From cores to stars

Abstract

Dense cores associated with low mass stars generally have mean density 3×10⁴ cm⁻³within 0.1 pc, and temperature 10 K, while cores of similar density forming more massive stars are generally larger and warmer, according to observations of molecular lines of NH₃, HC₃N, CS, and C¹⁸O. These cores are near virial equilibrium, taking into account both the thermal and nonthermal parts of the line width. They are associated with the youngest stars, having the reddest broadband spectra and the greatest column density of circumstellar gas and dust. Their nonthermal motions probably arise from magnetohydrodynamic waves. Their nonthermal and thermal kinetic energy density are comparable when their associated star has mass ≈ 2 M⊙; cores associated with less (more) massive stars are dominated by thermal (nonthermal) motions. Core maps are generally elongated, with aspect ratio ≈2, and a substantial fraction of cores with elongated maps are prolate. Core velocity gradients are of order one line width per map size, and correspond to rotational kinetic energy density less than 0.1 of the gravitational kinetic energy density. Core magnetic energy density is comparable to the nonthermal kinetic energy density, in the few cases where the Zeeman effect is detected. The nonthermal part of the line width in a core increases with the size of the map in the same line, as a power law with exponent 0.3–0.5. Equilibrium models of core-cloud systems, based on observed line widths and map sizes, indicate that cores in Orion A and B have star-forming mass infall rates up to an order of magnitude greater than in low-mass regions such as Taurus. Cores differ significantly in their typical size, velocity dispersion, and extinction from one molecular cloud complex to another. These differences correlate with the size and mass of the complex, and with the incidence of young stars in the complex.