2,750 research outputs found
Extended OH(1720 MHz) Maser Emission from Supernova Remnants
Compact OH(1720 MHz) masers have proven to be excellent signposts for the
interaction of supernova remnants with adjacent molecular clouds. Less
appreciated has been the weak, extended OH(1720 MHz) emission which accompanies
strong compact maser sources. Recent single-dish and interferometric
observations reveal the majority of maser-emitting supernova remnants have
accompanying regions of extended maser emission. Enhanced OH abundance created
by the passing shock is observed both as maser emission and absorption against
the strong background of the remnant. Modeling the observed OH profiles gives
an estimate of the physical conditions in which weak, extended maser emission
arises. I will discuss how we can realize the utility of this extended maser
emission, particularly the potential to measure the strength of the post-shock
magnetic field via Zeeman splitting over these large-scales.Comment: 5 Pages, 2 Figures, To appear in IAU 242, Astrophysical Masers and
Their Environments, eds. J. Chapman & W. Baa
VLA Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster
The Galactic center Arches stellar cluster, detected and studied until now
only in the near-infrared, is comprised of at least one hundred massive (M>20
Msun) stars. Here we report the detection at centimeter wavelengths of radio
continuum emission from eight radio sources associated with the cluster. Seven
of these radio sources have rising spectral indices between 4.9 and 8.5 GHz and
coincide spatially with the brightest stars in the cluster, as determine from
JHK photometry and Brackett alpha and Brackett Gamma spectroscopy. Our results
confirm the presence of powerful ionized winds in these stars. The eighth radio
source has a nonthermal spectrum and its nature is yet unclear, but it could be
associated with a lower mass young star in the cluster.Comment: 6 pages, 2 embedded figures, accepted to ApJLetter
High-resolution Observations of OH(1720 MHz) Masers Toward the Galactic Center
High-resolution VLA observations of 1720 MHz OH maser emission from Sgr A
East and the circumnuclear disk with spatial and spectral resolutions of
2\dasec5 1\dasec3 and 0.27 \kms are reported. This follow-up
observational study focuses on the recent discovery of a number of such OH
maser features and their intense circularly polarized maser lines detected
toward these Galactic center sources. The 1720 MHz maser line of OH arises from
collisionally excited gas behind a C-type shock and is an important diagnostic
of the interaction process that may occur between molecular clouds and
associated X-ray emitting shell-type supernova remnants. The present
observations have confirmed that the observed Stokes signal is due to
Zeeman splitting and that the OH masers are angularly broadened by the
scattering medium toward the Galactic center. The scale length of the magnetic
field fluctuations in the scattering medium toward the Galactic center is
estimated to be greater than 0.1-0.2 pc using the correlation of the position
angles of the scatter-broadened maser spots. In addition, the kinematics of the
maser spots associated with Sgr A East are used to place a 5 pc displacement
between this extended radio structure and the Galactic center.Comment: 13 pages, 2 Tables and 2 figures, to be published in Ap
Measurement of the Parallax of PSR B0950+08 Using the VLBA
A new technique has been developed to remove the ionosphere's distorting
effects from low frequency VLBI data. By fitting dispersive and non-dispersive
components to the phases of multi-frequency data, the ionosphere can be
effectively removed from the data without the use of {\em a priori} calibration
information. This technique, along with the new gating capability of the VLBA
correlator, was used to perform accurate astrometry on pulsar B0950+08,
resulting in a much improved measurement of this pulsar's proper motion
( mas/yr, mas/yr)
and parallax ( mas). This puts the pulsar at a distance of
parsecs, about twice as far as previous estimates, but in good
agreement with models of the electron density in the local bubble.Comment: 5 pages, Latex with AASTEX. Accepted for publication in Ap
The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from the site of
SN 1970G in the Sc galaxy M101. These observations are 31 years after the
supernova event, making SN 1970G the longest monitored radio supernova. With
flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm,
the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when
compared with the previously reported value of -0.56 +/- 0.11, taken in 1990.
The radio emission at 20 cm has decayed since the 1990 observations with a
power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties
of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal
Letter
NGC 3576 and NGC 3603: Two Luminous Southern HII Regions Observed at High Resolution with the Australia Telescope Compact Array
NGC 3576 (G291.28-0.71; l=291.3o, b=-0.7o) and NGC 3603 (G291.58-0.43;
l=291.6o, b=-0.5o) are optically visible, luminous HII regions located at
distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian
Telescope Compact Array (ATCA) observations of these two sources in the
continuum and the H90a, He90a, C90a and H113b recombination lines with an
angular resolution of 7" and a velocity resolution of 2.6 km/s. All four
recombination lines are detected in the integrated profiles of the two sources.
Broad radio recombination lines are detected in both NGC 3576 (DV_{FWHM}>= 50
km/s) and NGC 3603 (DV_{FWHM}>=70 km/s). In NGC 3576 a prominent N-S velocity
gradient (~30 km/s/pc) is observed, and a clear temperature gradient (6000 K to
8000 K) is found from east to west, consistent with a known IR color gradient
in the source. In NGC 3603, the H90a, He90a and the H113b lines are detected
from 13 individual sources. The Y^+ (He/H) ratios in the two sources range from
0.08+/-0.04 to 0.26+/-0.10. We compare the morphology and kinematics of the
ionized gas at 3.4 cm with the distribution of stars, 10 micron emission and
H_2O, OH, and CH_3OH maser emission. These comparisons suggest that both NGC
3576 and NGC 3603 have undergone sequential star formation.Comment: 24 pages, 12 Postscript figure
A Survey of Hydroxyl Toward Supernova Remnants: Evidence for Extended 1720 MHz Maser Emission
We present the results of GBT observations of all four ground-state hydroxyl
(OH) transitions toward 15 supernova remnants (SNRs) which show OH(1720 MHz)
maser emission. This species of maser is well established as an excellent
tracer of an ongoing interaction between the SNR and dense molecular material.
For the majority of these objects we detect significantly higher flux densities
with a single dish than has been reported with interferometric observations. We
infer that spatially extended, low level maser emission is a common phenomenon
that traces the large-scale interaction in maser-emitting SNRs. Additionally we
use a collisional pumping model to fit the physical conditions under which OH
is excited behind the SNR shock front. We find the observed OH gas associated
with the SNR interaction having columns less than approximately 10^17 per
square cm, temperatures of 20 to 125 K, and densities 10^5 per cubic cm.Comment: 24 pages, 23 figures, Accepted to ApJ, March 26, 2008; v2 - added
Figure 6, minor clarifications to text in Sections 3 and
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