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