4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Contributed Talk

Hans Harnack Haus -
FROM CLOUDS TO DENSE CORES 2

The Deuteration Clock for Massive Starless Cores

Speakers

  • Shuo KONG

Primary authors

Co-authors

Content

Initial conditions are crucial to understanding the formation of massive stars, which is still a mystery. One of the most debated points is whether massive star formation is a fast or slow process. Tan et al. (2013, ApJ, 779, 96, hereafter T13) discovered two massive starless cores C1-N and C1-S with ALMA. Their study suggests $\sim$mG magnetic field be present if the cores are virialized. Here we present astrochemical study with observation and modeling. We use deuterium fraction $D_{\rm frac}^{\rm N_2H^+}$ ($\equiv$ [$\rm N_2D^+$]/[$\rm N_2H^+$]) as chemical clock. We utilize the chemical model from Kong et al. (2015, ApJ, 804, 98), and explore the effects of different core collapsing rates relative to free-fall $\alpha_{\rm ff}$ ($\frac{{\rm d}n_{\rm H}}{{\rm d}t} = \alpha_{\rm ff}\frac{n_{\rm H}(t)}{t_{\rm ff}(t)}$). The chemical network includes spin states, deuterium, and freeze-out. For each $\alpha_{\rm ff}$, we explore models with different initial density, initial depletion, initial ortho-to-para $\rm H_2$ ratio, and cosmic-ray ionization rate. Then we compare output $D_{\rm frac}^{\rm N_2H^+}$, ${\rm N_2H^+}$, and ${\rm o}$-${\rm H_2D^+}$ with observational results collected from ALMA, CARMA, JCMT, IRAM 30m, and NRO 45m. Multi-transition fitting of ${\rm N_2D^+}$ and ${\rm N_2H^+}$ lines are performed in order to have the most accurate determination of the deuterium fraction so far possible. Comparisons between the observation and model predictions suggests that both C1-N and C1-S have collapsed at a rate at least 10 times slower than free-fall. This supports the dynamical study in T13, and indicates the potentially important role of magnetic fields in slowing down collapse. Depending on data delivery, we will also present initial results from several ALMA Cycle 2 projects that are related to massive starless cores.