2,020 research outputs found
Tidal disruption of inviscid protoplanets
Roche showed that equilibrium is impossible for a small fluid body synchronously orbiting a primary within a critical radius now termed the Roche limit. Tidal disruption of orbitally unbound bodies is a potentially important process for planetary formation through collisional accumulation, because the area of the Roche limit is considerably larger then the physical cross section of a protoplanet. Several previous studies were made of dynamical tidal disruption and different models of disruption were proposed. Because of the limitation of these analytical models, we have used a smoothed particle hydrodynamics (SPH) code to model the tidal disruption process. The code is basically the same as the one used to model giant impacts; we simply choose impact parameters large enough to avoid collisions. The primary and secondary both have iron cores and silicate mantles, and are initially isothermal at a molten temperature. The conclusions based on the analytical and numerical models are summarized
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Emerging Translational Opportunities in Comparative Oncology With Companion Canine Cancers: Radiation Oncology.
It is estimated that more than 6 million pet dogs are diagnosed with cancer annually in the USA. Both primary care and specialist veterinarians are frequently called upon to provide clinical care that improves the quality and/or quantity of life for affected animals. Because these cancers develop spontaneously in animals that often share the same environment as their owners, have intact immune systems and are of similar size to humans, and because the diagnostic tests and treatments for these cancers are similar to those used for management of human cancers, canine cancer provides an opportunity for research that simultaneously helps improve both canine and human health care. This is especially true in the field of radiation oncology, for which there is a rich and continually evolving history of learning from the careful study of pet dogs undergoing various forms of radiotherapy. The purpose of this review article is to inform readers of the potential utility and limitations of using dogs in that manner; the peer-reviewed literature will be critically reviewed, and current research efforts will be discussed. The article concludes with a look toward promising future directions and applications of this pet dog "model.
The Formation of Fragments at Corotation in Isothermal Protoplanetary Disks
Numerical hydrodynamics simulations have established that disks which are
evolved under the condition of local isothermality will fragment into small
dense clumps due to gravitational instabilities when the Toomre stability
parameter is sufficiently low. Because fragmentation through disk
instability has been suggested as a gas giant planet formation mechanism, it is
important to understand the physics underlying this process as thoroughly as
possible. In this paper, we offer analytic arguments for why, at low ,
fragments are most likely to form first at the corotation radii of growing
spiral modes, and we support these arguments with results from 3D hydrodynamics
simulations.Comment: 21 pages, 1 figur
A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity
measurements of 160 metal-poor stars observed with HIRES on the Keck 1
telescope. We report on variability and long-term velocity trends for each star
in our sample. We identify several long-term, low-amplitude radial-velocity
variables worthy of follow-up with direct imaging techniques. We place lower
limits on the detectable companion mass as a function of orbital period. Our
survey would have detected, with a 99.5% confidence level, over 95% of all
companions on low-eccentricity orbits with velocity semi-amplitude K > 100 m/s,
or M_p*sin(i) > 3.0 M_JUP*(P/yr)^(1/3), for orbital periods P< 3 yr. None of
the stars in our sample exhibits radial-velocity variations compatible with the
presence of Jovian planets with periods shorter than the survey duration. The
resulting average frequency of gas giants orbiting metal-poor dwarfs with -2.0
< [Fe/H] < -0.6 is f_p<0.67% (at the 1-sigma confidence level). We examine the
implications of this null result in the context of the observed correlation
between the rate of occurrence of giant planets and the metallicity of their
main-sequence solar-type stellar hosts. By combining our dataset with the
Fischer & Valenti (2005) uniform sample, we confirm that the likelihood of a
star to harbor a planet more massive than Jupiter within 2 AU is a steeply
rising function of the host's metallicity. However, the data for stars with
-1.0 < [Fe/H] < 0.0 are compatible, in a statistical sense, with a constant
occurrence rate f_p~1%. Our results can usefully inform theoretical studies of
the process of giant planet formation across two orders of magnitude in
metallicity.Comment: 59 pages, 7 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
Particle Dark Energy
We explore the physics of a gas of particles interacting with a condensate
that spontaneously breaks Lorentz invariance. The equation of state of this gas
varies from 1/3 to less than -1 and can lead to the observed cosmic
acceleration. The particles are always stable. In our particular class of
models these particles are fermions with a chiral coupling to the condensate.
They may behave as relativistic matter at early times, produce a brief period
where they dominate the expansion with w<0 today, and behave as matter at late
time. There are no small parameters in our models, which generically lead to
dark energy clustering and, depending on the choice of parameters, smoothing of
small scale power.Comment: 8 pages, 5 figures; minor update with added refs; version appearing
in Phys. Rev.
Northward range extension of Cyclinella tenuis Recluz
The venerid bivalve, C. tenuis, was described by Recluz (1852. Jour. de Conch., 3: 250) from Baie de la Pointe-a-Pitre, Guadeloupe in the Lesser Antilles. The northernmost occurrence of C. tenuis was reported as Cape Hatteras, North Carolina, by Dall (1889. U.S. Nat. Mus. Bull. 37, p. 56) and Johnson (1934. Proc. Boston Soc. Nat. Hist., 40: 48). (...
Spectral backscattering properties of marine phytoplankton cultures
The backscattering properties of marine phytoplankton, which are assumed to vary widely with differences in size, shape, morphology and internal structure, have been directly measured in the laboratory on a very limited basis. This work presents results from laboratory analysis of the backscattering properties of thirteen phytoplankton species from five major taxa. Optical measurements include portions of the volume scattering function (VSF) and the absorption and attenuation coefficients at nine wavelengths. The VSF was used to obtain the backscattering coefficient for each species, and we focus on intra- and interspecific variability in spectral backscattering in this work. Ancillary measurements included chlorophyll-a concentration, cell concentration, and cell size, shape and morphology via microscopy for each culture. We found that the spectral backscattering properties of phytoplankton deviate from theory at wavelengths where pigment absorption is significant. We were unable to detect an effect of cell size on the spectral shape of backscattering, but we did find a relationship between cell size and both the backscattering ratio and backscattering crosssection. While particulate backscattering at 555 nm was well correlated to chlorophyll-a concentration for any given species, the relationship was highly variable between species. Results from this work indicate that phytoplankton cells may backscatter light at significantly higher efficiencies than what is predicted by Mie theory, which has important implications for closing the underwater and remotely sensed light budget
A Keck/HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios
We describe a high-precision Doppler search for giant planets orbiting a
well-defined sample of metal-poor dwarfs in the field. This experiment
constitutes a fundamental test of theoretical predictions which will help
discriminate between proposed giant planet formation and migration models. We
present here details on the survey as well as an overall assessment of the
quality of our measurements, making use of the results for the stars that show
no significant velocity variation.Comment: 25 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Mastoparan-Induced Intracellular Ca2+ Fluxes May Regulate Cell-to-Cell Communication in Plants
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