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+2.3-1.3 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 mÅ] 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 Msolar <4.9×105 Msolar. 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.