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S. I. B. Cartledge (LSU)
An interesting dichotomy has arisen regarding the character of elemental abundance variation in the Galaxy: UV absorption line investigations of the ISM using GHRS have described a homogeneous medium within 500 pc, with only small abundance differences from sight line to sight line; however, stellar and nebular abundances exhibit large intrinsic variations, of order 0.1-0.15 dex, overlaid on subtler large-scale trends. With the goal of extending the study of Galactic ISM homogeneity and gaining insight into the origins of the contrast between these pictures of Galactic elemental abundance fluctuations, the UV absorption profiles for lines of krypton, oxygen, copper, germanium, magnesium, manganese, nickel, and phosphorus present in spectra from 47 ISM SNAP Survey sight lines up to 7 kpc in length have been examined.
This new data set dramatically increases the number of sight lines for which high quality high resolution data are available; for instance, the number of HST-observed sight lines with reliable krypton and oxygen abundances have been nearly tripled and quadrupled, respectively. The Kr/H abundance ratios for sight lines contained within the Orion Spur are remarkably uniform, exhibiting scatter smaller than column density measurement uncertainty. Based on gas-phase O/H ratios, this overall ISM homogeneity appears to extend to the nearby Carina/Sagittarius and Perseus Arms, since the observed O/H gas ratios are consistent with density-dependent depletion. However, the scatter in O/H data as a function of hydrogen mean sight line density is sufficient to accomodate the heliocentric distance effect, whereby low density sight lines extending beyond 750 pc exhibit somewhat higher gas-phase O/H ratios than shorter ones. Data for the remaining elements are consistent with these results, demonstrating that to the limits of current UV absorption line abundance measurement capabilities, the ISM is homogeneous on length scales over which stellar and nebular elemental abundances exhibit large intrinsic variations.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.