1,392 research outputs found
Asymmetry in the Spectrum of High-Velocity H2O Maser Emission Features in Active Galactic Nuclei
We suggest a mechanism for the amplification of high-velocity water-vapor
maser emission features from the central regions of active galactic nuclei. The
model of an emitting accretion disk is considered. The high-velocity emission
features originate in the right and left wings of the Keplerian disk. The
hyperfine splitting of the signal levels leads to an asymmetry in the spectral
profile of the water vapor maser line at a frequency of 22.235 GHz. We show
that the gain profile asymmetry must lead to an enhanced brightness of the
blueshifted high-velocity emission features compared to the redshifted ones.
Such a situation is observed in the source UGC 3789.Comment: 11 pages 3 figure
A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk
The water maser in the mildly active nucleus in the nearby galaxy NGC4258
traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the
technique of very long baseline interferometry, we have detected the proper
motions of these masers as they sweep in front of the central black hole at an
orbital velocity of about 1100 km/s. The average maser proper motion of 31.5
microarcseconds per year is used in conjunction with the observed acceleration
of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3
Mpc. This is the most precise extragalactic distance measured to date, and,
being independent of all other distance indicators, is likely to play an
important role in calibrating the extragalactic distance scale.Comment: 11 pages, 3 figures. Accepted for publication in Natur
Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2
We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in
ApJ (April 1, 2001 issue
The magnetic field along the jets of NGC 4258 as deduced from high frequency radio observations
We present 2.4" resolution, high sensitivity radio continuum observations of
the nearby spiral galaxy NGC 4258 in total intensity and linear polarization
obtained with the Very Large Array at 3.6 cm (8.44 GHz). The radio emission
along the northern jet and the center of the galaxy is polarized and allows
investigation of the magnetic field. Assuming energy-equipartition between the
magnetic field and the relativistic particles and distinguishing between (1) a
relativistic electron-proton jet and (2) a relativistic electron-positron jet,
we obtain average magnetic field strengths of about (1) 310\muG and (2) 90\muG.
The rotation measure is determined to range from -400 to -800 rad/m^2 in the
northern jet. Correcting the observed E-vectors of polarized intensity for
Faraday rotation, the magnetic field along the jet turns out to be orientated
mainly along the jet axis. An observed tilt with respect to the jet axis may
indicate also a toroidal magnetic field component or a slightly helical
magnetic field around the northern jet.Comment: 9 pages with 9 figures. Accepted for publication in A&
A Parallel Ocean Model With Adaptive Mesh Refinement Capability For Global Ocean Prediction
An ocean model with adaptive mesh refinement (AMR) capability is presented for simulating ocean circulation on decade time scales. The model closely resembles the LLNL ocean general circulation model with some components incorporated from other well known ocean models when appropriate. Spatial components are discretized using finite differences on a staggered grid where tracer and pressure variables are defined at cell centers and velocities at cell vertices (B-grid). Horizontal motion is modeled explicitly with leapfrog and Euler forward-backward time integration, and vertical motion is modeled semi-implicitly. New AMR strategies are presented for horizontal refinement on a B-grid, leapfrog time integration, and time integration of coupled systems with unequal time steps. These AMR capabilities are added to the LLNL software package SAMRAI (Structured Adaptive Mesh Refinement Application Infrastructure) and validated with standard benchmark tests. The ocean model is built on top of the amended SAMRAI library. The resulting model has the capability to dynamically increase resolution in localized areas of the domain. Limited basin tests are conducted using various refinement criteria and produce convergence trends in the model solution as refinement is increased. Carbon sequestration simulations are performed on decade time scales in domains the size of the North Atlantic and the global ocean. A suggestion is given for refinement criteria in such simulations. AMR predicts maximum pH changes and increases in CO{sub 2} concentration near the injection sites that are virtually unattainable with a uniform high resolution due to extremely long run times. Fine scale details near the injection sites are achieved by AMR with shorter run times than the finest uniform resolution tested despite the need for enhanced parallel performance. The North Atlantic simulations show a reduction in passive tracer errors when AMR is applied instead of a uniform coarse resolution. No dramatic or persistent signs of error growth in the passive tracer outgassing or the ocean circulation are observed to result from AMR
Self-gravitating warped discs around supermassive black holes
We consider warped equilibrium configurations for stellar and gaseous disks
in the Keplerian force-field of a supermassive black hole, assuming that the
self-gravity of the disk provides the only acting torques. Modeling the disk as
a collection of concentric circular rings, and computing the torques in the
non-linear regime, we show that stable, strongly warped precessing equilibria
are possible. These solutions exist for a wide range of disk-to-black hole mass
ratios , can span large warp angles of up to ,
have inner and outer boundaries, and extend over a radial range of a factor of
typically two to four. These equilibrium configurations obey a scaling relation
such that in good approximation \phidot/\Omega\propto M_d/M_{bh} where
\phidot is the (retrograde) precession frequency and is a
characteristic orbital frequency in the disk. Stability was determined using
linear perturbation theory and, in a few cases, confirmed by numerical
integration of the equations of motion. Most of the precessing equilibria are
found to be stable, but some are unstable. The main result of this study is
that highly warped disks near black holes can persist for long times without
any persistent forcing other than by their self-gravity. The possible relevance
of this to galactic nuclei is briefly discussed.Comment: 13 pages, 21 figures, published in MNRA
The Distribution of H2O Maser Emission in the Nucleus of NGC 4945
We present the first interferometer map of the water maser emission in the
active nucleus of NGC 4945. Although the declination of the galaxy is about -49
degrees, we were able to make the observations with the southernmost antennas
of the Very Long Baseline Array. Strong maser emission is present in three
velocity ranges, one near the systemic velocity and two shifted roughly
symmetrically by +/-(100-150) km/s. This is the first detection of highly
blue-shifted water emission in NGC 4945. We determined the position of the
maser to be RA(B1950)= 13 02 32.28 +/- 0.02 ; Dec(B1950)= -49 12 01.9 +/- 0.1.
The uncertainties in earlier estimates are at least several arcseconds. The
maser lies within 2'' (36 pc at a distance of 3.7 Mpc) of the peaks in 1.4 GHz
continuum and 1.6 micron emission from the nucleus. The mappable maser emission
is distributed roughly linearly over about 40 milliarcseconds (0.7 pc) at a
position angle of about 45 degrees, which is close to the 43 +/- 2 degree
position angle of the galactic disk. The red and blue-shifted emission
symmetrically stradle the systemic emission on the sky, which suggests material
in edge-on circular motion around a central object. The position-velocity
structure indicates a binding mass of about one million Suns, within a volume
of radius about 0.3 pc. This implies that the central engine radiates on the
order of 10% of its Eddington luminosity.Comment: 18 pages, including 5 Postscript figures. Accepted for publication in
ApJ Letter
Proper Motion of Water Masers Associated with IRAS 21391+5802: Bipolar Outflow and an AU-Scale Dusty Circumstellar Shell
We present VLBA observations of water maser emission associated with the
star-forming region IRAS 21391+5802, which is embedded in a bright rimmed
cometary globule in IC1396. The angular resolution of the maps is about 0.8
mas, corresponding to a spatial resolution of about 0.6 AU, at an estimated
distance of 750 pc. Proper motions are derived for 10 maser features identified
consistently over three epochs, which were separated by intervals of about one
month. The masers appear in four groups, which are aligned linearly on the sky,
roughly along a northeast-southwest direction, with a total separation of about
520 AU (about 0.7 arcseconds). The 3-D velocities of the masers have a maximum
value of about 42 km/s (about 9 AU/yr). The average error on the derived proper
motions is about 4 km/s. The overall pattern of proper motions is indicative of
a bipolar outflow. Proper motions of the masers in a central cluster, with a
projected extent of about 20 AU, show systematic deviations from a radial
outflow. However, we find no evidence of Keplerian rotation, as has been
claimed elsewhere. A nearly circular loop of masers lies near the middle of the
cluster. The radius of this loop is 1 AU and the line-of-sight velocities of
the masers in the loop are within 2 km/s of the systemic velocity of the
region. These masers presumably exist at the radial distance where significant
dust condensation occurs in the outflow emanating from the star.Comment: 15 pages, 4 figures. Accepted for publication in the Astrophysical
Journal. Version 2.12.00: Astrometric coordinates of maser revise
Proper-Motion Measurements with the VLA. II. Observations of Twenty-eight Pulsars
Using the Very Large Array, we have measured the proper motions of
twenty-eight radio pulsars. On average, the pulsars studied are fainter and
more distant than those studied in earlier work, reducing the selection biases
inherent in surveys restricted to the Solar neighborhood. The typical
measurement precision achieved is a few milliarcseconds per year, corresponding
to a few tens of kilometers per second for a pulsar a kiloparsec away. While
our results compare well with higher-precision measurements done using
very-long baseline interferometry, we find that several earlier proper motion
surveys appear to have reported overly optimistic measurement uncertainties,
most likely because of a failure to fully account for ionospheric effects. We
discuss difficulties inherent in estimating pulsar velocities from proper
motions given poorly constrained pulsar distances. Our observations favor a
distribution with 20% of pulsars in a low velocity component (sigma_1D = 99
km/s) and 80% in a high velocity component (sigma_1D = 294 km/s). Furthermore,
our sample is consistent with a scale height of pulsar birthplaces comparable
to the scale height of the massive stars that are their presumed progenitors.
No evidence is found in our data for a significant population of young pulsars
born far from the plane. We find that estimates of pulsar ages based on
kinematics agree well with the canonical spin-down age estimate, but agreement
is improved if braking indexes are drawn from a Gaussian distribution centered
at n=3 with width 0.8.Comment: 20 pages. Accepted for publication in the Astronomical Journa
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