The depletion of NO in pre-protostellar cores

Aims. 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. Methods. We observed the 150 GHz and 250 GHz transitions of NO and the 93 GHz transitions of N2H+ 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. Results. We find that NO behaves differently from N2H+ 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.