Joshua D. Destree

The abundance of interstellar fluorine and its implications

We report results from a survey of neutral fluorine (F I) in the interstellar medium. Data from FUSE were used to analyze 26 lines of sight lying in both the galactic disk and halo, including lines to Wolf-Rayet stars and through known supernova remnants. The equivalent widths of the fluorine resonance lines at 951.871 and 954.827 A were measured or assigned upper limits and combined with a nitrogen curve of growth to obtain F I column densities. These column densities were then used to calculate fluorine depletions. Comparisons are made to the previous study of F I by Federman and coworkers and implications for F I formation and depletion are discussed.

A Study of the ρ Ophiuchi Cloud: Mapping the Distribution and the Motions of Interstellar Gas

Guest Investigator results from Hipparcos, combined with high-resolution optical spectra obtained with the Anglo-Australian Observatory, Kitt Peak National Observatory, and McDonald Observatory, are used to create a three-dimensional map of the stars and several interstellar gas components in the ? Oph cloud complex. Many principal gas components are interspersed among the bright stars, which can be subdivided into three main groupings on the basis of right ascension and declination. At least one of these main gas components may persist over about 10 pc laterally, and there is evidence for an overall bulk movement of the gas, as determined from the observed radial velocities. We find the diffuse portion of the cloud complex to be clumpy, scattered over a distance range of ~110-150 pc. We find the most likely distance to the dense molecular cloud to be -->122 ? 8 pc.

The search for H- in astrophysical environments

The negative ion H− is widely understood to be important in many astrophysical environments, including the atmospheres of late-type stars like the Sun. However, the ion has never been detected spectroscopically outside the laboratory. A search for the far-ultraviolet autodetaching transitions of H− in interstellar and circumstellar matter seems to be the best hope for directly detecting this ion. We undertook a highly sensitive search using data from the FUSE instrument. We concentrated on two types of sight lines: planetary nebulae, where model calculations suggest a sufficient abundance of H− to be determined, and translucent clouds, where H− might form on dust grains as an intermediate step in molecular hydrogen formation. Upper limits on H− abundances were set.

A new FUSE survey of interstellar HD

We have used archival FUSE data to complete a survey of interstellar HD in 41 lines of sight with a wide range of extinctions. This follow-up to an earlier survey was made to further assess the utility of HD as a cosmological probe; to analyze the HD formation process; and to see what trends with other interstellar properties were present in the data. We employed the curve-of-growth method, supported by line profile fitting, to derive accurate column densities of HD. We find that the N(HD)/2N(H2) ratio is substantially lower than the atomic D/H ratio and conclude that the molecular ratio has no bearing on cosmology, because local processes are responsible for the formation of HD. Based on correlations with E(B − V) , H2, CO, and iron depletion, we find that HD is formed in the densest portion of the clouds; the slope of the log N(HD)/log N(H2) correlation is greater than 1.0, caused by the destruction rate of HD declining more slowly than that of H2; and, as a sidelight, that the depletions are density dependent.

SEARCHING FOR NAPHTHALENE CATION ABSORPTION IN THE INTERSTELLAR MEDIUM

Interstellar naphthalene cations (C10H+ 8) have been proposed by a study to be the carriers of a small number of diffuse interstellar bands (DIBs). Using an archive of high signal-to-noise spectra obtained at the Apache Point Observatory, we used two methods to test the hypothesis. Both methods failed to detect significant absorption at lab wavelengths of interstellar spectra with laboratory spectra. We thereby conclude that C10H+ 8 is not a DIB carrier in typical reddened sight lines.

An Ultraviolet Search for Interstellar CS

High- and medium-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) and the Goddard High Resolution Spectrograph (GHRS) were used to study the diatomic molecule CS through the C-X(0,0) band at 1401 A. The band was modeled to verify profile shape. The rest wavelength of the C-X band is refined to a value of 1400.88 A and a 3σ lower limit is set on the oscillator strength at 0.14 based on equivalent width upper limits of the A-X(0,0) CS Band at 2577 A. The strength of the 1401 A band is compared to other interstellar parameters and implications for CS formation and destruction are briefly discussed.

The Presence of Diffuse Interstellar Bands in the Spectra of Cool Stars

Data from the Kitt Peak National Observatory (KPNO) coude feed telescope were used to analyze the strength of three well-known diffuse interstellar bands (DIBs; 5780, 5797, and 6614 A). Ten mid-A, mid-F, and mid-G stars with moderate reddening (EB-V = 0.2-1.2) were observed, along with one reddened B0 dwarf for comparison. Synthetic spectra were calculated to remove the interference of stellar features. We report the detection of all three DIBs in A, F, and G stars. We also find the correlation of DIB strengths with E(B-V), in our sight lines, to be consistent with previous results from Herbig (1993), demonstrating that DIB strength is not dependent on the spectral type of the target star.

COSMIC ORIGINS SPECTROGRAPH OBSERVATIONS OF TRANSLUCENT CLOUDS: Cyg OB2 8A*

Data from the Cosmic Origins Spectrograph (COS) are presented for the first highly reddened target (Cyg OB2 8A) under the COS Science Teams guaranteed time allocation. Column densities of ionic, atomic, and molecular species are reported and implications are discussed. Data from Cyg OB2 8A demonstrate the ability to analyze highly reddened interstellar sight lines with the COS that were unavailable to previous UV instruments. Measured column densities indicate that the Cyg OB2 8A line of sight contains multiple diffuse clouds rather than a dominant translucent cloud.