188 research outputs found
Markov Process of Muscle Motors
We study a Markov random process describing a muscle molecular motor
behavior. Every motor is either bound up with a thin filament or unbound. In
the bound state the motor creates a force proportional to its displacement from
the neutral position. In both states the motor spend an exponential time
depending on the state. The thin filament moves at its velocity proportional to
average of all displacements of all motors. We assume that the time which a
motor stays at the bound state does not depend on its displacement. Then one
can find an exact solution of a non-linear equation appearing in the limit of
infinite number of the motors.Comment: 10 page
A Search for Small-Scale Clumpiness in Dense Cores of Molecular Clouds
We have analyzed HCN(1-0) and CS(2-1) line profiles obtained with high
signal-to-noise ratios toward distinct positions in three selected objects in
order to search for small-scale structure in molecular cloud cores associated
with regions of high-mass star formation. In some cases, ripples were detected
in the line profiles, which could be due to the presence of a large number of
unresolved small clumps in the telescope beam. The number of clumps for regions
with linear scales of ~0.2-0.5 pc is determined using an analytical model and
detailed calculations for a clumpy cloud model; this number varies in the
range: ~2 10^4-3 10^5, depending on the source. The clump densities range from
~3 10^5-10^6 cm^{-3}, and the sizes and volume filling factors of the clumps
are ~(1-3) 10^{-3} pc and ~0.03-0.12. The clumps are surrounded by inter-clump
gas with densities not lower than ~(2-7) 10^4 cm^{-3}. The internal thermal
energy of the gas in the model clumps is much higher than their gravitational
energy. Their mean lifetimes can depend on the inter-clump collisional rates,
and vary in the range ~10^4-10^5 yr. These structures are probably connected
with density fluctuations due to turbulence in high-mass star-forming regions.Comment: 23 pages including 4 figures and 4 table
Multi-frequency Studies of Massive Cores with Complex Spatial and Kinematic Structures
Five regions of massive star formation have been observed in various
molecular lines in the frequency range GHz. The studied regions
possess dense cores, which host young stellar objects. The physical parameters
of the cores are estimated, including kinetic temperatures ( K),
sizes of the emitting regions ( pc), and virial masses (). Column densities and abundances of various molecules are
calculated in the local thermodynamical equilibrium approximation. The core in
99.982+4.17, associated with the weakest IRAS source, is characterized by
reduced molecular abundances. Molecular line widths decrease with increasing
distance from the core centers (). For b\ga 0.1~pc, the dependences
are close to power laws (), where varies from
to , depending on the object. In four cores, the
asymmetries of the optically thick HCN(1--0) and HCO(1--0) lines indicate
systematic motions along the line of sight: collapse in two cores and expansion
in two others. Approximate estimates of the accretion rates in the collapsing
cores indicate that the forming stars have masses exceeding the solar mass.Comment: 18 pages, 7 figures, 6 table
Chemical differentiation in regions of high-mass star formation I. CS, dust and N2H^+ in southern sources
Aims. Our goals are to compare the CS, N2H+ and dust distributions in a
representative sample of high-mass star forming dense cores and to determine
the physical and chemical properties of these cores. Methods. We compare the
results of CS(5-4) and 1.2 mm continuum mapping of twelve dense cores from the
southern hemisphere presented in this work, in combination with our previous
N2H+(1-0) and CS(2-1) data. We use numerical modeling of molecular excitation
to estimate physical parameters of the cores. Results. Most of the maps have
several emission peaks (clumps). We derive basic physical parameters of the
clumps and estimate CS and N2H+ abundances. Masses calculated from LVG
densities are higher than CS virial masses and masses derived from continuum
data, implying small-scale clumpiness of the cores. For most of the objects,
the CS and continuum peaks are close to the IRAS point source positions. The
CS(5-4) intensities correlate with continuum fluxes per beam in all cases, but
only in five cases with the N2H+(1-0) intensities. The study of spatial
variations of molecular integrated intensity ratios to continuum fluxes reveals
that I(N2H+)/F{1.2} ratios drop towards the CS peaks for most of the sources,
which can be due to a N2H+ abundance decrease. For CS(5-4), the I(CS)/F{1.2}
ratios show no clear trends with distance from the CS peaks, while for CS(2-1)
such ratios drop towards these peaks. Possible explanations of these results
are considered. The analysis of normalized velocity differences between CS and
N2H+ lines has not revealed indications of systematic motions towards CS peaks.Comment: 13 pages, 5 figures, accepted by Astronomy and Astrophysic
N2H+(1-0) survey of massive molecular cloud cores
We present the results of N2H+(1-0) observations of 35 dense molecular cloud
cores from the northern and southern hemispheres where massive stars and star
clusters are formed. Line emission has been detected in 33 sources, for 28
sources detailed maps have been obtained. The optical depth of (23-12)
component toward peak intensity positions of 10 sources is ~ 0.2-1. In total,
47 clumps have been revealed in 26 sources. Integrated intensity maps with
aspect ratios < 2 have been fitted with a power-law radial distribution
convolved with the telescope beam. Mean power-law index is close to
unity corresponding to the density profile provided N2H+
excitation conditions do not vary inside these regions. Line widths of the
cores either decrease or stay constant with distance from the center. The ratio
of rotational to gravitational energy is too low for rotation to play a
significant role in the dynamics of the cores. A correlation between mean line
widths and sizes of clumps has been found.Comment: 17 pages, Late
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