1,962 research outputs found
Observations of a high-mass protostar in NGC 7538S
We present high angular resolution continuum observations of the high-mass
protostar NGC 7538S with BIMA and CARMA at 3 and 1.4 mm, VLA observations at
1.3, 2, 3.5 and 6 cm, and archive IRAC observations from the Spitzer Space
Observatory, which detect the star at 4.5, 5.8, and 8 m. The star looks
rather unremarkable in the mid-IR. The excellent positional agreement of the
IRAC source with the VLA free-free emission, the OH, CHOH, HO masers,
and the dust continuum confirms that this is the most luminous object in the
NGC 7538S core. The continuum emission at millimeter wavelengths is dominated
by dust emission from the dense cold cloud core surrounding the protostar.
Including all array configurations, the emission is dominated by an elliptical
source with a size of ~ 8" x 3". If we filter out the extended emission we find
three compact mm-sources inside the elliptical core. The strongest one, ,
coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm,
where we have sub-arcsecond resolution. The measured spectral index, ,
between 3 and 1.4 mm is ~ 2.3, and steeper at longer wavelengths, suggesting a
low dust emissivity or that the dust is optically thick. We argue that the dust
in these accretion disks is optically thick and estimate a mass of an accretion
disk or infalling envelope surrounding S to be ~ 60 solar masses.Comment: ApJ Accepted 2012, 13 pages, 9 figure
NGC7538 IRS1 - an ionized jet powered by accretion
Analysis of high spatial resolution VLA images shows that the free-free
emission from NGC7538 IRS1 is dominated by a collimated ionized wind. We have
re-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, and
measured separately the flux density from the compact bipolar core and the
extended (1.5" - 3") lobes. We find that the flux density of the core is
proportional to the frequency to the power of alpha, with alpha being about
0.7. The frequency dependence of the total flux density is slightly steeper
with alpha = 0.8. A massive optically thick hypercompact core with a steep
density gradient can explain this frequency dependence, but it cannot explain
the extremely broad recombination line velocities observed in this source.
Neither can it explain why the core is bipolar rather than spherical, nor the
observed decrease of 4% in the flux density in less than 10 years. An ionized
wind modulated by accretion is expected to vary, because the accretion flow
from the surrounding cloud will vary over time. BIMA and CARMA continuum
observations at 3 mm show that the free-free emission still dominates at 3 mm.
HCO+ J = 1 - 0 observations combined with FCRAO single dish data show a clear
inverse P Cygni profile towards IRS1. These observations confirm that IRS1 is
heavily accreting with an accretion rate of about 2 times 10(-4) solar masses
per year.Comment: Accepted for Astrophysical Journal Letter
A finite-state, finite-memory minimum principle, part 2
In part 1 of this paper, a minimum principle was found for the finite-state, finite-memory (FSFM) stochastic control problem. In part 2, conditions for the sufficiency of the minimum principle are stated in terms of the informational properties of the problem. This is accomplished by introducing the notion of a signaling strategy. Then a min-H algorithm based on the FSFM minimum principle is presented. This algorithm converges, after a finite number of steps, to a person - by - person extremal solution
PDA-BCJR algorithm for factorial hidden Markov models with application to MIMO equalisation
Publication in the conference proceedings of EUSIPCO, Florence, Italy, 200
Robustness results in LQG based multivariable control designs
The robustness of control systems with respect to model uncertainty is considered using simple frequency domain criteria. Results are derived under a common framework in which the minimum singular value of the return difference transfer matrix is the key quantity. In particular, the LQ and LQG robustness results are discussed
High Spectral and Spatial Resolution Observations of the PDR Emission in the NGC2023 Reflection Nebula with SOFIA and APEX
We have mapped the NGC 2023 reflection nebula in [CII] and CO(11--10) with
the heterodyne receiver GREAT on SOFIA and obtained slightly smaller maps in
13CO(3--2), CO(3--2), CO(4--3), CO(6--5), and CO(7--6) with APEX in Chile. We
use these data to probe the morphology, kinematics, and physical conditions of
the C II region, which is ionized by FUV radiation from the B2 star HD37903.
The [CII] emission traces an ellipsoidal shell-like region at a position angle
of ~ -50 deg, and is surrounded by a hot molecular shell. In the southeast,
where the C II region expands into a dense, clumpy molecular cloud ridge, we
see narrow and strong line emission from high-J CO lines, which comes from a
thin, hot molecular shell surrounding the [CII] emission. The [CII] lines are
broader and show photo evaporating gas flowing into the C II region. Based on
the strength of the [13CII] F=2--1 line, the [CII] line appears to be somewhat
optically thick over most of the nebula with an optical depth of a few. We
model the physical conditions of the surrounding molecular cloud and the PDR
emission using both RADEX and simple PDR models. The temperature of the CO
emitting PDR shell is ~ 90 -- 120 K, with densities of 10^5 -- 10^6 cm^-3, as
deduced from RADEX modeling. Our PDR modeling indicates that the PDR layer
where [CII] emission dominates has somewhat lower densities, 10^4 to a few
times 10^5 cm^-3Comment: Accepted by A&
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