249 research outputs found
Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
Context. In the framework of the interstellar polycyclic aromatic
hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a
fingerprint of single species in this class. Aims. We address the question of
detectability of low energy PAH vibrational bands, with respect to spectral
contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs).
Methods. We extend our extablished Monte-Carlo model of the photophysics of
specific PAHs in astronomical environments, to include rotational and
anharmonic band structure. The required molecular parameters were calculated in
the framework of the Density Functional Theory. Results. We calculate the
detailed spectral profiles of three low-energy vibrational bands of neutral
naphthalene, and four low-energy vibrational bands of neutral anthracene. They
are used to establish detectability constraints based on intensity ratios with
``classical'' AIBs. A general procedure is suggested to select promising
diagnostics, and tested on available Infrared Space Observatory data for the
Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the
far-IR is a challenging, but promising task, especially in view of the
forthcoming launch of the Herschel Space Observatory.Comment: 13 pages, 13 figures, accepted for publication in A&
Dehydrogenated polycyclic aromatic hydrocarbons and UV bump
Recent calculations have shown that the UV bump at about 217.5 nm in the
extinction curve can be explained by a complex mixture of PAHs in several
charge states. Other studies proposed that the carriers are a restricted
population made of neutral and singly-ionised dehydrogenated coronene molecules
(C24Hn, n less than 3), in line with models of the hydrogenation state of
interstellar PAHs predicting that medium-sized species are highly
dehydrogenated. To assess the observational consequences of the latter
hypothesis we have undertaken a systematic study of the electronic spectra of
dehydrogenated PAHs. We use our first results to see whether such spectra show
strong general trends upon dehydrogenation. We used state-of-the-art techniques
in the framework of the density functional theory (DFT) to obtain the
electronic ground-state geometries, and of the time- dependent DFT to evaluate
the electronic excited-state properties. We computed the absorption
cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and
cationic charge-states. Similar calculations were performed for other PAHs and
their fullydehydrogenated counterparts. pi electron energies are always found
to be strongly affected by dehydrogenation. In all cases we examined,
progressive dehydrogenation translates into a correspondingly progressive blue
shift of the main electronic transitions. In particular, the pi-pi* collective
resonance becomes broader and bluer with dehydrogenation. Its calculated energy
position is therefore predicted to fall in the gap between the UV bump and the
far-UV rise of the extinction curve. Since this effect appears to be
systematic, it poses a tight observational limit on the column density of
strongly dehydrogenated medium-sized PAHs.Comment: 5 pages, 7 figures, Astronomy & Astrophysics, in pres
Turbulence and Multiscaling in the Randomly Forced Navier Stokes Equation
We present an extensive pseudospectral study of the randomly forced
Navier-Stokes equation (RFNSE) stirred by a stochastic force with zero mean and
a variance , where is the wavevector and the dimension . We present the first evidence for multiscaling of velocity structure
functions in this model for . We extract the multiscaling exponent
ratios by using extended self similarity (ESS), examine their
dependence on , and show that, if , they are in agreement with those
obtained for the deterministically forced Navier-Stokes equation (NSE). We
also show that well-defined vortex filaments, which appear clearly in studies
of the NSE, are absent in the RFNSE.Comment: 4 pages (revtex), 6 figures (postscript
Large Scale Traces of Solar System Cold Dust on CMB Anisotropies
We explore the microwave anisotropies at large angular scales produced by the
emission from cold and large dust grains, expected to exist in the outer parts
of the Solar System, using a simple toy model for this diuse emission. Its
amplitude is constrained in the Far-IR by the COBE data and is compatible with
simulations found in the literature. We analyze the templates derived after
subtracting our model from the WMAP ILC 7 yr maps and investigate on the
cosmological implications of such a possible foreground. The anomalies related
to the low quadrupole of the angular power spectrum, the two-point correlation
function, the parity and the excess of signal found in the ecliptic plane are
significantly alleviated. An impact of this foreground for some cosmological
parameters characterizing the spectrum of primordial density perturbations,
relevant for on-going and future CMB anisotropy experiments, is found.Comment: Issue 2.0, Accepted for pub. in MNRAS, Apr 8th, 2011, (sub. Oct 4th,
2010); 10 pages, 6 Figures, 1 table; pdflatex with mn2e, AMS, natbib,
txfonts, graphic
Photonic superdiffusive motion in resonance line radiation trapping - partial frequency redistribution effects
The relation between the jump length probability distribution function and
the spectral line profile in resonance atomic radiation trapping is considered
for Partial Frequency Redistribution (PFR) between absorbed and reemitted
radiation. The single line Opacity Distribution Function [M.N. Berberan-Santos
et.al. J.Chem.Phys. 125, 174308 (2006)] is generalized for PFR and used to
discuss several possible redistribution mechanisms (pure Doppler broadening,
combined natural and Doppler broadening and combined Doppler, natural and
collisional broadening). It is shown that there are two coexisting scales with
a different behavior: the small scale is controlled by the intricate PFR
details while the large scale is essentially given by the atom rest frame
redistribution asymptotic. The pure Doppler and combined natural, Doppler and
collisional broadening are characterized by both small and large scale
superdiffusive Levy flight behaviors while the combined natural and Doppler
case has an anomalous small scale behavior but a diffusive large scale
asymptotic. The common practice of assuming complete redistribution in core
radiation and frequency coherence in the wings of the spectral distribution is
incompatible with the breakdown of superdiffusion in combined natural and
Doppler broadening conditions
Growing smooth interfaces with inhomogeneous, moving external fields: dynamical transitions, devil's staircases and self-assembled ripples
We study the steady state structure and dynamics of an interface in a pure
Ising system on a square lattice placed in an inhomogeneous external field. The
field has a profile with a fixed shape designed to stabilize a flat interface,
and is translated with velocity v_e. For small v_e, the interface is stuck to
the profile, is macroscopically smooth, and is rippled with a periodicity in
general incommensurate with the lattice parameter. For arbitrary orientations
of the profile, the local slope of the interface locks in to one of infinitely
many rational values (devil's staircase) which most closely approximates the
profile. These ``lock-in'' structures and ripples dissappear as v_e increases.
For still larger v_e the profile detaches from the interface which is now
characterized by standard Kardar-Parisi-Zhang (KPZ) exponents.Comment: 4 pages, 4 figures, published version, minor change
Open Clusters IC 4665 and Cr 359 and a Probable Birthplace of the Pulsar PSR B1929+10
Based on the epicyclic approximation, we have simulated the motion of the
young open star clusters IC 4665 and Collinder 359. The separation between the
cluster centers is shown to have been minimal 7 Myr ago, 36 pc. We have
established a close evolutionary connection between IC 4665 and the
Scorpius-Centaurus association -- the separation between the centers of these
structures was pc 15 Myr ago. In addition, the center of IC 4665
at this time was near two well-known regions of coronal gas: the Local Bubble
and the North Polar Spur. The star HIP 86768 is shown to be one of the
candidates for a binary (in the past) with the pulsar PSR B1929+10. At the
model radial velocity of the pulsar km s, a close
encounter of this pair occurs in the vicinity of IC 4665 at a time of -1.1 Myr.
At the same time, using currently available data for the pulsar B1929+10 at its
model radial velocity km s, we show that the hypothesis
of Hoogerwerf et al. (2001) about the breakup of the Oph--B1929+10
binary in the vicinity of Upper Scorpius (US) about 0.9 Myr ago is more
plausible.Comment: 19 pages, 8 figure
Scaling properties in off equilibrium dynamical processes
In the present paper, we analyze the consequences of scaling hypotheses on
dynamic functions, as two times correlations . We show, under general
conditions, that must obey the following scaling behavior , where the scaling variable is
and , two
undetermined functions. The presence of a non constant exponent
signals the appearance of multiscaling properties in the dynamics.Comment: 6 pages, no figure
Field Theory And Second Renormalization Group For Multifractals In Percolation
The field-theory for multifractals in percolation is reformulated in such a
way that multifractal exponents clearly appear as eigenvalues of a second
renormalization group. The first renormalization group describes geometrical
properties of percolation clusters, while the second-one describes electrical
properties, including noise cumulants. In this context, multifractal exponents
are associated with symmetry-breaking fields in replica space. This provides an
explanation for their observability. It is suggested that multifractal
exponents are ''dominant'' instead of ''relevant'' since there exists an
arbitrary scale factor which can change their sign from positive to negative
without changing the Physics of the problem.Comment: RevTex, 10 page
Recommended from our members
High Altitude Aerial Natural Gas Leak Detection System
The objective of this program was to develop and demonstrate a cost-effective and power-efficient advanced standoff sensing technology able to detect and quantify, from a high-altitude (> 10,000 ft) aircraft, natural gas leaking from a high-pressure pipeline. The advanced technology is based on an enhanced version of the Remote Methane Leak Detector (RMLD) platform developed previously by Physical Sciences Inc. (PSI). The RMLD combines a telecommunications-style diode laser, fiber-optic components, and low-cost DSP electronics with the well-understood principles of Wavelength Modulation Spectroscopy (WMS), to indicate the presence of natural gas located between the operator and a topographic target. The transceiver transmits a laser beam onto a topographic target and receives some of the laser light reflected by the target. The controller processes the received light signal to deduce the amount of methane in the laser's path. For use in the airborne platform, we modified three aspects of the RMLD, by: (1) inserting an Erbium-doped optical fiber laser amplifier to increase the transmitted laser power from 10 mW to 5W; (2) increasing the optical receiver diameter from 10 cm to 25 cm; and (3) altering the laser wavelength from 1653 nm to 1618 nm. The modified RMLD system provides a path-integrated methane concentration sensitivity {approx}5000 ppm-m, sufficient to detect the presence of a leak from a high capacity transmission line while discriminating against attenuation by ambient methane. In ground-based simulations of the aerial leak detection scenario, we demonstrated the ability to measure methane leaks within the laser beam path when it illuminates a topographic target 2000 m away. We also demonstrated simulated leak detection from ranges of 200 m using the 25 cm optical receiver without the fiber amplifier
- …