Deuterium Abundance

Following the detection of the 1420 MHz (21 cm) hyperfine line of hydrogen and its use in the mapping of the interstellar gas in the Galaxy, the detection of the similar ground state spin-flip transition of deuterium at 327 MHz (92 cm) was considered one of the most important efforts in radio astronomy. Its measurement constrains the photon to baryon ratio based upon the theory of nucleosynthesis during the big bang. Estimates of the primordial deuterium-to-hydrogen (D/H) abundance ratio from the ultraviolet Lyman alpha lines have differed by more than a factor of two. While most of the differences are now thought to be the result of astration and the depletion of deuterium by attachment to interstellar grains, the small isotope shift in the optical lines could lead to systematic errors. While the radio lines of hydrogen and deuterium are well separated, the deuterium radio line is extremely weak and most observers have only been able to set an upper limit on the D/H ratio.

Observations made with the Deuterium Array at Haystack Observatory resulted in a 6 sigma detection of the 327 MHz line of deuterium. An estimate of 21 ppm (3 ? error limits from 10 to 35 ppm) was derived for the D/H ratio towards anticenter, where the errors are ± 3 ? and include some uncertainty in conversion of the observed line amplitude to D/H ratio.