(Proposal ID) S16B-141

(PI) Buragohain, Mridusmita 

(Proposal Title)
Investigation of signatures from deuterium inclusion in astronomical PAHs

(Abstract)
'Depletion model of Deuterium (D)' suggests that a fraction of the 
interstellar deuterium (D) is preferably depleted onto dust grains and 
molecules which may explain the observed variation in D/H ratio along various 
lines of sight of our local galaxy. Interstellar Polycyclic Aromatic 
Hydrocarbon (PAH) molecules (astronomical PAHs) are regarded as a major 
reservoir of the interstellar D that may result into deuterated PAHs (PADs). 
4.4 and 4.65 mum features which arise due to the C-D stretching mode in PADs 
have been used as a standard tool to measure the deuterium fraction in PAHs, 
but recent results of AKARI observations indicate that the D/H ratio measured 
from those features are small, possibly due to the missing contribution in 
those features from the large PAHs. Laboratory experiments and theoretical 
quantum chemical calculations of deuterated PAHs show that the profiles 
corresponding to C-H oop bending mode (UIR 11.2mum) and C-C stretching mode 
(UIR 7.7 mum) are affected by the partial deuteration of PAHs. In this 
observational programme, we propose to test our hypothesis that the 
investigation of the profiles of UIR 7.7 mum and UIR 11.2 mum features can be 
used as an alternative tool to probe D/H ratio in astronomical PAHs, even 
applicable to large PAHs.