Water deuterium fractionation in the inner regions of two solar type protostars

Abstract

The [HDO]/[H2O] ratio is a crucial parameter for probing the history of water formation. So far, it has been measured for only three solar type protostars and yielded different results, possibly pointing to a substantially different history in their formation. In the present work, we report new interferometric observations of the HDO 4 2,2 - 4 2,3 line for two solar type protostars, IRAS2A and IRAS4A, located in the NGC1333 region. In both sources, the detected HDO emission originates from a central compact unresolved region. Comparison with previously published interferometric observations of the H218$O 3 1,3 - 2 2,0 line shows that the HDO and H$_2$O lines mostly come from the same region. A non-LTE LVG analysis of the HDO and H218$O line emissions, combined with published observations, provides a [HDO]/[H2O] ratio of 0.3 - 8 % in IRAS2A and 0.5 - 3 % in IRAS4A. First, the water fractionation is lower than that of other molecules such as formaldehyde and methanol in the same sources. Second, it is similar to that measured in the solar type protostar prototype, IRAS16293-2422, and, surprisingly enough, larger than that measured in NGC1333 IRAS4B. {The comparison of the measured values towards IRAS2A and IRAS4A with the predictions of our gas-grain model GRAINOBLE gives similar conclusions to those for IRAS 16293, arguing that these protostars {share} a similar chemical history, although they are located in different clouds.