The Deuteration Clock for Massive Starless Cores

¹ Dept. of Astronomy, University of Florida, Gainesville, Florida 32611, USA
² Depts. of Astronomy & Physics, University of Florida, Gainesville, Florida 32611, USA
³ Max-Planck-Institute for Extraterrestrial Physics, 85748 Garching, Germany
⁴ INAF – Osservatorio Astrosico di Arcetri, 50125 Florence, Italy

Abstract

To understand massive star formation requires study of its initial conditions. Two massive starless core candidates, C1-N & C1-S, have been detected in IRDC G028.37+00.07 in N₂D⁺(3–2) with ALMA. From their line widths, either the cores are subvirial and are thus young structures on the verge of near free-fall collapse, or they are threaded by ∼1 mG B-fields that help support them in near virial equilibrium and potentially have older ages. We modeled the deuteration rate of N₂H⁺ to constrain collapse rates of the cores. First, to measure their current deuterium fraction, D≡ [N₂D⁺]/[N₂H⁺], we observed multiple transitions of N₂H⁺ and N₂D⁺ with CARMA, SMA, JCMT, NRO 45 m and IRAM 30 m, to complement the ALMA data. For both cores we derived D ∼ 0.3, several orders of magnitude above the cosmic [D]/[H] ratio. We then carried out chemodynamical modeling, exploring how collapse rate relative to free-fall, α_ff, affects the level of D that is achieved from a given initial condition. To reach the observed D, most models require slow collapse with α_ff∼0.1, i.e., ∼1/10th of free-fall. This makes it more likely that the cores have been able to reach a near virial equilibrium state and we predict that strong B-fields will eventually be detected. The methods developed here will be useful for measurement of the pre-stellar core mass function.