here are the light curves of the Hubble-Sandage variables, Var C and Var 2, in M33. Their light curves are shown in Figure 3 of Humphreys and Davidson (1994). Var C varied irregularly by several tenths of a magnitude from the early 1920s to about 1937. It then increased by about to a maximum in 1947-48. It resembles, qualitatively, the variation of Cas from 1830 to its maximum in mid 1937, although Cas brightened by perhaps only half as much. Hubble and Sandage (1953) note that at maximum Var C had a spectral type somewhat later than F0, whereas at minimum the star had a colour index characteristic of a much hotter star. A recent eruption of Var C is described by Humphreys et al. (1988). They report that its spectrum at maximum suggested a temperature of about 7500 K. Var 2 was at maximum near 1925 and then declined by about to a minimum in the 1930s; it subsequently varied irregularly by a few tenths of a magnitude. Taken together, the light curves of Var C and Var 2 are strikingly similar to the variation of Cas, although the range from maximum to minimum for the LBVs is larger.

At maximum the LBVs are considerably cooler than at minimum. Presumably this is due to enhanced mass loss producing an optically thick, large shell, which radiates more energy at longer wavelengths. The variations of Cas are qualitatively similar. Either the rate of mass loss increased significantly or some matter was ejected from the star into the line of sight. In either case the resultant continuum energy distribution at optical wavelengths indicated a significantly lower temperature. The presence of He I 3889 implies this matter had a lower density than that of the atmosphere before the outburst. Thus there are sufficient similarities between the shell episode of Cas and the variability of LBVs to suggest that a similar physical process might be responsible in both cases.

Nevertheless, despite these apparent similarities, the shell phase of Cas cannot have arisen by the Stothers-Chin mechanism. Stothers (1996) investigated the dynamical stability of two 15 models, one not rotating and the other rotating uniformly at the Keplerian speed in the equatorial plane. In neither model was there any tendency for the outer envelope to become quasi-detached from the remainder of the star. Therefore, some physical process other than the Stothers-Chin mechanism is required, if the matter responsible for the shell phase of Cas was ejected from Cas itself.

© European Southern Observatory (ESO) 1997

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