Deuteration and evolution in the massive star formation process

Abstract

Context. An ever growing number of observational and theoretical evidence suggests that the deuterated fraction (column density ratio between a species containing D and its hydrogenated counterpart, D frac ) is an evolutionary indicator both in the low-and the highmass star formation process. However, the role of surface chemistry in these studies has not been quantified from an observational point of view. Aims. Because many abundant species, such as NH 3 , H 2 CO, and CH 3 OH, are actively produced on ice mantles of dust grains during the early cold phases, their D frac is expected to evolve differently from species formed only (or predominantly) in the gas, such as N 2 H + , HNC, HCN, and their deuterated isotopologues. The differences are expected to be relevant especially after the protostellar birth, in which the temperature rises, causing the evaporation of ice mantles. Methods. To compare how the deuterated fractions of species formed only in the gas and partially or uniquely on grain surfaces evolve with time, we observed rotational transitions of CH 3 OH, 13 CH 3 OH, CH 2 DOH, and CH 3 OD at 3 mm and 1.3 mm, of NH 2 D at 3 mm with the IRAM-30 m telescope, and the inversion transitions (1, 1) and (2, 2) of NH 3 with the GBT, towards most of the cores already observed in N 2 H + , N 2 D + , HNC, and DNC. Results. NH 2 D is detected in all but two cores, regardless of the evolutionary stage. D frac (NH 3 ) is on average above 0.1 and does not change significantly from the earliest to the most evolved phases, although the highest average value is found in the protostellar phase (∼0.3). Few lines of CH 2 DOH and CH 3 OD are clearly detected, and then only towards protostellar cores or externally heated starless cores. In quiescent starless cores, we have only one doubtful detection of CH 2 DOH. Conclusions. This work clearly confirms an expected different evolutionary trend of the species formed exclusively in the gas (N 2 D + and N 2 H + ) and those formed partially (NH 2 D and NH 3 ) or totally (CH 2 DOH and CH 3 OH) on grain mantles. It also reinforces the idea that D frac (N 2 H + ) is the best tracer of massive starless cores, while high values of D frac (CH 3 OH) seem fairly good tracers of the early protostellar phases, where the evaporation or sputtering of the grain mantles is most efficient.

Ongoing star formation in the protocluster IRAS 22134+5834

“…5 and 9). From single pointing observations, Fontani et al (2015) Fontani et al (2015) obtained agree with the number we derived for these three NH 2 D cores within an order of magnitude.…”

“…Single pointing observations toward massive star formation regions indicate that D frac (N 2 H + ) is high at the prestellar/cluster stage and then drops as the temperature of the cores increases and the protostellar objects form, similar to the low-mass regions Gerner et al 2015). On the other hand, D frac (NH 3 ) does not show significant differences between different evolutionary stages in high-mass star-forming cores (Fontani et al 2015).…”

“…Three deuterated ammonia ortho-NH 2 D clumps are detected surrounding the NIR cluster, and we name them NH 2 D-N (north), NH 2 D-E (east), and NH 2 D-S (south), respectively. NH 2 D-N is associated with NH 3 (1, 1) and N 2 H + (1-0) emission, NH 2 D-S is associated with the southern peak of HMSC-W, and NH 2 D-E sits close to the southeastern tail of the HMSC-E. Single-dish observations by Fontani et al (2015) detected ortho-NH 2 D emission toward the UCH region and HMSC-E with the same peak intensity of 0.05 K (corresponding 200 mJy with a beam size of ∼28 ), and this intensity is only one third of the intensity they detected toward HMSC-W. The 3σ limit for our ortho-NH 2 D observation is ∼273 mJy beam −1 (Table 3), and higher than the peak intensity Fontani et al (2015) detected toward the UCH region and HMSC-E. Strong C 2 H(N = 1-0) and CH 3 OH emission is also detected in the southeast of the NIR cluster and extended toward the "cavity" of the NIR cluster.…”

“…Fontani et al (2015) estimated ammonia column densities with a beam size of ∼32 and the assumed beam filling factor of 1, while the column densities of N 2 H + were derived with a much smaller beam size (∼9 ) and corrected for beam filling factor. We measured the size of HMSC-E and HMSC-W from our NH 3 (1, 1) integrated intensity map (Fig.…”

Exploring molecular complexity with ALMA (EMoCA): Deuterated complex organic molecules in Sagittarius B2(N2)

“…Even molecules containing several deuterium atoms have been detected (e.g., Parise et al 2004). Levels of deuteration above 10% have also been found for small molecules such as ammonia in high mass star-forming regions (Pillai et al 2007;Fontani et al 2015). Deuteration at somewhat higher temperatures (∼30-50 K), at which the H 2 D + channel is not efficient anymore, can also occur in the gas phase via CH 2 D + which is, like H 2 D + , produced by an exothermic reaction of CH + 3 with HD (e.g., Parise et al 2009).…”