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Subsections

3 Analysis and results


3.1 Line profile fitting

Interstellar Caii K and Nai D1 & D2 features were modelled using Gaussian fitting routines in the STARLINK package DIPSO (Howarth et al. 1996) to obtain radial velocity and equivalent width measurements. The results are summarised in Tables 2 and 3, and the observed spectra and corresponding fits are displayed in Figs. 2 and 3. The lower resolution of the red spectra precluded the measurements of reliable equivalent widths for the Nai D1 & D2 features, whilst Caii H is severely blended with stellar Hi and Hei lines; hence, these data are not discussed further. Additionally, due to the resolution of the ISIS spectra, we did not attempt to derive column densities and b-values for the HVC components.

  
\begin{figure}
\includegraphics[height=18cm,width=12cm]{7947.f2.eps}\end{figure} Figure 2: Observed spectra (histograms) and their respective fits (smooth lines) of the Caii K ($\lambda \sim 3933.66$ Å) line for the selected M 15 cluster stars. The stellar component is at $v_{\rm LSR}$ $\sim -100\,{\rm km\,s^{-1}}$, the low- and high-velocity interstellar components are at velocities $\sim 0$ and $ 70 {-} 80 \,{\rm km\,s^{-1}}$ respectively
  
\begin{figure}
\includegraphics[height=8.7cm,width=8.8cm]{7947.f3.eps}\end{figure} Figure 3: Spectra (histograms) and their respective fits (smooth lines) of the Nai D1 and D2 ($\lambda \sim 5891.91 \,
\mbox{and} \, 5889.95$ Å) lines for the selected M 15 cluster stars. The HVC position is indicated by dashed lines

3.2 Cluster membership

The stellar Caii K line was used to determine the radial velocities of the target stars. Table 2 shows that these stars have blue-shifted velocities in the range $v_{\rm LSR} = - 90$ to $- 100 \,{\rm km\,s^{-1}}$ which confirm their cluster membership (see, for example, Dull et al. 1997). We note that K-239, which is much brighter than our other targets, is most probably a PAGB star (see Durrel & Harris 1993), while K-084 exhibits strong stellar Nai D absorption (see Fig. 3) and possesses a spectrum compatible with an early A-type star.

3.3 Identification of HVC absorption

The sightlines show the HVC absorption in Caii K at velocities $v_{\rm HVC}
\sim 60 {-} 80 \,{\rm km\,s^{-1}}$ with equivalent widths that vary from 0.05 to 0.20 Å. However LSR velocities as high as $ \pm 10 \,{\rm km\,s^{-1}}$ for the local gas may be evidence for an incorrect assignment of the wavelength scale. Hence, in Tables 2 and 3 we also quote the difference, $v_{\rm rel}$, between the measured velocity of the HVC and the low velocity interstellar gas. The adoption of either $v_{\rm HVC}$ or $v_{\rm rel}$ in the subsequent analysis does not affect our principal conclusions (see Sect. 4).

The HVC was only detected in Nai D lines for four objects, and (as explained previously) only cloud velocities were reliably measured due to the low resolution of the red arm data. Within the limits of the uncertainties, there is a good agreement between the radial velocities measured for the HVC from Caii and Nai lines (see Tables 2 and 3). Additionally, these radial velocities are in good agreement with values obtained from optical spectra and radio Hi data by Langer et al. (1990) and Kennedy et al. (1998) towards the globular cluster M 15. Their observations were on a larger spatial scale than the present one, but show the same range in velocities.


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