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

Molecular Hydrogen Optical Depth Templates for FUSE Data Analysis

The calculation and use of molecular hydrogen optical depth templates to quickly identify and model molecular hydrogen absorption features longward of the Lyman edge at 912 Angstroms are described. Such features are commonly encountered in spectra obtained by the Far Ultraviolet Spectroscopic Explorer and also in spectra obtained by the Space Telescope Imaging Spectrograph, albeit less commonly. Individual templates are calculated containing all the Lyman and Werner transitions originating from a single rotational state (J'') of the 0th vibrational level (v'') of the ground electronic state. Templates are provided with 0.01 Angstrom sampling for doppler parameters ranging from 2 <= b <= 20 km s^-1 and rotational states 0 <= J'' <= 15. Optical depth templates for excited vibrational states are also available for select doppler parameters. Each template is calculated for a fiducial column density of log[N(cm^-2)] = 21 and may be scaled to any column less than this value without loss of accuracy. These templates will facilitate the determination of the distribution of molecular hydrogen column density as a function of rotational level. The use of these templates will free the user from the computationally intensive task of calculating profiles for a large number of lines and allow concentration on line profile or curve-of-growth fitting to determine column densities and doppler parameters. The templates may be downloaded freely from http://www.pha.jhu.edu/~stephan/h2ools2.htmlComment: 20 pages, 2 tables, 6 figures, submitted to PASP 02-04-2003 Accepted for publication on 03-05-2003 with revisions, including modified fg1, modifed fg6 to become fg2 to support improved error discussion. To appear in the June 2003 issue of the PAS

Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings

We report the discovery of optical ghosts generated when using Volume Phase Holographic (VPH) gratings in spectrographs employing the Littrow configuration. The ghost is caused by light reflected off the detector surface, recollimated by the camera, recombined by, and reflected from, the grating and reimaged by the camera onto the detector. This recombination can occur in two different ways. We observe this ghost in two spectrographs being developed by the University of Wisconsin - Madison: the Robert Stobie Spectrograph for the Southern African Large Telescope and the Bench Spectrograph for the WIYN 3.5m telescope. The typical ratio of the brightness of the ghost relative to the integrated flux of the spectrum is of order 10^-4, implying a recombination efficiency of the VPH gratings of order 10^-3 or higher, consistent with the output of rigorous coupled wave analysis. Any spectrograph employing VPH gratings, including grisms, in Littrow configuration will suffer from this ghost, though the general effect is not intrinsic to VPH gratings themselves and has been observed in systems with conventional gratings in non-Littrow configurations. We explain the geometric configurations that can result in the ghost as well as a more general prescription for predicting its position and brightness on the detector. We make recommendations for mitigating the ghost effects for spectrographs and gratings currently built. We further suggest design modifications for future VPH gratings to eliminate the problem entirely, including tilted fringes and/or prismatic substrates. We discuss the resultant implications on the spectrograph performance metrics.Comment: 13 pages, 8 figures, emulateapj style, accepted for publication in PAS

On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars

A sample of 59 sight lines to reddened Galactic OB stars was examined for correlations of the strength of the CO Fourth Positive (A - X) absorption band system with the ultraviolet interstellar extinction curve parameters. We used archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for comparison to parametric fits of the extinction curves from the literature. A strong correlation with the non-linear far-UV curvature term was found with greater absorption, normalized to E(B-V), being associated with more curvature. A weaker trend with the linear extinction term was also found. Mechanisms for enhancing CO in dust environments exhibiting high non-linear curvature are discussed.Comment: 10 pages, including 6 figures. LaTeX2e (emulateapj5.sty). To appear in ApJ, Sep 20, 200

Rocket and FUSE Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen

We present far-ultraviolet spectroscopy of the emission/reflection nebula IC 405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet Spectroscopic Explorer. Both data sets show a rise in the ratio of the nebular surface brightness to stellar flux (S/F_*) of approximately two orders of magnitude towards the blue end of the far-UV bandpass. Scattering models using simple dust geometries fail to reproduce the observed S/F_* for realistic grain properties. The high spectral resolution of the FUSE data reveals a rich fluorescent molecular hydrogen spectrum ~1000" north of the star that is clearly distinguished from the steady blue continuum. The S/F_* remains roughly constant at all nebular pointings, showing that fluorescent molecular hydrogen is not the dominant cause for the blue rise. We discuss three possible mechanisms for the ``Blue Dust'': differential extinction of the dominant star (HD 34078), unusual dust grain properties, and emission from nebular dust. We conclude that uncertainties in the nebular geometry and the degree of dust clumping are most likely responsible for the blue rise. As an interesting consequence of this result, we consider how IC 405 would appear in a spatially unresolved observation. If IC 405 was observed with a spatial resolution of less than 0.4 pc, for example, an observer would infer a far-UV flux that was 2.5 times the true value, giving the appearance of a stellar continuum that was less extinguished than radiation from the surrounding nebula, an effect that is reminiscent of the observed ultraviolet properties of starburst galaxies.Comment: 8 pages, 9 figures, using emulateapj, ApJ - accepte

Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis

We observed seven central stars of planetary nebulae (CSPN) in the Large Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE), and performed a model-based analysis of these spectra in conjunction with Hubble Space Telescope (HST) spectra in the UV and optical range to determine the stellar and nebular parameters. Most of the objects show wind features, and they have effective temperatures ranging from 38 to 60 kK with mass-loss rates of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich abundances typical of the [WC] class, and find its atmosphere to be iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in their nebulae, which may indicate a shocked environment. One of these (SMP LMC 62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap

Modeling H2 Fluorescence in Planetary Atmospheres with Partial Frequency Redistribution

We present the modeling of partial frequency redistribution (PRD) effects for the fluorescent emission lines of molecular hydrogen, the general computational approximations, and the applications to planetary atmospheres, as well as interstellar medium. Our model is applied to FUSE observations of Jupiter, Saturn, and reflection nebulae, allowing an independent confirmation of the H2 abundance and the structure of planetary atmospheres.Comment: To appear in Proc. Future Directions in Ultraviolet Spectroscopy, Annapolis, Oct. 20-22 200