264 research outputs found
New Statistical Results on the Angular Distribution of Gamma-Ray Bursts
We presented the results of several statistical tests of the randomness in
the angular sky-distribution of gamma-ray bursts in BATSE Catalog. Thirteen
different tests were presented based on Voronoi tesselation, Minimal spanning
tree and Multifractal spectrum for five classes (short1, short2, intermediate,
long1, long2) of gamma-ray bursts, separately. The long1 and long2 classes are
distributed randomly. The intermediate subclass, in accordance with the earlier
results of the authors, is distributed non-randomly. Concerning the short
subclass earlier statistical tests also suggested some departure from the
random distribution, but not on a high enough confidence level. The new tests
presented in this article suggest also non-randomness here.Comment: in GAMMA-RAY BURSTS 2007: Proceedings of the Santa Fe Conferenc
Discovery of molecular gas around HD 131835 in an APEX molecular line survey of bright debris disks
Debris disks are considered to be gas-poor, but recent observations revealed
molecular or atomic gas in several 10-40 Myr old systems. We used the APEX and
IRAM 30m radiotelescopes to search for CO gas in 20 bright debris disks. In one
case, around the 16 Myr old A-type star HD 131835, we discovered a new
gas-bearing debris disk, where the CO 3-2 transition was successfully detected.
No other individual system exhibited a measurable CO signal. Our Herschel Space
Observatory far-infrared images of HD 131835 marginally resolved the disk both
at 70 and 100m, with a characteristic radius of ~170 au. While in stellar
properties HD 131835 resembles Pic, its dust disk properties are
similar to those of the most massive young debris disks. With the detection of
gas in HD 131835 the number of known debris disks with CO content has increased
to four, all of them encircling young (40 Myr) A-type stars. Based on
statistics within 125 pc, we suggest that the presence of detectable amount of
gas in the most massive debris disks around young A-type stars is a common
phenomenon. Our current data cannot conclude on the origin of gas in HD 131835.
If the gas is secondary, arising from the disruption of planetesimals, then HD
131835 is a comparably young and in terms of its disk more massive analogue of
the Pic system. However, it is also possible that this system similarly
to HD 21997 possesses a hybrid disk, where the gas material is predominantly
primordial, while the dust grains are mostly derived from planetesimals.Comment: Accepted for publication in ApJ, 18 pages, 9 figures, 5 table
Circular Kinks on the Surface of Granular Material Rotated in a Tilted Spinning Bucket
We find that circular kinks form on the surface of granular material when the
axis of rotation is tilted more than the angle of internal friction of the
material. Radius of the kinks is measured as a function of the spinning speed
and the tilting angle. Stability consideration of the surface results in an
explanation that the kink is a boundary between the inner unstable and outer
stable regions. A simple cellular automata model also displays kinks at the
stability boundary
Metastability of a granular surface in a spinning bucket
The surface shape of a spinning bucket of granular material is studied using
a continuum model of surface flow developed by Bouchaud et al. and Mehta et al.
An experimentally observed central subcritical region is reproduced by the
model. The subcritical region occurs when a metastable surface becomes unstable
via a nonlinear instability mechanism. The nonlinear instability mechanism
destabilizes the surface in large systems while a linear instability mechanism
is relevant for smaller systems. The range of angles in which the granular
surface is metastable vanishes with increasing system size.Comment: 8 pages with postscript figures, RevTex, to appear in Phys. Rev.
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