The Deuterium Fraction in Massive Starless Cores and Dynamical Implications

We study deuterium fractionation in two massive starless/early-stage cores, C1-N and C1-S, in Infrared Dark Cloud G028.37+00.07, which was first identified by Tan et al. with ALMA. Line emission from multiple transitions of N2H+ and N2D+ were observed with the ALMA, CARMA, SMA, JCMT, NRO 45 m, and IRAM 30 m telescopes. By simultaneously fitting the spectra, we estimate the excitation conditions and deuterium fraction, {D}{frac}{{{N}}2{{{H}}}+} \equiv \quad [{{{N}}}2{{{D}}}+]/[{{{N}}}2{{{H}}}+], with values of {D}{frac}{{{N}}2{{{H}}}+} ≃ 0.2-0.7, several orders of magnitude above the cosmic [D]/[H] ratio. Additional observations of o-H2D+ are also presented that help constrain the ortho-to-para ratio of H2, which is a key quantity affecting the degree of deuteration. We then present chemodynamical modeling of the two cores, especially exploring the implications for the collapse rate relative to free-fall, αff. In order to reach the high level of observed deuteration of {{{N}}}2{{{H}}}+, we find that the most likely evolutionary history of the cores involves collapse at a relatively slow rate, ≲ one-tenth of free-fall