Abstract
Understanding the depletion of heavy elements is a fundamental step towards determining the structure of pre-protostellar cores just prior to collapse. We study the dependence of the NO abundance on position in the pre-protostellar cores L1544 and L183. We observed the 150 GHz and 250~GHz transitions of NO and the 93 GHz transitions of \NTHP \ towards L1544 and L183 using the IRAM 30 m telescope. We compare the variation of the NO column density with position in these objects with the H column density derived from dust emission measurements. We find that NO behaves differently from \NTHP \ and appears to be partially depleted in the high density core of L1544. Other oxygen-containing compounds are also likely to be partially depleted in dense-core nuclei. The principal conclusions are that: the prestellar core L1544 is likely to be 'carbon-rich'; the nitrogen chemistry did not reach equilibrium prior to gravitational collapse, and nitrogen is initially (at densities of the order of $10^4$~cm$^{-3}$) mainly in atomic form; the grain sticking probabilities of atomic C, N and, probably, O are significantly smaller than unity.
Highlights
It is accepted that most, if not all, C-containing species deplete on to dust grain surfaces in the central, high-density regions of prestellar cores
We find that NO behaves differently from N2H+ and appears to be partially depleted in the high density core of L1544
When one compares a map of C18O, for example, with that of the dust emission, one sees that the CO isotopomer traces only an outer shell and not the high-density interior1; this limits our understanding of prestellar cores, because the molecular line emission provides information on the kinematics
Summary
It is accepted that most, if not all, C-containing species deplete on to dust grain surfaces in the central, high-density regions of prestellar cores. This result throws doubt on much recent modelling of the chemistry of prestellar cores and poses the question of whether another more volatile and abundant form of nitrogen is responsible for the survival of N-containing species to high density Another possibility is that atomic nitrogen does not stick effectively to grain surfaces. Studies (Flower et al 2005, 2006), we have explored this suggestion and found that the best fit of the models to the observed abundances of NH3 and N2H+ is obtained when the sticking probabilities of both atomic N and O are low and, the mean grain surface area per H atom is a factor 5–10 lower than in the diffuse interstellar medium; this implies that considerable grain growth has occurred prior to the prestellar-core phase In all of this discussion, the abundance of oxygen-containing species, such as OH and water, has been somewhat neglected, and for good reason.
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