The Galactic Nature of High-Velocity Cloud Complex WB

Abstract

We have detected absorption lines from the high-velocity cloud (HVC) complex WB in the spectrum of the star HE 1048+0231. This detection sets an upper limit on the distance to the cloud of 8.8 kpc. Nondetection (at greater than 3 σ confidence) in the star HE 1138-1303 at 7.7 ± 0.2 kpc sets a probable lower limit. The equivalent width of the Ca II K line due to the HVC [Wλ(Ca II K) = 114.6 ± 4.4 mA] corresponds to a column density of N(Ca II) = (1.32 ± 0.05) × 1012 cm-2. Using an H I spectrum from the Leiden/Argentine/Bonn survey, we calculate N(Ca II)/N(H I) = (81 ± 16) × 10-9. These distance limits imply an H I mass limit of 3.8 × 105 M☉ < MH I < 4.9 × 105 M☉. The upper distance limit imposed by these observations shows that this HVC complex has a probable Galactic or circumgalactic origin. Future metallicity measurements will be able to confirm or refute this interpretation.