DIRECT DIAGNOSIS OF INFALL IN COLLAPSING PROTOSTARS .1. THE THEORETICAL IDENTIFICATION OF MOLECULAR-SPECIES WITH BROAD VELOCITY DISTRIBUTIONS

We have developed chemical models, including depletion on to grains, for the collapsing envelopes of protostars. Our primary goal has been to identify molecular species having broad velocity distributions, so that observations of their line profiles can be employed to diagnose the dynamics of protostar formation, and to yield information on the collapse age and depletion rate. Previously obtained data for NH3 emission lines, from a number of dark cores with embedded protostars, have shown no clear evidence for systematic collapse. Results from our model indicate that the collapse initiates very soon after the formation of an isothermal pressure-balanced sphere. The collapse is preceded by a period of exceptional quiescence. We demonstrate here that as depletion reduces the fractional abundance of NH3 in the accelerating gas of an infalling envelope and prevents the formation of broad wings on NH3 emission lines, the fractional abundances of some other species initially increase as the heavy elements become depleted, leading those species to have emission lines broader than those of NH3. CH is an observable species which, for a large variety of conditions, possesses a fairly constant or increasing fractional abundance in collapsing envelope gas in which depletion is occurring. High angular resolution observations of CH would be very desirable, but would require appropriate instrumentation on an array, or perhaps the Arecibo telescope. When the water abundance is sufficiently high in the outer envelope, so that many molecular ions are removed primarily in reactions with it rather than by dissociative recombination, the abundances of HCO+, N2H+ and H2S rise when collapse and depletion occur. Millimetre lines from these species can be observed with single-dish telescopes at higher angular resolution than can NH3, resulting in their lines having broader wings in spatially resolved sources than those of NH3, even when their abundances relative to NH3 do not increase substantially in the infall. HCO is a particularly interesting species, because its abundance ratio relative to HCO+ increases with growing density if the fractional abundance of gas-phase heavy metals remains constant. Also, HCO line emission can be observed with high angular resolution.