Optical spectroscopy of the M 15 intermediate velocity cloud ^*

¹ Astrophysics and Planetary Science Division, Department of Pure and Applied Physics, The Queen's University of Belfast, University Road, Belfast, BT7 1NN, UK
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³ Isaac Newton Group, Apartado de Correos 368, Santa Cruz de La Palma, Tenerife 38780, Canary Islands, Spain

Corresponding author: J. V. Smoker, jsmoker@eso.org

Received: 29 January 2002
Accepted: 14 March 2002

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 14.8. Seven of the observed stars are in the M 15 globular cluster, the remaining four being field 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 from ~0.09-0.20 Å, corresponding to log₁₀(N_Na 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 of ~2, respectively. Combining the current sightlines with previously obtained Na i data from Kennedy et al. ([CITE]b), 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 ( Å) and Ca ii ( Å) 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 a Na i/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 ( Å) towards 3 cluster stars, which have N(K i)/N(H i) ratios of ~0.5-310^-9.