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 &gt...; 7 high-J C18O transitions, and therefore we can calculate the total C18O column densities with great accuracy.
Publication
Instrument
HIFI_HifiPoint_dbs, HIFI_HifiMapping_raster
Temporal Coverage
2012-04-17T02:09:44Z/2013-03-08T14:31:50Z
Version
SPG v14.1.0
Mission Description
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
European Space Agency, 2013, Hifi Observations Of The C18O And C17O J = 5-4 To 15-14 Transitions In Hot Cores: A Direct Method To Obtain Total Column Densities, SPG v14.1.0, European Space Agency, https://doi.org/10.5270/esa-x6ym00i
