AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 30. Cosmological Probes and Parameters
Oral, Monday, May 31, 1999, 2:00-3:30pm, International Ballroom South

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[30.01] A third QSO absorption system with a low Deuterium/Hydrogen ratio

D. Kirkman (UCSD and Lucent Technologies, Bell Laboratories), D. Tytler (UC San Diego), S. Burles (UCSD and University of Chicago), D. Lubin, J. Omeara (UC San Diego)

We present high signal-to-noise ratio data on an extremely simple zabs ~3 partial Lyman limit absorption system. This absorber has only a single major velocity component, and unsaturated high order H I Lyman series lines. Hence, the parameters which describe the Hydrogen absorption are very well determined. We find excess absorption on the blue wing of the H I Ly\alpha line, near to the expected position of deuterium absorption. The excess absorption is slight, allowing us to set a firm upper limit to the amount of deuterium absorption in this absorber, D/H < 6 \times 10-5. However, the expected deuterium line is at the wrong wavelength to explain all of the excess absorption on the blue wing of the H I Ly\alpha line, and hence there must be contamination by additional H I interloper absorption. Because the parameters of the H I interloper and the D line will be somewhat degenerate, we can not place a lower limit on D/H in this absorption system. We show that this system probably has a very low metallicity, [C/H] < -2, so that our upper limit of D/H < 6 \times 10-5 should be representative of primordial D/H. All four of the known high redshift absorption line systems simple enough to provide useful limits on D are consistent with the low value of D/H = 3.4 ±0.5 \times 10-5 measured towards QSO 1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). Conversely, three of the four D/H limits are inconsistent with high D/H value proposed by Webb et al. (1997a, 1997b) towards QSO 1718+4807.

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