(Proposal ID) S17A-135

(PI) Goto, Miwa

(Proposal Title)
First Frost in the Pipe Nebula

(Abstract)
We started water ice survey in starless cores in the Pipe Nebula in 2015, 
using SpeX at the IRTF to better understand the early stages of the ice 
formation in relatively diffuse environment, where the empirical knowledge 
was close to nil. The water ice have been positively detected on 7 sources 
in the Pipe Nebula at 3.0 mu m. Their optical depths are, however, half as 
much as those in the nearby star forming regions with similar A_V. This is 
an encouraging sign that we might have indeed detected the ice formation 
actively in progress. Further investigation using NIRSPEC/Keck II this summer 
uncovered that the CO/water ice ratio was remarkably low, 6.5%, as opposed 
to ~30% typically found in the quiescent clouds in star forming regions. 
This is preliminary the first evidence of the quick conversion of CO to CO_2,
or to methanol, in the early phase of the ice formation. We would like to use 
IRCS with LGS/AO to collect CO ice spectra at 4.67 mu m in 15 line of sights 
in the interval between N(H_2O)=7 x 10^{17} and 2 x 10^{18}cm^{-2}. If the 
destruction of CO to CO_2 is really the case, we should see a rapid increase 
of the CO/H_2O ratio with N(H_2O), as the dust grains in the cloud cool down. 
The result will be compared to the dust temperature map from Herschel 
PACS/SPIRE.