88 research outputs found
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)
Water maser detections in southern candidates to post-AGB stars and Planetary Nebulae
We intended to study the incidence and characteristics of water masers in the
envelopes of stars in the post-AGB and PN evolutionary stages.
We have used the 64-m antenna in Parkes (Australia) to search for water maser
emission at 22 GHz, towards a sample of 74 sources with IRAS colours
characteristic of post-AGB stars and PNe, at declination . In our
sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB
stars or post-AGB candidates.
We have detected four new water masers, all of them in optically obscured
sources: three in PNe candidates (IRAS 12405-6219, IRAS 15103-5754, and IRAS
16333-4807); and one in a post-AGB candidate (IRAS 13500-6106). The PN
candidate IRAS 15103-5754 has water fountain characteristics, and it could be
the first PN of this class found.
We confirm the tendency suggested in Paper I that the presence of water
masers in the post-AGB phase is favoured in obscured sources with massive
envelopes. We propose an evolutionary scenario for water masers in the post-AGB
and PNe stages, in which ``water fountain'' masers could develop during
post-AGB and early PN stages. Later PNe would show lower velocity maser
emission, both along jets and close to the central objects, with only the
central masers remaining in more evolved PNe.Comment: 11 pages, 5 figures. Accepted by Astronomy and Astrophysic
ISOGAL Survey of Baade's Windows in the Mid-infrared
The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to
determine their stellar content and its possible bearing on the history of the
Galaxy. The NGC6522 and Sgr I Baade's Windows of low obscuration towards the
inner parts of the Bulge represent ideal places in which to calibrate and
understand the ISOGAL colour-magnitude diagrams.
The survey observations were made with the ISOCAM instrument of the ISO
satellite. The filter bands chosen were LW2(~7 microns) and LW3(~15 microns).
The results presented here show that most of the detected objects are late
M-type giants on the AGB, with a cut-off for those earlier than M3-M4. The most
luminous members of these two Bulge fields at 7 microns are the Mira variables.
However, it is evident that they represent the end of a sequence of increasing
15 micron dust emission which commences with M giants of earlier sub-type.
In observations of late-type giants the ISOCAM 15 micron band is mainly
sensitive to the cool silicate or aluminate dust shells which overwhelm the
photospheric emission. However, in ordinary M-giant stars, the 7 micron band is
not strongly affected by dust emission and may be influenced instead by
absorption. The nu2 band of water at 6.25 microns and the SiO fundamental at
7.9 microns are likely contribitors to this effect.
A group of late M stars has been found which vary little or not at all but
have infrared colours typical of well-developed dust shells. Their luminosities
are similar to those of 200-300 day Miras but they have slightly redder
[7]-[15] colours which form an extension of the ordinary M giant sequence.
The Mira dust shells show a mid-infrared [7]-[15] colour-period relation. ca
700 days.Comment: 13 pages 15 figure
Herschel/HIFI observations of O-rich AGB stars : molecular inventory
Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel
Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form
part of the guaranteed time key program HIFISTARS are presented. The aim of
this program is to study the dynamical structure, mass-loss driving mechanism,
and chemistry of the outflows from AGB stars as a function of chemical
composition and initial mass.
We used the HIFI instrument to observe nine AGB stars, mainly in the H2O and
high rotational CO lines We investigate the correlation between line
luminosity, line ratio and mass-loss rate, line width and excitation energy.
A total of nine different molecules, along with some of their isotopologues
have been identified, covering a wide range of excitation temperature. Maser
emission is detected in both the ortho- and para-H2O molecules. The line
luminosities of ground state lines of ortho- and para-H2O, the high-J CO and
NH3 lines show a clear correlation with mass-loss rate. The line ratios of H2O
and NH3 relative to CO J=6-5 correlate with the mass-loss rate while ratios of
higher CO lines to the 6-5 is independent of it. In most cases, the expansion
velocity derived from the observed line width of highly excited transitions
formed relatively close to the stellar photosphere is lower than that of lower
excitation transitions, formed farther out, pointing to an accelerated outflow.
In some objects, the vibrationally excited H2O and SiO which probe the
acceleration zone suggests the wind reaches its terminal velocity already in
the innermost part of the envelope, i.e., the acceleration is rapid.
Interestingly, for R Dor we find indications of a deceleration of the outflow
in the region where the material has already escaped from the star.Comment: 6 Figures in the main paper + 12 further figures in the appendix (to
be printed in electronic form) Accepted for publication by A&
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
The magnetic field toward the young planetary nebula K~3-35
K 3-35 is a planetary nebula (PN) where H2O maser emission has been detected,
suggesting that it departed from the proto-PNe phase only some decades ago.
Interferometric VLA observations of the OH 18 cm transitions in K~3-35 are
presented.OH maser emission is detected in all four ground state lines (1612,
1665, 1667, and 1720 MHz). All the masers appear blueshifted with respect to
the systemic velocity of the nebula and they have different spatial and
kinematic distributions.The OH 1665 and 1720 MHz masers appear spatially
coincident with the core of the nebula, while the OH 1612 and 1667 MHz ones
exhibit a more extended distribution. We suggest that the 1665 and 1720 masers
arise from a region close to the central star, possibly in a torus, while the
1612 and 1667 lines originate mainly from the extended northern lobe of the
outflow. It is worth noting that the location and velocity of the OH 1720 MHz
maser emission are very similar to those of the H2O masers (coinciding within
0.1" and ~2 km/s, respectively). We suggest that the pumping mechanism in the
H2O masers could be produced by the same shock that is exciting the OH 1720 MHz
transition. A high degree of circular polarization (>50%) was found to be
present in some features of the 1612, 1665, and 1720 MHz emission.For the 1665
MHz transition at ~ +18 km/s the emission with left and right circular
polarizations (LCP and RCP) coincide spatially within a region of ~0.03" in
diameter.Assuming that these RCP and LCP 1665 features come from a Zeeman pair,
we estimate a magnetic field of ~0.9 mG within 150 AU from the 1.3 cm continuum
peak. This value is in agreement with a solar-type magnetic field associated
with evolved stars.Comment: 6 pages, 2 tables, 4 figures, ApJ accepte
Accurate OH maser positions from the SPLASH pilot region
We report on high spatial resolution observations, using the Australia Telescope Compact Array (ATCA), of ground-state OH masers. These observations were carried out toward 196 pointing centers previously identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) pilot region, between Galactic longitudes of 334 and 344 and Galactic latitudes of -2 and +2. Supplementing our data with data from the MAGMO (Mapping the Galactic Magnetic field through OH masers) survey, we find maser emission toward 175 of the 196 target fields. We conclude that about half of the 21 nondetections were due to intrinsic variability. Due to the superior sensitivity of the followup ATCA observations, and the ability to resolve nearby sources into separate sites, we have identified 215 OH maser sites toward the 175 fields with detections. Among these 215 OH maser sites, 111 are new detections. After comparing the positions of these 215 maser sites to the literature, we identify 122 (57%) sites associated with evolved stars (one of which is a planetary nebula), 64 (30%) with star formation, two sites with supernova remnants, and 27 (13%) of unknown origin. The infrared colors of evolved star sites with symmetric maser profiles tend to be redder than those of evolved star sites with asymmetric maser profiles, which may indicate that symmetric sources are generally at an earlier evolutionary stage. © 2016. The American Astronomical Society. All rights reserved
A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph
We present a catalogue of positions, magnitudes and velocities for 3300
emission-line objects found by the Planetary Nebula Spectrograph in a survey of
the Andromeda Galaxy, M31. Of these objects, 2615 are found likely to be
planetary nebulae (PNe) associated with M31. Initial results from this survey
include: the likely non-existence of Andromeda VIII; a universal PN luminosity
function, with the exception of a small amount of obscuration, and a small
offset in normalization between bulge and disk components; very faint
kinematically-selected photometry implying no cut-off in the disk to beyond 4
scalelengths and no halo population in excess of the bulge out to 10 effective
bulge radii; disk kinematics that show significant dispersion and asymmetric
drift out to large radii, consistent with a warm flaring disk; and no sign of
any variation in kinematics with PN luminosity, suggesting that PNe arise from
a fairly uniform population of old stars.Comment: Accepted by MNRAS. 23 pages, 37 figures. A full resolution version is
available at http://www.astro.rug.nl/~pns/pns_pub.htm
MALT-45: a 7 mm survey of the southern Galaxy - I. Techniques and spectral line data
We present the first results from the MALT-45 (Millimetre Astronomer's Legacy Team-45 GHz) Galactic Plane survey. We have observed 5 square degrees (l = 330°â335°, b = ±0 â . 5) for spectral lines in the 7 mm band (42â44 and 48â49 GHz), including CS (1â0), class I CH3OH masers in the 7(0,7)â6(1,6) A+ transition and SiO (1â0) v = 0, 1, 2, 3. MALT-45 is the first unbiased, large-scale, sensitive spectral line survey in this frequency range. In this paper, we present data from the survey as well as a few intriguing results; rigorous analyses of these science cases are reserved for future publications. Across the survey region, we detected 77 class I CH3OH masers, of which 58 are new detections, along with many sites of thermal and maser SiO emission and thermal CS. We found that 35 class I CH3OH masers were associated with the published locations of class II CH3OH, H2O and OH masers but 42 have no known masers within 60 arcsec. We compared the MALT-45 CS with NH3 (1,1) to reveal regions of CS depletion and high opacity, as well as evolved star-forming regions with a high ratio of CS to NH3. All SiO masers are new detections, and appear to be associated with evolved stars from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). Generally, within SiO regions of multiple vibrational modes, the intensity decreases as v = 1, 2, 3, but there are a few exceptions where v = 2 is stronger than v = 1
Egg load evolution in parasitoids
Parasitic wasps lay their eggs in or on other insects. Allocation of resources to reproduction (eggs) and survival (life span) should maximize reproductive success, but stochasticity in the number of hosts encountered precludes an exact match of allocation with reproductive opportunity. We study optimal egg loads using a general model for pro-ovigenic parasitoids (which only mature eggs before adult life) and a dynamic programming model for synovigenic parasitoids (which can mature additional eggs during adult life). We find that stochasticity has a major effect on optimal egg loads and that the patchy distribution of hosts is the major source of stochasticity in reproductive opportunities. In a stochastic world, the further "relative habitat quality" (expected number of oviposition opportunities gained by reducing reproductive investment by one egg) is below unity (as we argue it generally is), the more the optimal egg load exceeds the expected number of hosts. In unusually rich environments, however, stochasticity decreases optimal egg loads. Although synovigenic parasitoids can adjust to the conditions they encounter, they too have to invest for an uncertain future. Because they can replenish their egg supply throughout life, the incidence of egg limitation is even lower than in pro-ovigenic parasitoids
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