Engineering topics

Leger, A.

Publications and source records attributed to Leger, A..

9 records

NASA NTRS2013

A Possible Future for Space-Based Interferometry

We address the question of space interferometry following the recent outcome of the science themes selection by ESA for the L2/L3 missions slots. We review the current context of exoplanetary sciences and its impact for an interferometric mission. We argue that space interferometry will make a major step forward when the scientific communities interested in this technique will merge their efforts into a coherent technology development plan.

interferometry↗

NASA NTRS1994

Detection of the overtone of the 3.3 micron emission feature in IRAS 21282+5050

The 1.6-1.8 micron spectrum of the planetary nebula, IRAS 21282+5050, a strong emitter of the unidentified interstellar bands, contains a 0.02 micron wide eimission feature centered at 1.680 micron, which is well matched by laboratory spectra of the 0-2 CH stretching mode in polycyclic aromatic hydrocarbons (PAHs). We identify the new feature as the overtone of the well-known 3.3 micron band. In view of the high excitation required for emission in this band, the identification indicates that the emission is by free molecules rather than molecular moieties in solid dust grains. Modeling of the intensity ratio of the 2-0 to 1-0 band implied that the PAHs emitting in these bands contain about 60 carbon atoms. It is inferred that the nu = 2-1 hot band of the CH stretching mode occurs at about 3.43 micron and contributes to the long-wavelength shoulder of the 3.40 micron feature. The main 3.40 micron feature probably is due to aliphatic sidegroups on PAH molecules.

Geballe, T. R.↗

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

Search for polarization of the 3.3 and 11.3-micron interstellar emission features

Results are reported from a search for linear polarization of the 3.3 and 11.3-micron emission features in the Orion ionization front, NGC 7027, HD 44179 (the Red Rectangle), NGC 2023, BD + 30 deg 3639, and Elias 1. Recent theoretical calculations have predicted that if these features are due to PAHs they may be linearly polarized in many astronomical situations. The most sensitive measurements of and upper limits on the feature polarization are 1 percent at 3.3 microns and 3 percent at 11.3 microns.

Sellgren, K.↗

NASA NTRS1986

Identification of polycyclic aromatic hydrocarbons

The nature of the Very Small Grains evidenced by K. Sellgren (1985) is discussed. Thsir stability suggests that they are graphitic material and specifically Polycyclic Aromatic Hydrocarbons (PAHs). The expected infrared emission of a typical PAH, coronene, gives an impressive spectroscopic agreement with the five observed Unidentified IR Emission Features, leading to an unambiguous identification. Those PAHs are the most abundant organic molecules detected to date (f is approximately .00001).

Leger, A.↗

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.↗

NASA NTRS1986

Pregraphitic molecules and the red-rectangle emission

In light of the recent identification of an infrared emission band at 3.3, 6.2, 7.7, 8.6, and 11 microns as emissions from pregraphitic molecules, the possibility that a large broad emission bump in the red part of the spectrum of the Red-Rectangle (AFGL 918) nebula is luminescence from the same molecules was investigated. The abundance, luminescence yield of the carrier, and the phosphorescence is discussed.

Dhendecourt, L.↗

NASA NTRS1986

Are aromatic hydrocarbons the carriers of the diffuse interstellar bands in the visible?

Large Polycyclic Aromatic Hydrocarbons (PAH) are the likely origin of the Unidentified Infrared Emission Feature. The molecules or their ions are also attractive candidates for the carriers of the Diffuse Interstellar Bands (DIBs) in the visible range. The PAHs have optically active transitions in the visible spectrum; have the ability to survive the ultraviolet photons in the Diffuse Interstellar Medium; and are the most abundant among the detected molecular species after H2 and CO. In particular, PAHs are better candidates than the long carbon chains that were proposed previously.

Leger, A.↗

NASA NTRS1985

Desorption from interstellar grains

Different desorption mechanisms from interstellar grains are considered to resolve the conflict between the observed presence of gaseous species in molecular clouds and their expected depletion onto grains. The physics of desorption is discussed with particular reference to the process of grain heating and the specific heat of the dust material. Impulsive heating by X-rays and cosmic rays is addressed. Spot heating of the grains by cosmic rays and how this can lead to desorption of mantles from very large grains is considered. It is concluded that CO depletion on grains will be small in regions with A(V) less than five from the cloud surface and n(H) less than 10,000, in agreement with observations and in contrast to expectations from pure thermal equilibrium. Even in very dense and obscured regions and in the absence of internal ultraviolet sources, the classical evaporation of CO or N2 and O2-rich mantles by cosmic rays is important.

Leger, A.↗