Up: High-velocity interstellar clouds towards
Our primary motivation in obtaining these spectra was to study the HVC towards
M 15 on a small angular scale, and identify suitable stars for further studies at high
spectral resolution. In Fig. 4 we have plotted the HVC equivalent
widths and velocities
versus the galactic latitude b for a sample of eight stars in a
strip (i.e. the stars
between the dotted lines in Fig. 1). For the relative
velocities
, we have a taken an uncertainty of
. Since all
the measured velocities do not have uncertainties greater than
, and this value
corresponds to half a pixel, it seems quite reasonable to assign this
uncertainty to
. On this small scale, the equivalent widths
vary randomly from 0.05 to 0.20 Å, and the velocities from
65 to
, with mean values of
Å and
.
An inspection of Fig. 4 reveals that any variations in these quantities over
small scales are probably on the order of or less than the measurement errors.
Higher quality data are needed to quantify the magnitude of these variations. In
particular, closely spaced pairings of sight lines such as the
K-064, K-084, K-143 combination (identified in Fig. 4) provide the
possibility of measuring HVC absorption over scales of
.These data could then be combined with high spatial resolution Hi
maps to provide reliable estimates of Hi column density at similar
locations. In this work, we have shown that absorption is detectable across the
face of the cluster and that many of these sight lines are suitable for such
studies.
Such observations will also allow us to investigate the fractal structure
of the HVC on scale-sizes of a few arcseconds.
According to
Vogelaar & Wakker (1994) all structural details
will disappear if the resolution is not sufficient to resolve individual
cloudlets.
![\begin{figure}
\vspace*{3mm}
\includegraphics[height=11cm,width=7.2cm]{7947.f4.eps}\end{figure}](/articles/aas/full/1999/02/ds7947/Timg45.gif) |
Figure 4:
Caii equivalent widths and velocities of the HVC as a function of
galactic latitude b for the subset of stars indicated in Fig. 1 |
Also the cloudlets within HVCs have been suggested as sites for star formation
in the halo via cloudlet-cloudlet collisions
(Dyson & Hartquist 1983). This
mechanism has been invoked to explain the existence of several early-type stars
far from the plane of the Galaxy, whose kinematics and z-distances are
incompatible with disc ejection
(Keenan 1992). However these computations are
extremely model-dependent (see, for example,
Christodoulou et al. 1997) and
require as input detailed information on the sizes, number density and physical
conditions of HVC cloudlets, which are currently very uncertain
(Wakker & van Woerden 1997), and that our future observations should provide.
Up: High-velocity interstellar clouds towards
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