EDP Sciences
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Volume 387, Number 3, June I 2002
Page(s) 1057 - 1065
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20020408

A&A 387, 1057-1065 (2002)
DOI: 10.1051/0004-6361:20020408

Optical spectroscopy of the M 15 intermediate velocity cloud

J. V. Smoker1, F. P. Keenan1, N. Lehner2 and C. Trundle1, 3

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

(Received 29 January 2002 / Accepted 14 March 2002 )

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=13.3-14.8. 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 D 2 ranging from ~0.09-0.20 Å, corresponding to log 10( $N_{\rm Na}$ cm -2) ~11.8-12.5, and Na I/H I column density ratios (neglecting the H II component) ranging from ~(1-3) $\times$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. (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 ~2 $\times$1 arcmin. One cluster star, M 15 ZNG-1, was also observed in the Ca I ( $\lambda_{\rm air}=4226.728$ Å) and Ca II ( $\lambda_{\rm air}=3933.663$ 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 ( $\lambda_{\rm air}=7698.974$ Å) towards 3 cluster stars, which have N(K I)/ N(H I) ratios of ~0.5-3 $\times$10 -9.

Key words: ISM: clouds -- ISM: abundances -- ISM: individual objects: M 15 IVC -- ISM: individual objects: complex gp

Offprint request: J. V. Smoker, jsmoker@eso.org

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