Kenneth R. Sembach

High-Resolution Ultraviolet Observations of the Highly Ionized Interstellar Gas toward Radio Loops I and IV*

We present new Goddard High-Resolution Spectrograph (GHRS) echelle observations of the high ionization lines of Si IV, C IV, and N V toward HD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernova remnants. Absorption along the path to HD 119608 makes it possible to study energetic processes that may result in the flow of gas into the Galactic halo. The data have a resolution (FWHM) of ≈ 3.5 km s-1 and S/N ratios of 30:1-50:1. The integrated high ion column densities log N = 13.57 ± 0.02, 14.48 ± 0.06, and 13.45 ± 0.07 for Si IV, C IV, and N V, respectively, imply a factor of 2-4 enhancement in the amount of highly ionized gas compared to average sight lines through the Galactic disk and halo. The integrated high ion column density ratios, N(C IV)/N(Si IV) = 8.1 ± 1.1 and N(C IV)/N(N V) = 10.7 ± 2.1, are also several times larger than normal. These high ion results suggest the absorption is influenced by passage of the sight line through the center of Loop IV. The HD 119608 C IV absorption profile has a bimodal velocity structure indicative of an expanding shell; we tentatively derive an expansion velocity of 16 km s-1 for Radio Loop IV. A detailed analysis of the high ion profile structure indicates that multiple types of highly ionized gas with a range of properties exist along this sight line. We also reexamine the high ionization properties of the QSO 3C 273 sight line using new intermediate-resolution (FWHM ≈ 20 km s-1) GHRS data. We obtain log N = 14.49 ± 0.03 and 13.87 ± 0.06 for C IV and N V, respectively. The C IV column density, which is 0.12 dex smaller than earlier estimates, leads to somewhat smaller ionic ratios than previously determined. We find N(C IV)/N(Si IV) = 5.1 ± 0.6 and N(C IV)/N(N V) = 4.2 ± 0.6. However, as for HD 119608, the high ion column densities toward 3C 273 are larger than normal by factors of 2-4. The 3C 273 high ion absorption profiles are much broader than those seen toward HD 119608 and other sight lines near the center of Loop IV. The larger line widths may result because the sight line passes through the turbulent edge of Loop IV as well as the X-ray and radio continuum emission regions of the North Polar Spur. We have compiled a list of the highest quality IUE and GHRS high ion measurements available for interstellar sight lines through the disk and halo and find the following median averaged results: N(C IV)/N(Si IV) = 3.8 ± 1.9 and N(C IV)/N(N V) = 4.0 ± 2.4. These ratios are lower than those found for four Loop IV sight lines. We suggest a model for the production of highly ionized gas in Loop IV in which the contributions from turbulent mixing layers and conductive interfaces/SNR bubbles to the total high ion column densities are approximately equal. Much of the high ion absorption toward HD 119608 and 3C 273 may occur within a highly fragmented medium within the remnant or the outer cavity walls of the remnant.

A critical analysis of interstellar Zn and Cr as galactic abundance benchmarks for quasar absorbers

We present interstellar gas-phase abundances of zinc and chromium in the Milky Way for comparison with the abundances of these elements in damped Ly-alpha systems at redshifts near z approximately 2. We make use of both IUE (International Ultraviolet Explorer) and GHRS (Goddard High Resolution Spectrograph) archival information in deriving the Milky Way values and find that the average difference in (Zn/H) and (Cr/Zn) between the Milky Way and damped Ly-alpha systems at large N(H1) is about 0.9 dex. The Zn/H values we find for the Milky Way are approximately -0.2 to -0.7 dex below solar and are due in large part to depletion of Zn onto dust grains. The amount of this gas-phase depletion is variable and depends upon the H1 column density and molecular fraction of the gas. Milky Way sight lines having low molecular fractions have values of (Zn/H) within approximately 0.2 dex of the solar value. Combined with larger values of (Cr/Zn) found for the damped Ly-alpha systems, this result confirms that measures of (Zn/H) should accurately reflect the lower gas-phase abundances of Zn found in these systems.

High-Resolution Observations of Interstellar C2 toward ζ Ophiuchi

We present high-resolution (λ/Δλ ≈ 540,000) absorption-line observations of two weak C2 lines in the 2-0 band of the Phillips system in the direction of ζ Ophiuchi. These high signal-to-noise ratio (S/N ≈ 230-380) data were obtained at the McDonald Observatory 2.7 m telescope using the coude echelle spectrograph in double-pass mode. The C2 line profiles are resolved into two components, having a spacing of ≈ 1.14 km s-1, as has been seen for other molecular species such as CN, CO, and CH. Unlike these molecules, however, the strengths of the two narrow C2 components are very similar. We briefly discuss the implications these results have for understanding the chemical behavior of diatomic molecules in diffuse interstellar clouds.

IUE observations of highly ionized gas toward distant stars in the Milky Way

Combined high-dispersion IUE spectra of interstellar Si IV, C, IV, and N V absorption along distnat lines toward early type stars in the Galactic disk and low halo are discussed. The highly ionized species have complex profiles and exhibit stronger absorption toward distant low latitude stars than toward high latitude stars. Absorption along low latitude directions is often broadened substantially (up to 100 km S6-1 in some cases) by Galactic rotation. Along these sight lines the Si IV, C IV, and N V profiles are more similar in shape to each other than to those of A1 III, which is a tracer of photoionized interstellar gas. The observed column density ratios of the high ions, <N(C IV)/N(Si IV)≥3.6±1.3 and <N(N IV)/N(N V)≥4.6±2.7, are similar in widely different regions within the Galaxy. The similarities in the high ion profile shape along individual sight lines and integrated column density ratios along differing sight lines suggest a common origin for these species in collisionally ionized gas associated with a global process such as a Galactic fountain.

Highly Ionized Gas in High Velocity Clouds — The PG 1259+593 Sight Line Through Complex C

We present FUSE and STIS observations of highly ionized gas associated with Complex C, along the line of sight towards PG 1259+593. We compare medium resolution (FWHM ≈ 20 km s-1) FUSE spectra of O VI with STIS observations (FWHM ≈ 7 km s-1) of CIV and SiIV. We also examine the inter-relationships between these high ions and some low ionization species, namely OI, SiII, and CII. We find that the high ion absorption can be modeled by absorption components at similar velocities to those seen in the neutral gas. The observed ionic ratios are consistent with the production of CIV, Si IV and O VI in one or more conductive interfaces centered at Complex C velocities. In this scenario we envisage the production of O VI in transition-temperature (T ≈ 3 x 105 K) gas at the interface between the neutral component of Complex C and a surrounding hot medium.

The Metallicity and Dust Content of HVC287+22+240: Evidence for a Magellanic Clouds Origin

We present evidence that the properties of the high-velocity cloud catalogued as #187 by Wakker & van Woerden (1991) (HVC287+22+240) can be explained if it is part of a leading arm counterpart of the Magellanic Stream, and we use this to argue that the HVC originated in the Magellanic Clouds after a tidal interaction.

Detection of Highly Ionized Galactic HVCs Toward Markarian 509

We present Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Mrk 509 (1 = 36, b = −30) in the 1233–1268 A and 1521–1558 A spectral regions at a resolution of 15 km s−1. We detect high velocity C IV absorption between −340 and −170 km s−1(LSR), but find no corresponding Si II 1526 A or N V 1238 A absorption. The high velocity C IV absorption has sub-structure that can be modeled with two Gaussian components centered at −227 and −283 km s−1 with N(C IV)/N(Si II) > 1.3 and > 5.1 and N(C IV)/N(N V) > 3.0 and > 6.6, respectively. We obtained sensitive observations of the H I 21 cm emission toward Mrk 509 with the NRAO 140 foot telescope. We find no detectable 21 cm emission at the velocities of the C IV-HVCs to a 4cr limit of N(H I) 4.9×1017 cm−2, but mapping of the 21 cm emission in the area around Mrk 509 indicates that H I-HVCs with velocities between −270 and −300 km s−1 arelocated within about 1.5 degrees of the sight line. The C IV-HVC absorption appears to probe the ionized boundaries of the H I-HVCs. Ionization model calculations indicate that it is likely that these C IV-HVCs are photoionized by extragalactic background radiation rather than by collisional ionization within a recombining, hot gas. These C IV high velocity clouds have ionization properties more closely resembling those of the high ionization quasar metal line systems than those of previously studied sight lines through the Milky Way disk and halo.

Probing the Outer Galactic Halo with the GHRS

The Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) has been used to obtain intermediate resolution (15 to 20 km/s) spectra of selected Milky Way absorption lines through the entire halo toward bright AGNs and QSOs. The objects so far observed include NGC 3783 (l = 287. b = 23), H 1821+643 (l = 94, b = 27), Fairall 9 (l = 295, b = −58), and Mrk 509 (l = 36,b = −30). This short contribution summarizes the key results.