WATER DEUTERIUM FRACTIONATION IN THE INNER REGIONS OF TWO SOLAR-TYPE PROTOSTARS

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 NGC 1333 region. In both sources, the detected HDO emission originates from a central compact unresolved region. A comparison with previously published interferometric observations of the (H2O)-O-18 3(1,3)-2(2,0) line shows that the HDO and H2O lines mostly come from the same region. A non-LTE large velocity gradient analysis of the HDO and (H2O)-O-18 line emissions, combined with published observations, provides an [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 NGC 1333 IRAS4B. The comparison of the measured values toward 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.