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 from laboratory data. This threshold, which can be expressed as a critical ultraviolet exposure [E](c) similar or equal to 10 J cm(-2), is reached after exposure of dust to the standard interstellar radiation field for similar to 10(6) yr when A(V)(0)=2.3 mag. The conversion from HAC 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 HAC-aC conversion. We also conclude that HAC should contain an appreciable quantity of structural H2O, which is distinct from condensed H2O ice. Infrared absorption due to this structural H2O should be detectable near 2.9 and 5.9 mu m.