63 research outputs found
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
A catalogue of soft X-ray sources in the galactic center region
We present a catalogue of 107 point-like X-ray sources derived from a
systematic analysis of all the ROSAT PSPC observations of the galactic center
region performed in 1992-1993. Besides SgrA*, the massive black hole at the
galactic center, 41 X-ray sources have been positionally associated with
already classified objects. Twenty are identified with foreground stars and
five with known Low Mass X-ray Binaries. The majority of the sources in our
catalogue still remains unidentified. They are hard and/or severely absorbed
and probably represent a large population of X-ray binaries located in the
galactic center region, accreting at low accretion rates, and still largely
unknown.Comment: 11 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics Main Journa
From pre- to young Planetary Nebulae: radio continuum variability
Searching for variability, we have observed a sample of hot post-AGB stars
and young Planetary Nebulae candidates with the Very Large Array at 4.8, 8.4,
and 22.4 GHz. The sources had been previously detected in the radio continuum,
which is a proof that the central stars have started ionising their
circumstellar envelopes and an increase in radio flux with time can be expected
as a result of the progression of the ionisation front. Such a behaviour has
been found in IRAS 18062+2410, whose radio modelling has allowed us to
determine that its ionised mass has increased from 10^{-4} to 3.3 10^{-4} M_sun
in 8 years and its envelope has become optically thin at lower frequencies.
Different temporal behaviours have been found for three other sources. IRAS
17423-1755 has shown a possibly periodic pattern and an inversion of its radio
spectral index, as expected from a varying stellar wind. We estimate that the
radio flux arises from a very compact region around the central star (10^{15}
cm) with an electron density of 2 10^6 cm^{-3}. IRAS 22568+6141 and 17516-2525
have decreased their radio flux densities of about 10% per year over 4 years.
While a linear increase of the flux density with time points out to the
progression of the ionisation front in the envelope, decreases as well as
quasi-periodic patterns may indicate the presence of unstable stellar
winds/jets or thick dusty envelopes absorbing ionising photons.Comment: 9 pages, 5 figures, accepted for publication on MNRA
Vagal nerve stimulation started just prior to reperfusion limits infarct size and no-reflow
Vagal nerve stimulation (VNS) started prior to, or during, ischemia has been shown to reduce infarct size. Here, we investigated the effect of VNS when started just prior to, and continued during early, reperfusion on infarct size and no-reflow and studied the underlying mechanisms. For this purpose, swine (13 VNS, 10 sham) underwent 45 min mid-LAD occlusion followed by 120 min of reperfusion. VNS was started 5 min prior to reperfusion and continued until 15 min of reperfusion. Area at risk, area of no-reflow (% of infarct area) and infarct size (% of area at risk), circulating cytokines, and regional myocardial leukocyte influx were assessed after 120 min of reperfusion. VNS significantly reduced infarct size from 67 ± 2 % in sham to 54 ± 5 % and area of no-reflow from 54 ± 6 % in sham to 32 ± 6 %. These effects were accompanied by reductions in neutrophil (~40 %) and macrophage (~60 %) infiltration in the infarct area (all p < 0.05), whereas systemic circulating plasma levels of TNFα and IL6 were not affected. The degree of cardioprotection could not be explained by the VNS-induced bradycardia or the VNS-induced decrease in the double product of heart rate and left ventricular systolic pressure. In the presence of NO-synthase inhibitor LNNA, VNS no longer attenuated infarct size and area of no-reflow, which was paralleled by similarly unaffected regional leukocyte infiltration. In conclusion, VNS is a promising novel adjunctive therapy that limits reperfusion injury in a large animal model of acute myocardial infarction
Discrete Source Survey of 6 GHz OH emission from PNe & pPNe and first 6 GHz images of K 3-35
The aim of this study is to investigate the physical properties of molecular
envelopes of planetary nebulae in their earliest stages of evolution. Using the
100m telescope at Effelsberg, we have undertaken a high sensitivity discrete
source survey for the first excited state of OH maser emission (J=5/2, 2PI3/2
at 6GHz) in the direction of planetary and proto-planetary nebulae exhibiting
18cm OH emission (main and/or satellite lines), and we further validate our
detections using the Nan\c{c}ay radio telescope at 1.6-1.7GHz and MERLIN
interferometer at 1.6-1.7 and 6GHz. Two sources have been detected at 6035MHz
(5cm), both of them are young (or very young) planetary nebulae. The first one
is a confirmation of the detection of a weak 6035MHz line in Vy 2-2. The second
one is a new detection, in K 3-35, which was already known to be an exceptional
late type star because it exhibits 1720MHz OH emission. The detection of
6035MHz OH maser emission is confirmed by subsequent observations made with the
MERLIN interferometer. These lines are very rarely found in evolved stars. The
1612MHz masers surround but are offset from the 1720 and 6035MHz masers which
in turn lie close to a compact 22GHz continuum source embedded in the optical
nebula.Comment: 9 pages, 7 figures, published in A&
OH spectral evolution of oxygen-rich late-type stars
We investigated the main-line spectral evolution with shell thickness of
oxygen rich AGB stars. The study is based on a sample of 30 sources distributed
along the IRAS colour-colour diagram. The sources were chosen to trace the
Miras with thick shells and the whole range of OH/IR stars. The Miras exhibit a
1665 MHz emission strength comparable to that at 1667 MHz. Even though the
Miras of the study have quite thick shells, their spectral characteristics in
both main lines attest to a strong heterogeneity in their OH shell with, in
particular, the presence of significant turbulence and acceleration. The
expansion velocity has been found to be about the same at 1665 and 1667 MHz,
taking into account a possible velocity turbulence of 1-2km/s at the location
of the main-line maser emission. An increase in the intensity ratio 1667/1665
with shell thickness has been found. A plausible explanation for such a
phenomenon is that competitive gain in favour of the 1667 MHz line increases
when the shell is getting thicker. There is an evolution in the spectral
profile shape with the appearance of a substantial inter-peak signal when the
shell is getting thicker. Also, inter-peak components are found and can be as
strong as the external standard peaks when the shell is very thick. This trend
for an increase of the signal in between the two main peaks is thought to be
the result of an increase of the saturation with shell thickness. All sources
but two - a Mira and an OH/IR star from the lower part of the colour-colour
diagram - are weakly polarized. The strong polarization observed for those two
particular objects is thought to be the result of perturbations in their
shells.Comment: 19 pages, 12 figures, accepted for publication in A&
First detection of CO lines in a water fountain star
[Context] Water fountain stars are very young post-AGB stars with high
velocity water maser jets. They are the best objects to study the onset of
bipolar jets from evolved stars due to their young dynamical ages. [Methods] We
use the Arizona Radio Observatory 10m telescope to observe the CO J=2-1 line
and compare the line parameters with that of masers. [Results] We report the
detection of 12CO and 13COJ=2-1 lines from IRAS 16342-3814. The inferred 12CO
mass loss rate is an order of magnitude lower than the infrared and OH mass
loss rates, indicating a very cold and thick O-rich circumstellar envelope
around the star. We also find a 12CO expansion velocity of Vexp = 46 +- 1 km/s
that is too high for an AGB wind and confirm the systemic velocity of 44 +- 1
km/s. In addition we measure a very low 12CO/13CO line ratio of 1.7.
[Conclusions] The first detection of CO lines has provided a new way to
investigate the water fountain stars. Given the high expansion velocity of the
CO gas and its relation to maser velocities, we infer that the CO emission
region is co-located with the OH mainline masers in the warm base of the
optical bipolar lobes, while the high velocity OH1612MHz and H2O masers are
located in the side walls and at the farthest ends of the bipolar lobes,
respectively. Further observations are highly desired to understand the very
low 12CO/13CO line ratio.Comment: 5 pages, 1 figure, accepted by A&A Lette
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
Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars, post-AGB
objects, and planetary nebulae using data collected mainly from the MSX
catalogue. Magnitudes and colour indices are compared with those calculated
from a grid of synthetic spectra that describe the post-AGB evolution beginning
at the onset of the superwind. We find that carbon stars and OH/IR objects form
two distinct sequences in the (K-[8.3])x([8.3]-[14.7]) MSX colour diagram.
OH/IR objects are distributed in two groups: the bluest ones are crowded near
[14.7]-[21.3]=1 and [8.3]-[14.7]=2, and a second, redder group is spread over a
large area in the diagram, where post-AGB objects and planetary nebulae are
also found. High mass-loss rate OH/IR objects, post-AGB stars, and planetary
nebulae share the same region in the (K-[8.3])x([8.3]-[14.7]) and
[14.7]-[21.3]x([8.3]-[14.7]) colour-colour diagrams. This region in the diagram
is clearly separated from a bluer one where most OH/IR stars are found. We use
a grid of models of post-AGB evolution, which are compared with the data. The
gap in the colour-colour diagrams is interpreted as the result of the rapid
trajectory in the diagram of the stars that have just left the AGB.Comment: 11 pages, 5 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics, Also available at: http://www.astro.iag.usp.br/~macie
Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: A Correlation between Turnoff Width and Early Escape Velocity
We present a color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1-2 Gyr old) star clusters in the Magellanic Clouds, including eight clusters for which new data were obtained. We find that all star clusters in our sample feature extended main-sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate the dynamical evolution of clusters with and without initial mass segregation. Our main results are (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages of \ufffd1.35 Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity v esc, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters; and (3) we find that v esc \ufffd 15 km s-1 out to ages of at least 100 Myr for all clusters featuring eMSTOs, and v esc \ufffd 12 km s-1 at all ages for two lower-mass clusters in the same age range that do not show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12-15 km s-1 is consistent with wind velocities of intermediate-mass asymptotic giant branch stars and massive binary stars in the literature. \ufffd 2014. The American Astronomical Society. All rights reserved
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