| Description |
Using CO to trace the total H2 column density in molecular clouds is a common practice. This practice, however, can be fraught with difficulties. First of all, CO is often optically thick, especially towards the highest column density regions in molecular clouds (where stars are born) and so the analysis of CO emission requires complicated radiative transfer modelling. Second, the conversion from CO to H2 relies on an often unknown conversion factor and so a canonical value of 1:10,000 is usually assumed. This is especially problematic in cold (T < 20 K) dense gas, in which CO can be depleted onto dust grains. However, in warm gas surrounding massive or even low mass protostars (so called hot cores.), depletion can be circumvented and the rarer isotopologues (13CO, C18O and C17O) are optically thin enough that they can be used as column density tracers. We propose to use Herschel/HIFI to directly derive total C18O and C17O column densities in a number of high mass protostars. The method we will use offers an unprecedented opportunity to derive this fundamental quantity in a model independent fashion. The basic idea is simple. For an optically thin line the observed integrated emission is proportional to the column density in the upper state. This quantity can be derived without any assumptions regarding density or temperature. If you observe enough transitions of C18O one can simply estimate the total column from summing all the observed states and correcting for the missing population. In high mass star forming regions, the high densities and temperatures mean that the higher-J states can be significantly populated and an estimate of the total column density based on only a few low energy transitions can be seriously in error. With HIFI, we have access to > 7 high-J C18O transitions, and therefore we can calculate the total C18O column densities with great accuracy. |
