UPS SGR (Upsilon Sagittarii). What a strange star is seemingly obscure Upsilon Sagittarii. Well to the northeast of the Little Milk Dipper that makes up much of classical Sagittarius (in fact the most northerly Sagittarian stary with a Greek letter) and near the western edge of the Milky Way, fifth magnitude (4.61, but changing by perhaps a tenth) Ups Sgr hardly seems to belong to the constellation at all. Yet for all its hidden nature, it reigns as number one of its odd type as the brightest of the rare "extremely hydrogen deficient binaries" (known in the trade as "HdB" stars). It's classed all over the place, often as a binary with a composite spectrum: as a peculiar B2 dwarf plus an A2 supergiant shell star (implying a surrounding disk); as F2 plus B8; as a B0 supergiant; and most recently as a class A supergiant, the class actually varying with time. Clearly it can't be pinned down, in part because of the star's unusual chemical composition, in part because of the confusing disk. Doppler motions in the spectrum show that Ups Sgr is indeed a double in a 137.9-day orbit with perhaps a 10 percent eccentricity. The companion, however, is apparently invisible: the weird dual spectra are both coming from the one visible star, Ups Sgr itself! Ignoring the companion, adopting a temperature from the literature of 12,600 Kelvin, assuming no dimming by interstellar dust (which would be surprising, given the star's distance of 1800 light years give or take 225), we get a luminosity of 7200 times that of the Sun, a radius of 18 solar, and a mass of 8 Suns. Adopting a magnitude of interstellar absorption would raise the luminosity to 18,000 Suns, the radius to 28 solar, and the mass to 10 times that of the Sun. The system seems to be surrounded by an envelope or disk, which gives rise to a false stellar "surface" and contributes to the spectral uncertainty and variation. The visible component is estimated to be 100 times brighter (in the visual spectral realm) than the invisible companion (which can be seen in the deep ultraviolet). Analysis of the shifting spectral absorptions suggest, however, that of the two the invisible star is 60 percent the more massive! Constraining the orbital tilt at 50 degrees to the plane of the sky with observations of the disk gives a mean separation of 1.25 Astronomical Units along with a mass of 5.45 Suns for the visible star and 8.6 for the invisible one, but the mass-sum could be much higher or lower (up to 50 Suns for a 30- degree orbit), suggesting that the crude analysis above may not be far off the mark. Ups Sgr proper is VERY deficient in hydrogen and similarly rich in helium, but is also enriched in carbon and nitrogen. The star seems to be a supergiant stripped of much of its outer envelope. The idea is that it is tidally distorted by its more massive but largely unseen mate. Filling its teardrop- shaped "Roche lobe" (the zero-gravity surface caused by the combined gravities of the pair), it is now transferring what is left of its outer layers to the companion, the gas flow severely messing up the spectrum. In that sense it is similar perhaps to Beta Lyrae. What will happen to the system is not clear, but an eventual supernova explosion would not be out of line. (This story is based largely on work by P. Koubsky et al., Astronomy and Astrophysics, vol. 459, p. 849, 2006 and by M. Netolicky et al., Astronomy and Astrophysics, vol. 499, p. 827, 2009.)
Written by Jim Kaler 9/23/11. Return to STARS.