(Proposal ID) S16B-118

(PI) Goto, Miwa

(Proposal Title)
Is the Pipe frozen with Water? or CO?

(Abstract)
We have started ice mapping program in 2015 in the starless cores in the Pipe 
Nebula, using SpeX at the IRTF, in order to gain empirical insights into the 
formation of water in the earliest phase of the cloud evolution. We have 
collected 45 spectra, 7 out of which show clear absorption feature of water 
ice at 3.0mum. The extinction threshold of the water ice was A_V=5.2mag, and 
significantly larger than the canonical threshold known in Taurus (3.1mag). 
In other words, the water ice in the Pipe Nebula is much less than is expected 
from the given A_V. The lower abundance of water ice in the Pipe Nebula 
implies that ice formation is incomplete and actively in progress right now. 
If that is the case, our prediction is that we should see CO ice at 4.7mum on 
the low-extinction line of sight (A_V<5mag) where the water ice is {not} 
detected. If our photo-process model is correct, CO ice formation starts 
earlier at smaller A_V than water ice, as opposed to what is commonly believed 
at larger A_V(> 5-10mag). We would like to use NIRSPEC at KeckII to obtain CO 
ice spectra at 4.7mum, which was not possible with SpeX and IRTF for the lack 
of sensitivity.