2002A&A...387.1057S

We present echelle spectrograph observations in the NaD lines, at resolutions of 6.2-8.5km/s, for 11 stars located in the line-of-sight to the M15 intermediate velocity cloud (IVC), which has a radial velocity of ~+70km/s in the Local Standard of Rest. This cloud is a part of IVC Complex gp. The targets range in magnitude from m_V=13.3-14.8. Seven of the observed stars are in the M15 globular cluster, the remaining four being field stars. Three of the observed cluster stars are located near a peak in intensity of the IVC HI column density as observed at a resolution of ∼1 arcmin. Intermediate velocity gas is detected in absorption towards 7 stars, with equivalent widths in NaD₂ ranging from ∼0.09-0.20Å, corresponding to log₁₀(N_Na cm^–2)∼11.8-12.5, and NaI/HI column density ratios (neglecting the HII component) ranging from ~(1-3)x10^–8. Over scales ranging from 30 arcsec to 1 arcmin, the Nai column density and the NaI/HI ratio varies by upto 70 per cent and a factor of ∼2, respectively. Combining the current sightlines with previously obtained NaI data from Kennedy et al. (1998MNRAS.297..849K), the NaI/HI column density ratio over cluster sightlines varies by upto a factor of ∼25, when using HI data of resolution ∼2x1arcmin. One cluster star, M15 ZNG-1, was also observed in the CaI (λ_air=4226.728Å) and CaII (λ_air=3933.663Å) lines. A column density ratio N(CaI)/N(CaII)<0.03 was found, typical of values seen in the warm ionised interstellar medium. Towards this sightline, the IVC has a NaI/CaII column density ratio of ∼0.25, similar to that observed in the local interstellar medium. Finally, we detect tentative evidence for IV absorption in KI (λ_air=7698.974Å) towards 3 cluster stars, which have N(KI)/N(HI) ratios of ∼0.5-3x10^–9.