Up: High-velocity interstellar clouds towards
Subsections
The programme stars were chosen from the UBV photometry of
Buonanno et al. (1983)
and
Battistini et al. (1985), using the following
selection criteria:
Table 1:
Sample of stars in the nearby field of M 15 used to search for HVC
absorption
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![\begin{figure}
\vspace*{1mm}
\includegraphics[height=9.5cm,width=6.5cm]{7947.f1.eps}\vspace*{-2mm}\end{figure}](/articles/aas/full/1999/02/ds7947/Timg11.gif) |
Figure 1:
Programme stars observed in the globular cluster M 15.
Stars within the region enclosed by the dotted lines have their
equivalent widths and radial velocities plotted in Fig. 4 |
- Stars should be B-type, to provide suitable continua and relatively
clean spectra with few stellar lines. Hence, assuming a reddening,
(Buonanno et al. 1983) towards M 15, we have selected all objects with
. - Stars should be sufficiently bright to allow high resolution, high
signal-to-noise spectra to be obtained using 4 m class telescopes.
We have imposed a faint magnitude limit V=17. This criterion
excludes the fainter sdO/early sdB stars; hence, our targets mainly
comprise blue horizontal branch (BHB) and Post-Asymptotic Giant Branch (PAGB)
objects.
- Stars should also be members of the M 15 globular cluster. Foreground stars
were eliminated by identifying objects with stellar radial velocities that were
not consistent with the nominal cluster velocity
(Dull et al. 1997).
- Finally, priority was given to stars with separations of less than
to allow studies on small spatial scales.
Table 1 lists the positions and magnitudes of stars that were found to meet
these criteria, and their positions are plotted in Fig. 1. The
star identifications
in Table 1 correspond to those in Table II of
Buonanno et al. (1983)
and Battistini et al. (1985). Observations of K-127
were serendipitously obtained with K-080, and N-1 with K-143 and K-154.
The observations were carried out using the ISIS spectrograph on the
4.2 m William Herschel Telescope at La Palma Observatory on
16/17 August 1997. The blue and red arms were operated with the H2400B lines/mm
grating (Loral
CCD) and R1200R lines/mm grating (Tek
CCD), respectively.
The spectra were centered on 3960 Å for the Caii K and H lines (blue arm)
and 5900 Å for the Nai D lines (red arm).
This instrumental configuration resulted in a
FWHM velocity resolution of
for the Nai data and
for the Caii data,
determined from CuAr arc lines used for wavelength calibration. The slit width was
, with a scale of
per pixel for the blue arm and
per pixel for the red arm.
Multiple exposures were made for each star, interspersed with CuAr arc line
calibration exposures. Bias and flat field exposures were made at the start and the
end of each night.
The raw CCD images were reduced using standard techniques within the
NOAO IRAF package
(Morris & Privett 1995). The spectra were wavelength
calibrated, and cross-correlation techniques were employed for multiple
spectra of the same object to ensure that there was no shift between successive
exposures. Images were then co-added, using weights determined by their
continuum signal-to-noise levels. Typically, co-added spectra have a signal-to-noise
ratio (S/N) of about 50 for the red data, and in the range 7 - 20
for the blue data. The spectra were then shifted to
the Dynamical LSR
(Mihalas & Binney 1981), using corrections obtained from the
STARLINK RV
(Wallace & Clayton 1996) package.
Although the Caii K and H lines are free of telluric absorption features,
the Nai D lines show evidence of contamination. However, a comparison
of a synthetic telluric spectrum with the observational data demonstrated
that only the continuum level was affected, and that the telluric lines would not
affect the measurement of HVC radial velocities. Therefore we did not remove
these telluric lines from the data.
Table 2:
Equivalent width and radial velocity data for both the stellar and
interstellar Caii K lines
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Table 3:
Radial velocity data for the interstellar Nai D lines
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