Magnetic Properties of Star-forming Dense Cores
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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, indicating nearly critical field strengths. The field strength B pos is correlated with column density N as Bpos ∝ Np 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, with less scatter. Relations between virial mass M V , magnetic critical mass M B , and Alfvén amplitude match the observed range of M/M B for cores observed to be nearly virial,0.5 ≲ M/Mv ≲ 2, with moderate Alfvén amplitudes, 0.1 ≲ σB/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 ∝ n2/3 because M 1/3 is nearly constant compared to the range of n 2/3. The observed core fields that follow B ∝ n2/3 need not be much weaker than gravity, in contrast to core fields that follow B ∝ n2/3 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.