1,041 research outputs found
A Parallax-based Distance Estimator for Spiral Arm Sources
The spiral arms of the Milky Way are being accurately located for the first
time via trigonometric parallaxes of massive star forming regions with the
BeSSeL Survey, using the Very Long Baseline Array and the European VLBI
Network, and with the Japanese VERA project. Here we describe a computer
program that leverages these results to significantly improve the accuracy and
reliability of distance estimates to other sources that are known to follow
spiral structure. Using a Bayesian approach, sources are assigned to arms based
on their (l,b,v) coordinates with respect to arm signatures seen in CO and HI
surveys. A source's kinematic distance, displacement from the plane, and
proximity to individual parallax sources are also considered in generating a
full distance probability density function. Using this program to estimate
distances to large numbers of star forming regions, we generate a realistic
visualization of the Milky Way's spiral structure as seen from the northern
hemisphere.Comment: 25 pages with 16 figures; to appear in Ap
Trigonometric Parallaxes of Massive Star Forming Regions: G012.88+0.48 and W33
We report trigonometric parallaxes for water masers in the G012.88+0.48
region and in the massive star forming complex W33 (containing G012.68--0.18,
G012.81--0.19, G012.90--0.24, G012.90--0.26), from the Bar and Spiral Structure
Legacy (BeSSeL) survey using the Very Long Baseline Array. The parallax
distances to all these masers are consistent with kpc,
which locates the W33 complex and G012.88+0.48 in the Scutum spiral arm. Our
results show that W33 is a single star forming complex at about two-thirds the
kinematic distance of 3.7 kpc. The luminosity and mass of this region, based on
the kinematic distance, have therefore been overestimated by more than a factor
of two. The spectral types in the star cluster in W33\,Main have to be changed
by 1.5 points to later types.Comment: 9 pages, 11 figures, 2 tables; accepted for publication at A&
Molecular clouds and galactic spiral structure
Galactic CO line emission at 115 GHz was surveyed in order to study the distribution of molecular clouds in the inner galaxy. Comparison of this survey with similar H1 data reveals a detailed correlation with the most intense 21 cm features. To each of the classical 21 cm H1 spiral arms of the inner galaxy there corresponds a CO molecular arm which is generally more clearly defined and of higher contrast. A simple model is devised for the galactic distribution of molecular clouds. The modeling results suggest that molecular clouds are essentially transient objects, existing for 15 to 40 million years after their formation in a spiral arm, and are largely confined to spiral features about 300 pc wide
HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud
We report HI 21-cm line observations of the AGB star X Her obtained with the
Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI
emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope
of the star. The HI distribution exhibits a head-tail morphology, similar to
those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24
pc) in the plane of the sky, along the direction of the star's space motion. We
also detect a velocity gradient of ~6.5 km/s across the envelope, consistent
with the HI tracing a turbulent wake that arises from the motion of a
mass-losing star through the ISM. GBT mapping of a 2x2deg region around X Her
reveals that the star lies (in projection) near the periphery of a much larger
HI cloud that also exhibits signatures of ISM interaction. The properties of
the cloud are consistent with those of compact high-velocity clouds. Using CO
observations, we have placed an upper limit on its molecular gas content of
N_H2<1.3e20 cm^-2. Although the distance to the cloud is poorly constrained,
the probability of a chance coincidence in position, velocity, and apparent
position angle of space motion between X Her and the cloud is extremely small,
suggesting a possible physical association. However, the large HI mass of the
cloud (~>2.4~M_sun) and the blueshift of its mean velocity relative to X Her
are inconsistent with an origin tied directly to stellar ejection. (abridged)Comment: Accepted to AJ; 47 pages, 15 figures; version with full resolution
figures available at
http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_XHer.pd
The 3D inelastic analysis methods for hot section components
Advanced 3-D inelastic structural/stress analysis methods and solution strategies for more accurate and yet more cost-effective analysis of combustors, turbine blades, and vanes are being developed. The approach is to develop four different theories, one linear and three higher order with increasing complexities including embedded singularities. Progress in each area is reported
High-Mass Star Formation in the Outer Scutum-Centaurus Arm
The Outer Scutum-Centaurus (OSC) spiral arm is the most distant molecular
spiral arm in the Milky Way, but until recently little was known about this
structure. Discovered by Dame and Thaddeus (2011), the OSC lies 15 kpc
from the Galactic Center. Due to the Galactic warp, it rises to nearly
4 above the Galactic Plane in the first Galactic quadrant, leaving it
unsampled by most Galactic plane surveys. Here we observe HII region candidates
spatially coincident with the OSC using the Very Large Array to image radio
continuum emission from 65 targets and the Green Bank Telescope to search for
ammonia and water maser emission from 75 targets. This sample, drawn from the
WISE Catalog of Galactic HII Regions, represents every HII region candidate
near the longitude-latitude (l,v) locus of the OSC. Coupled with their
characteristic mid-infrared morphologies, detection of radio continuum emission
strongly suggests that a target is a bona fide HII region. Detections of
associated ammonia or water maser emission allow us to derive a kinematic
distance and determine if the velocity of the region is consistent with that of
the OSC. Nearly 60% of the observed sources were detected in radio continuum,
and over 20% have ammonia or water maser detections. The velocities of these
sources mainly place them beyond the Solar orbit. These very distant high-mass
stars have stellar spectral types as early as O4. We associate high-mass star
formation at 2 new locations with the OSC, increasing the total number of
detected HII regions in the OSC to 12.Comment: 14 pages text and tables + 10 pages supplemental figure
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