Donald G. York

High-resolution spectroscopy of the Z = 1.79 absorption-line system toward B2 1225 + 317

The results of Multiple Mirror Telescope echelle spectroscope scans of species in the z = 1.79 system toward the quasar B2 1225 + 317 are reported. The campaign was undertaken to compare quasar absorption-line systems with gas absorption lines in other galaxies. Relative column densities obtained for Zn II, Si II, Mg II, Fe II and H I were similar to those observed for the interstellar medium. Data analyses with photoionization models for the radiation field above the Lyman limit yielded upper and lower limits to the specific intensity at 1000 A that were consistent with the UV radiation field in the halo of a spiral galaxy.

Echelle spectroscopy with a charge-coupled device /CCD/

Observations made with an RCA thinned back illuminated CCD on the echelle spectrograph of the Kitt Peak 4-meter Mayall telescope are discussed. With emphasis on interstellar line measurements in bright and faint background sources, the system efficiency, spectrum format, achieved signal-to-noise, interference fringes, and overall performance are discussed. A signal-to-noise of > 100/1 has been achieved at 0.2 Å resolution in two hours on a 13.2 mag star. Projected CCD improvements should yield signal-to-noise ratios of 20/1 for 18th magnitude objects (V) in 2 hours. With appropriate gratings, 1500 Å of continuous spectrum can be obtained.

Far-ultraviolet extinction in σ Scorpii

It was found earlier from OAO-2 data (Bless and Savage, 1972) that considerable variability with direction in space is present in both the shape and level (relative toB-V color excess) of the interstellar extinction curve in the far ultraviolet. The star σ Sco was shown to be a case of extremely low UV extinction, but there was some question of whether this could be due to scattered nebular light entering the large entrance slit of the Wisconsin spectrometer aboard OAO-2. We have obtained UV data on σ Sco usingCopernicus (OAO-3), which has an entrance slit on the order of 103 times smaller in projected area than that of OAO-2, so that the contribution to the signal from scattered nebular light would be correspondingly smaller. We find very good agreement with the extinction curve of Bless and Savage, confirming the low UV extinction in the line of sight to σ Sco. The curve is extended down to 100 Å, showing a continued rise towards short wavelengths.

Abundance Determinations for Interstellar Gas

The results of general abundance studies, using the Copernicus satellite are reviewed. The general pattern of depletion of some elements such as Si, Al, Mn, Fe, and Mg, near the Sun is described. Techniques for obtaining abundances for various species are discussed, as are the aspects of the analysis which are uncertain. Variations in gas phase abundance from region to region are found for Fe and Si, similar to variations now known to exist for Ca and Ti based on visible spectra. A new generation of space instruments, with higher efficiency and resolution, in particular for 920 A < λ < 1400 A, can provide detailed interstellar line profiles, thus putting abundance determinations based on equivalent widths on a more firm footing, and yield more detailed information reflecting the make up and binding energies of interstellar dust particles as well as regional changes in the products of nucleonsynthesis. The most important wavelengths and techniques relevant to these future studies are tabulated and discussed.

ORBITING ASTRONOMICAL OBSERVATORY – COPERNICUS

SUMMARY Of the three observatories planned in NASAs OAO program, one, OAO-3, is still in orbit and producing scientifically useful data. The prime experiment is the Princeton telescope spectrometer. A second experiment, an X-ray telescope operated by the University College of London, uses 10% of the total observatory time. Following a brief history of the OAO program, a description is given of the Princeton telescope with its 80-cm primary mirror, and of the spectrometer, which yields a resolution of up to 0.05 A. The spacecraft guidance system is also described. This system initially points the observatory to within a few arcminutes of the target, places the 0.3-arcsec slit on the star in less than 3 min, and holds on the star for up to 50 min with errors less than 0.05 arcsec. The main scientific results are described under the following categories: (1) the widespread presence of molecular hydrogen; (2) the search for other molecules, including detection of CO; (3) the nature of the interstellar medium as inferred from the detection of various atomic lines; (4) the study of chromospheres in late type stars; and (5) the study of mass loss in binaries and single stars. Operation of the satellite has become quite routine. Some 70 targets have been observed at high and intermediate resolution from 1000 to 1400 A and from 1700 to 3000 A. On a few bright objects, slight extensions of these wavelength regions have been possible. The main problems scientifically are removal of the unexpectedly high background signal due to particle counts in the conventional photomultipliers and to stray light in the vented photomultipliers used in the region from 1000 to 1400 A.