Magnetic Properties of Star-forming Dense Cores

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 less than or similar to M/MB less than or similar to 3 <i , indicating nearly critical field strengths. The field strength B (pos) is correlated with column density N as B-pos proportional to Np <i , where p=1.05 +/- 0.08 <i , and with density n as B-pos proportional to nq <i , where q = 0.66 +/- 0.05 <i . 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 Alfven amplitude sigma(B)/B-M/M (B) for cores observed to be nearly virial, 0.5 less than or similar to M/MV less than or similar to 2 <i , with moderate Alfven amplitudes, 0.1 less than or similar to sigma(B)/B less than or similar to 0.4 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 proportional to N M/M (B) is nearly constant compared to the range of N, and B proportional to n(2/3) M (1/3) is nearly constant compared to the range of n (2/3). The observed core fields that follow B proportional to n(2/3) <i need not be much weaker than gravity, in contrast to core fields that follow B proportional to 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.