Primordial deuterium abundance at z=2.504 towards Q1009+2956
  The z_abs = 2.504 Lyman Limit absorption system (LLS) towards Q1009+2956 has
previously been used to estimate the primordial deuterium abundance. Since the
initial measurement by Burles & Tytler, this quasar has been observed
extensively with the Keck telescope, providing a substantial increase in
signal-to-noise (from 60 to 147 at continuum level of Ly-alpha at z_abs=2.504).
We explore a set of different models for the absorption system and find that
the deuterium feature is contaminated by Ly-alpha absorption from a low column
density H I cloud. This significantly limits precision to which one can
estimate the D/H ratio in this LLS. Our final result for this system D/H
=2.48^{+0.41}_{-0.35}*10^{-5} has the same relative uncertainty of 17% as the
previous estimate by Burles & Tytler despite the far higher signal-to-noise of
our dataset. A weighted mean of 13 D/H measurements available in the literature
(including our result) gives the primordial deuterium abundance of (D/H)_p =
(2.545 +/- 0.025)*10^{-5} and the corresponding baryon density of the Universe
of Omega_b h^2 = 0.02174 +/- 0.00025 marginally inconsistent with the 2015
Planck CMB data.
