1,144 research outputs found
Exploiting pattern transformation to tune phononic band gaps in a two-dimensional granular crystal
The band structure of a two-dimensional granular crystal composed of silicone rubber and polytetrafluoroethylene (PTFE) cylinders is investigated numerically. This system was previously shown to undergo a pattern transformation with uniaxial compression by Göncü et al. [Soft Matter 7, 2321 (2011)]. The dispersion relations of the crystal are computed at different levels of deformation to demonstrate the tunability of the band structure, which is strongly affected by the pattern transformation that induces new band gaps. Replacement of PTFE particles with rubber ones reveals that the change of the band structure is essentially governed by pattern transformation rather than particles¿ mechanical properties
Large N gauge theories and topological cigars
We analyze the conjectured duality between a class of double-scaling limits
of a one-matrix model and the topological twist of non-critical superstring
backgrounds that contain the N=2 Kazama-Suzuki SL(2)/U(1) supercoset model. The
untwisted backgrounds are holographically dual to double-scaled Little String
Theories in four dimensions and to the large N double-scaling limit of certain
supersymmetric gauge theories. The matrix model in question is the auxiliary
Dijkgraaf-Vafa matrix model that encodes the F-terms of the above
supersymmetric gauge theories. We evaluate matrix model loop correlators with
the goal of extracting information on the spectrum of operators in the dual
non-critical bosonic string. The twisted coset at level one, the topological
cigar, is known to be equivalent to the c=1 non-critical string at self-dual
radius and to the topological theory on a deformed conifold. The spectrum and
wavefunctions of the operators that can be deduced from the matrix model
double-scaling limit are consistent with these expectations.Comment: 34 page
Radio observations of the cool gas, dust, and star formation in the first galaxies
We summarize cm through submm observations of the host galaxies of z ~ 6
quasars. These observations reveal the cool molecular gas (the fuel for star
formation), the warm dust (heated by star formation), the fine structure line
emission (tracing the CNM and PDRs), and the synchrotron emission. Our results
imply active star formation in ~ 30% of the host galaxies, with star formation
rates ~ 10^3 M_sun/year, and molecular gas masses ~ 10^10 M_sun. Imaging of the
[CII] emission from the most distant quasar reveals a 'maximal starburst disk'
on a scale ~ 1.5 kpc. Gas dynamical studies suggest a departure of these
galaxies from the low-z M_{BH} -- M_{bulge} relation, with the black holes
being, on average, 15 times more massive than expected. Overall, we are
witnessing the co-eval formation of massive galaxies and supermassive black
holes within 1 Gyr of the Big Bang.Comment: First Stars and Galaxies: Challenges in the Next Decade, AIP, 2010;
Austin TX (eds Whelan, Bromm, Yoshida); 7 page
A radiation driven implosion model for the enhanced luminosity of protostars near HII regions
Context. Molecular clouds near the H II regions tend to harbor more luminous
protostars. Aims. Our aim in this paper is to investigate whether or not
radiation-driven implosion mechanism enhances luminosity of protostars near
regions of high-ionizing fluxes. Methods. We performed numerical simulations to
model collapse of cores exposed to UV radiation from O stars. We investigated
dependence of mass loss rates on the initial density profiles of cores and
variation of UV fluxes. We derived simple analytic estimates of accretion rates
and final masses of protostars. Results. Radiation-driven implosion mechanism
can increase accretion rates of protostars by 1-2 orders of magnitude. On the
other hand, mass loss due to photo-evaporation is not large enough to have a
significant impact on the luminosity. The increase of accretion rate makes
luminosity 1-2 orders higher than those of protostars that form without
external triggering. Conclusions. Radiation-driven implosion can help explain
the observed higher luminosity of protostars in molecular clouds near H II
regions.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Quasars in the MAMBO blank field survey
Our MAMBO 1.2 mm blank field imaging survey of ~0.75 sqd has uncovered four
unusually bright sources, with flux densities between 10 and 90 mJy, all
located in the Abell 2125 field. The three brightest are flat spectrum radio
sources with bright optical and X-ray counterparts. Their mm and radio flux
densities are variable on timescales of months. Their X-ray luminosities
classify them as quasars. The faintest of the four mm bright sources appears to
be a bright, radio-quiet starburst at z~3, similar to the sources seen at lower
flux densities in the MAMBO and SCUBA surveys. It may also host a mildly
obscured AGN of quasar-like X-ray luminosity. The three non-thermal mm sources
imply an areal density of flat spectrum radio sources higher by at least 7
compared with that expected from an extrapolation of the lower frequency radio
number counts.Comment: 8 pages, 7 figures. Accepted for publication by A&
Thermodynamics of string black hole with hyperscaling violation
In this paper, we start with black brane and construct specific space-time
which violates hyperscaling. In order to obtain the string solution we apply
Null-Melvin-Twist and -reduction. By using the difference action method we
study thermodynamics of system to obtain Hawking-Page phase transition. In
order to have hyperscaling violation we need to consider
In that case the free energy is always negative and our solution is thermal
radiation without a black hole. Therefore we find that there is not any
Hawking-Page transition. Also, we discuss the stability of system and all
thermodynamical quantities.Comment: 12 pages. Accepted for publication in EPJ
Phase transition and hyperscaling violation for scalar Black Branes
We investigate the thermodynamical behavior and the scaling symmetries of the
scalar dressed black brane (BB) solutions of a recently proposed, exactly
integrable Einstein-scalar gravity model [1], which also arises as
compactification of (p-1)-branes with a smeared charge. The extremal, zero
temperature, solution is a scalar soliton interpolating between a conformal
invariant AdS vacuum in the near-horizon region and a scale covariant metric
(generating hyperscaling violation on the boundary field theory)
asymptotically. We show explicitly that for the boundary field theory this
implies the emergence of an UV length scale (related to the size of the brane),
which decouples in the IR, where conformal invariance is restored. We also show
that at high temperatures the system undergoes a phase transition. Whereas at
small temperature the Schwarzschild-AdS BB is stable, above a critical
temperature the scale covariant, scalar-dressed BB solution, becomes
energetically preferred. We calculate the critical exponent z and the
hyperscaling violation parameter of the scalar-dressed phase. In particular we
show that the hyperscaling violation parameter is always negative. We also show
that the above features are not a peculiarity of the exact integrable model of
Ref.[1], but are a quite generic feature of Einstein-scalar and
Einstein-Maxwell-scalar gravity models for which the squared-mass of the scalar
field is positive and the potential vanishes exponentially as the scalar field
goes to minus infinity.Comment: 20 pages, 4 figures. In the revised version it has been pointed out
that the Einstein-scalar gravity model considered in the paper also arises as
compactification of black p-branes with smeared charge
Size determination of the Centaur Chariklo from millimeter-wavelength bolometer observations
Using the Max-Planck Millimeter Bolometer Array (MAMBO) at the IRAM 30m
telescope we detected emission at 250 GHz from the Centaur Chariklo (1997
CU26). The observed continuum flux density implies a photometric diameter of
273 km. The resulting geometric albedo is 0.055, somewhat higher than expected
from a comparison with most of the other few Centaurs and cometary nuclei for
which such data are available.Comment: 4 pages, 1 Postscript figure, to appear in Astronomy & Astrophysic
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