7,814 research outputs found
Near-Infrared Photometric Variability of Stars Toward the Chamaeleon I Molecular Cloud
We present the results of a J, H, and K_s photometric monitoring campaign of
a 0.72 x 6 sq deg. area centered on the Chamaeleon I star forming region. Data
were obtained on 15 separate nights over a 4 month time interval using the
2MASS South telescope. Out of a total of 34,539 sources brighter than the
photometric completeness limits (J=16.0, H=15.2, K_s=14.8), 95 exhibit
near-infrared variability in one or more bands. The variables can be grouped
into a population of bright, red objects that are associated with the
Chamaeleon I association, and a population of faint, blue variables that are
dispersed over the full 6 deg of the survey and are likely field stars or older
pre-main-sequence stars unrelated to the present-day Chamaeleon I molecular
cloud. Ten new candidate members of Chamaeleon I, including 8 brown dwarf
candidates, have been identified based on variability and/or near-infrared
excess emission in the J-H vs. H-K_s color-color-diagram. We also provide a
compendium of astrometry and J, H, and K_s photometry for previously identified
members and candidate members of Chamaeleon I.Comment: To appear in AJ; see
http://www.astro.caltech.edu/~jmc/variables/cham1
Universal Dripping and Jetting in a Transverse Shear Flow
One particularly efficient approach to making emulsions having monosized droplets is to push a fluid through an orifice into a transverse flow of a second immiscible fluid. We find that, at an intermediate particle Reynolds number, the final droplet size can be readily computed using a simple force balance. Remarkably like the well-known dripping faucet, this system displays both dripping and jetting behavior, controlled by the capillary, Weber and Ohnesorge numbers of the relevant fluids, and interesting nonlinear behavior such as period doubling near the transition between these two regimes
The Nature of the Giant Outbursts in the Bursting Pulsar GRO J 1744-28
We investigate the possible role of an accretion disk instability in
producing the giant outbursts seen in GRO J1744-28. Specifically, we study the
global, time dependent evolution of the Lightman-Eardley instability which can
develop near the inner edge of an accretion disk when the radiation pressure
becomes comparable to the gas pressure. Broadly speaking, our results are
compatible with earlier works by Taam \& Lin and by Lasota \& Pelat. The
uniqueness of GRO J1744-28 appears to be associated with the constraint that,
in order for outbursts to occur, the rate of accretion at the inner edge must
be within a narrow range just above the critical accretion rate at which
radiation pressure is beginning to become significant.Comment: 11 pages in .tex file, 4 Postscript figures, .tex file uses
aasms.sty; Ap. J. L. 1996, in pres
Scheduling time-critical graphics on multiple processors
This paper describes an algorithm for the scheduling of time-critical rendering and computation tasks on single- and multiple-processor architectures, with minimal pipelining. It was developed to manage scientific visualization scenes consisting of hundreds of objects, each of which can be computed and displayed at thousands of possible resolution levels. The algorithm generates the time-critical schedule using progressive-refinement techniques; it always returns a feasible schedule and, when allowed to run to completion, produces a near-optimal schedule which takes advantage of almost the entire multiple-processor system
Performability: a retrospective and some pointers to the future
As computing and communication systems become physically and logically more complex, their evaluation calls for continued innovation with regard to measure definition, model construction/solution, and tool development. In particular, the performance of such systems is often degradable, i.e., internal or external faults can reduce the quality of a delivered service even though that service, according to its specification, remains proper (failure-free). The need to accommodate this property, using model-based evaluation methods, was the raison d'etre for the concept of performability. To set the stage for additional progress in its development, we present a retrospective of associated theory, techniques, and applications resulting from work in this area over the past decade and a half. Based on what has been learned, some pointers are made to future directions which might further enhance the effectiveness of these methods and broaden their scope of applicability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30223/1/0000615.pd
Gut microbiota and brain function: an evolving field in neuroscience
There is a growing appreciation of the importance of gut microbiota to health and disease. This has been driven by advances in sequencing technology and recent findings demonstrating the important role of microbiota in common health disorders such as obesity. Moreover, the potential role of gut microbiota in influencing brain function, behavior, and mental health has attracted the attention of neuroscientists and psychiatrists. At the 29th International College of Neuropsychopharmacology (CINP) World Congress held in Vancouver, Canada, in June 2014, a group of experts presented the symposium, âGut microbiota and brain function: Relevance to psychiatric disordersâ to review the latest findings in how gut microbiota may play a role in brain function, behavior, and disease. The symposium covered a broad range of topics, including gut microbiota and neuroendocrine function, the influence of gut microbiota on behavior, probiotics as regulators of brain and behavior, and imaging the gut-brain axis in humans. This report provides an overview of these presentations
On the M_V(peak) Versus Orbital Period Relation for Dwarf Nova Outbursts
We present computations for the accretion disk limit cycle model in an
attempt to explain the empirical relation for dwarf nova outbursts between the
peak visual absolute magnitude and orbital period found by Warner. For longer
period systems one sees intrinsically brighter outbursts. This is accounted for
in the limit cycle model by the scaling with radius of the critical surface
density which triggers the dwarf nova outbursts. During the
storage phase of the instability the accretion disk mass must be less than some
maximum value, a value which scales with radius and therefore orbital period.
When the instability is triggered and the accumulated mass is redistributed
into a quasi steady state disk in outburst, the resultant peak optical flux
from the disk is a measure of the total mass which was stored in quiescence. We
compute light curves for a range in outer disk radius (or equivalently, orbital
period), and find that our peak values of are within mag of the
observed relation (peak)(h) for 2\la
P_{orbital}(h) \la 8.Comment: 15 pages, 5 figure
Optical Spectroscopy of the Surface Population of the rho Ophiuchi Molecular Cloud: The First Wave of Star Formation
We present the results of optical spectroscopy of 139 stars obtained with the
Hydra multi-object spectrograph. The objects extend over a 1.3 square degree
area surrounding the main cloud of the rho Oph complex. The objects were
selected from narrowband images to have H alpha in emission. Using the presence
of strong H alpha emission, lithium absorption, location in the
Hertzsprung-Russell diagram, or previously reported x-ray emission, we were
able to identify 88 objects as young stars associated with the cloud. Strong H
alpha emission was confirmed in 39 objects with line widths consistent with
their origin in magnetospheric accretion columns. Two of the strongest
emission-line objects are young, x-ray emitting brown dwarf candidates with M8
spectral types. Comparisons of the bolometric luminosities and effective
temperatures with theoretical models suggest a medianage for this population of
2.1 Myr which is signifcantly older than the ages derived for objects in the
cloud core. It appears that these stars formed contemporaneously with low mass
stars in the Upper Scorpius subgroup, likely triggered by massive stars in the
Upper-Centaurus subgroup.Comment: 35 pages of postscript which includes seven figures (some of which
are multi-panel) and four postscript tables. Astronomical Journal (in press
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