Exploring Molecular Complexity with ALMA (EMoCA): Deuterated complex
  organic molecules in Sagittarius B2(N2)
  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 high mass star forming regions. We use a spectral line survey
performed with ALMA to search for deuterated complex organic molecules toward
the hot molecular core Sgr B2(N2). Population diagrams and integrated intensity
maps are constructed to fit rotational temperatures and emission sizes for each
molecule. Column densities are derived by modelling the full spectrum under the
LTE assumption. The results are compared to predictions of two astrochemical
models that treat the deuteration process. We report the detection of CH2DCN
toward Sgr B2(N2) with a deuteration level of 0.4%, and tentative detections of
CH2DOH, CH2DCH2CN, the chiral molecule CH3CHDCN, and DC3N with levels in the
range 0.05%-0.12%. A stringent deuteration upper limit is obtained for CH3OD
(<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 CH2DOCHO. Ethyl cyanide is less deuterated than methyl cyanide by
at least a factor five. 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. 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.
