Christoforos Mallouris

Intermediate- and High-Velocity Ionized Gas toward ζ Orionis*

We combine near-UV spectra obtained with the Hubble Space Telescope GHRS echelle with far-UV spectra obtained with the Interstellar Medium Absorption Profile Spectrograph and Copernicus to study the abundances and physical conditions in the predominantly ionized gas seen at high velocity (� 105 km s � 1 dvd � 65 km s � 1 ) and at intermediate velocity (� 60 km s � 1 dvd � 10 km s � 1 ) along the line of sight to the starOri. We have high-resolution (FWHM � 3:3 4: 5k m s � 1 ) and/or high signal-to-noise ratio spec- tra for at least two significant ions of C, N, Al, Si, S, and Fe—enabling accurate estimates for both the total N(H ii) and the elemental depletions. C, N, and S have essentially solar relative abundances; Al, Si, and Fe appear to be depleted by about 0.8, 0.3-0.4, and 0.95 dex, respectively, relative to C, N, and S. While various ion ratios would be consistent with collisional ionization equilibrium (CIE) at temperatures of 25,000-80,000 K, the widths of individual high-velocity absorption components indicate that T � 9000 � 2000 K—so the gas is not in CIE. Analysis of the C ii fine-structure excitation equilibrium, at that temperature, yields estimates for the densities (nenH � 0:1 0: 2c m � 3 ), thermal pressures (2nHT � 2000 4000 cm � 3 K), and thicknesses (0.5-2.7 pc) characterizing the individual clouds. We compare the abundances and physical properties derived for these clouds with those found for gas at similar velocities toward 23 Ori andCMa and also with several different models for shocked gas. While the shock models can reproduce some features of the observed line profiles and some of the observed ion ratios, there are also significant differences between the models and the data. The measured depletions suggest that roughly 10% of the Al, Si, and Fe originally locked in dust in the preshock medium may have been returned to the gas phase, consistent with recent predictions for the destruction of silicate dust in a 100 km s � 1 shock. The observed near-solar gas-phase abundance of carbon, however, appears to be inconsistent with the predicted longer timescales for the destruction of graphite grains. Subject headings: ISM: abundances — ISM: general — stars: individual (� Orionis, � Canis Majoris) — ultraviolet: ISM

Far Ultraviolet Spectroscopic Explorer Observations of Interstellar Gas toward the Small Magellanic Cloud Star Sk 108

We discuss the interstellar absorption lines found in Far Ultraviolet Spectroscopic Explorer spectra of the Wolf-Rayet binary Sk 108, which is located in the northeastern part of the main bar of the Small Magellanic Cloud. The spectra cover the 988-1187 ? wavelength range at a resolution of about 12,000 and a signal-to-noise ratio of 20-40. We use detailed component information from higher resolution near-UV and optical spectra to model the far-UV lines of similarly distributed species. Both the Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, although a significant fraction of the SMC gas is ionized. The column densities of P II, S II, and Ar I are consistent with essentially solar ratios, relative to N(Zn II), in both the Galactic and SMC gas; the column density of N I remains somewhat uncertain. Molecular hydrogen is present in the Galactic gas, with properties similar to those found in low mean density Galactic lines of sight and in the Galactic gas toward several other LMC and SMC stars. We report a tentative detection of H2 in the SMC gas for J = 1 and 3, with rotational level populations consistent with an excitation temperature on the order of 1000 K?similar to the H2 found in diffuse Galactic gas toward ? Puppis. Strong absorption from N III, S III, and Fe III has revealed a significant ionized component, particularly in the SMC; O VI is present, but relatively weak, especially in the Galactic gas. The N(C )/N(O ) ratio varies somewhat within the SMC, suggesting that several processes may contribute to the observed high ion abundances.

The Complete Far Ultraviolet Spectroscopic Explorer Spectrum of the Small Magellanic Cloud Star Sk 108

The complete FUSE spectrum of the SMC Wolf-Rayet binary star Sk 108 is presented. The spectrum covers the 905-1186 A wavelength range at a resolution of about 12,500 and a S/N of 25-50. Detailed component information from higher resolution, UV and optical spectra is used to model the far-UV absorption lines of the ISM in the Galaxy and SMC, and special attention is given to the accuracy of the wavelength scale. Improved FUSE measurements, compared to those previously published, of column densities of Ar, N, P, and H2 and new measurements of O are presented. Relative abundances for P and the four α-particle elements—Ar, O, S, and Si—appear to be at solar values, relative to Zn, in the SMC, even though they may be depleted in the Galaxy. Fe is depleted in both the SMC and the Galaxy, but at higher levels in the Galaxy. This implies that, compared to the Galactic environment, the SMC interstellar clouds either exhibit a lower dust-to-metal ratio or that the dust grains are destroyed (with some remaining dust grains which contain Fe) or that there is an enhancement of α-elements. Furthermore, the overall N abundance appears to be truly deficient in the SMC. In addition, empirical oscillator strengths for a series of Fe II lines are derived, using the predetermined column densities for the Fe II components from GHRS data.