Up: High-velocity interstellar clouds towards
Subsections
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}](/articles/aas/full/1999/02/ds7947/Timg29.gif) |
Figure 2:
Observed spectra (histograms) and their respective fits (smooth lines)
of the Caii K ( Å) line
for the selected M 15 cluster stars. The stellar component is at
, the low-
and high-velocity interstellar components are at velocities and
respectively |
![\begin{figure}
\includegraphics[height=8.7cm,width=8.8cm]{7947.f3.eps}\end{figure}](/articles/aas/full/1999/02/ds7947/Timg31.gif) |
Figure 3:
Spectra (histograms) and their respective fits (smooth lines) of
the Nai D1 and D2 ( Å) lines for the selected
M 15 cluster stars. The HVC position is indicated by dashed lines |
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
to
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.
The sightlines show the HVC absorption in Caii K at velocities
with equivalent widths that vary from 0.05 to 0.20 Å.
However LSR velocities as high as
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,
,
between the measured velocity of the HVC and the low velocity interstellar gas.
The adoption of either
or
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.
Up: High-velocity interstellar clouds towards
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