Chemical study of intermediate-mass (IM) Class 0 protostars: CO depletion and N_2H~+ deuteration

摘要

Aims. We are carrying out a physical and chemical study of the protostellar envelopes in a representative sample of IM Class 0 protostars. In our first paper we determined the physical structure (density-temperature radial profiles) of the protostellar envelopes. Here, we study the CO depletion and N _2H~+ deuteration. Methods. We observed the millimeter lines of C~(18)O, C~(17)O, N_2H~+ and N _2D~+ towards the protostars using the IRAM 30m telescope. Based on these observations, we derived the C~(18)O, N_2H ~+ and N_2D~+ radial abundance profiles across their envelopes using a radiative transfer code. In addition, we modeled the chemistry of the protostellar envelopes. Results. All the C~(18)O 1→0 maps are well fit when assuming that the C~(18)O abundance decreases inwards within the protostellar envelope until the gas and dust reach the CO evaporation temperature, ≈ 20-25 K, where the CO is released back to the gas phase. The N_2H~+ deuterium fractionation in Class 0 IMs is N_2D~+ /N_2H~+ = 0.005-0.014, two orders of magnitude higher than the elemental D/H value in the interstellar medium, but a factor of 10 lower than in prestellar clumps. Chemical models account for the C~(18)O and N_2H~+ observations if we assume the CO abundance is a factor of ～2 lower than the canonical value in the inner envelope. This could be the consequence of the CO being converted into CH_3OH on the grain surfaces prior to the evaporation and/or the photodissociation of CO by the stellar UV radiation. The deuterium fractionation is not fitted by chemical models. This discrepancy is very likely caused by the simplicity of our model that assumes spherical geometry and neglects important phenomena like the effect of bipolar outflows and UV radiation from the star. More important, the deuterium fractionation is dependent on the ortho-to-para H_2 ratio, which is not likely to reach the steady-state value in the dynamical time scales of these protostars.