EPoS Contribution
EPoS Contribution
Extracting the Effect of Low-Mass Star Formation on Deuterium Fractionation

Daiki Shibata
The University of Tokyo, Tokyo, Japan
It is well known that deuterium fractionation in molecules is enhanced in cold evolved starless cores due to the time evolutionally effect and the CO depletion. Hence, it has extensively been employed to trace the evolutionary stages of starless cores. On the other hand, it is still controversial how the deuteirum fractionation ratio changes after the onset of star formation. When a protostar is formed in a dense core, the temperature is raised up in the vicinity of the protostar. Since the equilibrium deuterium fractionation ratio is lower for higher temperature, the deuterium fractionation ratio should start to decrease near the protostar toward the new equilibrium ratio at the elevated temperature. In order to explore this effect observationally, we have investigated the spacial distribution of the deuterium fractionation ratios for a few molecules toward two representative low mass star-forming regions, L1551 (Class I) and IRAS16293-2422 (Class 0). With the Nobeyama 45 m telescope, we have observed the J=1-0 lines of DCO+, H13CO+, DNC, HN13C, N2D+ and N2H+. We have conducted 5-points strip observations centred at the protostar position. For L1551, the DCO+/H13CO+ ratio toward the protostar position is found to be decreased in comparison with those of other positions. On the other hand, the DNC/HN13C ratio does not show such a central dip. This can be interpreted as follows. For ionic spiecies such as DCO+ and H13CO+, the dissociative electron recombination is their main destruction mechanism, and the timescale is a few 100 years. In contrast, the major destruction pathway for the neutral species like DCN is the ionic destruction by H+, H3+, and He+, and the timescale is as long as 10^{4} - 10^{5}. Therefore, the DCO+/H13CO+ ratio quickly decreases after the temperature rise, whereas the DNC/HN13C ratio remains for a while as it was in the cold starless stage. However, we have not been able to find decrease of the N2D+/N2H+ in L1551 in contrast to the above prediction. This seems to be due to depletion of N2H+ (and N2D+) in the vicinity of the protostar. Since N2H+ (and N2D+) is destructed by CO which is evaporated from the dust grains in a warm region, the N2H+ (and N2D+) line would mainly trace the cold envelope. Hence, the N2D+/N2H+ ratio does not show significant decrease toward the protostar position due to overwhelming contribution of the cold envelope. The effect of the cold envelope is found to be more significant in the Class 0 object, IRAS16293-2422. Toward this source, even the DCO+/H13CO+ ratio does not decrease toward the protostar position, as well as the DNC/HN13C and N2D+/N2H+ ratios. These results mean that it is essential to reduce the effect of the cold envelope in order to study the change in deuterium fractionation due to star formation. This would be possible by observing high excitation lines or by observing the lines with high spatial resolution. If we can properly extract the deuterium fractionation ratios of neutral spiecies toward the protostar, we would be able to learn the physical condition of the parent core just before the onset of star formation. This would be a new approach to investigate an origin of the diversity of the star formation.
Nami Sakai, The University of Tokyo, Japan
Tomoya Hirota, National Astronomical Observatory of Japan, Japan
Satoshi Yamamoto, The University of Tokyo, Japan