An Unprecedented Change in the Spectrum of S Doradus: As Cool as It Gets

Abstract

ABSTRACTRecent observations of S Doradus, the archetype of luminous blue variables (LBVs), reveal that the star’s optical spectrum now resembles an F‐type supergiant, with a rich complex of absorption lines. Despite nearly 50 years of spectroscopic monitoring, such a spectrum has never before been seen for S Dor despite numerous occasions when the star was equally bright. However, such F‐type spectra have been seen in other LBVs, including Var B in M33 during a recent outburst, and in η Carina during an outburst in 1893. The singly ionized metal lines arise in a layer moving away from the star (toward us) at 50 km s−1, consistent with the lines forming in a “pseudophotosphere” originating in the stellar wind. The temperature suggested by the F‐type spectrum is as cool as an LBV can get. Our CCD UBV photometry shows that the star has brightened only 0.3 mag in V since 1996, at which time it showed an emission‐line spectrum characteristic of LBVs at minimum. The dereddened B−V color is consistent with an effective temperature suggested by the spectrum (7500 K), but there is a pronounced UV excess as evidenced by U−B. Why the behavior of S Dor has suddenly changed remains a mystery.