160 research outputs found
The distance to a star forming region in the Outer arm of the Galaxy
We performed astrometric observations with the VLBA of WB89-437, an H2O maser
source in the Outer spiral arm of the Galaxy. We measure an annual parallax of
0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2
kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the
heliocentric distance is considerably smaller than the kinematic distance of
8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135
degrees. We also measured the full space motion of the object and find a large
peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion
explains the large error in the kinematic distance estimate. We also find that
WB89-437 has the same rotation speed as the LSR, providing more evidence for a
flat rotation curve and thus the presence of dark matter in the outer Galaxy.Comment: The Astrophysical Journal, accepted, 16 pages, 4 Figure
Distance of W3(OH) by VLBI annual parallax measurement
The most powerful tool for measuring distances within our Galaxy is the
annual parallax. We carried out phase-referencing VLBI observations of HO
masers in the star forming region W3(OH) with respect to the extragalactic
continuum source ICRF 0244+624 to measure their absolute proper motions. The
measured annual parallax is 0.484 0.004 milli-arcseconds which
corresponds to a distance of 2.07^{+0.01}_{-0.02}$ kpc from the sun. This
distance is consistent with photometric and kinematic distances from previous
observations.Comment: Proceedings of the 7th European VLBI Network Symposium (October 12-15
2004, Toledo, Spain), eds. Bachiller, R., Colomer, F., Desmurs, J. F., & de
Vicente, P., 4 pages, 4 figures, needs evn2004.cl
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
A Trigonometric Parallax of Sgr B2
We have measured the positions of water masers in Sgr B2, a massive star
forming region in the Galactic center, relative to an extragalactic radio
source with the Very Long Baseline Array. The positions measured at 12 epochs
over a time span of one year yield the trigonometric parallax of Sgr B2 and
hence a distance to the Galactic center of Ro=7.9 (+0.8/-0.7) kpc. The proper
motion of Sgr B2 relative to Sgr A* suggests that Sgr B2 is about 0.13 kpc
nearer than the Galactic center, assuming a low-eccentricity Galactic orbit.Comment: Submitted to ApJ; 4 tables; 3 figures. Version 2 corrects Fig. 2
which was missing some dat
Trigonometric Parallaxes of Massive Star Forming Regions: III. G59.7+0.1 and W 51 IRS2
We report trigonometric parallaxes for G59.7+0.1 and W 51 IRS2, corresponding
to distances of 2.16^{+0.10}_{-0.09} kpc and 5.1^{+2.9}_{-1.4} kpc,
respectively. The distance to G59.7+0.1 is smaller than its near kinematic
distance and places it between the Carina-Sagittarius and Perseus spiral arms,
probably in the Local (Orion) spur. The distance to W 51 IRS2, while subject to
significant uncertainty, is close to its kinematic distance and places it near
the tangent point of the Carina-Sagittarius arm. It also agrees well with a
recent estimate based on O-type star spectro/photometry. Combining the
distances and proper motions with observed radial velocities gives the full
space motions of the star forming regions. We find modest deviations of 5 to 10
km/s from circular Galactic orbits for these sources, both counter to Galactic
rotation and toward the Galactic center.Comment: 16 pages, 6 figures; to appear in the Astrophysical Journa
Parallaxes of Star Forming Regions in the Outer Spiral Arm of the Milky Way
We report parallaxes and proper motions of three water maser sources in
high-mass star-forming regions in the Outer Spiral Arm of the Milky Way. The
observations were conducted with the Very Long Baseline Array as part of Bar
and Spiral Structure Legacy Survey and double the number of such measurements
in the literature. The Outer Arm has a pitch angle of 14.9 +/- 2.7 deg and a
Galactocentric distance of 14.1 +/- 0.6 kpc toward the Galactic anticenter. The
average motion of these sources toward the Galactic center is 10.7 +/- 2.1 km/s
and we see no sign of a significant fall in the rotation curve out to 15 kpc
from the Galactic center. The three-dimensional locations of these star-forming
regions are consistent with a Galactic warp of several hundred parsecs from the
plane.Comment: 19 pages, 7 figures, Accepted for publication in Ap
Trigonometric Parallaxes of Massive Star-Forming Regions. IX. The Outer Arm in the First Quadrant
We report a trigonometric parallax measurement with the Very Long Baseline
Array for the water maser in the distant high-mass star-forming region
G75.30+1.32. This source has a heliocentric distance of 9.25+-0.45 kpc, which
places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above
the Galactic plane and is associated with a substantial HI enhancement at the
border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc,
G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped
toward the North Galactic Pole. While the star-forming region has an
instantaneous Galactic orbit that is nearly circular, it displays a significant
motion of 18 km/s toward the Galactic plane. The present results, when combined
with two previous maser studies in the Outer arm, yield a pitch angle of about
12 degrees for a large section of the arm extending from the first quadrant to
the third.Comment: 19 pages, 5 figures, 4 tables, accepted by The Astrophysical Journa
Trigonometric Parallaxes of Massive Star Forming Regions: IV. G35.20-0.74 and G35.20-1.74
We report trigonometric parallaxes for the high-mass star forming regions
G35.20-0.74 and G35.20-1.74, corresponding to distances of 2.19 (+0.24 -0.20)
kpc and 3.27 (+0.56 -0.42) kpc, respectively. The distances to both sources are
close to their near kinematic distances and place them in the
Carina-Sagittarius spiral arm. Combining the distances and proper motions with
observed radial velocities gives the locations and full space motions of the
star forming regions. Assuming a standard model of the Galaxy, G35.20-0.74 and
G35.20-1.74 have peculiar motions of ~13 km/s and ~16 km/s counter to Galactic
rotation and ~9 km/s toward the North Galactic Pole.Comment: 16 pages, 8 figure
Trigonometric Parallaxes of 6.7 GHz Methanol Masers
Emission from the 6.7 GHz methanol maser transition is very strong, is
relatively stable, has small internal motions, and is observed toward numerous
massive star-forming regions in the Galaxy. Our goal is to perform
high-precision astrometry using this maser transition to obtain accurate
distances to their host regions. Eight strong masers were observed during five
epochs of VLBI observations with the European VLBI Network between 2006 June,
and 2008 March. We report trigonometric parallaxes for five star-forming
regions, with accuracies as good as as. Distances to these
sources are kpc for ON 1, kpc
for L 1206, kpc for L 1287,
kpc for NGC 281-W, and kpc for S 255. The distances and
proper motions yield the full space motions of the star-forming regions hosting
the masers, and we find that these regions lag circular rotation on average by
17 km s, a value comparable to those found recently by similar
studies.Comment: 17 pages, 21 figures, 5 tables, accepted for publication in A&A,
corrected typo
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