8 research outputs found
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The acceleration and dissolution of stars moving through the blackbody radiation of a collapsing universe.
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Toward a New Geometric Distance to the Active Galaxy NGC 4258. II. Centripetal Accelerations and Investigation of Spiral Structure
We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is the determination of a new geometric maser distance to NGC 4258, accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range , indicating an origin within 13° of the disk midline (the perpendicular to the line of sight [LOS] to the black hole). Comparison of the projected positions of high-velocity emission in VLBI images with those derived from acceleration data provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range , consistent with emission originating from a concavity where the thin, warped disk is tangent to the LOS. A trend in accelerations of low-velocity emission, as a function of Doppler velocity, may be associated with disk geometry and orientation or with the presence of spiral structure.Astronom
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Toward a New Geometric Distance To the Active Galaxy NGC 4258. III. Final Results and The Hubble Constant
We report a new geometric maser distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is , a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is .Astronom
A Class of Interstellar OH Masers Associated with Protostellar Outflows
Using the Very Large Array, we have detected weak OH maser emission near the
Turner-Welch protostellar source in the W3 OH region. Unlike typical
interstellar OH masers, which are associated with ultra-compact HII regions,
our measured positions and proper motions (from Very Long Baseline
Interferometry) indicate that these OH masers are associated with a bipolar
outflow traced by strong H2O masers. These OH masers may be part of a class of
interstellar OH masers that are associated with very young stars which have yet
to, or may never, create ultra-compact HII regions. This class of OH masers
appears to form near the edges of very dense material (within which H2O masers
form), where total densities drop precipitously and interstellar UV radiation
is sufficient to dissociate the H2O molecules. Observations of this class of OH
masers may be an important way to probe the distribution of this important
molecule in interstellar shocks at arcsecond resolution or better
Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: I. Data
We present full-polarization VLBA maps of the ground-state, main-line, 2 Pi
3/2, J = 3/2 OH masers in 18 Galactic massive star-forming regions. This is the
first large polarization survey of interstellar hydroxyl masers at VLBI
resolution. A total of 184 Zeeman pairs are identified, and the corresponding
magnetic field strengths are indicated. We also present spectra of the NH3
emission or absorption in these star-forming regions. Analysis of these data
will be presented in a companion paper.Comment: 111 pages, including 42 figures and 21 tables, to appear in ApJ
Interstellar Hydroxyl Masers in the Galaxy. II. Zeeman Pairs and the Galactic Magnetic Field
We have identified and classified Zeeman pairs in the survey by Argon, Reid,
& Menten of massive star-forming regions with 18 cm (2 Pi 3/2, J = 3/2) OH
maser emission. We have found a total of more than 100 Zeeman pairs in more
than 50 massive star-forming regions. The magnetic field deduced from the
Zeeman splitting has allowed us to assign an overall line-of-sight magnetic
field direction to many of the massive star-forming regions. Combining these
data with other data sets obtained from OH Zeeman splitting, we have looked for
correlations of magnetic field directions between star-forming regions
scattered throughout the Galaxy. Our data do not support a uniform,
Galactic-scale field direction, nor do we find any strong evidence of magnetic
field correlations within spiral arms. However, our data suggest that in the
Solar neighborhood the magnetic field outside the Solar circle is oriented
clockwise as viewed from the North Galactic Pole, while inside the Solar circle
it is oriented counterclockwise. This pattern, including the magnetic field
reversal near the Sun, is in agreement with results obtained from pulsar
rotation measures.Comment: 37 pages AASTeX, including 6 figures, to appear in Ap