Digging into the Interior of Hot Cores with ALMA (DIHCA). V. Deuterium Fractionation of Methanol

Abstract

We have observed the ${}^{13}$CH${}_3$OH $5_1-4_1$ A${}^{+}$, ${}^{13}$CH${}_3$OH $14_1-13_2$ A${}^{-}$, and CH${}_2$DOH $8_{2,6}-8_{1,7}$ $e_0$ lines toward 24 high-mass star-forming regions by using Atacama Large Millimeter/submillimeter Array (ALMA) with an angular resolution of about 0${}^{\prime \prime }$.3. This resolution corresponds to a linear scale of 400-1600 au, allowing us to resolve individual cores properly. We detected the ${}^{13}$CH${}_3$OH and CH${}_2$DOH emission near the continuum peaks in many of these regions. From the two ${}^{13}$CH${}_3$OH lines, we calculated the temperature toward the ${}^{13}$CH${}_3$OH peaks, and confirm that the emission traces hot ($>$100 K) regions. The $N$(CH${}_2$DOH)/$N$(${}^{12}$CH${}_3$OH) ratio in the observed high-mass star-forming regions is found to be lower than that in low-mass star-forming regions. We have found no correlation between the $N$(CH${}_2$DOH)/$N$(${}^{13}$CH${}_3$OH) or $N$(CH${}_2$DOH)/$N$(${}^{12}$CH${}_3$OH) ratios and either temperatures or distance to the sources, and have also found a source-to-source variation in these ratios. Our model calculations predict that the $N$(CH${}_2$DOH)/$N$(${}^{12}$CH${}_3$OH) ratio in hot cores depends on the duration of the cold phase; the shorter the cold phase, the lower the deuterium fractionation in the hot cores. We have suggested that the lower $N$(CH${}_2$DOH)/$N$(${}^{12}$CH${}_3$OH) ratio in high-mass star-forming regions compared to that in low-mass star-forming regions is due to the shorter duration of the cold phase and that the diversity in the $N$(CH${}_2$DOH)/$N$(${}^{12}$CH${}_3$OH) ratio in high-mass star-forming regions is due to the diversity in the length of the cold prestellar phase, and not the time that the objects have been in the hot core phase.

Accepted for publication in ApJ (49 pages)

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