WATER ICE FORMATION ON INTERSTELLAR CARBON DUST - WET HAC (WHAC)

A quantitative threshold for the radiative conversion of hydrogenated amorphous carbon (HAC) to amorphous carbon (aC) has been identified fr om laboratory data. This threshold, which can be expressed as a critic al ultraviolet exposure [E](c) similar or equal to 10 J cm(-2), is rea ched after exposure of dust to the standard interstellar radiation fie ld for similar to 10(6) yr when A(V)(0)=2.3 mag. The conversion from H AC to aC involves a change in surface structure, which may inhibit the bonding of gaseous molecules when A(V) < A(V)(0). This suggests that the threshold for ice formation on dust grains may be related to the H AC-aC conversion. We also conclude that HAC should contain an apprecia ble quantity of structural H2O, which is distinct from condensed H2O i ce. Infrared absorption due to this structural H2O should be detectabl e near 2.9 and 5.9 mu m.