1,616 research outputs found
Stellar Limb-Darkening Coefficients for CoRot and Kepler
Transiting exoplanets provide unparalleled access to the fundamental
parameters of both extrasolar planets and their host stars. We present
limb-darkening coefficients (LDCs) for the exoplanet hunting CoRot and Kepler
missions. The LDCs are calculated with ATLAS stellar atmospheric model grids
and span a wide range of Teff, log g, and metallically [M/H]. Both CoRot and
Kepler contain wide, nonstandard response functions, and are producing a large
inventory of high-quality transiting lightcurves, sensitive to stellar limb
darkening. Comparing the stellar model limb darkening to results from the first
seven CoRot planets, we find better fits are found when two model intensities
at the limb are excluded in the coefficient calculations. This calculation
method can help to avoid a major deficiency present at the limbs of the 1D
stellar models.Comment: Accepted for publication in A&A. 4 pages, 2 figures, 2 tables. Full
versions of tables 1 and 2 containing limb-darkening coefficients available
at http://vega.lpl.arizona.edu/~sing
Accretion in the Early Kuiper Belt II. Fragmentation
We describe new planetesimal accretion calculations in the Kuiper Belt that
include fragmentation and velocity evolution. All models produce two power law
cumulative size distributions, N_C propto r^{-q}, with q = 2.5 for radii less
than 0.3-3 km and q = 3 for radii exceeding 1-3 km. The power law indices are
nearly independent of the initial mass in the annulus, the initial eccentricity
of the planetesimal swarm, and the initial size distribution of the
planetesimal swarm. The transition between the two power laws moves to larger
radii as the initial eccentricity increases. The maximum size of objects
depends on their intrinsic tensile strength; Pluto formation requires a
strength exceeding 300 erg per gram. Our models yield formation timescales for
Pluto-sized objects of 30-40 Myr for a minimum mass solar nebula. The
production of several `Plutos' and more than 10^5 50 km radius Kuiper Belt
objects leaves most of the initial mass in 0.1-10 km radius objects that can be
collisionally depleted over the age of the solar system. These results resolve
the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.Comment: to appear in the Astronomical Journal (July 1999); 54 pages including
7 tables and 13 figure
Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks
We have investigated evolution of magneto-rotational instability (MRI) in
protoplanetary disks that have radially non-uniform magnetic field such that
stable and unstable regions coexist initially, and found that a zone in which
the disk gas rotates with a super-Keplerian velocity emerges as a result of the
non-uniformly growing MRI turbulence. We have carried out two-dimensional
resistive MHD simulations with a shearing box model. We found that if the
spatially averaged magnetic Reynolds number, which is determined by widths of
the stable and unstable regions in the initial conditions and values of the
resistivity, is smaller than unity, the original Keplerian shear flow is
transformed to the quasi-steady flow such that more flattened (rigid-rotation
in extreme cases) velocity profile emerges locally and the outer part of the
profile tends to be super-Keplerian. Angular momentum and mass transfer due to
temporally generated MRI turbulence in the initially unstable region is
responsible for the transformation. In the local super-Keplerian region,
migrations due to aerodynamic gas drag and tidal interaction with disk gas are
reversed. The simulation setting corresponds to the regions near the outer and
inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of
dead zone, as well as the inner edge, would be a favorable site to accumulate
dust particles to form planetesimals and retain planetary embryos against type
I migration.Comment: 28 pages, 11figures, 1 table, accepted by Ap
DEVELOPMENT AND VALIDATION OF UV SPECTROPHOTOMETRIC METHODS FOR DETERMINATION OF MEGLUMINE IN BULK
UV, first, second and third derivative spectrophotometric methods have been developed for the determination of meglumine. The solutions of standard and sample were prepared in distilled water. For the first method i.e. calibration curve UV spectrophotometric method, the quantitative determination of the drug was carried at 254 nm and the linearity range was found to be 10 – 60 µg/ml. For the first, second, third derivative spectrophotometric methods the drug was determined at 247 nm, 216 nm, 266 nm with the linearity range 10 – 60 µg /ml. The calibration graphs constructed at their wavelength of determination were found to be linear for UV and derivative spectrophotometric methods. All the proposed methods have been extensively validated. There was no significant difference between the performance of the proposed methods regarding the mean values and standard deviations
Size-selective concentration of chondrules and other small particles in protoplanetary nebula turbulence
Size-selective concentration of particles in a weakly turbulent
protoplanetary nebula may be responsible for the initial collection of
chondrules and other constituents into primitive body precursors. This paper
presents the main elements of this process of turbulent concentration. In the
terrestrial planet region, both the characteristic size and size distribution
of chondrules are explained. "Fluffier" particles would be concentrated in
nebula regions which were at a lower gas density and/or more intensely
turbulent. The spatial distribution of concentrated particle density obeys
multifractal scaling}, suggesting a close tie to the turbulent cascade process.
This scaling behavior allows predictions of the probability distributions for
concentration in the protoplanetary nebula to be made. Large concentration
factors (>10^5) are readily obtained, implying that numerous zones of particle
density significantly exceeding the gas density could exist. If most of the
available solids were actually in chondrule sized particles, the ensuing
particle mass density would become so large that the feedback effects on gas
turbulence due to mass loading could no longer be neglected. This paper
describes the process, presenting its basic elements and some implications,
without including the effects of mass loading.Comment: 34 pages, 7 figures; in press for Astrophys. J; expected Jan 01 2001
issu
Collisional Cascades in Planetesimal Disks I. Stellar Flybys
We use a new multiannulus planetesimal accretion code to investigate the
evolution of a planetesimal disk following a moderately close encounter with a
passing star. The calculations include fragmentation, gas and
Poynting-Robertson drag, and velocity evolution from dynamical friction and
viscous stirring. We assume that the stellar encounter increases planetesimal
velocities to the shattering velocity, initiating a collisional cascade in the
disk. During the early stages of our calculations, erosive collisions damp
particle velocities and produce substantial amounts of dust. For a wide range
of initial conditions and input parameters, the time evolution of the dust
luminosity follows a simple relation, L_d/L_{\star} = L_0 / [alpha +
(t/t_d)^{beta}]. The maximum dust luminosity L_0 and the damping time t_d
depend on the disk mass, with L_0 proportional to M_d and t_d proportional to
M_d^{-1}. For disks with dust masses of 1% to 100% of the `minimum mass solar
nebula' (1--100 earth masses at 30--150 AU), our calculations yield t_d approx
1--10 Myr, alpha approx 1--2, beta = 1, and dust luminosities similar to the
range observed in known `debris disk' systems, L_0 approx 10^{-3} to 10^{-5}.
Less massive disks produce smaller dust luminosities and damp on longer
timescales. Because encounters with field stars are rare, these results imply
that moderately close stellar flybys cannot explain collisional cascades in
debris disk systems with stellar ages of 100 Myr or longer.Comment: 33 pages of text, 12 figures, and an animation. The paper will appear
in the March 2002 issue of the Astronmomical Journal. The animation and a
copy of the paper with full resolution figures are at S. Kenyon's planet
formation website: http://cfa-www.harvard.edu/~kenyon/p
Exotic smooth structures and symplectic forms on closed manifolds
We give a short proof of the (known) result that there are no Kaehler
structures on exotic tori. This yields a negative solution to a problem posed
by Benson and Gordon. W discuss the symplectic version of the problem and
analyze results which yield an evidence for the conjecture that there are no
symplectic structures on exotic tori.Comment: AMSLaTeX, 16 pages, a new version. A survey of the symplectic version
of the problem is adde
Accretion in the Early Kuiper Belt I. Coagulation and Velocity Evolution
We describe planetesimal accretion calculations in the Kuiper Belt.
Our evolution code simulates planetesimal growth in a single annulus and
includes velocity evolution but not fragmentation. Test results match analytic
solutions and duplicate previous simulations at 1 AU.
In the Kuiper Belt, simulations without velocity evolution produce a single
runaway body with a radius of 1000 km on a time scale inversely proportional to
the initial mass in the annulus. Runaway growth occurs in 100 Myr for 10 earth
masses and an initial eccentricity of 0.001 in a 6 AU annulus centered at 35
AU. This mass is close to the amount of dusty material expected in a minimum
mass solar nebula extrapolated into the Kuiper Belt.
Simulations with velocity evolution produce runaway growth on a wide range of
time scales. Dynamical friction and viscous stirring increase particle
velocities in models with large (8 km radius) initial bodies. This velocity
increase delays runaway growth by a factor of two compared to models without
velocity evolution. In contrast, collisional damping dominates over dynamical
friction and viscous stirring in models with small (80--800 m radius) initial
bodies. Collisional damping decreases the time scale to runaway growth by
factors of 4--10 relative to constant velocity calculations. Simulations with
minimum mass solar nebulae, 10 earth masses, reach runaway growth on time
scales of 20-40 Myr with 80 m initial bodies, 50-100 Myr with 800 m bodies, and
75-250 Myr for 8 km initial bodies. These growth times vary linearly with the
mass of the annulus but are less sensitive to the initial eccentricity than
constant velocity models.Comment: 45 pages of text (including 5 tables), 31 pages of figur
Palliative Endoscopic Salvage of a Functionally Obstructed Gastrojejunostomy - Report of Technique
BACKGROUND: Gastric outlet obstruction secondary to foregut gastrointestinal malignancies can be managed with a variety of medical, endoscopic, and surgical options. Laparoscopic gastrojejunostomy is an option for those patients who are able to tolerate an operation as a long-term palliative option. This operation may be associated with some significant postoperative technical and nontechnical complications, including delayed gastric emptying. This paper describes an incision-less, endoscopic option that we propose can be used to salvage a functionally obstructed gastrojejunostomy.
CASE DESCRIPTION: A 57-year old male patient had a history of pancreatic adenocarcinoma causing gastric outlet obstruction and underwent a previously created surgical gastrojejunostomy at an outside hospital. His procedure was complicated by anastomotic leak and essentially persistent obstructive symptoms secondary to delayed gastric emptying. Though his anastomosis was demonstrably patent, these symptoms were thought to be secondary to a functional obstruction at the gastrojejunostomy. After repeated workups and many failed attempts to treat these symptoms, he ultimately underwent endoscopic placement of an uncovered colonic stent into the efferent limb of his gastrojejunostomy. This allowed for preferential drainage of gastric contents down the efferent limb, and improvement of his delayed gastric emptying.
CONCLUSIONS: In a select group of patients with advanced foregut malignancy, and with high re-operative risks, salvage endoscopic stenting may be useful in the palliation of symptoms from a functionally obstructed gastrojejunostomy
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