Spectroscopy of giant stars in the Pyxis globular cluster (vol 112, pg 1305, 2000)

The Pyxis globular cluster is a recently discovered globular cluster that l ies in the outer halo (R-gc similar to 40 kpc) of the Milky Way. Pyxis lies along one of the proposed orbital planes of the Large Magellanic Cloud (LM C), and it has been proposed to be a detached LMC globular cluster captured by the Milky Way. We present the first measurement of the radial velocity of the Pyxis globular cluster based on spectra of six Pyxis giant stars. Th e mean heliocentric radial velocity is similar to 36 km s(-1), and the corr esponding velocity of Pyxis with respect to a stationary observer at the po sition of the Sun is similar to -191 km s(-1). This radial velocity is a la rge enough fraction of the cluster's expected total space velocity-assuming that it is bound to the Milky Way-that it allows strict limits to be place d on the range of permissible transverse velocities that Pyxis could have i n the case that it still shares or nearly shares on orbital pole with the L MC. We can rule out that Pyxis is on a near circular orbit if it is Magella nic debris, but we cannot rule out an eccentric orbit associated with the L MC. We have calculated the range of allowed proper motions for the Pyxis gl obular cluster that result in the cluster having an orbital pole within 15 degrees of the present orbital pole of the LMC and that are consistent with our measured radial velocity, but verification of the tidal capture hypoth esis must await proper motion measurement from the Space Interferometry Mis sion or Hubble Space Telescope. A spectroscopic metallicity estimate of [Fe /H] = -1.4 +/- 0.1 is determined for Pyxis from several spectra of its brig htest giant; this is consistent with photometric determinations of the clus ter metallicity from isochrone fitting.