Observations and chemistry of interstellar refractory elements

Abstract

New searches are reported for eight refractory element compounds, bringing to 21 the total number of simple species of the elements Si, P, Mg, Na, Al, Fe, Ti, and Ca not observed to low abundance limits in dense warm molecular clouds. Only SiO and PN are detected, which together suggest a depletion factor of 1000 for both Si and P in star-forming regions. If such a factor applies to other refractory elements, then compounds of Mg and Na (and probably Al and Fe) should be easily detected. To understand the apparent inconsistency, we examine the chemistry of these elements, starting with their incorporation into refractory grain cores in circumstellar envelopes, following their fate through the warm diffuse and cold diffuse phases of the ISM, then the gas-grain interactions in cold dense molecular clouds, and finally the desorption of refractory ice mantles and subsequent gas-phase chemistry in warm dense regions of massive star formation