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Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| The distance to Complex M and the Intermediate Velocity Arch
The Intermediate Velocity Arch is an association of Hi clouds lyingwithin the broad limits l~110 deg-210 deg,b~50 deg-80 deg. We usehigh-resolution optical, and single-dish 21-cm, observations of 10early-type halo star sightlines in this region to establish distancelimits (0.4<=z<=3.5kpc) for seven IVCs that are part of the Arch.We re-examine a previous determination of upper and lower distancelimits on the IVC Complex M using the closely aligned halo starsBD+382182 and HD93521. Our Hi spectra show no evidence of emission fromComplex M, leading us to conclude that, while an upper distance limit ofz<=3.5 kpc may be established, no useful lower distance determinationcan be made. Our new Hi observations allow new limits to be establishedon the metallicity of Complex M, and show C, O and Si variations withrespect to solar abundances in the range +/-0.3 dex.
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