2,097 research outputs found
Multifluid, Magnetohydrodynamic Shock Waves with Grain Dynamics II. Dust and the Critical Speed for C Shocks
This is the second in a series of papers on the effects of dust on
multifluid, MHD shock waves in weakly ionized molecular gas. We investigate the
influence of dust on the critical shock speed, v_crit, above which C shocks
cease to exist. Chernoff showed that v_crit cannot exceed the grain
magnetosound speed, v_gms, if dust grains are dynamically well coupled to the
magnetic field. We present numerical simulations of steady shocks where the
grains may be well- or poorly coupled to the field. We use a time-dependent,
multifluid MHD code that models the plasma as a system of interacting fluids:
neutral particles, ions, electrons, and various ``dust fluids'' comprised of
grains with different sizes and charges. Our simulations include grain inertia
and grain charge fluctuations but to highlight the essential physics we assume
adiabatic flow, single-size grains, and neglect the effects of chemistry. We
show that the existence of a phase speed v_phi does not necessarily mean that C
shocks will form for all shock speeds v_s less than v_phi. When the grains are
weakly coupled to the field, steady, adiabatic shocks resemble shocks with no
dust: the transition to J type flow occurs at v_crit = 2.76 v_nA, where v_nA is
the neutral Alfven speed, and steady shocks with v_s > 2.76 v_nA are J shocks
with magnetic precursors in the ion-electron fluid. When the grains are
strongly coupled to the field, v_crit = min(2.76 v_nA, v_gms). Shocks with
v_crit < v_s < v_gms have magnetic precursors in the ion-electron-dust fluid.
Shocks with v_s > v_gms have no magnetic precursor in any fluid. We present
time-dependent calculations to study the formation of steady multifluid shocks.
The dynamics differ qualitatively depending on whether or not the grains and
field are well coupled.Comment: 43 pages with 17 figures, aastex, accepted by The Astrophysical
Journa
The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2
We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33
in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the
James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to
be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI]
emission offset towards the direction of the center of the HII region. In
addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII
region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on
morphology where the [CI] emission is strongest on the side of the cloud facing
the center of the HII region, and not detected at all on the opposite side This
suggests that the sources of the incident flux creating C from the dissociation
of CO are the massive stars of the HII region. The lowest line ratios are
similar to what is observed in Galactic molecular clouds, while the highest are
similar to starburst galaxies and other regions of intense star formation. The
column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement
with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table
On the hierarchical classification of G Protein-Coupled Receptors
Motivation: G protein-coupled receptors (GPCRs) play an important role in many physiological systems by transducing an extracellular signal into an intracellular response. Over 50% of all marketed drugs are targeted towards a GPCR. There is considerable interest in developing an algorithm that could effectively predict the function of a GPCR from its primary sequence. Such an algorithm is useful not only in identifying novel GPCR sequences but in characterizing the interrelationships between known GPCRs.
Results: An alignment-free approach to GPCR classification has been developed using techniques drawn from data mining and proteochemometrics. A dataset of over 8000 sequences was constructed to train the algorithm. This represents one of the largest GPCR datasets currently available. A predictive algorithm was developed based upon the simplest reasonable numerical representation of the protein's physicochemical properties. A selective top-down approach was developed, which used a hierarchical classifier to assign sequences to subdivisions within the GPCR hierarchy. The predictive performance of the algorithm was assessed against several standard data mining classifiers and further validated against Support Vector Machine-based GPCR prediction servers. The selective top-down approach achieves significantly higher accuracy than standard data mining methods in almost all cases
The Ratio of Ortho- to Para-H2 in Photodissociation Regions
We discuss the ratio of ortho- to para-H2 in photodissociation regions
(PDRs). We draw attention to an apparent confusion in the literature between
the ortho-to-para ratio of molecules in FUV-pumped vibrationally excited
states, and the H2 ortho-to-para abundance ratio. These ratios are not the same
because the process of FUV-pumping of fluorescent H2 emission in PDRs occurs
via optically thick absorption lines. Thus, gas with an equilibrium ratio of
ortho- to para-H2 equal to 3 will yield FUV-pumped vibrationally excited
ortho-to-para ratios smaller than 3, because the ortho-H2 pumping rates are
preferentially reduced by optical depth effects. Indeed, if the ortho and para
pumping lines are on the ``square root'' part of the curve-of-growth, then the
expected ratio of ortho and para vibrational line strengths is the square root
of 3, ~ 1.7, close to the typically observed value. Thus, contrary to what has
sometimes been stated in the literature, most previous measurements of the
ratio of ortho- to para-H2 in vibrationally excited states are entirely
consistent with a total ortho-to-para ratio of 3, the equilibrium value for
temperatures greater than 200 K. We present an analysis and several detailed
models which illustrate the relationship between the total ratios of ortho- to
para-H2 and the vibrationally excited ortho-to-para ratios in PDRs. Recent
Infrared Space Observatory (ISO) measurements of pure rotational and
vibrational H2 emissions from the PDR in the star-forming region S140 provide
strong observational support for our conclusions.Comment: 23 pages (including 5 figures), LaTeX, uses aaspp4.sty, accepted for
publication in Ap
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
Rice-based Cropping Systems - ICRISAT's Experience
Rice is the most important staple food crop in Asia, where about 90% of the world's rice
production is located. Rice covers 81 m ha in South and Southeast Asia, where approximately
30% of the cropped land is irrigated and double-cropped throughout the year. With
the increase in population and a continuous decline in the average farm size (0.5-2.0 ha),
there is a growing need to put the existing land base to better use. The prevailing practice
of monocropping rice and then leaving the fields fallow after the harvest, could prove
disastrous because of its inability to meet the increasing demand for rice and grain
legumes
Complex Langevin Equation and the Many-Fermion Problem
We study the utility of a complex Langevin (CL) equation as an alternative
for the Monte Carlo (MC) procedure in the evaluation of expectation values
occurring in fermionic many-body problems. We find that a CL approach is
natural in cases where non-positive definite probability measures occur, and
remains accurate even when the corresponding MC calculation develops a severe
``sign problem''. While the convergence of CL averages cannot be guaranteed in
principle, we show how convergent results can be obtained in three examples
ranging from simple one-dimensional integrals over quantum mechanical models to
a schematic shell model path integral.Comment: 19 pages, 10 PS figures embedded in tex
A pair of planets around HD 202206 or a circumbinary planet?
Long-term precise Doppler measurements with the CORALIE spectrograph reveal
the presence of a second planet orbiting the solar-type star HD202206. The
radial-velocity combined fit yields companion masses of m_2\sini = 17.4 M_Jup
and 2.44 M_Jup, semi-major axes of a = 0.83 AU and 2.55 AU, and eccentricities
of e = 0.43 and 0.27, respectively. A dynamical analysis of the system further
shows a 5/1 mean motion resonance between the two planets. This system is of
particular interest since the inner planet is within the brown-dwarf limits
while the outer one is much less massive. Therefore, either the inner planet
formed simultaneously in the protoplanetary disk as a superplanet, or the outer
Jupiter-like planet formed in a circumbinary disk. We believe this singular
planetary system will provide important constraints on planetary formation and
migration scenarios.Comment: 9 pages, 14 figures, accepted in A&A, 12-May-200
HST STIS Ultraviolet Spectral Evidence for Outflow in Extreme Narrow-line Seyfert 1 Galaxies: I. Data and Analysis
We present HST STIS observations of two extreme NLS1s, IRAS 13224-3809 and 1H
0707-495. The spectra are characterized by very blue continua, broad, strongly
blueshifted high-ionization lines (including \ion{C}{4} and \ion{N}{5}), and
narrow, symmetric intermediate- (including \ion{C}{3}], \ion{Si}{3}],
\ion{Al}{3}) and low-ionization (e.g., \ion{Mg}{2}) lines centered at their
rest wavelengths. The emission-line profiles suggest that the high-ionization
lines are produced in a wind, and the intermediate- and low-ionization lines
are produced in low-velocity gas associated with the accretion disk or base of
the wind. In this paper, we present the analysis of the spectra from these two
objects; in a companion paper we present photoionization analysis and a toy
dynamical model for the wind. The highly asymmetric profile of \ion{C}{4}
suggests that it is dominated by emission from the wind, so we develop a
template for the wind from the \ion{C}{4} line. We model the bright emission
lines in the spectra using a combination of this template, and a narrow,
symmetric line centered at the rest wavelength. We also analyzed a comparison
sample of HST spectra from 14 additional NLS1s, and construct a correlation
matrix of emission line and continuum properties. A number of strong
correlations were observed, including several involving the asymmetry of the
\ion{C}{4} line.Comment: 26 pages, 10 figures, accepted for publication in ApJ with no change
Warm Molecular Gas Traced with CO J=7->6 in the Galaxy's Central 2 Parsecs: Dynamical Heating of the Circumnuclear Disk
We present an 11 arcsec resolution map of the central two parsecs of the
Galaxy in the CO J =7->6 rotational transition. The CO emission shows rotation
about Sgr A*, but also evidence for non-circular turbulent motion and a clumpy
morphology. We combine our dataset with available CO measurements to model the
physical conditions in the disk. We find that the molecular gas in the region
is both warm and dense, with T~200-300 K, n_H2~50,000-70,000 cm^-3. The mass of
warm molecular gas we measure in the central two parsecs is at least 2000
M_solar, about 20 times the UV-excited atomic gas mass, ruling out an UV
heating scenario for the molecular material. We compare the available spectral
tracers with theoretical models and conclude that molecular gas is heated with
magneto-hydrodynamic shocks with v~10-20 kms and B~0.3-0.5 mG. Using the
conditions derived with the CO analysis, we include the other important
coolants--neutral oxygen and molecular hydrogen--to estimate the total cooling
budget of the molecular material. We derive a mass to luminosity ratio of 2-3
M_solar/ L_solar, which is consistent with the total power dissipated via
turbulent decay in 0.1 pc cells with v_rms~15 kms. These size and velocity
scales are comparable to the observed clumping scale and the velocity
dispersion. At this rate, the material near Sgr A* its dissipating its orbital
energy on an orbital timescale, and cannot last for more than a few orbits. Our
conclusions support a scenario in which the features near Sgr A* such as the
CND and northern arm are generated by infalling clouds with low specific
angular momentum.Comment: 31 pages, including 5 figures, accepted for publication in Ap
- …