OH/IR stars are oxygen-rich, cool giants that lose
matter at the end of their evolution, in the so-called asymptotic
giant branch (AGB) superwind phase (Renzini 1981).
The rate at which they lose mass is high
(
), but the expansion velocity is relatively
low (10 to 30 
). The outflow appears in the form of
a circumstellar envelope (CSE) with a chemical composition that varies
with radial distance from the star.
The composition
is determined by, for instance,
temperature and ambient UV radiation (see Olofsson 1994).
The dust in the outflow absorbs
the stellar radiation and reemits in the infrared; the spectrum
typically extends from 4 to 40 
with a peak at 10 to 20 
.
This radiation pumps an OH maser (Elitzur et al. 1976)
that forms in a thin shell, on
the inside of which H
O molecules are dissociated into OH and H,
on the outside OH into O and H.
Various OH lines show maser emission, but we are interested in the
strongest, at 1612 MHz, that has an easily recognisable, double-peaked
line profile.
OH/IR stars represent a wide range of stellar masses;
almost all low and intermediate mass (1 to 6 
) stars enter this phase
at the end of their life.
Little is known about the duration of the AGB superwind phase,
but it is thought to depend upon main-sequence mass, and
present estimates indicate 
yr
(Whitelock & Feast 1993; Vassiliadis & Wood 1993).
Since this is only a short time compared to the total lifetime of
the star, the objects are relatively rare.
In addition to AGB stars, stellar OH 1612 MHz maser emission is also
detected
from a small number of more massive red supergiant stars (Cohen 1989).
We refer to Habing (1996) for
an extensive review of the properties of OH/IR stars.
Since their discovery in 1968 (Wilson & Barett), OH/IR stars
have become favourite objects for studying very different
processes, including, amongst others, stellar evolution and
the dynamical behaviour of our Galaxy (Habing 1993).
The OH/IR stars (and related objects such
as Miras and protoplanetary nebulae (PPNe) (Kwok 1993 and references
therein))
are ideal tracers of the galactic potential for a number of reasons.
Firstly, the 1612 MHz line (
18 cm) is not influenced by interstellar
extinction, which might otherwise cause a bias in the
observed surface density in
certain directions because of different optical depths.
Secondly, the two narrow peaks of the spectrum yield a
very accurate stellar velocity, which
is a necessary piece of knowledge in the hunt for the potential.
Thirdly, the OH/IR stars have progenitors with
a wide range of main-sequence masses
and therefore they have a wide range of ages ( 
1 to 8 Gyr), while
they are all in the same, late, stage of stellar evolution.
Such a sample is therefore relatively
dynamically relaxed and homogeneous and representative of the stellar
content of the Galaxy.
Finally, the emission resulting from a maser
causes the 1612 MHz line of OH/IR stars to be
strong and this enables us to acquire a statistically meaningful sample in
a practically meaningful timespan.
In this article we discuss observations (Sect. 2) and reduction
(Sect. 3 and Appendix A) of a sample of OH/IR stars
(and related objects) in the inner Galaxy (Sect. 4), between
and 
.
We will address this region throughout this article, slightly
megalomaniacally, as the Bulge region.
The observations were part of a larger survey with the Australia Telescope
Compact Array (ATCA) and the Very Large Array (VLA) of the region between
and 
, the
complete results of which will be presented in due course.
A statistical analysis of the sample, partly through comparison
with relevant existing data, is presented (Sect. 5).
Morphology, astrophysics, kinematics and the
dynamical distribution of the
sample will be discussed in subsequent articles.