The Deuterium Fraction in Massive Starless Cores and Dynamical Implications

TL;DR

This study investigates deuterium fractionation in massive starless cores using multi-telescope observations, revealing high deuteration levels and suggesting slow collapse rates in early star formation stages.

Contribution

It provides new measurements of deuterium fraction in massive cores and links these to chemodynamical models indicating slow collapse.

Findings

Deuterium fraction $D_{frac}^{N_2H^+}$ is 0.2–0.7, much higher than cosmic ratios.

High deuteration levels imply cores are collapsing at less than 10% of free-fall speed.

Observations of o-H$_2$D$^+$ help constrain the ortho-to-para H$_2$ ratio.

Abstract

We study deuterium fractionation in two massive starless/early-stage cores C1-N and C1-S in Infrared Dark Cloud (IRDC) G028.37+00.07, first identified by Tan et al. (2013) with ALMA. Line emission from multiple transitions of $\rm N_2H^+$ and $\rm N_2D^+$ were observed with the ALMA, CARMA, SMA, JCMT, NRO 45m and IRAM 30m telescopes. By simultaneously fitting the spectra, we estimate the excitation conditions and deuterium fraction, $D_{\rm frac}^{\rm N_2H^+} \equiv [\rm N_2D^+]/[N_2H^+]$, with values of $D_{\rm frac}^{\rm N_2H^+} \simeq 0.2$--$0.7$, several orders of magnitude above the cosmic [D]/[H] ratio. Additional observations of o-H$_2$D$^+$ are also presented that help constrain the ortho-to-para ratio of $\rm H_2$, which is a key quantity affecting the degree of deuteration. We then present chemodynamical modeling of the two cores, exploring especially the implications for the collapse rate relative to free-fall, $\alpha_{\rm ff}$. In order to reach the high level of observed deuteration of $\rm N_2H^+$, we find that the most likely evolutionary history of the cores involves collapse at a relatively slow rate, $\lesssim1/10$th of free-fall.