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.