Molecular Line Observations of Carbon-Chain-Producing Regions L1495B and L1521B

Abstract

We present the first comprehensive study on physical and chemical properties of quiescent starless cores L1495B and L1521B, which are known to be rich in carbon-chain molecules like the cyanopolyyne peak of TMC-1 and L1521E. We have detected radio spectral lines of various carbon-chain molecules such as CCS, C$_{3}$S, C$_{4}$H, HC$_{3}$N, and HC$_{5}$N. On the other hand, the NH$_{3}$ lines are weak and the N$_{2}$H$^{+}$ lines are not detected. According to our mapping observations of the HC$_{3}$N, CCS, and C$_{3}$S lines, the dense cores in L1495B and L1521B are compact with the radius of 0.063 and 0.044 pc, respectively, and have a simple elliptical structure. The distributions of CCS seem to be different from those of well-studied starless cores, L1498 and L1544, where the distribution of CCS shows a shell-like structure. Since the H$^{13}$CO$^{+}$, HN$^{13}$C, and C$^{34}$S lines are detected in L1495B and L1521B, the densities of these cores are high enough to excite the NH$_{3}$ and N$_{2}$H$^{+}$ lines. Therefore, the abundances of NH$_{3}$ and N$_{2}$H$^{+}$ relative to carbon-chain molecules are apparently deficient, as observed in L1521E. We found that longer carbon-chain molecules such as HC$_{5}$N and C$_{4}$H are more abundant in TMC-1 than L1495B and L1521B, while those of sulfur-bearing molecules such as C$^{34}$S, CCS, and C$_{3}$S are comparable. Both distributions and abundances of the observed molecules of L1495B and L1521B are quite similar to those of L1521E, strongly suggesting that L1495B and L1521B is in a very early stage of physical and chemical evolution.