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

Abstract

Context. Deuteration is a powerful tracer of the history of the cold prestellar phase in star-forming regions. Apart from methanol, little is known about deuterium fractionation of complex organic molecules in the interstellar medium, especially in regions forming high-mass stars. Aims. Our goal is to detect deuterated complex organic molecules toward the high mass star-forming region Sagittarius B2 (Sgr B2) and derive the level of deuteration for these molecules. Methods. We use a complete 3-mm spectral line survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to search for deuterated complex organic molecules toward the hot molecular core Sgr B2(N2). We constructed population diagrams and integrated intensity maps to fit rotational temperatures and emission sizes for each molecule. Column densities are derived by modeling the full spectrum under the assumption of local thermodynamic equilibrium. We compare the results to predictions of two astrochemical models that treat the deuteration process. Results. We report the detection of CH 2 DCN toward Sgr B2(N2) with a deuteration level of 0.4%, and tentative detections of CH 2 DOH, CH 2 DCH 2 CN, the chiral molecule CH 3 CHDCN, and DC 3 N with levels in the range 0.05%-0.12%. A stringent deuteration upper limit is obtained for CH 3 OD (<0.07%). Upper limits in the range 0.5-1.8% are derived for the three deuterated isotopologues of vinyl cyanide, the four deuterated species of ethanol, and CH 2 DOCHO. Ethyl cyanide is less deuterated than methyl cyanide by at least a factor five. The [CH 2 DOH]/[CH 3 OD] abundance ratio is higher than 1.8. It may still be consistent with the value obtained in Orion KL. Except for methyl cyanide, the measured deuteration levels lie at least a factor four below the predictions of current astrochemical models. The deuteration levels in Sgr B2(N2) are also lower than in Orion KL by a factor of a few up to a factor ten. Conclusions. The discrepancy between the deuteration levels of Sgr B2(N2) and the predictions of chemical models, and the difference between Sgr B2(N2) and Orion KL may both be due to the higher kinetic temperatures that characterize the Galactic center region compared to nearby clouds. Alternatively, they may result from a lower overall abundance of deuterium itself in the Galactic center region by up to a factor ten.

Spectroscopic study and astronomical detection of doubly13C-substituted ethyl cyanide

“…The 12 C 13 C/ 13 C 13 C ratio derived for ethyl cyanide is very similar to the 12 C 13 C/ 13 C 13 C ratio tentatively obtained for HC 3 N in the same source (∼24; see Belloche et al 2016) and to the 12 C/ 13 C ratio derived for methanol and ethanol (∼25; see Müller et al 2016). However, the difference on the order of 25% between the 12 C 12 C/ 12 C 13 C and 12 C 13 C/ 13 C 13 C ratios of ethyl cyanide is a priori surprising.…”

“…The two ratios would be reconciled with a value of 28.6 if the column densities of the singly 13 Csubstituted isotopologues were higher by a factor 1.12, which would correspond to a vertical increment of 0.11 in their population diagrams (see Figs. 7b,8b, and 9b of Belloche et al 2016). Such an increment is not completely excluded, but it seems to be marginally consistent with the observed spectra, given the large number of detected transitions and the fact that a few of these are already somewhat overestimated by the current model.…”

“…They may be underestimated. (c) For these species, the analysis was performed in Belloche et al (2016). 13 C atoms have been securely detected in the interstellar medium.…”

“…Second, some lines plotted in the population diagrams are somewhat more contamined by other species than what we require to qualify them as detected. Still we show them in the population diagrams because we can account for this contamination thanks to our full LTE model that contains all of the species identified so far (see Belloche et al 2016). Table 7 lists the results of the rotational temperature fits for the three doubly 13 C-substituted isotopologues, along with the results previously obtained for C 2 H 5 CN and the three singly 13 C-substituted isotopologues .…”

The millimeter and sub-millimeter rotational spectrum of triple 13C-substituted ethyl cyanide

“…The rotational spectra of deuterated, mono-13 C, and -15 N containing isotopologs of ethyl cyanide (Margulès et al , 2016Richard et al 2012;Heise et al 1974), as well as vibrationally excited ethyl cyanide (Gibb et al 2000b;Heise et al 1981;Mehringer et al 2004;Daly et al 2013;Belloche et al 2013Belloche et al , 2016 have also been investigated and successfully detected in the ISM.…”