Peculiar Carbon-Chain Chemistry in Low-Mass Star Forming Regions

¹ Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
² Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 181-8588, Japan
³ National Radio Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Abstract

We have found low-mass star-forming regions with extremely high abundances of carbon-chain molecules. Those are L1527 in Taurus and IRAS 15398-3359 in Lupus. In these sources, carbon-chain molecules would be regenerated in a lukewarm region near the protostar, triggered by the evaporation of the CH₄ ice. This is new carbon-chain chemistry (Warm Carbon-Chain Chemistry: WCCC) in contrast to the conventional one applied to cold starless cores. Our interferometric observation shows a steep abundance increase of the carbon-chain molecules inward of a radius of 500 − 1000 AU from the protostar. This size corresponds to the temperature range of 20–30 K, which is close to the evaporation temperature of CH₄ (25 K). This result is therefore consistent with the WCCC. The discovery of the WCCC sources demonstrates chemical diversity of low-mass star-forming regions. In particular, a remarkable contrast can be seen between the WCCC and the hot corino chemistry known in IRAS 16293-2422. A possible origin for the diversity would be the time scale of the starless-core phase; a shorter contraction time would result in the WCCC. Relatively low deuterium fractionation ratios in L1527 also support this scenario. If so, chemical compositions will tell us an important clue to understand the source-to-source variation of star-formation processes.