13,279 research outputs found
Scaling relations of supersonic turbulence in star-forming molecular clouds
We present a direct numerical and analytical study of driven supersonic MHD
turbulence that is believed to govern the dynamics of star-forming molecular
clouds. We describe statistical properties of the turbulence by measuring the
velocity difference structure functions up to the fifth order. In particular,
the velocity power spectrum in the inertial range is found to be close to E(k)
\~ k^{-1.74}, and the velocity difference scales as ~ L^{0.42}. The
results agree well with the Kolmogorov--Burgers analytical model suggested for
supersonic turbulence in [astro-ph/0108300]. We then generalize the model to
more realistic, fractal structure of molecular clouds, and show that depending
on the fractal dimension of a given molecular cloud, the theoretical value for
the velocity spectrum spans the interval [-1.74 ... -1.89], while the
corresponding window for the velocity difference scaling exponent is [0.42 ...
0.78].Comment: 17 pages, 6 figures include
Magnetic fields at the periphery of UCHII regions from carbon recombination line observations
Several indirect evidences indicate a magnetic origin for the non-thermal
width of spectral lines observed toward molecular clouds. In this letter, I
suggest that the origin of the non-thermal width of carbon recombination lines
(CRLs) observed from photo-dissociation regions (PDRs) near ultra-compact \HII\
regions is magnetic and that the magnitude of the line width is an estimate of
the \alfven speed. The magnetic field strengths estimated based on this
suggestion compare well with those measured toward molecular clouds with
densities similar to PDR densities. I conclude that multi-frequency CRL
observations have the potential to form a new tool to determine the field
strength near star forming regions.Comment: To appear in ApJ Letter
A Prediction of Brown Dwarfs in Ultracold Molecular Gas
A recent model for the stellar initial mass function (IMF), in which the
stellar masses are randomly sampled down to the thermal Jeans mass from
hierarchically structured pre-stellar clouds, predicts that regions of
ultra-cold CO gas, such as those recently found in nearby galaxies by Allen and
collaborators, should make an abundance of Brown Dwarfs with relatively few
normal stars. This result comes from the low value of the thermal Jeans mass,
considering that the hierarchical cloud model always gives the Salpeter IMF
slope above this lower mass limit. The ultracold CO clouds in the inner disk of
M31 have T~3K and pressures that are probably 10 times higher than in the solar
neighborhood. This gives a mass at the peak of the IMF equal to 0.01 Msun, well
below the Brown Dwarf limit of 0.08 Msun. Using a functional approximation to
the IMF, the ultracold clouds would have 50% of the star-like mass and 90% of
the objects below the Brown Dwarf limit. The brightest of the Brown Dwarfs in
M31 should have an apparent, extinction-corrected K-band magnitude of ~21 mag
in their pre-main sequence phase.Comment: 13 pages, 2 figures, to be published in Astrophysical Journal, Vol
522, September 10, 199
HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
It has been recently argued that the HCN J=1--0 line emission may not be an
unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous
Infrared Galaxies (LIRGs: ) and HCO J=1--0
may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting
doubt into earlier claims supporting the former as a good tracer of such gas
(Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3
observations of four such galaxies are presented, revealing a surprisingly wide
excitation range for their dense gas phase that may render the J=1--0
transition from either species a poor proxy of its mass. Moreover the
well-known sensitivity of the HCO abundance on the ionization degree of the
molecular gas (an important issue omitted from the ongoing discussion about the
relative merits of HCN and HCO as dense gas tracers) may severely reduce
the HCO abundance in the star-forming and highly turbulent molecular gas
found in LIRGs, while HCN remains abundant. This may result to the decreasing
HCO/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and
casts doubts on the HCO rather than the HCN as a good dense molecular gas
tracer. Multi-transition observations of both molecules are needed to identify
the best such tracer, its relation to ongoing star formation, and constrain
what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical
Journa
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
COST OF ORGANIC PORK PRODUCTION: A SEASONAL ANALYSIS AND NEEDED PRICE PREMIUM FOR CONTINUOUS PRODUCTION
Niche markets of agricultural products are experiencing rapid growth. One such market is organic pork. Organic production typically demands specific production conditions that can be cost increasing. This study evaluates the cost of organic pork production, seasonal differences in costs, and premiums necessary to induce continuous organic pork production. In the past few years, niche marketing has been a rapidly growing phenomenon in agriculture. One area that has experienced dramatic growth is the production of organic products. Organic production typically brings with it specific conditions on how the product is produced. These can impact the cost structure of the business and in turn, needed premiums for economic production. One of these niche markets that are growing rapidly is the market for organic pork. A major issue in organic pork production is the differing cost structures across the seasons of the year which relates, in part, to the types of production systems allowed or disallowed. Studies have shown that consumers are very conscientious of product price, freshness, and availability. In the production of organic pork, we are able to demonstrate that there are cost savings to the producer by producing a seasonal product over a continuous product. While this allows for a lower cost for the producer, it causes an uneven pig flow problem throughout the vertical chain which affects packers, marketers, and consumers. The packers are affected by having their plants at full capacity only part of the year. For the consumer, there are certain times of the year when the product is in surplus and other times when it is in shortage causing prices to fluctuate greatly. This implies that both the consumer and the packer may have an incentive to induce the producer to provide a continuous supply of organic pork to the market. This study has two objectives. The first is to provide a detailed analysis of the producer's cost of producing organic pork in a seasonal and continuous production system. The second objective is to develop a premium structure that could induce a producer to adopt a continuous production system. The study addresses the issue by examining the increase of costs involved in expanding a seasonal (summer only farrowing) organic pork production system to continuous production of organic hogs. Production costs differ by production system and season of the year. Organic pork production cost per hundred pounds is projected to be 63.88 per hundred pounds. The continuous system has farrowing occurring in both the winter and summer seasons. With the current premium structures for organic pork, there are more hogs being produced using summer farrowing than by winter farrowing. Consumers prefer a more uniform supply of fresh pork. To foster a more uniform supply of fresh organic pork throughout the year, premiums received by producers need to reflect the seasonal production costs differences. Our results show that if the producer is paid the continuous system's cost of production of 7.47 beyond the seasonal price must be paid to the producer for producing hogs in the winter, and a premium of 9.13 per hundred pounds over the seasonal production costs to induce the producer to produce a continuous supply of hogs.Marketing,
Self-Interacting Dark Matter Halos and the Gravothermal Catastrophe
We study the evolution of an isolated, spherical halo of self-interacting
dark matter (SIDM) in the gravothermal fluid formalism. We show that the
thermal relaxation time, , of a SIDM halo with a central density and
velocity dispersion of a typical dwarf galaxy is significantly shorter than its
age. We find a self-similar solution for the evolution of a SIDM halo in the
limit where the mean free path between collisions, , is everywhere
longer than the gravitational scale height, . Typical halos formed in this
long mean free path regime relax to a quasistationary gravothermal density
profile characterized by a nearly homogeneous core and a power-law halo where
. We solve the more general time-dependent problem and
show that the contracting core evolves to sufficiently high density that
inevitably becomes smaller than in the innermost region. The core
undergoes secular collapse to a singular state (the ``gravothermal
catastrophe'') in a time , which is longer than the
Hubble time for a typical dark matter-dominated galaxy core at the present
epoch. Our model calculations are consistent with previous, more detailed,
N-body simulations for SIDM, providing a simple physical interpretation of
their results and extending them to higher spatial resolution and longer
evolution times. At late times, mass loss from the contracting, dense inner
core to the ambient halo is significantly moderated, so that the final mass of
the inner core may be appreciable when it becomes relativistic and radially
unstable to dynamical collapse to a black hole.Comment: ApJ in press (to appear in April), 12 pages. Extremely minor changes
to agree with published versio
Phase behaviour and particle-size cutoff effects in polydisperse fluids
We report a joint simulation and theoretical study of the liquid-vapor phase
behaviour of a fluid in which polydispersity in the particle size couples to
the strength of the interparticle interactions. Attention is focussed on the
case in which the particles diameters are distributed according to a fixed
Schulz form with degree of polydispersity . The coexistence
properties of this model are studied using grand canonical ensemble Monte Carlo
simulations and moment free energy calculations. We obtain the cloud and shadow
curves as well as the daughter phase density distributions and fractional
volumes along selected isothermal dilution lines. In contrast to the case of
size-{\em independent} interaction strengths (N.B. Wilding, M. Fasolo and P.
Sollich, J. Chem. Phys. {\bf 121}, 6887 (2004)), the cloud and shadow curves
are found to be well separated, with the critical point lying significantly
below the cloud curve maximum. For densities below the critical value, we
observe that the phase behaviour is highly sensitive to the choice of upper
cutoff on the particle size distribution. We elucidate the origins of this
effect in terms of extremely pronounced fractionation effects and discuss the
likely appearance of new phases in the limit of very large values of the
cutoff.Comment: 12 pages, 15 figure
UV Interstellar Absorption Lines towards the Starburst Dwarf Galaxy NGC 1705
Archival Goddard High Resolution Spectrograph low-resolution spectra of NGC
1705, with wavelength ranges 1170.3 to 1461.7 A and 1453.5 to 1740.1 A and a
velocity resolution of about 100 km\s, have been used to derive the velocity
structure and equivalent widths of the absorption lines of Si II 1190.42,
1260.42, 1304.37 and 1526.71 A, S II 1253 , Al II 1670.79 Aand Fe II 1608.45 A
in this sightline. Three relatively narrow absorption components are seen at
LSR velocities --20 km/s, 260 km/sand 540 km/s. Arguments are presented to show
these absorption features are interstellar rather than stellar in origin based
on a comparison with the C III 1175.7 A absorption feature. We identify the
--20 km/s component with Milky Way disk/halo gas and the 260 km/s component
with an isolated high-velocity cloud HVC 487. This small HVC is located about
10 degrees from the H I gas which envelops the Magellanic Clouds and the
Magellanic Stream (MS). The (Si/H) ratio for this HVC is > 0.6 (Si/H)solar
which together with velocity agreement, suggests association with the
Magellanic Cloud and MS gas. H-alpha emission line kinematics of NGC 1705 show
the presence of a kpc-scale expanding supershell of ionized gas centered on the
central nucleus with a blue-shifted emission component at 540 km/s (Meurer et
al. 1992). We identify the 540 km/s absorption component seen in the GHRS
spectra with the front side of this expanding, ionized supershell. The most
striking feature of this component is strong Si II and Al II absorption but
weak Fe II 1608 A absorption. The low Fe II column density derived is most
likely intrinsic since it cannot be accounted for by ionization corrections or
dust depletion. Due to their shallow gravitational potential wells, dwarf
galaxies have small gravitational binding energies and are vulnerable to largeComment: 15 pages, LaTEX, 1 figure. Accepted for publication in Astrophysical
Journal Letter
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