Abundant Carbon-Chain Molecules toward the Low-Mass Protostar IRAS 04368+2557 in L1527

We have detected the high-excitation lines of carbon-chain molecules such as C₄H₂ (J = 10_0,10–9_0,9), C₄H (N = 9–8, F₁, F₂), l-C₃H₂ (4_1,3-3_1,2), and CH₃CCH (J = 5–4, K = 2) toward a low-mass star-forming region, L1527. In particular, the F₁ line of C₄H is as strong as 1.7 K (T_MB). The rotational temperature of C₄H₂ is determined to be 12.3 ± 0.8 K, which is higher than that in TMC-1 (3.8 K). Furthermore, the column density of C₄H₂ is derived to be about 1/4 of that in TMC-1, indicating that carbon-chain molecules are abundant in L1527 for a star-forming region. Small mapping observations show that the C₄H, C₄H₂, and c-C₃H₂ emissions are distributed from the infalling envelope to the inner part. Furthermore, we have detected the lines of C₅H, HC₇N, and HC₉N in the 20 GHz region. Since the carbon-chain molecules are generally deficient in star-forming cores, the above results cannot simply be explained by the existing chemical models. The following hypothesis is proposed. If the timescale of the prestellar collapse in L1527 were shorter than those of the other star-forming cores, the carbon-chain molecules could survive in the central part of the core. In addition, regeneration processes of the carbon-chain molecules due to star formation activities would play an important role. Evaporation of CH₄ from the grain mantles would drive the regeneration processes. The present observations show new chemistry in a warm and dense region near the protostars, which is named "warm carbon-chain chemistry (WCCC)."