84 research outputs found
Detection of CO+ toward the reflection nebula NGC 7023
We have detected CO+ toward the photon-dominated region (PDR) associated with
the reflection nebula NGC 7023. This is the first detection of CO+ in the
vicinity of a Be star. A CO+ column density of ~ 3E11 cm-2 has been derived
toward the PDR peak. We have, however, not detected CO+ in a well shielded
clump of the adjacent molecular cloud, where the CO+/HCO+ abundance ratio is at
least 100 times lower than in the PDR. Our results show, for the first time,
that CO^+ column densities as large as ~ 3E11 cm-2 can be produced in regions
with incident UV fields of just a few 1E3 (in units of Habing field) and
densities of ~ 1E5 cm-3. Furthermore, since the ionization potential of CO is
larger than 13.6 eV, our data rule out the direct photoionization of CO as a
significant CO+ formation mechanism.Comment: 14 pages, 1 Postscript figure, uses aasms4.sty. to appear in
Astrophysical Journal Letter
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei
We present self-consistent models of the vertical structure and emergent
spectrum of AGN accretion disks. The central object is assumed to be a
supermassive Kerr black hole. We demonstrate that NLTE effects and the effects
of a self-consistent vertical structure of a disk play a very important role in
determining the emergent radiation, and therefore should be taken into account.
In particular, NLTE models exhibit a largely diminished H I Lyman discontinuity
when compared to LTE models, and the He II discontinuity appears strongly in
emission for NLTE models. Consequently, the number of ionizing photons in the
He II Lyman continuum predicted by NLTE disk models is by 1 - 2 orders of
magnitude higher than that following from the black-body approximation. This
prediction has important implications for ionization models of AGN broad line
regions, and for models of the intergalactic radiation field and the ionization
of helium in the intergalactic medium.Comment: 11 pages; 2 postscript figures; LaTeX, AASPP4 macro; to appear in the
Astrophysical Journal (Letters
Detection of CO+ in the nucleus of M82
We present the detection of the reactive ion CO+ towards the prototypical
starburst galaxy M82. This is the first secure detection of this short-lived
ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the
inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+]
ratio had only been previously measured towards the atomic peak in the
reflection nebula NGC7023. This detection corroborates that the molecular gas
reservoir in the M82 disk is heavily affected by the UV radiation from the
recently formed stars. Comparing the column densities measured in M82 with
those found in prototypical Galactic photon-dominated regions (PDRs), we need
\~20 clouds along the line of sight to explain our observations. We have
completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR
chemical model successfully explains the [CO+]/[HCO+] ratios measured in the
M~82 nucleus but fails by one order of magnitude to explain the large measured
CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to
reconcile the chemical model and the observations.Comment: 12 pages, 2 figure
Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
Photodissociation Region (PDR) models are computed over a wide range of
physical conditions, from those appropriate to giant molecular clouds
illuminated by the interstellar radiation field to the conditions experienced
by circumstellar disks very close to hot massive stars. These models use the
most up-to-date values of atomic and molecular data, the most current chemical
rate coefficients, and the newest grain photoelectric heating rates which
include treatments of small grains and large molecules. In addition, we examine
the effects of metallicity and cloud extinction on the predicted line
intensities. Results are presented for PDR models with densities over the range
n=10^1-10^7 cm^-3 and for incident far-ultraviolet radiation fields over the
range G_0=10^-0.5-10^6.5, for metallicities Z=1 and 0.1 times the local
Galactic value, and for a range of PDR cloud sizes. We present line strength
and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158
micron, [O I] 63 and 145 micron, [C I] 370 and 609 micron, CO J=1-0, J=2-1,
J=3-2, J=6-5 and J=15-14, as well as the strength of the far-infrared
continuum. These plots will be useful for the interpretation of Galactic and
extragalactic far infrared and submillimeter spectra observable with ISO,
SOFIA, SWAS, FIRST and other orbital and suborbital platforms. As examples, we
apply our results to ISO and ground based observations of M82, NGC 278, and the
Large Magellenic Cloud.Comment: 54 pages, 20 figures, accepted for publication in The Astrophysical
Journa
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
CN and HCN in Dense Interstellar Clouds
We present a theoretical investigation of CN and HCN molecule formation in
dense interstellar clouds. We study the gas-phase CN and HCN production
efficiencies from the outer photon-dominated regions (PDRs) into the opaque
cosmic-ray dominated cores. We calculate the equilibrium densities of CN and
HCN, and of the associated species C+, C, and CO, as functions of the
far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with
hydrogen particle densities from 10^2 to 10^6 cm^-3. We study clouds that are
exposed to FUV fields with intensities 20 to 2*10^5 times the mean interstellar
FUV intensity. We assume cosmic-ray H2 ionization rates ranging from 5*10^-17
s^-1, to an enhanced value of 5*10^-16 s^-1. We also examine the sensitivity of
the density profiles to the gas-phase sulfur abundance.Comment: Accepted for publication in ApJ, 33 pages, 8 figure
Photoevaporation of Clumps in Photodissociation Regions
We present the results of an investigation of the effects of Far Ultraviolet
(FUV) radiation from hot early type OB stars on clumps in star-forming
molecular clouds. Clumps in Photodissociation regions (PDRs) undergo external
heating which, if rapid, creates strong photoevaporative mass flows off the
clump surfaces, and drives shocks into the clumps, compressing them to high
densities. The clumps lose mass on relatively short timescales. The evolution
of an individual clump is found to be sensitive to its initial colunm density,
the temperature of the heated surface and the ratio of the ``turn-on time''
of the heating flux on a clump to its initial sound crossing-time
.
In this paper, we use spherical 1-D numerical hydrodynamic models as well as
approximate analytical models to study the evolution of turbulence-generated
and pressure-confined clumps in PDRs. Turbulent clumps evolve so that their
column densities are equal to a critical value determined by the local FUV
field, and typically have short photoevaporation timescales,
years for a 1 M clump in a typical star-forming region. Clumps that
are confined by an interclump medium may either get completely photoevaporated,
or may preserve a shielded core with a warm, dissociated, protective shell that
absorbs the incident FUV flux. We compare our results with observations of some
well-studied PDRs: the Orion Bar, M17SW, NGC 2023 and the Rosette Nebula. The
data are consistent with both interpretations of clump origin, with a slight
indication for favouring the turbulent model for clumps over pressure-confined
clumps.Comment: To appear in the Astrophysical Journa
Disorder Effects on Exciton-Polariton Condensates
The impact of a random disorder potential on the dynamical properties of Bose
Einstein condensates is a very wide research field. In microcavities, these
studies are even more crucial than in the condensates of cold atoms, since
random disorder is naturally present in the semiconductor structures. In this
chapter, we consider a stable condensate, defined by a chemical potential,
propagating in a random disorder potential, like a liquid flowing through a
capillary. We analyze the interplay between the kinetic energy, the
localization energy, and the interaction between particles in 1D and 2D
polariton condensates. The finite life time of polaritons is taken into account
as well. In the first part, we remind the results of [G. Malpuech et al. Phys.
Rev. Lett. 98, 206402 (2007).] where we considered the case of a static
condensate. In that case, the condensate forms either a glassy insulating phase
at low polariton density (strong localization), or a superfluid phase above the
percolation threshold. We also show the calculation of the first order spatial
coherence of the condensate versus the condensate density. In the second part,
we consider the case of a propagating non-interacting condensate which is
always localized because of Anderson localization. The localization length is
calculated in the Born approximation. The impact of the finite polariton life
time is taken into account as well. In the last section we consider the case of
a propagating interacting condensate where the three regimes of strong
localization, Anderson localization, and superfluid behavior are accessible.
The localization length is calculated versus the system parameters. The
localization length is strongly modified with respect to the non-interacting
case. It is infinite in the superfluid regime whereas it is strongly reduced if
the fluid flows with a supersonic velocity.Comment: chapter for a book "Exciton Polaritons in Microcavities: New
Frontiers" by Springer (2012), the original publication is available at
http://www.springerlink.co
Microscopic Theory of Scattering of Weak Electromagnetic Radiation by a Dense Ensemble of Ultracold Atoms
Based on the developed quantum microscopic theory, the interaction of weak
electromagnetic radiation with dense ultracold atomic clouds is described in
detail. The differential and total cooperative scattering cross sections are
calculated for monochromatic radiation as particular examples of application of
the general theory. The angular, spectral, and polarization properties of
scattered light are determined. The dependence of these quantities on the
sample size and concentration of atoms is studied and the influence of
collective effects is analyzed
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