7,522 research outputs found
Low energy effective gravitational equations on a Gauss-Bonnet brane
We present effective gravitational equations at low energies in a
-symmetric braneworld with the Gauss-Bonnet term. Our derivation is based
on the geometrical projection approach, and we solve iteratively the bulk
geometry using the gradient expansion scheme. Although the original field
equations are quite complicated due to the presence of the Gauss-Bonnet term,
our final result clearly has the form of the Einstein equations plus correction
terms, which is simple enough to handle. As an application, we consider
homogeneous and isotropic cosmology on the brane. We also comment on the
holographic interpretation of bulk gravity in the Gauss-Bonnet braneworld.Comment: 10 pages, v2: minor clarification
Incompressible fluid inside an astrophysical black hole?
It is argued that under natural hypothesis the Fermions inside a black hole
formed after the collapse of a neutron star could form a non compressible fluid
(well before reaching the Planck scale) leading to some features of integer
Quantum Hall Effect. The relations with black hole entropy are analyzed.
Insights coming from Quantum Hall Effect are used to analyze the coupling with
Einstein equations. Connections with some cosmological scenarios and with
higher dimensional Quantum Hall Effect are shortly pointed out.Comment: 30 pages, 2 figures. Accepted for publication on Physical Review D:
references added, typos corrected, test polishe
HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
It has been recently argued that the HCN J=1--0 line emission may not be an
unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous
Infrared Galaxies (LIRGs: ) and HCO J=1--0
may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting
doubt into earlier claims supporting the former as a good tracer of such gas
(Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3
observations of four such galaxies are presented, revealing a surprisingly wide
excitation range for their dense gas phase that may render the J=1--0
transition from either species a poor proxy of its mass. Moreover the
well-known sensitivity of the HCO abundance on the ionization degree of the
molecular gas (an important issue omitted from the ongoing discussion about the
relative merits of HCN and HCO as dense gas tracers) may severely reduce
the HCO abundance in the star-forming and highly turbulent molecular gas
found in LIRGs, while HCN remains abundant. This may result to the decreasing
HCO/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and
casts doubts on the HCO rather than the HCN as a good dense molecular gas
tracer. Multi-transition observations of both molecules are needed to identify
the best such tracer, its relation to ongoing star formation, and constrain
what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical
Journa
Detection of Neutral Carbon in the M 31 Dark Cloud D478
Emission from the 492 GHz CI tranition was detected towards the dark cloud
D478 in M31. Using existing 12CO and 13CO measurements, models for the gas
properties of D478 are discussed. The observed CO and C line ratios can be
explained by two-component models (dense cores and tenuous envelopes);
single-density models appear less likely. The models indicate temperatures
T(kin) = 10 K. The beam-averaged C column density is 0.3 - 0.8 times that of
CO, whereas the total carbon to hydrogen ratio N(C)/N(H) = 5-3 times 10**-4.
The resulting CO-to-H2 conversion factor X is about half that of the Solar
Neighbourhood. With temperatures of about 10 K and projected mass densities of
5-10 M(sun)/pc**2 there appears to be no need to invoke the presence of very
cold and very massive clouds. Rather, D478 appears to be comparable to Milky
Way dark cloud complexes such as the Taurus-Auriga dark cloud complex.Comment: 7 Pages, 1 Figure; accepted by A&
[CII] 158m and [NII] 205m emission from IC 342 - Disentangling the emission from ionized and photo-dissociated regions
Aims: We investigate how much of the [CII] emission in the nucleus of the
nearby spiral galaxy IC 342 is contributed by PDRs and by the ionized gas. We
examine the spatial variations of starburst/PDR activity and study the
correlation of the [CII] line with the [NII] 205{\textmu}m emission line coming
exclusively from the HII regions. Methods: We present small maps of [CII] and
[NII] lines recently observed with the GREAT receiver on board SOFIA. In
particular we present a super-resolution method to derive how unresolved,
kinematically correlated structures in the beam contribute to the observed line
shapes. Results: We find that the emission coming from the ionized gas shows a
kinematic component in addition to the general Doppler signature of the
molecular gas. We interpret this as the signature of two bi-polar lobes of
ionized gas expanding out of the galactic plane. We then show how this requires
an adaptation of our understanding of the geometrical structure of the nucleus
of IC~342. Examining the starburst activity we find ratios
between 400 and 1800 in energy units.
Applying predictions from numerical models of HII and PDR regions to derive the
contribution from the ionized phase to the total [CII] emission we find that
35-90% of the observed [CII] intensity stems from the ionized gas if both
phases contribute. Averaged over the central few hundred parsec we find for the
[CII] contribution a HII-to-PDR ratio of 70:30. Conclusions: The ionized gas in
the center of IC 342 contributes more strongly to the overall [CII] emission
than is commonly observed on larger scales and than is predicted. Kinematic
analysis shows that the majority of the [CII] emission is related to the strong
but embedded star formation in the nuclear molecular ring and only marginally
emitted from the expanding bi-polar lobes of ionized gas.Comment: 20 pages spectra available online:
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ submitted to and accepted by
A&
Comments on photonic shells
We investigate in detail the special case of an infinitely thin static
cylindrical shell composed of counter-rotating photons on circular geodetical
paths separating two distinct parts of Minkowski spacetimes--one inside and the
other outside the shell--and compare it to a static disk shell formed by null
particles counter-rotating on circular geodesics within the shell located
between two sections of flat spacetime. One might ask whether the two cases are
not, in fact, merely one
Bevalac calibration of the SOFIE range and hodoscope detectors
The scintillating optical fiber isotope experiment (SOFIE) is a Cerenkov-dE/dx-Range experiment which was developed initially for balloon flight to study the isotopic composition of cosmic rays in the iron region. The electronic range and hodoscope detectors use scintillating optical fibers to image the tracks of stopping charged particles and to determine their trajectory. The particle range is determined and used together with a Cerenkov measurement to determine the mass of the stopping particle. Preliminary results of a Bevalac calibration performed in August, 1984 with a prototype of the balloon flight instrument, to study the measurement precision in range and trajectory which could be attained with this detector are described
The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2
We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33
in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the
James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to
be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI]
emission offset towards the direction of the center of the HII region. In
addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII
region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on
morphology where the [CI] emission is strongest on the side of the cloud facing
the center of the HII region, and not detected at all on the opposite side This
suggests that the sources of the incident flux creating C from the dissociation
of CO are the massive stars of the HII region. The lowest line ratios are
similar to what is observed in Galactic molecular clouds, while the highest are
similar to starburst galaxies and other regions of intense star formation. The
column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement
with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table
Qualitative Viscous Cosmology
The Full (non--truncated) Israel--Stewart theory of bulk viscosity is applied
to dissipative FRW spacetimes. Dimensionless variables and dimensionless
equations of state are used to write the Einstein--thermodynamic equations as a
plane autonomous system and the qualitative behaviour of this system is
determined. Entropy production in these models is also discussed.Comment: 13 pages, REVTeX, accepted for publication in Physical Review
Kinetic theory for nongeodesic particle motion: Selfinteracting equilibrium states and effective viscous fluid pressures
The particles of a classical relativistic gas are supposed to move under the
influence of a quasilinear (in the particle four-momenta), self-interacting
force inbetween elastic, binary collisions. This force which is completely
fixed by the equilibrium conditions of the gas, gives rise to an effective
viscous pressure on the fluid phenomenological level. Earlier results
concerning the possibility of accelerated expansion of the universe due to
cosmological particle production are reinterpreted. A phenomenon such as power
law inflation may be traced back to specific self-interacting forces keeping
the particles of a gas universe in states of generalized equilibrium.Comment: 16 pages, latex, uses ioplppt.sty. To appear in Class. Quantum Gra
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