90 research outputs found
Very Large Array Detection of the 36 GHz Zeeman Effect in DR21W Revisited
We report on the observation of the 36 GHz methanol maser line in the star
forming region DR21W to accurately measure the Zeeman effect. The reported
Zeeman signature by Fish et al. (2011) became suspicious after an instrumental
effect was discovered in the early days of the Very Large Array Wide-band
Digital Architecture (WIDAR) correlator commissioning. We conclude that the
previously reported magnetic field strength of 58 mG ((1.7 Hz/mG)/z) is
instrumental in nature and thus incorrect. With the improved performance of the
array, we now deduce a 3 sigma limit of -4.7 to +0.4 mG ((1.7 Hz/mG)/z) for the
line-of-sight component of the magnetic field strength in DR21W.Comment: 6 pages, 1 figure, accepted for publication in Ap
GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1
We present global very long baseline interferometry observations of the first excited-state hydroxyl (OH) masers in the massive star-forming region Onsala 1 (ON 1). The 29 masers detected are nearly all from the 6035 MHz transition and nearly all are identifiable as Zeeman pair components. The 6030 and 6035 MHz masers are coincident with previously published positions of ground-state masers to within a few milliarcseconds, and the magnetic fields deduced from Zeeman splitting are comparable. The 6.0 GHz masers in ON 1 are always found in close spatial association with 1665 MHz OH masers, in contrast to the situation in the massive star-forming region W3(OH), suggesting that extreme high density OH maser sites (excited-state masers with no accompanying ground-state maser, as seen in W3(OH)) are absent from ON 1. The large magnetic field strength among the northern, blueshifted masers is confirmed. The northern masers may trace an outflow or be associated with an exciting source separate from the other masers, or the relative velocities of the northern and southern masers may be indicative of expansion and rotation. High angular resolution observations of nonmasing material will be required in order to understand the complex maser distribution in ON 1.National Science Foundation (U.S.
Non-equilibrium chemistry and dust formation in AGB stars as probed by SiO line emission
We have performed high spatial resolution observations of SiO line emission
for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers.
Detailed radiative transfer modelling suggests that there are steep chemical
gradients of SiO in their circumstellar envelopes. The emerging picture is one
where the radial SiO abundance distribution starts at an initial high
abundance, in the case of M-stars consistent with LTE chemistry, that
drastically decreases at a radius of ~1E15 cm. This is consistent with a
scenario where SiO freezes out onto dust grains. The region of the wind with
low abundance is much more extended, typically ~1E16 cm, and limited by
photodissociation. The surpisingly high SiO abundances found in carbon stars
requires non-equilibrium chemical processes.Comment: 2 pages, 1 figure. To be published in the proceedings of the
conference "Why Galaxies Care about AGB Stars", held in Vienna, August 7-11,
2006; F. Kerschbaum, C. Charbonnel, B. Wing eds, ASP Conf.Ser. in pres
86 GHz SiO maser survey of late-type stars in the Inner Galaxy. IV. SiO emission and infrared data for sources in the Scutum and Sagittarius-Carina arms, 20 deg < l < 50 deg
We present an 86 GHz SiO (v = 1, J = 2 ---> 1) maser search toward late-type
stars located within |b|<0.5 deg and 20 deg < l < 50 deg. This search is an
extension at longer longitudes of a previously published work. We selected 135
stars from the MSX catalog using color and flux criteria and detected 92 (86
new detections). The detection rate is 68%, the same as in our previous study.
The last few decades have seen the publication of several catalogs of point
sources detected in infrared surveys (MSX, 2MASS, DENIS, ISOGAL, WISE, GLIMPSE,
AKARI, and MIPSGAL). We searched each catalog for data on the 444 targets of
our earlier survey and for the 135 in the survey reported here. We confirm
that, as anticipated, most of our targets have colors typical of oxygen-rich
asymptotic giant branch (AGB) stars. Only one target star may have already left
the AGB. Ten stars have colors typical of carbon-rich stars, meaning a
contamination of our sample with carbon stars <=1.7%.Comment: 13 pages, 6 Figures, A&A accepte
Finding evolved stars in the inner Galactic disk with Gaia
The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey will provide
positions and line-of-sight velocities of ~20,000 evolved, maser bearing stars
in the Galactic plane. Although this Galactic region is affected by optical
extinction, BAaDE targets may have Gaia cross-matches, eventually providing
additional stellar information. In an initial attempt to cross-match BAaDE
targets with Gaia, we have found more than 5,000 candidates. Of these, we may
expect half to show SiO emission, which will allow us to obtain velocity
information. The cross-match is being refined to avoid false positives using
different criteria based on distance analysis, flux variability, and color
assessment in the mid- and near-IR. Once the cross-matches can be confirmed, we
will have a unique sample to characterize the stellar population of evolved
stars in the Galactic bulge, which can be considered fossils of the Milky Way
formation.Comment: To appear in the Proceedings of the IAU Symposium No. 330:
"Astrometry and Astrophysics in the Gaia sky
86 GHz SiO masers in Galactic Centre OH/IR stars
We present results on a search for 86.243 GHz SiO (J = 2 -- 1, v = 1) maser
emission toward 67 OH/IR stars located near the Galactic Centre. We detected 32
spectral peaks, of which 28 correspond to SiO maser lines arising from the
envelopes of these OH/IR stars. In OH/IR stars, we obtained an SiO maser
detection rate of about 40%. We serendipitously detected two other lines from
OH/IR stars at 86.18 GHz, which could be due to a CCS-molecule transition at
86.181 GHz or probably to an highly excited OH molecular transition at 86.178
GHz. The detection rate of 86 GHz maser emission is found to be about 60% for
sources with The Midcourse Space Experiment (MSX) A - E < 2.5 mag; but it drops
to 25% for the reddest OH/IR stars with MSX A - E > 2.5 mag. This supports the
hypothesis by Messineo et al. (2002) that the SiO masers are primarily found in
relatively thinner circumstellar material.Comment: 4 figures, 14 page
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
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