We present Spitzer observations of a sample of 12 starless cores selected to have prominent 24 µm shadows .
The Spitzer images show 8 and 24 µm shadows and in some cases 70 µm shadows ; these spatially resolved absorption features trace the densest regions of the cores .
We have carried out a ^ { 12 } CO ( 2-1 ) and ^ { 13 } CO ( 2-1 ) mapping survey of these cores with the Heinrich Hertz Telescope ( HHT ) .
We use the shadow features to derive optical depth maps .
We derive molecular masses for the cores and the surrounding environment ; we find that the 24 µm shadow masses are always greater than or equal to the molecular masses derived in the same region , a discrepancy likely caused by CO freeze–out onto dust grains .
We combine this sample with two additional cores that we studied previously to bring the total sample to 14 cores .
Using a simple Jeans mass criterion we find that \sim 2 / 3 of the cores selected to have prominent 24 µm shadows are collapsing or near collapse , a result that is supported by millimeter line observations .
Of this subset at least half have indications of 70 µm shadows .
All cores observed to produce absorption features at 70 µm are close to collapse .
We conclude that 24 µm shadows , and even more so the 70 µm ones , are useful markers of cloud cores that are approaching collapse .