CARBON-CHAIN AND ORGANIC MOLECULES AROUND VERY LOW LUMINOSITY PROTOSTELLAR OBJECTS OF L1521F-IRS AND IRAM 04191+1522

We have observed dense gas around the Very Low Luminosity Objects (VeLLOs) L1521F-IRS and IRAM 04191+1522 in carbon-chain and organic molecular lines with the Nobeyama 45 m telescope. Toward L1521F-IRS, carbon-chain lines of CH3CCH (5(0)-4(0)), C4H (17/2-15/2), and C3H2 (2(12)-1(01)) are 1.5-3.5 times stronger than those toward IRAM 04191+1522, and the abundances of the carbon-chain molecules toward L1521F-IRS are 2-5 times higher than those toward IRAM 04191+1522. Mapping observations of these carbon-chain molecular lines show that in L1521F the peak positions of these carbon-chain molecular lines are different from each other and there is no emission peak toward the VeLLO position, while in IRAM 04191+1522 these carbon-chain lines are as weak as the detection limits, except for the C3H2 line. The observed chemical differentiation between L1521F and IRAM 04191+1522 suggests that the evolutionary stage of L1521F-IRS is younger than that of IRAM 04191+1522, consistent with the extent of the associated outflows seen in the (CO)-C-13 (1-0) line. The non-detection of the organic molecular lines of CH3OH (6(-2)-7(-1) E) and CH3CN (6(0)-5(0)) implies that the warm (similar to 100 K) molecular-desorbing region heated by the central protostar is smaller than similar to 100 AU toward L1521F-IRS and IRAM 04191+1522, suggesting the young age of these VeLLOs. We propose that the chemical status of surrounding dense gas can be used to trace the evolutionary stages of VeLLOs.