S. R. Federman

Interstellar carbon monoxide toward zeta Ophiuchi

Interstellar CO A-X bands in the spectrum of zeta Oph were recorded at high Sound-to-Noise (S/N) with grating G160M of the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Isotopic fractionation of CO is severe: CO-12/CO-13 = 167, C(16)O/C(18)O approximately equal to 1550 and C(16)O/C(17)O is greater than 5900 are found where C-12/C-13 = 70, O-16/O-18 = 500, and O-16.O-17 = 2600 are observed or expected. Standard models of the zeta Oph cloud predict CO-12/CO-13 is less than or approximately 70. The higher observed ratio suggests that photodissociation of CO, not the isotopic charge exchange reaction ((13)C(+) + CO reversible reaction (12)C(+) + (13)(CO), is the dominant influence on the CO-12/CO-13 ratio.

Hubble Space Telescope Survey of Interstellar 12CO/13CO in the Solar Neighborhood

We examine 20 diffuse and translucent Galactic sight lines and extract the column densities of the 12CO and 13CO isotopologues from their ultraviolet A-X absorption bands detected in archival Space Telescope Imaging Spectrograph data with ?/?? ? 46,000. Five more targets with Goddard High-Resolution Spectrograph data are added to the sample that more than doubles the number of sight lines with published Hubble Space Telescope observations of 13CO. Most sight lines have 12CO-to-13CO isotopic ratios that are not significantly different from the local value of 70 for 12C/13C, which is based on millimeter-wave observations of rotational lines in emission from CO and H2CO inside dense molecular clouds, as well as on results from optical measurements of CH+. Five of the 25 sight lines are found to be fractionated toward lower 12C/13C values, while three sight lines in the sample are fractionated toward higher ratios, signaling the predominance of either isotopic charge exchange or selective photodissociation, respectively. There are no obvious trends of the 12CO-to-13CO ratio with physical conditions such as gas temperature or density, yet 12CO/13CO does vary in a complicated manner with the column density of either CO isotopologue, owing to varying levels of competition between isotopic charge exchange and selective photodissociation in the fractionation of CO. Finally, rotational temperatures of H2 show that all sight lines with detected amounts of 13CO pass through gas that is on average colder by 20 K than the gas without 13CO. This colder gas is also sampled by CN and C2 molecules, the latter indicating gas kinetic temperatures of only 28 K, enough to facilitate an efficient charge exchange reaction that lowers the value of 12CO/13CO.

Hubble Space Telescope observations of C2 molecules in diffuse interstellar clouds

Interstellar C2 F-X (1342 A) and D-X (2313 A) bands in the spectrum of zeta Oph were detected using the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST). The total C2 column density is (1.79 +/- 0.06) 10(exp 13)/sq cm for an adopted f-value of 0.0545 for the 2313 A band of the Mulliken (D-X) system. Relative f-values for the 0-0 F-X, 0-0 D-X, and 2-0 A-X (Phillips) bands are derived by combining ultraviolet and near-infrared spectra: f(sub 00 sup FX)/f(sub 00 sup DX) = 1.83 +/- 0.18 and f(sub 20 sup AX)/f(sub 00 sup DX) = 0.0226 +/- 0.0029. For the Mulliken system, lines are detected up to a rotational level J double prime = 24. The relative populations along the rotational ladder are shown to be consistent with the physical and environmental conditions suggested by other diagnostics. Interstellar C2 molecules were detected towards zeta Per (N(C2) = (0.80 +/- 0.23) 10(exp 13)) but not towards Beta(sup 1), pi, and omega(sup 1) Sco(N(C2) less than or equal to 0.17 x 10(exp 13)/sq cm.

Vibrationally excited H2, HCl and NO+ in the diffuse clouds toward zeta Ophiuchi

Absorption lines from vibrationally excited H2 and from HCl were detected in the ultraviolet spectrum of Zeta Oph with the Hubble Space Telescope (HST). Improved upper limits on NO(+) absorption were obtained as well. The data were analyzed with an updated version of a chemical model for diffuse clouds (van Dishoeck & Black 1986) as a guide to understanding this gaseous environment. The comparisons suggest that the flux of ultraviolet radiation impinging on the cloud surface is approximately 1-2 times the average interstellar radiation field, which is lower than once believed.

Ultraviolet Detection of Interstellar 12C17O and the CO Isotopomeric Ratios toward X Persei

We report the detection of fully resolved absorption lines of A-X bands from interstellar 12C17O and 12C18O, through high-resolution spectroscopy of X Persei with the Space Telescope Imaging Spectrograph*. The first ultraviolet measurement of an interstellar 12C17O column density shows that its isotopomeric ratio is 12C16O/12C17O = 8700 ± 3600. Simultaneously, the second ultraviolet detection of interstellar 12C18O establishes its isotopomeric ratio at 3000 ± 600. These ratios are about five times higher than local ambient oxygen isotopic ratios in the ISM. Such severe fractionation of rare species shows that both 12C17O and 12C18O are destroyed by photodissociation, whereas 12C16O avoids destruction through self-shielding. This is to be contrasted with our ratio of 12C16O/13C16O = 73 ± 12 toward X Per, which is indistinguishable from 12C/13C, the result of a balance between the photodissociation of 13C16O and its preferential formation via the isotope exchange reaction between CO and C+.

Detection of boron, cobalt, and other weak interstellar lines toward Zeta Ophiuchi

Numerous weak lines from interstellar atomic species toward Zeta Ophiuchi were observed with the Goddard High-Resolution Spectrograph. Of particular note are the first interstellar detection of cobalt and the detection of boron in this sight line. These measurements provide estimates for the amount of depletion for the two elements. Boron, a volatile, and cobalt, a refractory element, display the depletion pattern found by Savage et al. (1992). The abundance of phosphorus in the H II region associated with the star was obtained from a detection of P III. Additional weak lines from S I, C I, Ni II, and Cu II were detected for the first time; these lines provide the basis for refinements in oscillator strength and column density. Analysis of the neutral sulfur data indicates that the atomic gas is more widely distributed than the molecular material in the main component.

The 11B/10B Ratio of Local Interstellar Diffuse Clouds*

The isotopic ratio 11B/10B of gas in diffuse interstellar clouds toward ? Oph, ? Ori, and ? Sco is measured using HST/Goddard High Resolution Spectrograph echelle spectra of the B II 1362 ? line. To within the errors of measurement, the three lines of sight give identical results for a mean value of 11B/10B = 3.4 ? 0.7, a value quite similar to the solar system ratio of 4.05. These results show that the latter value is not highly anomalous and that a ratio higher than 2.5, as predicted for boron produced from spallation reactions controlled by high-energy cosmic rays, most probably requires a general explanation. The observed ratio is consistent with additional boron production either by spallation by low-energy (Orion) cosmic rays or by neutrino-induced spallation of carbon in Type II supernovae. The total abundance of B in the diffuse clouds is a factor of 5 less than the meteoritic value. This depletion of B is consistent with that found for Cu and Ga, two elements with a condensation temperature similar to B.

Far Ultraviolet Spectroscopic Explorer Measurements of Interstellar Fluorine

The source of fluorine is not well understood, although core-collapse supernovae, Wolf-Rayet stars, and asymptotic giant branch stars have been suggested. A search for evidence of the ν-process during Type II supernovae is presented. Absorption from interstellar F I is seen in spectra of HD 208440 and HD 209339A acquired with the Far Ultraviolet Spectroscopic Explorer. In order to extract the column density for F I from the line at 954 A, absorption from H2 has to be modeled and then removed. Our analysis indicates that for H2 column densities less than about 3 × 1020 cm-2, the amount of F I can be determined from λ954. For these two sight lines, there is no clear indication for enhanced F abundances resulting from the ν-process in a region shaped by past supernovae.The source of fluorine is not well understood, although core-collapse supernovae, Wolf-Rayet stars, and asymptotic giant branch stars have been suggested. A search for evidence of the nu process during Type II supernovae is presented. Absorption from interstellar F I is seen in spectra of HD 208440 and HD 209339A acquired with the Far Ultraviolet Spectroscopic Explorer. In order to extract the column density for F I from the line at 954 A, absorption from H2 has to be modeled and then removed. Our analysis indicates that for H2 column densities less than about 3 x 10^20 cm^-2, the amount of F I can be determined from lambda 954. For these two sight lines, there is no clear indication for enhanced F abundances resulting from the nu process in a region shaped by past supernovae.

Ultraviolet Transitions of Low Condensation Temperature Heavy Elements and New Data for Interstellar Arsenic, Selenium, Tellurium, and Lead

We report on the first interstellar detections of transitions of the heavy elements As II and Se II along with 2 σ upper limits for Te II and Pb II. We also present a line list of the strongest ultraviolet transitions for an additional eight heavy elements, including rest wavelengths and a compilation of existing transition oscillator strengths along with new theoretical values for many species. All these transitions fall within the accessible UV range of the Goddard High Resolution Spectrograph. With two exceptions, all these elements are characterized by relatively low condensation temperatures (T c ≤720 K) and are therefore expected to be only lightly depleted onto dust

Fractionation of CO in the diffuse clouds toward Zeta Ophiuchi

An analysis of CO A-X bands in diffuse clouds toward Zeta Ophiuchi is presented. The results provide isotopic ratios that constitute the strongest observational evidence for highly localized isotopic fractionation of CO in diffuse portions of an interstellar cloud. The value of the fractionation implies that selective photodissociation is the controlling influence of the fractionation. The molecules excitation temperature is discussed and a lower limit to the O-16/O-18 ratio is provided. The absence of CO lines from the gas that provides broad CH+ and CH lines commonly attributed to warm gas behind a shock front is addressed.