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 Bpos are estimated from the dispersion of polarization directions, following Davis, Chandrasekhar, and Fermi (DCF). Their ratio of mass to magnetic critical mass is $0.5\lesssim M/{M}_{B}\lesssim 3$ , indicating nearly critical field strengths. The field strength Bpos is correlated with column density N as ${B}_{\mathrm{pos}}\propto {N}^{p}$ , where $p=1.05\pm 0.08$ , and with density n as ${B}_{\mathrm{pos}}\propto \ {n}^{q}$ , where $q=0.66\pm 0.05$ . These magnetic properties are consistent with those derived from Zeeman studies, with less scatter. Relations between virial mass MV, magnetic critical mass MB, and Alfvén amplitude ${\sigma }_{B}/B$ match the observed range of M/MB for cores observed to be nearly virial, $0.5\lesssim M/{M}_{V}\lesssim 2$ , with moderate Alfvén amplitudes, $0.1\lesssim {\sigma }_{B}/B\lesssim 0.4$ . 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, $B\propto N$ because M/MB is nearly constant compared to the range of N, and $B\propto {n}^{2/3}$ because M1/3 is nearly constant compared to the range of n2/3. The observed core fields that follow $B\propto {n}^{2/3}$ need not be much weaker than gravity, in contrast to core fields that follow $B\propto {n}^{2/3}$ owing to spherical contraction at constant mass. Instead, the nearly critical values of M/MB suggest that the observed core fields are nearly as strong as possible, among values that allow gravitational contraction.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2021
- DOI:
- 10.3847/1538-4357/abf4c8
- arXiv:
- arXiv:2104.02597
- Bibcode:
- 2021ApJ...917...35M
- Keywords:
-
- Interstellar magnetic fields;
- Star formation;
- Star-forming regions;
- 845;
- 1569;
- 1565;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication, The Astrophysical Journal