A Dramatic Change in the H-alpha Emission and Continuum Flux of S Doradus

ATel #2560; Noel D. Richardson (Georgia State University), Douglas R. Gies (Georgia State University)
on 14 Apr 2010; 15:30 UT
Credential Certification: Douglas R. Gies (gies@chara.gsu.edu)

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S Dor is a prototypical Luminous Blue Variable and is one of the brightest objects in the Large Magellanic Cloud. The star's photometric history over the last century is described by van Genderen et al. (1997) in terms of a very-long-term and a quasi-periodic (7 year) variation. The faintest V-magnitude (V=11.3) was observed in 1965, while the brightest V-magnitude (V=8.8) was observed in 2006 by the All Sky Automated Survey (Pojmanski 2002). The star is currently fading in brightness.

We are obtaining spectroscopic observations of S Dor with the CTIO 1.5 m telescope and RC spectrograph (resolving power of 2000 around H-alpha). We began an H-alpha spectroscopic survey of this and other bright LBVs two years ago, and we generally obtain a spectrum every 2-3 weeks. We now have collected a total of 28 spectra of S Dor. The data reduction process is described by Richardson et al. (2010).

The H-alpha emission line equivalent width of S Doradus exhibited only a modest increase over the first 1.5 years, but during the last eight months the emission has tripled in strength. The fast increase of the H-alpha emission was preceded by a fading in the V-band flux. The ASAS photometry data show that the star is the now fainter than at any time over the last nine years. Current AAVSO CCD observations indicate that the magnitude has reached V=9.83 (2010 Mar 08). Temporal plots of the contemporary H-alpha equivalent width and V-magnitude are available on-line at http://www.chara.gsu.edu/~richardson/SDor/

The increase in emission line strength during fading episodes has occurred before (Thackeray 1965; Wolf & Kaufer 1997) and may be due to the emergence of UV continuum light, as occurs during minima of the Galactic analog AG Car (Viotti et al. 1984). The current event offers us the first opportunity in over a decade to explore this unusual state of S Dor. We encourage both spectroscopic and multi-color photometric observations to document the line and continuum variations.

References:
Pojmanski,G. 2002, Acta Astron., 52, 397
Richardson, N. D., Gies, D. R., Henry, T. J., Fernandez-Lajus, E., Okazaki, A. T. 2010, AJ, 139, 1534
Thackeray, A. D. 1965, MNRAS, 129, 169
van Genderen, A. M., Sterken, C., de Groot, M. 1997, A&A, 318, 81
Wolf, B., & Kaufer, A. 1997, in Luminous Blue Variables, ed. K. Davidson, A. F. J. Moffat, & H. J. G. L. M. Lamers (Dordrecht: Kluwer), 91
Viotti, R., Altamore, A., Barylak, M., Cassatella, A., Gilmozzi, R., & Rossi, C. 1984, in Future of Ultraviolet Astronomy based on Six Years of IUE Research (NASA CP-2349), ed. J. M. Mead, R. D. Chapman, & Y. Kondo (Washington, D.C: NASA), 231