Rotational and Cyclical Variability in gamma Cassiopeiae. II. Fifteen Seasons

TL;DR

This study analyzes 15 seasons of photometric data of gamma Cassiopeiae, revealing persistent cyclical and rotational variability, a mass-loss outburst, and no detectable binary period variation, advancing understanding of Be star disk dynamics.

Contribution

It provides long-term observational evidence of variability patterns, disk instability effects, and magnetic modulation in gamma Cas, with detailed analysis of cycle behavior and outburst events.

Findings

Long-term cycles of 70 days with amplitude and period variations.

Mass-loss outburst in 2010 affected cycle amplitude.

No detectable photometric signature of the 203.55-day binary period.

Abstract

The B0.5IVe star gam Cas is of great interest because it is the prototype of a small group of classical Be stars having hard X-ray emission of unknown origin. We discuss results from ongoing B and V observations of the gam Cas star-disk system acquired with an APT during the observing seasons 1997-2011. In an earlier study, Smith, Henry, & Vishniac showed that light variations in gam Cas are dominated by a series of comparatively prominent cycles with amplitudes of 0.02-0.03 mag and lengths of 2-3 months, superimposed on a 1.21-day periodic signal some five times smaller, which they attributed to rotation. The cycle lengths clustered around 70 days. Changes in both cycle length and amplitude were observed from year to year. These authors also found the V-band cycles to be 30-40% larger than the B-band cycles. In the present study we find continued evidence for these variability patterns and for the bimodal distribution of the B/V amplitude ratios in the long cycles. During the 2010 observing season, gam Cas underwent a mass loss event (outburst), as evidenced by the brightening and reddening seen in our new photometry. This episode coincided with a waning of the amplitude in the ongoing cycle. The Be outburst ended the following year, and the light-curve amplitude returned to pre-outburst levels. This behavior reinforces the interpretation that cycles arise from a global disk instability. Remarkably, we also find that both the amplitude and the asymmetry of the rotational waveform changed over the years. We review arguments for this modulation arising from transits of a surface magnetic disturbance. Finally, to a limit of 5 mmag, we find no evidence for any photometric variation corresponding to the gam Cas binary period, 203.55 days, or to the first few harmonics.