An Ammonia Spectral Map of the L1495-B218 Filaments in the Taurus Molecular Cloud. II. CCS and HC_7N Chemistry and Three Modes of Star Formation in the Filaments

Abstract

We present deep CCS and HC_7N observations of the L1495-B218 filaments in the Taurus molecular cloud obtained using the K-band focal plane array on the 100 m Green Bank Telescope. We observed the L1495-B218 filaments in CCS J_N = 2_1–1_0 and HC_7N J = 21−20 with a spectral resolution of 0.038 km s^(−1) and an angular resolution of 31''. We observed strong CCS emission in both evolved and young regions and weak emission in two evolved regions. HC_7N emission is observed only in L1495A-N and L1521D. We find that CCS and HC_7N intensity peaks do not coincide with NH_3 or dust continuum intensity peaks. We also find that the fractional abundance of CCS does not show a clear correlation with the dynamical evolutionary stage of dense cores. Our findings and chemical modeling indicate that the fractional abundances of CCS and HC_7N are sensitive to the initial gas-phase C/O ratio, and they are good tracers of young condensed gas only when the initial C/O is close to solar value. Kinematic analysis using multiple lines, including NH_3, HC_7N, CCS, CO, HCN, and HCO^+, suggests that there may be three different star formation modes in the L1495-B218 filaments. At the hub of the filaments, L1495A/B7N has formed a stellar cluster with large-scale inward flows (fast mode), whereas L1521D, a core embedded in a filament, is slowly contracting because of its self-gravity (slow mode). There is also one isolated core that appears to be marginally stable and may undergo quasi-static evolution (isolated mode).