Magnetic Properties of Star-Forming Dense Cores
Abstract
Magnetic and energetic properties are presented for 17 dense cores within a few hundred pc of the Sun. Their plane-of-sky field strengths are estimated from the dispersion of polarization directions, following Davis, Chandrasekhar and Fermi (DCF). Their ratio of mass to magnetic critical mass is 0.5-3, indicating nearly critical field strengths. The field strength Bpos is correlated with column density N as Bpos~Np, where p=1.05+-0.08, and with density n as Bpos~nq, where q=0.66+-0.05. These magnetic properties are consistent with those derived from Zeeman studies (Crutcher et al. 2010), with less scatter. Relations between virial mass MV, magnetic critical mass MB, and Alfven amplitude sigmaB/B match the observed range of M/MB for cores observed to be nearly virial, with M/MV=0.5-2, with moderate Alfven amplitudes, and with sigmaB/B=0.1-0.4. The B-N and B-n correlations in the DCF and Zeeman samples can be explained when such bound, Alfvenic, and nearly-critical cores have central concentration and spheroidal shape. For these properties, B~N because M/MB is nearly constant compared to the range of N, and B~n(2/3) because M(1/3) is nearly constant compared to the range of n(2/3). The observed core fields which follow B~n(2/3) need not be much weaker than gravity, in contrast to core fields which follow B~n(2/3) due to spherical contraction at constant mass (Mestel 1966). Instead, the nearly critical values of M/MB suggest that the observed core fields are nearly as strong as possible, among values which allow gravitational contraction.