291 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
GHG emissions of green coffee production : toward a standard methodology for carbon footprinting : report
In this project, the scope for product specific rules for carbon footprinting of (green) coffee is investigated and a proposal is drafted for further work toward actual definition and implementation of such a standard
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
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)
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
Exciting Maser Science with New Instruments - the Promise of the EVLA
In the near future, the Expanded Very Large Array (EVLA) will allow surveys
for maser sources with unprecedented sensitivity, spectral coverage and
spectroscopic capabilities. In particular, comprehensive surveys for many maser
species with simultaneous sensitive continuum imaging and absorption studies
will give a comprehensive radio picture of star formation in the Galactic plane
and elsewhere. Very efficient EVLA surveys for H2O megamasers in Active Galacic
Nuclei will be possible to practically arbitrary redshifts.
EVLA and Atacama Large Millimeter Array (ALMA) studies of H2O and SiO masers
will serve as high resolution probes of the innermost envelopes of oxygen-rich
evolved stars and HCN masers of carbon-rich stars.
Farther in the future, the Square Kilometer Array (SKA) promises the
detection of OH gigamasers at all conceivable redshifts and maser astrometry
with unprecedented accuracy.Comment: 10 pages, incl. 4 figures, iaus.cls, to appear the Procedings of IAU
Symp. 242 (Astrophysical masers and their environments) eds. J. Chapman & W.
Baan Replaced version with a few typos correcte
Complexity Results for Modal Dependence Logic
Modal dependence logic was introduced recently by V\"a\"an\"anen. It enhances
the basic modal language by an operator =(). For propositional variables
p_1,...,p_n, =(p_1,...,p_(n-1);p_n) intuitively states that the value of p_n is
determined by those of p_1,...,p_(n-1). Sevenster (J. Logic and Computation,
2009) showed that satisfiability for modal dependence logic is complete for
nondeterministic exponential time. In this paper we consider fragments of modal
dependence logic obtained by restricting the set of allowed propositional
connectives. We show that satisfibility for poor man's dependence logic, the
language consisting of formulas built from literals and dependence atoms using
conjunction, necessity and possibility (i.e., disallowing disjunction), remains
NEXPTIME-complete. If we only allow monotone formulas (without negation, but
with disjunction), the complexity drops to PSPACE-completeness. We also extend
V\"a\"an\"anen's language by allowing classical disjunction besides dependence
disjunction and show that the satisfiability problem remains NEXPTIME-complete.
If we then disallow both negation and dependence disjunction, satistiability is
complete for the second level of the polynomial hierarchy. In this way we
completely classify the computational complexity of the satisfiability problem
for all restrictions of propositional and dependence operators considered by
V\"a\"an\"anen and Sevenster.Comment: 22 pages, full version of CSL 2010 pape
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
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