234 research outputs found
Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152
We present visible and near IR images of the compact HII region Sh 152. Some
of these images reveal the presence of Extended Red Emission (ERE) around 698
nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2
micron. Other images show the near infrared (7-12 micron) continuous emission
of the nebula. The ERE emission is found to coincide with the ionized region
and significantly differ from the UIRBs location. Also some evidence is found
in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the
colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ;
available in html format at http://www.obs-hp.fr/preprints.htm
The Photophysics of the Carrier of Extended Red Emission
Interstellar dust contains a component which reveals its presence by emitting
a broad, unstructured band of light in the 540 to 950 nm wavelength range,
referred to as Extended Red Emission (ERE). The presence of interstellar dust
and ultraviolet photons are two necessary conditions for ERE to occur. This is
the basis for suggestions which attribute ERE to an interstellar dust component
capable of photoluminescence. In this study, we have collected all published
ERE observations with absolute-calibrated spectra for interstellar
environments, where the density of ultraviolet photons can be estimated
reliably. In each case, we determined the band-integrated ERE intensity, the
wavelength of peak emission in the ERE band, and the efficiency with which
absorbed ultraviolet photons are contributing to the ERE. The data show that
radiation is not only driving the ERE, as expected for a photoluminescence
process, but is modifying the ERE carrier as manifested by a systematic
increase in the ERE band's peak wavelength and a general decrease in the photon
conversion efficiency with increasing densities of the prevailing exciting
radiation. The overall spectral characteristics of the ERE and the observed
high quantum efficiency of the ERE process are currently best matched by the
recently proposed silicon nanoparticle (SNP) model. Using the experimentally
established fact that ionization of semiconductor nanoparticles quenches their
photoluminescence, we proceeded to test the SNP model by developing a
quantitative model for the excitation and ionization equilibrium of SNPs under
interstellar conditions for a wide range of radiation field densities.Comment: 42 p., incl. 8 fig. Accepted for publication by Ap
The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing the
extended red emission (ERE) in many dusty interstellar environments remains
unidentified. Several competing models are more or less able to match the
observed broad, unstructured ERE band. We now constrain the character of the
ERE carrier further by determining the wavelengths of the radiation that
initiates the ERE. Using the imaging capabilities of the Hubble Space
Telescope, we have resolved the width of narrow ERE filaments appearing on the
surfaces of externally illuminated molecular clouds in the bright reflection
nebula NGC 7023 and compared them with the depth of penetration of radiation of
known wavelengths into the same cloud surfaces. We identify photons with
wavelengths shortward of 118 nm as the source of ERE initiation, not to be
confused with ERE excitation, however. There are strong indications from the
well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic
cirrus that the photon flux with wavelengths shortward of 118 nm is too small
to actually excite the observed ERE, even with 100% quantum efficiency. We
conclude, therefore, that ERE excitation results from a two-step process. While
none of the previously proposed ERE models can match these new constraints, we
note that under interstellar conditions most polycyclic aromatic hydrocarbon
(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV
and that the electronic energy level structure of PAH di-cations is consistent
with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations
deserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap
Extended Red Emission in the "Evil Eye" Galaxy (NGC4826)
We obtained low-resolution, long-slit 5300--9100 A spectroscopy of NGC4826 (a
nearby galaxy with an absorbing dust lane (DL) asymmetrically placed across its
bulge, associated with several HII regions) with a slit encompassing its bulge,
positioned across its nucleus. The wavelength-dependent effects of absorption
and scattering by dust in the lane are evident when comparing the observed
stellar SEDs of pairs of positions symmetrically located with respect to the
nucleus, one on the DL side and one on the opposite side of the bulge, by
assuming that the intrinsic ISRF is axi-symmetric. We analyzed these SED ratios
through the multiple-scattering radiative transfer model of Witt and Gordon and
we discovered strong residual Extended Red Emission (ERE) from a region of the
DL within a distance of 13 arcsec from the nucleus, adjacent to a broad, bright
HII region. ERE is an established phenomenon in the literature interpreted as
originating from photoluminescence by nanometer-sized clusters, illuminated by
UV/optical photons. The complex radial variation of the ERE band-integrated
intensity and of the ERE-to-scattered light band-integrated intensity ratio
with the optical depth of the model derived for the DL and with the strength
and hardness of the illuminating ISRF is reproduced consistently through the
theoretical interpretation of the photophysics of the ERE carrier by Smith and
Witt. When examined within the context of ERE observations in a variety of
Galactic dusty environments (e.g. the diffuse ISM, reflection nebulae,
planetary nebulae and the Orion Nebula), we conclude that the ERE photon
conversion efficiency in NGC4826 is as high as found elsewhere, but that the
size of the actively luminescing nanoparticles there is about twice as large as
those thought to exist in the Galactic diffuse ISM.Comment: 57 pages, 11 Postscript figures, accepted for publication in Ap
Implementation of an Optimal First-Order Method for Strongly Convex Total Variation Regularization
We present a practical implementation of an optimal first-order method, due
to Nesterov, for large-scale total variation regularization in tomographic
reconstruction, image deblurring, etc. The algorithm applies to -strongly
convex objective functions with -Lipschitz continuous gradient. In the
framework of Nesterov both and are assumed known -- an assumption
that is seldom satisfied in practice. We propose to incorporate mechanisms to
estimate locally sufficient and during the iterations. The mechanisms
also allow for the application to non-strongly convex functions. We discuss the
iteration complexity of several first-order methods, including the proposed
algorithm, and we use a 3D tomography problem to compare the performance of
these methods. The results show that for ill-conditioned problems solved to
high accuracy, the proposed method significantly outperforms state-of-the-art
first-order methods, as also suggested by theoretical results.Comment: 23 pages, 4 figure
Preservation of Piecewise Constancy under TV Regularization with Rectilinear Anisotropy
A recent result by Lasica, Moll and Mucha about the -anisotropic
Rudin-Osher-Fatemi model in asserts that the solution is
piecewise constant on a rectilinear grid, if the datum is. By means of a new
proof we extend this result to . The core of our proof consists
in showing that averaging operators associated to certain rectilinear grids map
subgradients of the -anisotropic total variation seminorm to
subgradients
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