1201.1785 (Audrey Coutens et al.)

A study of deuterated water in the low-mass protostar IRAS16293-2422    [PDF]

Audrey Coutens, Charlotte Vastel, Emmanuel Caux, Cecilia Ceccarelli, Sandrine Bottinelli, Laurent Wiesenfeld, Alexandre Faure, Yohann Scribano, Claudine Kahane

Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS16293-2422 and learn more about the water formation mechanisms by the determination of the HDO/H2O abundance ratio. A spectral survey of the source IRAS16293-2422 has been carried out in the framework of the CHESS Herschel Key program with the HIFI instrument, allowing the detection of numerous HDO lines. Other transitions were previously observed with ground-based telescopes in the framework of TIMASSS. The spherical Monte Carlo radiative transfer code RATRAN has been used to reproduce the observed line profiles of HDO assuming an abundance jump, corresponding to the sublimation of the molecules trapped on the icy grain mantles in the hot corino. To determine the H2O abundance throughout the envelope, a similar study has been applied to the H2-18O observed lines, as the H2O main isotope lines are contaminated by the outflows. We derive an inner HDO abundance of 1.7e-7 and an outer HDO abundance of 8e-11. To reproduce the HDO absorption lines, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. The HDO/H2O ratio is ~1.4-5.8% in the hot corino whereas it is ~0.2-2.2% in the outer envelope. It is estimated at ~4.8% in the added absorbing layer. Although it is clearly higher than the cosmic D/H abundance, the HDO/H2O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similar ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol which formed later once the CO molecules have depleted on the grains.

View original: http://arxiv.org/abs/1201.1785