856 research outputs found
The Formation of Low-Mass Binary Star Systems Via Turbulent Fragmentation
We characterize the infall rate onto protostellar systems forming in
self-gravitating radiation-hydrodynamic simulations. Using two dimensionless
parameters to determine disks' susceptability to gravitational fragmentation,
we infer limits on protostellar system multiplicity and the mechanism of binary
formation. We show that these parameters give robust predictions even in the
case of marginally resolved protostellar disks. We find that protostellar
systems with radiation feedback predominately form binaries via turbulent
fragmentation, not disk instability, and we predict turbulent fragmentation is
the dominant channel for binary formation for low-mass stars. We clearly
demonstrate that systems forming in simulations including radiative feedback
have fundamentally different parameters than those in purely hydrodynamic
simulations.Comment: 11 pages, 10 figures, accepted to Ap
TurbuStat: Turbulence Statistics in Python
We present TurbuStat (v1.0): a Python package for computing turbulence
statistics in spectral-line data cubes. TurbuStat includes implementations of
fourteen methods for recovering turbulent properties from observational data.
Additional features of the software include: distance metrics for comparing two
data sets; a segmented linear model for fitting lines with a break-point; a
two-dimensional elliptical power-law model; multi-core fast-fourier-transform
support; a suite for producing simulated observations of fractional Brownian
Motion fields, including two-dimensional images and optically-thin HI data
cubes; and functions for creating realistic world coordinate system information
for synthetic observations. This paper summarizes the TurbuStat package and
provides representative examples using several different methods. TurbuStat is
an open-source package and we welcome community feedback and contributions.Comment: Accepted in AJ. 21 pages, 8 figure
The Cop Number of the One-Cop-Moves Game on Planar Graphs
Cops and robbers is a vertex-pursuit game played on graphs. In the classical
cops-and-robbers game, a set of cops and a robber occupy the vertices of the
graph and move alternately along the graph's edges with perfect information
about each other's positions. If a cop eventually occupies the same vertex as
the robber, then the cops win; the robber wins if she can indefinitely evade
capture. Aigner and Frommer established that in every connected planar graph,
three cops are sufficient to capture a single robber. In this paper, we
consider a recently studied variant of the cops-and-robbers game, alternately
called the one-active-cop game, one-cop-moves game or the lazy-cops-and-robbers
game, where at most one cop can move during any round. We show that Aigner and
Frommer's result does not generalise to this game variant by constructing a
connected planar graph on which a robber can indefinitely evade three cops in
the one-cop-moves game. This answers a question recently raised by Sullivan,
Townsend and Werzanski.Comment: 32 page
On the Role of Disks in the Formation of Stellar Systems: A Numerical Parameter Study of Rapid Accretion
We study rapidly accreting, gravitationally unstable disks with a series of
global, three dimensional, numerical experiments using the code ORION. In this
paper we conduct a numerical parameter study focused on protostellar disks, and
show that one can predict disk behavior and the multiplicity of the accreting
star system as a function of two dimensionless parameters which compare the
disk's accretion rate to its sound speed and orbital period. Although
gravitational instabilities become strong, we find that fragmentation into
binary or multiple systems occurs only when material falls in several times
more rapidly than the canonical isothermal limit. The disk-to-star accretion
rate is proportional to the infall rate, and governed by gravitational torques
generated by low-m spiral modes. We also confirm the existence of a maximum
stable disk mass: disks that exceed ~50% of the total system mass are subject
to fragmentation and the subsequent formation of binary companions.Comment: 16 pages, 12 figures, submitte
Assessing the Impact of Astrochemistry on Molecular Cloud Turbulence Statistics
We analyze hydrodynamic simulations of turbulent, star-forming molecular
clouds that are post-processed with the photo-dissociation region
astrochemistry code 3D-PDR. We investigate the sensitivity of 15 commonly
applied turbulence statistics to post-processing assumptions, namely variations
in gas temperature, abundance and external radiation field. We produce
synthetic CO(1-0) and CI(P-P) observations and
examine how the variations influence the resulting emission distributions. To
characterize differences between the datasets, we perform statistical
measurements, identify diagnostics sensitive to our chemistry parameters, and
quantify the statistic responses by using a variety of distance metrics. We
find that multiple turbulent statistics are sensitive not only to the chemical
complexity but also to the strength of the background radiation field. The
statistics with meaningful responses include principal component analysis,
spatial power spectrum and bicoherence. A few of the statistics, such as the
velocity coordinate spectrum, are primarily sensitive to the type of tracer
being utilized, while others, like the delta-variance, strongly respond to the
background radiation field. Collectively, these findings indicate that more
realistic chemistry impacts the responses of turbulent statistics and is
necessary for accurate statistical comparisons between models and observed
molecular clouds.Comment: 27 pages, 21 figures, accepted to Ap
Cluster J Mycobacteriophages: Intron Splicing in Capsid and Tail Genes
Bacteriophages isolated on Mycobacterium smegmatis mc2155 represent many distinct genomes sharing little or no DNA sequence similarity. The genomes are architecturally mosaic and are replete with genes of unknown function. A new group of genomes sharing substantial nucleotide sequences constitute Cluster J. The six mycobacteriophages forming Cluster J are morphologically members of the Siphoviridae, but have unusually long genomes ranging from 106.3 to 117 kbp. Reconstruction of the capsid by cryo-electron microscopy of mycobacteriophage BAKA reveals an icosahedral structure with a triangulation number of 13. All six phages are temperate and homoimmune, and prophage establishment involves integration into a tRNA-Leu gene not previously identified as a mycobacterial attB site for phage integration. The Cluster J genomes provide two examples of intron splicing within the virion structural genes, one in a major capsid subunit gene, and one in a tail gene. These genomes also contain numerous free-standing HNH homing endonuclease, and comparative analysis reveals how these could contribute to genome mosaicism. The unusual Cluster J genomes provide new insights into phage genome architecture, gene function, capsid structure, gene mobility, intron splicing, and evolution. © 2013 Pope et al
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
An Investigation into the Oil Transport and Starvation of Piston-Ring Pack
In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant film thickness, friction and total power loss. The model takes into account starvation caused by reverse flow at the conjunctional inlet wedge, and applied to a ring pack, comprising a compression and scraper ring. Inlet boundaries are calculated for an engine cycle of a four-cylinder, four-stroke gasoline engine operating at 1500 r/min with conditions pertaining to the New European Drive Cycle. The analysis shows the two main sources of starvation: first, due to a physical lack of inlet meniscus and second, due to reverse flow at the inlet wedge significantly affecting the prevailing conditions from the generally assumed idealised boundary conditions. Such an approach has not hitherto been reported in literature
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