R.D. Davies, PASA, 14 (1), 117.
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Interstellar Medium Physics
The Queen's University Belfast-Jodrell Bank optical-IR programme addresses questions about physical conditions in the ISM. HI emission data are taken alonglines of sight to stars whose absorption spectra are of particular interest, for example lying in the direction of intermediate velocity clouds (IVCs) or high velocity clouds (HVCs). The -71 km 
IVC/HVC lying in front of the globular cluster M13 has been investigated in this way using Echelle spectra from the NaI D line and Ca II K line observations of stars in M13 along with 12' resolution HI spectra taken with the Lovell Telescope (Shaw et al. 1996). N(Na I)/N(HI) ratios in the clouds lying in front of M13 lie in the range 
, while the N(CaII)/N(HI) ratios are 
. Temperatures in the compact clouds are a few hundred Kelvin while the warmer, more extensive, gas is at 
K. Ionization conditions can be determined for the clouds.
Similar investigations have been made of the conditions in the large gas concentration (1300 
), in the direction of Perseus at 
, 
(Trapero et al. 1995). HI temperatures in this cloud reach as low as 30K. Systematic velocity changes across its surface of several km are found. A distance for the cloud of 
pc was determined from the presence or not of corresponding absorption features in the spectra of stars lying this direction.
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