A limit on the metallicity of compact high-velocity clouds

There is a fortuitous coincidence in the positions of the quasar Ton S210 and the compact H I high-velocity cloud CHVC 224.0-83.4-197 on the sky. Using Far Ultraviolet Spectroscopic Explorer observations of the metal line absorption in this cloud and sensitive H I 21 cm emission observations obtained with the multibeam system at Parkes Observatory, we determine a metallicity of (O/H)<0.46 solar at a confidence of 3 sigma. The metallicity of the high-velocity gas is consistent with either an extragalactic or Magellanic Cloud origin but is not consistent with a location inside the Milky Way unless the chemical history of the gas is considerably different from that of the interstellar medium in the Galactic disk and halo. Combined with measurements of highly ionized species (C III and O VI) at high velocities, this metallicity limit indicates that the cloud has a substantial halo of ionized gas; there is as much ionized gas as neutral gas directly along the Ton S210 sight line. We suggest several observational tests that would improve the metallicity determination substantially and help to distinguish between possible origins for the high-velocity gas. Additional observations of this sight line would be valuable, since the number of compact HVCs positioned in front of background sources bright enough for high-resolution absorption-line studies is extremely limited.