Submillimetre Studies of Pre-stellar and Starless Cores in the Ophiuchus, Taurus and Cepheus Molecular Clouds

Abstract

We identify, and characterise the properties of, starless cores in the Ophiuchus, Taurus and Cepheus molecular clouds. We identify sources in SCUBA-2 850-μm emission, and determine temperatures by fitting a modified blackbody model to their spectral energy distributions, measured using SCUBA-2, PACS and SPIRE data. We construct convolution kernels to accurately bring SCUBA-2 and Herschel data to a common resolution. Masses are calculated using best-fit temperatures and measured 850-μm flux densities.
In Ophiuchus, the mass distribution of starless cores is consistent with the expected shape of the Core Mass Function (CMF). We determine core masses from C18O and N2H+ measurements, and find some evidence for high-density N2H+ freezeout. Virial analysis, including external pressure, shows that most cores are either bound or virialised. Gravitational potential and external pressure energies are found to be typically of a similar order of magnitude, with some variation between regions. Non-thermal linewidths decrease between C18O-traced and N2H+-traced material, indicating dissipation of turbulence. Core properties vary with region, and hence we infer a south-west to north-east evolutionary gradient.
In Taurus, we identify starless cores in SCUBA-2 850- μm emission, Herschel 500-μm emission, and Herschel 500-μm emission filtered to remove large-scale structure. Cores detected and characterised using unfiltered Herschel 500-μm data have higher densities and temperatures than their equivalents in SCUBA-2 emission. SCUBA-2 detects only the densest starless cores relative to the filtered Herschel data, due to a surface-brightness sensitivity limit, as both populations have similar ranges in temperature. Virial analysis shows that the SCUBA-2 cores are pressure-confined and that almost all are virially bound in the absence of an internal magnetic field. The magnetic field strengths required to bring our cores into virial equilibrium are consistent with those measured in dense gas in Taurus.
In Cepheus, we compare starless cores in the regions L1147/58, L1172/74, L1251 and L1228. Region CMFs generally show sub-Salpeter power-law indices. L1147/58 and L1228 have a high ratio of cores to protostars; L1251 and L1174 have a low ratio, suggesting that the latter are active sites of star formation, while in the former, star formation proceeds quiescently. Core external pressures are estimated; all but one of our cores are pressure-confined. We find a power-law relation between gravitational potential and external pressure energies. Cores which obey this relation are strongly pressure-dominated; those which do not are candidates for gravitational collapse.
Core temperatures and masses in each cloud are similar. Cores in Ophiuchus are significantly smaller and denser than in other regions. Ophiuchus shows strong evidence for clustering: a non-uniform surface density of sources, and small nearest-neighbour distances between sources. Taurus is a dispersed region, while Cepheus is intermediate. Ophiuchus shows the most variation of core properties with location. Cores in Taurus are extremely homogeneous; cores in Cepheus show a wide range of properties, but little correlation of properties with location.
We present a new analytical model for the evolution of starless cores. We find that not all pressure-confined and virially-bound cores will become gravitationally bound, with many instead collapsing to virial equilibrium. Hence, we state that only gravitationally bound starless cores can be definitively considered to be prestellar.