Magnetic Properties of Star-forming Dense Cores

Abstract

Magnetic and energetic properties are presented for 17 dense cores within a few hundred parsecs of the Sun. Their plane-of-sky field strengths B pos 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 ≲ M / M B ≲ 3 , indicating nearly critical field strengths. The field strength B pos is correlated with column density N as B pos ∝ N p , where p = 1.05 ± 0.08 , and with density n as B pos ∝ n q , where q = 0.66 ± 0.05 . These magnetic properties are consistent with those derived from Zeeman studies, with less scatter. Relations between virial mass M V , magnetic critical mass M B , and Alfvén amplitude σ B / B match the observed range of M / M B for cores observed to be nearly virial, 0.5 ≲ M / M V ≲ 2 , with moderate Alfvén amplitudes, 0.1 ≲ σ B / B ≲ 0.4 . The B  −  N and B  −  n correlations in the DCF and Zeeman samples can be explained when such bound, Alfvénic, and nearly critical cores have central concentration and spheroidal shape. For these properties, B ∝ N because M / M B 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 that follow B ∝ n 2 / 3 need not be much weaker than gravity, in contrast to core fields that follow B ∝ n 2 / 3 owing to spherical contraction at constant mass. Instead, the nearly critical values of M / M B suggest that the observed core fields are nearly as strong as possible, among values that allow gravitational contraction.