Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region

Abstract

(Abridged) Context. Core condensation is a critical step in the\nstar-formation process, but is still poorly characterized observationally.\nAims. We have studied the 10 pc-long L1495/B213 complex in Taurus to\ninvestigate how dense cores have condensed out of the lower-density cloud\nmaterial. Results. From the N$_2$H$^+$ emission, we identify 19 dense cores,\nsome starless and some protostellar. They are not distributed uniformly, but\ntend to cluster with relative separations on the order of 0.25 pc. From the\nC$^{18}$O emission, we identify multiple velocity components in the gas. We\nhave characterized them by fitting gaussians to the spectra, and by studying\nthe distribution of the fits in position-position-velocity space. In this\nspace, the C$^{18}$O components appear as velocity-coherent structures, and we\nhave identified them automatically using a dedicated algorithm (FIVe: Friends\nIn Velocity). Using this algorithm, we have identified 35 filamentary\ncomponents with typical lengths of 0.5 pc, sonic internal velocity dispersions,\nand mass-per-unit-length close to the stability threshold of isothermal\ncylinders at 10 K. Core formation seems to have occurred inside the filamentary\ncomponents via fragmentation, with a small number of fertile components with\nlarger mass-per-unit-length being responsible for most cores in the cloud. At\nlarge scales, the filamentary components appear grouped into families, which we\nrefer to as bundles. Conclusions. Core formation in L1495/B213 has proceeded by\nhierarchical fragmentation. The cloud fragmented first into several pc-scale\nregions. Each of these regions later fragmented into velocity-coherent\nfilaments of about 0.5 pc in length. Finally, a small number of these filaments\nfragmented quasi-statically and produced the individual dense cores we see\ntoday.\n