201 research outputs found
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&
Diversity of chemistry and excitation conditions in the high-mass star forming complex W33
The object W33 is a giant molecular cloud that contains star forming regions
at various evolutionary stages from quiescent clumps to developed H II regions.
Since its star forming regions are located at the same distance and the primary
material of the birth clouds is probably similar, we conducted a comparative
chemical study to trace the chemical footprint of the different phases of
evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment
(APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We
detected 27 transitions of 10 different molecules in the APEX data and 52
transitions of 16 different molecules in the SMA data. The chemistry on scales
larger than 0.2 pc, which are traced by the APEX data, becomes more
complex and diverse the more evolved the star forming region is. On smaller
scales traced by the SMA data, the chemical complexity and diversity increase
up to the hot core stage. In the H II region phase, the SMA spectra resemble
the spectra of the protostellar phase. Either these more complex molecules are
destroyed or their emission is not compact enough to be detected with the SMA.
Synthetic spectra modelling of the HCO transitions, as detected with the
APEX telescope, shows that both a warm and a cold component are needed to
obtain a good fit to the emission for all sources except for W33 Main1. The
temperatures and column densities of the two components increase during the
evolution of the star forming regions. The integrated intensity ratios
NH(32)/CS(65) and
NH(32)/HCO(43) show clear trends as a
function of evolutionary stage, luminosity, luminosity-to-mass ratio, and
H peak column density of the clumps and might be usable as chemical
clocks.Comment: 66 pages, 28 figures, 8 tables, accepted for publication at A&
The Parallax of W43: a Massive Star Forming Complex near the Galactic Bar
We report trigonometric parallax measurements of masers in the massive star
forming complex W43 from VLBA observations as part of the BeSSeL Survey. Based
on measurements of three 12 GHz methanol maser sources (G029.86-00.04,
G029.95-00.01 and G031.28+00.06) and one 22 GHz water maser source
(G031.58+00.07) toward W43, we derived a distance of kpc
to W43. By associating the masers with CO molecular clouds, and associating the
clouds kinematically with CO longitude-velocity spiral features, we assign W43
to the Scutum spiral arm, close to the near end of the Galactic bar. The
peculiar motion of W43 is about 20 km/s toward the Galactic Center and is very
likely induced by the gravitational attraction of the bar.Comment: 23 pages, 16 figures, accepted for publication in Ap
Electrostatic Screen for Transport of Martian and Lunar Regolith
The Martian and Lunar Regolith contain fine particulate including those in the size range from 0.5 to 200 micron [1-2]. Martian dust can be transported and deposited by Aeolian processes, including "Dust Devils". Due to the ultra high vacuum (10e-12 Torr), transport of dust on the Moon is solely a result of collision/ballistic motion. Dust obscuration of solar cells is one of the primary factors limiting the duration of Martian missions, including the Mars Exploration Rovers. Dust contamination in vacuum seals is one of the primarily factors that limited lunar excursions during the Apollo missions. Controlled transportation of dust on Mars and the Moon is important for many reasons, including both contamination mitigation and in situ resource utilization (ISRU). Since both the monopole and dipole electrostatic moments result in non-trivial forces on particles in an electrostatic field, dust particles, whether charged or not, can be transported by electrostatic fields. In the electrostatic screen, alternating waveforms of voltage applied to patterned grids of electrodes will transport dust. The authors will show that the canonical methods for transporting dust via electrostatic screen can be readily applied to transport of Martian and Lunar regolith. Experiments have been performed in ambient, low humidity, Martian, and Lunar conditions. Screen parameters have been examined for application to each regolith, such as grid spacing, trace width, grid voltage, pulse pattern, pulse frequency, and coating type. The authors have also developed an electrostatic screen based on optically transparent conductors that can be placed over solar arrays, windows, visors, lenses, etc
Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
Over 100 trigonometric parallaxes and proper motions for masers associated
with young, high-mass stars have been measured with the BeSSeL Survey, a VLBA
key science project, the EVN, and the Japanese VERA project. These measurements
provide strong evidence for the existence of spiral arms in the Milky Way,
accurately locating many arm segments and yielding spiral pitch angles ranging
from 7 to 20 degrees. The widths of spiral arms increase with distance from the
Galactic center. Fitting axially symmetric models of the Milky Way with the 3-D
position and velocity information and conservative priors for the solar and
average source peculiar motions, we estimate the distance to the Galactic
center, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To,
to be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2
+/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a
"universal" spiral galaxy form for the rotation curve, we estimate the thin
disk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not
highly correlated and are relatively insensitive to different forms of the
rotation curve. Adopting a theoretically motivated prior that high-mass star
forming regions are in nearly circular Galactic orbits, we estimate a global
solar motion component in the direction of Galactic rotation, Vsun = 14.6 +/-
5.0 km/s. While To and Vsun are significantly correlated, the sum of these
parameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the
angular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro
= 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates
of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their
Galactic orbits, significantly reducing the uncertainty in tests of
gravitational radiation predicted by general relativity.Comment: 38 pages, 6 tables, 6 figures; v2 fixed typos and updated pulsar
section; v3 replaced fig 2 (wrong file
Techniques for Accurate Parallax Measurements for 6.7-GHz Methanol Masers
The BeSSeL Survey is mapping the spiral structure of the Milky Way by
measuring trigonometric parallaxes of hundreds of maser sources associated with
high-mass star formation. While parallax techniques for water masers at high
frequency (22 GHz) have been well documented, recent observations of methanol
masers at lower frequency (6.7 GHz) have revealed astrometric issues associated
with signal propagation through the ionosphere that could significantly limit
parallax accuracy. These problems displayed as a "parallax gradient" on the sky
when measured against different background quasars. We present an analysis
method in which we generate position data relative to an "artificial quasar" at
the target maser position at each epoch. Fitting parallax to these data can
significantly mitigate the problems and improve parallax accuracy
Singly- and doubly-deuterated formaldehyde in massive star-forming regions
Deuterated molecules are good tracers of the evolutionary stage of
star-forming cores. During the star formation process, deuterated molecules are
expected to be enhanced in cold, dense pre-stellar cores and to deplete after
protostellar birth. In this paper we study the deuteration fraction of
formaldehyde in high-mass star-forming cores at different evolutionary stages
to investigate whether the deuteration fraction of formaldehyde can be used as
an evolutionary tracer. Using the APEX SEPIA Band 5 receiver, we extended our
pilot study of the =32 rotational lines of HDCO and DCO to
eleven high-mass star-forming regions that host objects at different
evolutionary stages. High-resolution follow-up observations of eight objects in
ALMA Band 6 were performed to reveal the size of the HCO emission and to
give an estimate of the deuteration fractions HDCO/HCO and DCO/HDCO at
scales of 6" (0.04-0.15 pc at the distance of our targets). Our
observations show that singly- and doubly deuterated HCO are detected
toward high-mass protostellar objects (HMPOs) and ultracompact HII regions
(UCHII regions), the deuteration fraction of HCO is also found to decrease
by an order of magnitude from the earlier HMPO phases to the latest
evolutionary stage (UCHII), from 0.13 to 0.01. We have not detected
HDCO and DCO emission from the youngest sources (high-mass starless cores,
HMSCs). Our extended study supports the results of the previous pilot study:
the deuteration fraction of formaldehyde decreases with evolutionary stage, but
higher sensitivity observations are needed to provide more stringent
constraints on the D/H ratio during the HMSC phase. The calculated upper limits
for the HMSC sources are high, so the trend between HMSC and HMPO phases cannot
be constrained.Comment: 15 pages, 4 figures, 4 tables, accepted for publication in A&
Anomalous peculiar motions of high-mass young stars in the Scutum spiral arm
We present trigonometric parallax and proper motion measurements toward 22
GHz water and 6.7 GHz methanol masers in 16 high-mass star-forming regions.
These sources are all located in the Scutum spiral arm of the Milky Way. The
observations were conducted as part of the Bar and Spiral Structure Legacy
(BeSSeL) survey. A combination of 14 sources from a forthcoming study and 14
sources from the literature, we now have a sample of 44 sources in the Scutum
spiral arm, covering a Galactic longitude range from 0 to 33. A
group of 16 sources shows large peculiar motions of which 13 are oriented
toward the inner Galaxy. A likely explanation for these high peculiar motions
is the combined gravitational potential of the spiral arm and the Galactic bar.Comment: 27 pages, 52 figures, 4 tables, accepted for publication in A&
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