148 research outputs found
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
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
The Pattern Speed of the OH/IR Stars in the Milky Way
We show how the continuity equation can be used to determine pattern speeds
in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine &
Weinberg in the context of external galaxies, requires projected positions,
, and line-of-sight velocities for a spatially complete sample of
relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the
radial direction (), then the quantity that is measured is , where is the pattern speed of
the non-axisymmetric feature, is the distance of the Sun from the
Galactic centre and is the tangential motion of the LSR,
including the circular velocity. We use simple models to assess the reliability
of the method for measuring a single, constant pattern speed of either a bar or
spiral in the inner MWG. We then apply the method to the OH/IR stars in the
ATCA/VLA OH 1612 MHz survey of Sevenster et al, finding
km/s, if . Assuming further that kpc and \kms, this gives km/s/kpc with a possible
systematic error of perhaps 10 km/s/kpc. The non-axisymmetric feature for which
we measure this pattern speed must be in the disc of the MWG.Comment: 15 pages, 13 figures. Accepted to MNRA
The pattern speed of the OH/IR stars in the Milky Way
We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, (l,b), and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction (uLSR), then the quantity that is measured is ΔV≡ΩpR0-VLSR, where Ωp is the pattern speed of the non-axisymmetric feature, R0 is the distance of the Sun from the Galactic centre and VLSR is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612-MHz survey of Sevenster et al., finding ΔV=252±41 km s-1, if uLSR=0. Assuming further that R0=8 kpc and VLSR=220 km s-1, this gives Ωp=59±5 km s-1 kpc-1 with a possible systematic error of perhaps 10 km s−1 kpc−1. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MW
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
Polarization properties of OH masers in AGB and post-AGB stars
Context: Ground-state OH maser emission from late-type stars is usually
polarized and remains a powerful probe of the magnetic field structure in the
outer regions of circumstellar envelopes if observed with high angular and
spectral resolutions. Observations in all four Stokes parameters are quite
sparse and this is the most thorough, systematic study published to date.
Aims: We aim to determine polarization properties of OH masers in an
extensive sample of stars that show copious mass loss and search for candidate
objects that are well-suited for high angular resolution studies.
Methods: Full-polarization observations of the OH 1612 and 1667 MHz maser
transitions were carried out for a sample of 117 AGB and post-AGB stars.
Several targets were also observed in the 1665 MHz line.
Results: Polarized features occur in more than 75% of the sources in the
complete sample and there is no intrinsic difference in the occurrence of
polarized emission between the three classes of objects of different infrared
characteristics. The highest fractional polarization occurs for the post-AGB+PN
and the Mira+SR classes at 1612 and 1667 MHz, respectively. Differences in the
fractional polarization between the sources at different evolutionary stages
appear to be related to depolarization caused by blending. The alignment of the
polarization angles at the extreme sides of the shell implies a regular
structure of the magnetic field of a strength of 0.3-2.3 mG.
Conclusions: Polarized OH maser features are widespread in AGB and post-AGB
stars. The relationship between the circular and linear fractional
polarizations for a representative sample are consistent with the standard
models of polarization for the Zeeman splitting higher than the Doppler line
width, whereas the polarized features are the sigma components.Comment: Accepted for publication in Astronomy and Astrophysics. A version
with appendices (Tab. A.1 and Fig. B.1) can be downloaded from
http://paulo.astro.uni.torun.pl/~pw/arXiv_
In vitro fertilisation when normal sperm morphology is less than fifteen per cent: Results of in vitro fertilisation and embryo transfer at H. F. Verwoerd Hospital, Pretoria
The outcome of in vitro fertilisation and embryo transfer in 90 couples where the husband's normal sperm morphology was less than 15% were analysed. Based on the percentage of morphologically normal spermatozoa the patients were divided into three groups: group A - normal morphological features 0 - 5%; group B - 6 - 10%; and group C - 11 - 14%. A control group had normal morphological features ≥ 15%. The fertilisation rate and number of embryos transferred was not significantly different in these groups. However, the pregnancy rate per embryo transfer was significantly different for groups A and B compared with group C and the control group (0% and 5,9% for groups A and B v. 13,9% and 18,3% for group C and the control group, respectively). It is concluded that when the normal sperm morphology is less than 11% the prospect of a pregnancy is poor
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
Search for radiative pumping lines of OH masers: I. The 34.6um absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long been thought
to be pumped by 34.6um photons. Only recently, the Infrared Space Observatory
has made possible spectroscopic observations which enable the direct
confirmation of this pumping mechanism in a few cases. To look for the presence
of this pumping line, we have searched the Infrared Space Observatory Data
Archive and found 178 spectra with data around 34.6um for 87 galactic 1612MHz
masers. The analysis performed showed that the noise level and the spectral
resolution of the spectra are the most important factors affecting the
detection of the 34.6um absorption line. Only 5 objects from the sample (3 red
supergiants and 2 galactic center sources) are found to show clear 34.6um
absorption (all of them already known) while two additional objects only
tentatively show this line. The 3 supergiants show similar pump rates and their
masers might be purely radiatively pumped. The pump rates of OH masers in late
type stars are found to be about 0.05, only 1/5 of the theoretical value of
0.25 derived by Elitzur (1992). We have also found 16 maser sources which,
according to the analysis assuming Elitzur's pump rate, should show the 34.6
m absorption line but do not. These non-detections can be tentatively
explained by far-infrared photon pumping, clumpy nature of the OH masing region
or a limb-filling emission effect in the OH shell.Comment: 11 pages, 8 figures, 3 table
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