摘要：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 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. The [CH2DOH]/[CH3OD] 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.
