241 research outputs found
Kinematics of OB-associations and the new reduction of the Hipparcos data
The proper motions of OB-associations computed using the old (Hipparcos 1997}
and new (van Leeuwen 2007) reductions of the Hipparcos data are in a good
agreement with each other. The Galactic rotation curve derived from an analysis
of line-of-sight velocities and proper motions of OB-associations is almost
flat in the 3-kpc neighborhood of the Sun. The angular rotation velocity at the
solar distance is Omega_0=31 +/-1 km s-1 kpc-1. The standard deviation of the
velocities of OB-associations from the rotation curve is sigma=7.2 km s-1. The
distance scale for OB associations (Blaha & Humphreys 1989) should be shortened
by 10-20%. The residual velocities of OB-associations calculated for the new
and old reductions differ, on average, by 3.5 km s-1. The mean residual
velocities of OB-associations in the stellar-gas complexes depend only slightly
on the data reduction employed.Comment: 9 pages, 4 figures, accepted for publication in MNRA
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
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Review of existing literature on methodologies to model non- linearity, thresholds and irreversibility in high-impact climate change events in the presence of environmental tipping points
VLA+PT Astrometry of 46 Radio Stars
We have used the Very Large Array (VLA), linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 46 radio stars in
the International Celestial Reference Frame (ICRF). Positions were obtained in
the ICRF directly through phase referencing of the stars to nearby ICRF quasars
whose positions are accurate at the 0.25 mas level. Radio star positions are
estimated to be accurate at the 10 mas level, with position errors approaching
a few milli-arcseconds for some of the stars observed. Our measured positions
were combined with previous measurements taken from as early as 1978 to obtain
proper motion estimates for all 46 stars with average uncertainties of ~1.7
mas/yr. We compared our radio star positions and proper motions with the
Hipparcos Catalogue data, and find consistency in the reference frames produced
by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the
reference frame orientation angles at our mean epoch of 2003.78. No significant
spin is found between our radio data frame and the Hipparcos Celestial
Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr
around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr.
Thus, our results are consistent with a non-rotating Hipparcos frame with
respect to the ICRF.Comment: 29 pages, 8 figures, accepted for publication in the Astronomical
Journa
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