The deuterium to hydrogen abundance ratio toward a fourth QSO: HS 0105+1619

We report the measurement of the primordial D/H abundance ratio toward QSO HS 0105+1619. The column density of the neutral hydrogen in the z similar o r equal to 2.536 Lyman limit system is high, log N-HI = 19.422 +/- 0.009 cm (-2), allowing for the deuterium to be seen in five Lyman series transition s. The measured value of the D/H ratio toward QSO HS 0105 + 1619 is found t o be D/H = 2.54 +/- 0.23 x 10(-5). The metallicity of the system showing D/ H is found to be similar or equal to 0.01 solar, indicating that the measur ed D/H is the primordial D/H within the measurement errors. The gas that sh ows D/H is neutral, unlike previous D/H systems that were more highly ioniz ed. Thus, the determination of the D/H ratio becomes more secure since we a re measuring it in different astrophysical environments, but the error is l arger because we now see more dispersion between measurements. Combined wit h prior measurements of D/H, the best D/H ratio is now D/H = 3.0 +/- 0.4 x 10(-5), which is 10% lower than the previous value. The new values for the baryon-to-photon ratio and baryonic matter density derived from D/H are eta = 5.6 +/- 0.5 x 10(-10) and Omega (b), h(2) = 0.0205 +/- 0.0018, respectiv ely.