Optical spectroscopy of the M 15 intermediate velocity cloud

Abstract

We present echelle spectrograph observations in the Na D lines, at resolutions of 6.2 8.5 km s 1 , for 11 stars located in the line-of-sight to the M 15 intermediate velocity cloud (IVC), which has a radial velocity of+70 km s 1 in the Local Standard of Rest. This cloud is a part of IVC Complex gp. The targets range in magnitude from mV =1 3:3 14.8. Seven of the observed stars are in the M 15 globular cluster, the remaining four being eld stars. Three of the observed cluster stars are located near a peak in intensity of the IVC H i column density as observed at a resolution of 1 arcmin. Intermediate velocity gas is detected in absorption towards 7 stars, with equivalent widths in Na D2 ranging from0.09 0.20 A, corresponding to log10(NNa cm 2 ) 11.8 12.5, and Na i/H i column density ratios (neglecting the H ii component) ranging from (1 3)10 8 . Over scales ranging from 30 arcsec to 1 arcmin, the Na i column density and the Na i/H i ratio varies by upto 70 per cent and a factor of2, respectively. Combining the current sightlines with previously obtained Na i data from Kennedy et al. (1998b), the Na i/H i column density ratio over cluster sightlines varies by upto a factor of 25, when using H i data of resolution 21 arcmin. One cluster star, M 15 ZNG-1, was also observed in the Ca i (air = 4226:728 A) and Ca ii (air = 3933:663 A) lines. A column density ratio N(Ca i)/N(Ca ii) < 0.03 was found, typical of values seen in the warm ionised interstellar medium. Towards this sightline, the IVC has aN ai/Ca ii column density ratio of 0.25, similar to that observed in the local interstellar medium. Finally, we detect tentative evidence for IV absorption in K i (air = 7698:974 A) towards 3 cluster stars, which have N(K i)/N(H i )r atios of0.5 310 9 .