Molecular Hydrogen in Orion as Observed by the Far Ultraviolet Spectroscopic Explorer

Diffuse far-ultraviolet stellar emission scattered by dust grains has been observed in a region near the Orion Nebula. In addition to the scattered stellar continuum, emission and absorption features produced by molecular hydrogen have been identified. In this Letter, we present an analysis of this absorption and fluorescent emission from molecular hydrogen in Orion. We model the spectra obtained with the Far Ultraviolet Spectroscopic Explorer using optical depth templates and a fluorescent emission code. These results are surprising because previous studies have found little ultraviolet absorption from H_2 in this region, and the emission is coming from a seemingly empty part of the nebula. We find that the emission fills in the observed absorption lines where the two overlap. These data support the claim that fluorescent excitation by ultraviolet photons is the primary mechanism producing the near-infrared emission spectrum observed in the outer regions of the Orion Nebula.Comment: 4 pages, 2 figures, uses emulate apj. ApJL - accepte

How Well Do We Know the Beta-Decay of 16N and Oxygen Formation in Helium Burning

We review the status of the 12C(a,g)16O reaction rate, of importance for stellar processes in a progenitor star prior to a super-nova collapse. Several attempts to constrain the p-wave S-factor of the 12C(a,g)16O reaction at Helium burning temperatures (200 MK) using the beta-delayed alpha-particle emission of 16N have been made, and it is claimed that this S-factor is known, as quoted by the TRIUMF collaboration. In contrast reanalyses (by G.M. hale) of all thus far available data (including the 16N data) does not rule out a small S-factor solution. Furthermore, we improved our previous Yale-UConn study of the beta- delayed alpha-particle emission of \n16 by improving our statistical sample (by more than a factor of 5), improving the energy resolution of the experiment (by 20%), and in understanding our line shape, deduced from measured quantities. Our newly measured spectrum of the beta-delayed alpha-particle emission of 16N is not consistent with the TRIUMF('94) data, but is consistent with the Seattle('95) data, as well as the earlier (unaltered !) data of Mainz('71). The implication of this discrepancies for the extracted astrophysical p-wave s-factor is briefly discussed.Comment: 6 pages, 4 figures, Invited Talk, Physics With Radioactive Beams, Puri, India, Jan. 12-17, 1998, Work Supported by USDOE Grant No. DE-FG02-94ER4087

Spitzer Mapping of PAHs and H2 in Photodissociation Regions

The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs) and the lowest pure rotational states of molecular hydrogen (H2); two species which are probes of the physical properties of gas and dust in intense UV radiation fields. We utilize the high angular resolution of the Infrared Spectrograph on the Spitzer Space Telescope to construct spectral maps of the PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023, NGC 2023 and IC 63. We present spatially resolved maps of the physical properties, including the H2 ortho-to-para ratio, temperature, and G_o/n_H. We also present evidence for PAH dehydrogenation, which may support theories of H2 formation on PAH surfaces, and a detection of preferential self-shielding of ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2 column densities of ~10^20 cm^-2, G_o/n_H ~ 0.1 - 0.8, and ortho-to-para ratios of ~ 1.8. We find that while the average of each of these properties is consistent with previous single value measurements of these PDRs, when available, the addition of spatial resolution yields a diversity of values with gas temperatures as high as 1500 K, column densities spanning ~ 2 orders of magnitude, and extreme ortho-to-para ratios of 3.Comment: 14 figure

The MMI cash-futures spread on October 19, 1987

Includes bibliographical references (p. 29)

SSR-Based genetic variability assessment of Tunisian local citrus rootstocks : [P22]

In the framework of the 6th PCRD European Project 'CIBEWU', the genetic diversity of Tunisian Citrus rootstocks germplasm was assessed using microsatellite markers (SSR). This work started with a prospection in order to identify the different rootstocks used all over the country, focusing on those which are the most adaptable to abiotic stress (mainly salinity). Sour Orange is by far the most widely used in Tunisia. However, we have also observed other rootstocks in some aged plantations and in southern Tunisia. Our survey has showed that these rootstocks are being used since a long time in the oasis of southern Tunisia and were preferred than Sour Orange by fanners due to their adaptation to the particular conditions of the oasis. We have collected a total set of 206 accessions, on which, 20 SSR markers were performed. This work generated interpretable results for all markers and a total of 121 alleles were revealed. Little variability was observed among Sour Orange (Citrus aurantium L.) rootstocks, while specific alleles were observed for the other rootstocks such as Limes, lemons and oranges. The genetic diversity assessment of the Citrus local rootstocks resources allowed the estimation of the variability either within the Sour Orange and between Sour Orange and the other rootstocks analysed. This work plays a role in the preservation of local resources biodiversity. Prospection, collection and identification, object of this study will avoid the loss of important local resources diversity. Indeed, even if the Sour Orange is threatened by the spread of Citrus Tristeza Virus over the Mediterranean Basin, it constitutes a good local material, very well adapted to local abiotic stress conditions. The Sour Orange diversity with the other local rootstocks form a genetic heritage of great importance for further rootstock breeding project and has to be preserved. (Texte intégral