A Study of Cosmically Abundant Elements with UV Absorption Line Data

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Session 8 -- General ISM, Dust
Display presentation, Monday, 9:20-6:30, Pauley Room

[8.17] A Study of Cosmically Abundant Elements with UV Absorption Line Data

Ulysses J. Sofia, Blair D. Savage (Univ. Wisconsin-Madison)

We use high resolution ultraviolet absorption line data obtained with the Goddard High Resolution Spectrograph to obtain accurate column densities for the most cosmically abundant elements in the interstellar medium. These elements are O, C, N, Mg,Si, Fe, and S, each which has one or more resonance transitions for its dominant neutral-hydrogen cloud ion in the wavelength region observed by the instrument. The observations used for this study were in the sightlines toward $\zeta$ Ophiuchi, $\xi$ Persei, and $\mu$ Columbae. These lines of sight have the most complete wavelength coverage at 3.5 km/sec resolution with the GHRS. The column densities for the ions were determined from the integrated optical depth method, from line profile fitting, or from the continuum reconstruction method when the absorption has Lorentzian wings. depending upon the strength of the transition and the relative radial velocities of the absorbing components. Extreme care was taken to ensure that each component of the absorption was evaluated separately. When a column density could be determined from more than one transition of an ion, a comparison of the column densities was used to assess the inaccuracies associated with the atomic constants of the transitions. In some cases we make recommendations for using new f values for weak transitions as determined by this empirical method. The accurately determined column densities are used to explore the degree to which a given element is incorporated into the interstellar grains for each neutral- hydrogen absorbing component. This process involves evaluation of the ionization occurring in the components. It also requires a knowledge of the reference abundance ratio of each element to hydrogen in the components; this ratio is not always the same as the solar abundance ratio. We will discuss the similarities and differences for the elemental incorporation into the dust in neutral-hydrogen regions with different characteristics. Work supported by the NASA grant NAG5-1852.

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