Engineering topics

Desert, F. X.

Publications and source records attributed to Desert, F. X..

10 records

NASA NTRS1992

Modelling of IR emission of interstellar clouds. I - Emission of isolated clouds and dust abundance variations

A model for the IR emission and extinction properties of small dust particles is used here to compute the IR emissions in the IRAS photometric bands for a set of isolated, spherical, and nonhomogeneous clouds heated by the local interstellar radiation field. It is predicted that the IR limb brightening (LB) in the IRAS photometric bands caused by selective absorption of UV photons of different energies in the cloud generally happens for central extinctions over 4 mag at 12 microns and greater than 10 mag at 100 microns and intermediate extinctions for the other IRAS bands. The surface brightness in the four IRAS bands is limited to about 0.4, 0.6, 3, and 10 MJy/sr at 12, 25, 60, and 100 microns respectively when no strong density discontinuity is present at the cloud edge.

Bernard, J. P.↗

NASA NTRS1990

Detection of the 3.3-micron feature in two starburst galaxies

This paper reports the detection of the 3.3-micron emission feature in the center of two external galaxies: IC 694 (interacting with NGC 3690) and NGC 4194 (a merger). This feature has been previously detected in various galactic and extragalactic objects and is thought to be due to very small grains or large molecules that probably belong to the Polycyclic Aromatic Hydrocarbon (PAH) family. Its presence, as well as the IRAS colors, strongly suggest that these galaxies are dominated by starbursts rather than active nuclei. From publishing data and the present observations, the brightness of the feature in different galaxies is studied. A simple model of radiative transfer shows that the 3.3-micron feature brightness of a given galaxy allows the determination of the unreddened surface brightness of the galaxy stellar content. In galaxies with relatively large extinction, the 3.3-micron feature (and the other PAH related features) is therefore a useful spatial indicator of star-formation activity in their centers.

Dennefeld, M.↗

NASA NTRS1989

Photo-thermo-dissociation. I - A general mechanism for destroying molecules

Photothermodissociation (PTD) of an isolated molecule is the name of the process when a photon is absorbed by an electronic transition, the energy is transferred to the vibrations (hot molecule) and then an atom or a fragment is ejected. A statistical physics treatment is presented that goes beyond the thermal approximation and fully takes into account the isolation of the system. A model is given that permits an explicit calculation of the ejection rates of hydrogen and carbon fragments from aromatic molecules as a function of their size and internal energy. Then, the lifetime of species against PTD in different astronomical radiation fields will result when a precise dynamical evolution of their internal energy can be established.

Leger, A.↗

NASA NTRS1988

An all-sky search for molecular cirrus clouds

An all-sky map of IR-excess gas has been produced by subtracting the H I component from the IR brightness. The large IR-excess residuals that are spatially connected are used to define IR-excess clouds. Also, nearby molecular clouds at high Galactic latitude and clouds with local heating sources or a larger dust-to-gas ratio than average are identified. The map and a catalog of 516 significant clouds are presented and statistically analyzed.

Desert, F. X.↗

NASA NTRS1988

The link between IRAS spectra and near-infrared emission features in external galaxies

The relationship in external galaxies between the presence of the near-infrared (NIR) emission features attributed to polycyclic aromatic hydrocarbon (PAH) molecules, and the far-infrared (FIR) properties as observed by IRAS, is investigated. It is found that whenever the NIR features are absent in a galaxy, the FIR spectrum displays an enhancement at shorter wavelengths relative to normal galaxies. This enhancement is always associated with a strong activity in the galactic nucleus. Some Seyfert galaxies do not exhibit such an infrared signature and therefore they are probably energetically dominated by star-formation processes. Finally, the importance of hard UV photons and of the hot medium in the narrow line region of active nuclei is emphasized in relation to the survival of the PAH molecules. In this frame, the absence of PAHs in the galactic center could be taken as evidence for the presence of an active nucleus.

Desert, F. X.↗

NASA NTRS1988

Fractal structure of interstellar cirrus

The paper investigates the fractal structure of some of the infrared cirrus discovered by IRAS. The clouds studied are characterized by a perimeter with fractal dimension 1.26 + or - 0.04 using the method of Lovejoy. This dimension was found to be essentially constant from region to region. Methods of calculating this dimension are compared and the relation of this quantity to some other observable cloud properties is discussed.

Bazell, D.↗

NASA NTRS1988

Small grains and IRAS colors

The paper studies how infrared colors of dust emission from the interstellar medium vary with the energy density of the radiation field on the basis of IRAS observation of the California Nebula. The data suggest that color variations result from a combinatin of equilibrium emission from large grains, and nonequilibrium emission from small grains, with destruction of the small grains emitting at 12 microns at high energy density; it is estimated that 80 percent of these small particles are destroyed for an energy density in ultraviolet photons larger than 50 times that of the average interstellar radiation field in the solar neighborhood. In a color-color diagram, I(v)(60 microns)/I(v)(100 microns) versus I(v)(12 microns)/I(v)(25 microns), the California Nebula measurements at various distances to the ionizing star Zeta Per follow a sequence similar to that of galaxies. This result shows that the position of a galaxy along this sequence is a measure of the intensity of the radiation field in the regions responsible for the infrared emission.

Boulanger, F.↗

NASA NTRS1986

The molecular content of the nearby galaxy from IRAS and HI observations

Because infrared emission is a very good tracer of mass at high latitudes, by combining it with HI observations it provides a convenient though indirect way of observing the spatial distribution of molecular material. Moreover, these observations will premit placing limits on the fraction of total infrared luminosity emitted by dust associated with molecular and atomic hydrogen clouds. A preliminary result from the study of the correlation between HI column density and 100 micron infrared flux density as measured by the IRAS satellite is reported. The ratio F100/W(HI) = R has an average value of roughty 17 KJy/sr/(K km/s) over the whole sky. Bright regions in the FIR such as the Galactic plane and HII regions are excluded from the data. The histogram of the number of pixels vs R has a strong peak near 17 (same units as before) and is asymmetric about this mean value, having a tail at higher values of R. This basic shape is fairly independent of the region of the sky we observe. The peak confirms the general correlation between infrared emission and HI column density reported previously. One way to explain the shape of the distribution is to assume a constant dust to gas mass ratio and a constant interstellar radiation field and associate points in the tail with molecular clouds. In this case the ratio R is higher for points in the tail because it does not account for the column density of molecular hydrogen.

Bazell, D.↗

NASA NTRS1986

Grain temperature fluctuations - A key to infrared spectra

Interstellar grains of radius less than 003 microns are known to have a fluctuating temperature because their internal energy is comparable to the energy of UV and visible photons. The exact resultant grain temperature distribution is given by studying the multiphoton absorption processes. This distribution satisfies an homogeneous integral equation which can be solved by an iteration technique. These small grains generally emit at shorter wavelengths than big cold grians and therefore could help in the interpretation of the infrared spectra (1-1000 microns) of various astrophysical objects.

Desert, F. X.↗

NASA NTRS1986

Nature of very small grains - PAH molecules or silicates?

The predictions of the model of Puget et al. (1985) for the emission from Very Small Grains (VSGs) including both graphitic and silicate components are compared with published 8-13-micron observations of astronomical sources. The VSGs are found to be mainly graphitic and an upper limit is placed on the relative mass of silicates based on lack of the 9.7-micron silicate emission feature on M 82 and NGC 2023. This dissymetry in the composition of VSGs supports the suggestion that they are formed in grain-grain collisions where the behaviors of graphite and silicate grains are expected to be quite different.

Desert, F. X.↗