PSI VEL (Psi Velorum). Almost lost among the vast number of stars
of huge Argo (formally, Argo Navis, the
Ship of the Argonauts), bright-fourth magnitude (3.60) Psi Velorum
recommends itself three ways. First, it's the most northerly star
of the classic outline of Vela the Sails
(the other parts of Argo being Puppis, the
Stern, and Carina, the Keel), lying just
barely to the south of the dim modern constellation Antlia (the Air Pump). At a distance of only 61 light
years, it's also one of the closest of Vela's (for that matter
Argo's) classic stars. It's also a very close orbiting double that consists of a pair
of warm class F stars (traditionally given as F3 and F0 subgiants,
sometimes as F2 subgiant plus F0 hydrogen-fusing dwarf, but see
below). As seen from Earth separated by under a second of arc, the
two go around each other every 33.95 years at an average physical
separation of 16.0 Astronomical Units, a fairly high eccentricity
taking them between 9 and 23 AU apart. They were last closest
together in 2003, and will be again in 2037. The orbital
parameters entered into Kepler's Laws yield a combined mass 3.6
times that of the Sun. Psi Vel A and B have respective magnitudes
of 4.1 and 4.6, the latter varying between 4.5 and 5.1 (with no
known period), the brighter F3 star having a stated temperature of
7130 Kelvin, the fainter F0, but hotter, component a probable
temperature of 7400 Kelvin. Respective luminosities then come in
at 6 and 3.8 solar, radii at 1.6 and 1.2 solar, and masses both
close to 1.5 times that of the Sun. Though
classified as subgiants (stars that have given up core hydrogen
fusion or are very close to it), both are clearly dwarfs, and in
fact not that old. With a combined mass of 3.0 solar, there is
some discrepancy with the orbital results, implying some orbital
uncertainty. Spinning with a fast projected equatorial velocity of
156 kilometers per second, Psi Vel A rotates in under half a day!
But now things get a bit confusing, as one source suggests that Psi
Vel A is actually the warmer F0 star and the Psi Vel B the cooler
F3 star, which actually makes a bit more sense from an evolutionary
point of view. No matter, however, as the masses come out about
the same, the small discrepancy with the orbital solution not
resolved.
Written by Jim Kaler 5/02/08. Return to STARS.