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 with an angular resolution of about 0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 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 (&gt;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.