Interstellar titanium in the galactic halo

We present observations of Ti~II~$lambda 3384$ absorption towards 15 distant stars in the Galactic halo and the Magellanic Clouds. These new data extend existing surveys of the distribution of Ti$^+$ to larger distances from the plane of the Galaxy than sampled previously, allowing the scale height of titanium to be determined for the first time. We find $h_{Ti^+} = 1.5 pm 0.2$~kpc, a value which although greater than those of other tracers of neutral gas, is not as large as had been suspected. We interpret the extended distribution of Ti$^+$ as an indication that its severe depletion in interstellar clouds in the disk is reduced at the lower densities prevailing in the halo. The data are consistent with a simple power-law dependence of the Ti abundance on the ambient density, with exponent $k simeq -1$. If the model is correct, it implies that refractory elements like Ti are fully returned to the gas phase at distances beyond $sim 1$ kpc from the plane of the Galaxy.Comment: Accepted for publication in ApJ, due to appear 1 April 1995. 23 pages including 5 figures; uuencoded compressed postscript; RG0-21