90 research outputs found
First analysis of an OH survey of inner Galaxy: evidence for a stellar Bar
Part of a large survey of the inner galactic Plane ( and total) in the OH 1612MHz line in search for OH/IR
stars is analysed. We find strong evidence for a central m=2 distortion based
on geometrical considerations. The observed deviation from axisymmetry cannot
be explained by lopsidedness and agrees with other recent models of the
galactic Bar on length, inclination and axis ratio.Comment: 4 pages, 140 kB postscript, to appear in proceedings of IAU
colloquium 157 "Barred Galaxies
Something about the structure of the Galaxy
We analyse a sample of 507 evolved stars in the inner galactic Plane. We
derive average ages for subsets of this sample and use those sets as beacons
for the evolution of the Galaxy. In the Bulge the oldest OH/IR stars in the
plane are 7.5 Gyr, in the Disk 2.7 Gyr. The vertical distribution of almost all
AGB stars in the Disk is found to be nearly exponential, with scaleheight
increasing from 100 pc for stars of \lsim 1 Gyr to 500 pc for stars of \gsim 5
Gyr. There may be a small, disjunct population of OH/IR stars. The radial
distribution of AGB stars is dictated by the metallicity gradient. Unequivocal
morphological evidence is presented for the existence of a central Bar, but
parameters can be constrained only for a given spatial-density model. Using a
variety of indicators, we identify the radii of the inner ultra-harmonic (2.5
kpc) and corotation resonance (3.5 kpc). We show that the 3-kpc arm is likely
to be an inner ring, as observed in other barred galaxies, by identifying a
group of evolved stars that is connected to the 3-kpc HI filament. Also, using
several observed features, we argue that an inner-Lindblad resonance exists, at
1-1.5 kpc. The compositions of OH/IR populations within 1 kpc from the
galactic Centre give insight into the bar-driven evolution of the inner
regions. We suggest that the Bar is 8 Gyr old, relatively weak (SAB) and
may be in a final stage of its existence.Comment: 18 pages, 15 figures, TeX, accepted for publication in MNRA
A shock-excited OH maser in a post-AGB envelope ?
We have observed a sample of OH 1612-MHz masing objects in all four OH
ground--state transitions with the Australia Telescope Compact Array. One
likely post-AGB object is found to emit in the 1612-MHz, 1665-MHz and 1720-MHz
transitions. We discuss the evidence that this object may be an early post-AGB
object and the possibility for such a circumstellar envelope to harbour a
1720-MHz maser. We argue that during a very brief period, just after the star
has left the thermally-pulsing phase of the AGB and the wind velocity starts to
increase, post-AGB objects might show \sath emission. The best objects to
search for such emission would be those that are masing at 1612 MHz and 1665
MHz, but not at 1667 MHz nor in the 22-GHz H2O transition.Comment: 4 pages, 2 figures, LaTeX, accepted for publication in ApJ Letter
The distribution of maser stars in the inner Milky Way: the effect of a weak, rotating bar
We derive the distribution of maser stars in the inner Milky Way (MW) based on an analysis of lV-diagrams (lVd) for two samples of maser stars: 771 OH/IR stars and 363 SiO-maser stars. They are all close to the plane of the MW and have long. from -45 to +45deg. The two lVds are compared and found to be very similar. They also compare well with the lVd of interstellar CO, but there are significant differences in detail between the stellar lVds and that of the ISM. Based on the qualitative discussion we divide the lVds into seven areas. In each area we compare the number of stars observed with those predicted by an assumed set of orbits in a galactic potential. This potential is axially symmetric but a weak rotating bar has been added. We conclude that the maser stars move on almost circular orbits outside of about 3.5 kpc, but that the orbits become more and more elongated when one goes deep inside our MW. We find a strong effect of the Corotation (CR) resonance (res) at 3.3 kpc, we see a small but noticeable effect of the Outer Lindblad res at 5 kpc and no effect of the Inner Lindblad res at r=0.8 kpc. We find a set of 6 groups of orbits that together predict counts in agreement with the counts of stars observed. We then calculate the trajectory of each orbit and so find the distribution of the maser stars in the plane of the MWG. This distribution has two new (but not unexpected) features. The first is a bar-like distribution within 2 kpc from the GC outlined. These orbits explain the high-vel stars near l=0deg in the forbidden and the permitted quadrants. The second feature are two "croissant"-like voids in the distribution close to the CR radius (3.3 kpc), which are the consequence of the presence of the CR res. We find excellent agreement with an earlier reconstruction by Sevenster (1999)
OH-selected AGB and post-AGB stellar objects II.Blue versus red evolution off the AGB
Using objects found in a systematic survey of the galactic Plane in the
1612-MHz OH line, we discuss in detail two ``sequences'' of post-AGB evolution,
a red and a blue. We argue that the red and the blue groups separate by initial
mass at 4Msun, based on evolutionary-sequence turn-off colours, spectral energy
distributions, outflow velocities and scaleheight. The higher-mass (blue)
objects may have earlier AGB termination. The lower-mass (red) objects undergo
very sudden reddening for IRAS colour R21\sim1.2; these sources must all
undergo a very similar process at AGB termination. The transition colour
corresponds to average initial masses of 1.7Msun. A combined IRAS-MSX colour
proves a very sensitive tool to distinguish lower-mass, early post-AGB objects
from sources still on the AGB and also to distinguish more evolved post-AGB
objects from star-forming regions. The high-mass blue objects are the likely
precursors of bipolar planetary nebulae, whereas the low-mass red objects will
evolve into elliptical planetary nebulae.Comment: 12 pages, LaTex, 7 figures (1 colour), AJ (accepted
OH-selected AGB and post-AGB objects I.Infrared and maser properties
Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz
OH line, new light is cast on the infrared properties of evolved stars on the
TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS
colours, largely fail to distinguish early post-AGB stages. A two-colour
diagram from narrower-band MSX flux densities, with bimodal distributions,
provides a better tool to do the latter. Four mutually consistent selection
criteria for OH-masing red PPNe are given, as well as two for early post-AGB
masers and one for all post--AGB masers, including the earliest ones. All these
criteria miss a group of blue, high-outflow post-AGB sources with 60-mum
excess; these will be discussed in detail in Paper II. The majority of post-AGB
sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly
low outflow velocities, although the fractions of single peaks and irregular
spectra may vary with age and mass. The OH flux density shows a fairly regular
relation with the stellar flux and the envelope optical depth, with the maser
efficiency increasing with IRAS colour R21. The OH flux density is linearly
correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted
Distribution functions for evolved stars in the inner galactic Plane
We present dynamical distribution functions for evolved stars in the inner galactic plane. We use an axisymmetric, two-component Stackel potential that satisfies recent constraints on the galactic potential, amongst others a slightly declining local rotation curve. We show that this potential is adequate to model stellar-kinematic samples with radial extent ranging from to the first three projected moments provides a very good global representation of the data but fails to reproduce the central dispersion, the central apparent scaleheight and the cylindrical rotation at intermediate longitudes. All these features are fitted well by a three--integral model. We discuss various properties of the 2I- and 3I models and the implications for galactic structure. A somewhat thicker disk component is needed to explain the distribution of older AGB stars in the plane; this component also fits the kinematics at higher latitudes better. We find that the Disk and the Bulge, as traced by AGB stars, are very similar dynamically and could well be one and the same component. There is a dynamically distinct component in the inner 100 pc of the Bulge, however
The ATCA/VLA OH 1612 MHz survey. III. Northern Galactic Plane
We present observations of the region between 5deg < l < 45deg and |b| <
3deg, in the OH 1612 MHz line, taken from 1993 to 1995 with the VLA. These
observations are the last part of a larger survey, covering |l| < 45deg and |b|
< 3deg, with the ATCA and the VLA. The region was systematically observed on a
30'x30' grid in (l,b) and the resulting coverage was 92%, with 965 pointings.
We found 286 OH--masing objects, 161 of which are new detections and 207 have
reliable IRAS point-source identifications. The outflow velocity was determined
for 276 sources. A total of 766 sources were detected in the combined ATCA/VLA
survey, of which 29 were detected in two regions of the survey. The source
tables and spectra may be downloaded from
http://msowww.anu.edu.au/~msevenst/pubs.html In this article we analyse the
data statistically and give identifications with known sources where possible.
The efficiency of this VLA survey is 75% of that of the ATCA Bulge survey. This
efficiency was determined by comparing the detections in the region where the
two surveys overlap. The completeness- and error characteristics are similar,
though less homogeneous, except for the much larger errors in the flux
densities. The relatively large surface number density found in the northern
disk, suggests that we can see the Bar extending to higher longitudes on this
side of the galactic Centre.Comment: 9 pages, 9 figs, LaTeX, A&A (accepted), table+spectra on htt
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