(Proposal ID) S14B-156

(PI) Goto Miwa

(Proposal Title)
 When and how the ice evaporates in hot cores

(Abstract)
We propose high-resolution M- and L-band spectroscopy of young high-mass stars 
with IRCS to shed lights on the transient evaporation process of molecular ice 
in hot cores. Laboratory experiments indicate that the molecules evaporate 
from dust grains in a few discrete episodes, because of the interplay with the 
water ice that undergoes phase changes, crystallization, and evaporation. The 
infrared absorption spectroscopy is now able to constrain the excitation 
temperature (T_{ex}) of molecules with the accuracy better than plus_minus10~K 
using 10--50 rotational-vibrational transitions in the same time. In the dense 
gas typical in hot cores, T_{ex} is a good proxy of the gas temperature. In 
our previous study we found that the CO ice column density is a function of 
T_{ex}(CO). The additional observation of gas and solid CO, which are 
observable in the same time, along the line of sights toward relatively "cool" 
hot cores will reinforce the correlation found above to understand how the 
water ice could delay the desorption of CO. The comparison of T_{ex}(CO) with 
those of other small molecules of various binding energies such as C_2H_2, 
HCN, NH_3, and H_2CO, all observable simultaneously at 3--4~mum, will allow 
us to understand the sequential desorption of molecules from the surface layer 
better.