COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3116 Konkoly Observatory Budapest 3 December 1987 HU ISSN 0374-0676 RAPID VARIATION IN H-ALPHA EMISSION OF GAMMA CASSIOPEIAE The well known Be star Gamma Cassiopeiae has been an object of intense study through over a century. Apart from the major phases namely, Be, Be shell, and normal B star with time scales of decades, the star also shows rapid changes in the visible spectrum. The time scales of these variations may be of the order of months, days and even a few minutes. Slettebak and Snow (1978) found simultaneous emission events in the H-alpha line and ultraviolet Si IV and Mg II lines lasting for a duration of about 100 minutes. Rapid profile variations on a time scale of a few minutes have been detected for H-beta by Hutchings (1976) and Doazan (1976). These changes are believed to be erratic. Reynolds and Slettebak (1980) have found definite evidence of the change in emission intensity on the time scale of months for four Be stars, though night to night changes, and changes on an individual night are less certain. In this communication, we report the scanner observations taken at the Uttar Pradesh State Observatory, on four nights in October-November 1986. Appreciable changes in H-alpha line are seen. The observations were secured at the Cassegrain focus of the 104-cm reflector through a spectrum scanner and standard d.c. recording technique. The dispersion at the focal plane of the spectrophotometer is 70 A/mm. An entrance aperture of 2 mm (equivalent to 30 arc sec) and an exit slit of 0.4 mm have been used. The tracings were read out near the H-alpha line at 2 mm intervals on the chart paper, which corresponds to nearly 13 A in wavelength scale. A free hand continuum was drawn on both sides of the H-alpha line, and the emission intensity in terms of continuum Ilambda/Ic was read out at points covering the entire line. Figure 1 shows the Ilambda/Ic versus lambda plot. In this plot the position of the peak on the red side of the line is designated as origin of wavelength scale. Figure 1 clearly shows the sequence of changes in the shape of the H-alpha line during the period of these observations. [FIGURE 1] Figure 1. Spectral scans of H-alpha line on four nights On October 30, the violet component of the line is the stronger one. On November 5 and 6, the red component has become slightly stronger compared to the violet. On November 10, however, the violet component has become very faint with respect to the red one. In fact in one scan on November 10, at 17:33 UT, the violet component is entirely absent. There seems to be an extended underlying absorption instead. The reality of this absorption feature may, however, be uncertain. In order to estimate the changes in the total, emission intensity of the H-alpha line, we have planimetered the area under the emission curves of each scan. Table I shows these values along with the equivalent widths of each scan. It is apparent from Table I that the emission intensity was maximum at 14.2A on 30 October 1986. The same seems to have increased to an average of about 11.4 A on 10 November 1986. Table I. Measured intensities of the H-alpha line Date UT Area under the curve Equivalent (cmE2) width (lambda) 1986 October 30 0.742 35.6 14.2 November 5 0.647 30.1 12.0 November 5 0.653 26.8 10.7 November 6 0.633 23.2 9.3 November 6 0.653 20.0 8.0 November 10 0.724 31.5 12.6 November 10 0.731 28.6 11.4 November 10 0.740 25.2 10.1 The hydrogen emission mechanism in Be stars is the ionization of hydrogen atom both from the ground, and excited states, followed by recombination to the excited states and then cascading to lower excited states. The amount of recombination radiation in a particular line depends on the hydrogen particle density (Ni, Ne) and the emitting volume. Fresh ejection of material is required to increase the emitting volume having the appropriate particle density for the rapid changes observed in the shape of the H-alpha line and its total strength. This ejection of stellar matter could occur from, random points along the equatorial regions of the rotating stellar disc, causing changes in the particle density as well as the emitting volume density. Due to this matter taking part in the rotation also, the region of the increased particle density would present differently to the line of sight, causing the changes in the shape of the line. The particle density in the emitting regions of the Be stars are believed to be 10E10-10E12 cmE-3 (Hirata and Kogure, 1984). An increase in the density to twice the mean value would mean addition of some 10E11cmE-3 particles, or an addition of some 10E13 gram of hydrogen. Changes of this magnitude have not been reported earlier for gamma Cas. This situation may therefore represent a particular phase of activity of the star. We acknowledge with thanks the useful discussions held with Dr. B.S. Rautela. S.C. JOSHI, R.K. SRIVASTAVA and J.B. SRIVASTAVA Uttar Pradesh State Observatory, Manora Peak, Naini Tal, 263 129 INDIA. References: Doazan, V., 1976, in Proc. IAU Symp. 70, Be and Shell Stars ed. A. Slettebak, D. Reidel Publ. Co. Dordrecht, Holland, p. 37. [BIBCODE 1976IAUS...70...37D ] Hirata, R. and Kogure, T., 1984, Bull. Astron. Soc. India, 12, No. 2, 109 [BIBCODE 1984BASI...12..109H ] Hutchings, J.B., 1976, in Proc. IAU Symp. 70, Be and Shell Stars ed. A. Slettebak, D. Reidel Publ. Co., Dordrecht, Holland, p. 13. [BIBCODE 1976IAUS...70...13H ] Reynolds, R.C. and Slettebak, A., 1980, Publ. Astron. Soc. Pacific 92, 472 [BIBCODE 1980PASP...92..472R ] Slettebak, A. and Snow, T.P. 1985, Astrophys J. Lett. 224, L127 [BIBCODE 1978ApJ...224L.127S ]