M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption

• Published in: The Astronomical journal 2006-04, Vol.131 (4), p.2105-2113
• Main Authors: Humphreys, Roberta M, Jones, Terry J, Polomski, Elisha, Koppelman, Michael, Helton, Andrew, McQuinn, Kristen, Gehrz, Robert D, Woodward, C. E, Wagner, R. Mark, Gordon, Karl, Hinz, Joannah, Willner, S. P
• Format: Article
• Language: English
• ISSN: 1538-3881; 0004-6256; 1538-3881
• DOI: 10.1086/500811

Variable A in M33 is a member of a rare class of highly luminous, evolved stars near the upper luminosity boundary that show sudden and dramatic shifts in apparent temperature due to the formation of optically thick winds in high mass loss episodes. Recent optical and infrared spectroscopy and imaging reveal that its 'eruption,' begun in ~1950, has ended, having lasted 45 yr. Our current observations show major changes in its wind from a cool, dense envelope to a much warmer state surrounded by low-density gas with rare emission lines of Ca II, [Ca II], and K I. Its spectral energy distribution has unexpectedly changed, especially at the long wavelengths, with a significant decrease in its apparent flux, while the star remains optically obscured. We conclude that much of its radiation is now escaping out of our line of sight. We attribute this to the changing structure and distribution of its circumstellar ejecta, corresponding to the altered state of its wind as the star recovers from a high mass loss event.