AAS 196th Meeting, June 2000
Session 35. High Resolution Spectroscopy at Visible and Ultraviolet Wavelengths
Topical Session Oral, Wednesday, June 7, 2000, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, 4:15-6:00pm, Highland A/K

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[35.01] Observing with the Ultra-High-Resolution Facility at the Anglo-Australian Telescope: Structure of Diffuse Interstellar Bands

S.J. Fossey, I.A. Crawford (University College London)

The Ultra-High-Resolution Facility (UHRF) at the Anglo-Australian Telescope (AAT) is an echelle spectrograph designed to achieve a maximum resolving power of R~ 106. At this spectral resolution of 0.3 km s-1, the UHRF is able to resolve fully the intrinsic profiles of the narrowest absorption lines in the cool, diffuse interstellar medium. The reliable measurement of the velocity-dispersion parameters of atomic and molecular absorption lines in diffuse clouds allows strict upper limits to be set on the gas kinetic temperature and the turbulent velocity within a cloud. The thermal and turbulent-velocity contributions to the line width can be separated, under certain assumptions, by observing species differing in atomic mass, such as Na\,{\sc i} and K\,{\sc i}, or by determining the kinetic temperature of the gas from the rotational excitation of C2, for example. The line widths and line-of-sight velocities of molecular species such as CH, CH+, CN, and NH yield important parameters for unravelling the physical structure and chemistry of diffuse clouds.

The UHRF has also been used at resolving powers up to R~ 600000 to record the intrinsic profiles of several diffuse interstellar bands along lines of sight with simple diffuse-cloud velocity structure. The diffuse bands at 5797Å\ and 6614Å\ have been found to be composed of partially resolved components which might be interpreted as the rotational branch structure of electronic transitions in gas-phase molecules. In the \lambda5797 diffuse band, detailed fine structure has been discovered, providing an exacting test against which theoretical or laboratory candidates can be assessed. Candidates which have recently been proposed to account for some of the diffuse bands include C7-, for which laboratory spectra have been published, and a model of isotopic substitution in large, highly symmetric molecules containing ~40--100 carbon atoms. These proposals are reviewed in the light of the UHRF diffuse-band profiles.

This work has been supported by the UK Particle Physics and Astronomy Research Council.

The author(s) of this abstract have provided an email address for comments about the abstract: sjf@star.ucl.ac.uk

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