Self-similarity of the magnetic field at different scales: The case of G31.41+0.31
Journal article, 2024
Aims. To study the magnetic field morphology toward the four (proto)stars A, B, C, and D contained in G31.41+0.31 and examine whether the self-similarity observed at core scales (1 '' and 0.'' 24 resolution) still holds at circumstellar scales, we carried out ALMA observations of the polarized dust continuum emission at 1.3 mm and 3.1 mm at an angular resolution of similar to 0.'' 068 (similar to 250 au), sufficient to resolve the envelope emission of the embedded protostars.
Methods. We used ALMA to perform full polarization observations at 233 GHz (Band 6) and 97.5 GHz (Band 3) with a synthesized beam of 0.'' 072 x 0.'' 064. We carried out polarization observations at two different wavelengths to confirm that the polarization traces magnetically aligned dust grains and is not due to dust self-scattering.
Results. The polarized emission and the direction of the magnetic field obtained at the two wavelengths are basically the same, except for an area between the embedded sources C and B. In such an area, the emission at 1.3 mm could be optically thick and affected by dichroic extinction. In the rest of the core, the similarity of the emission at the two wavelengths suggests that the polarized emission is due to magnetically aligned grains. The polarized emission has been successfully modeled with a poloidal field with a small toroidal component on the order of 10% of the poloidal component, with a position angle phi = -63 degrees, an inclination i = 50 degrees, and a mass-to-flux ratio lambda = 2.66. The magnetic field axis is oriented perpendicular to the NE-SW velocity gradient detected in the core. The strength of the plane-of-the-sky component of the mean magnetic field, estimated using both the Davis-Chandrasekhar-Fermi and the polarization-intensity gradient methods, is in the range similar to 10-80 mG, for a density range 1.4 x 10(7)-5 x 10(8) cm(-3). The mass-to-flux ratio is in the range lambda similar to 1.9-3.0, which suggests that the core is "supercritical". The polarization-intensity gradient method indicates that the magnetic field cannot prevent gravitational collapse inside the massive core. The collapse in the external part of the core is (slightly) sub-Alfvenic and becomes super-Alfvenic close to the center.
Conclusions. Dust polarization measurements from large core scales to small circumstellar scales, in the hot molecular core G31.41+0.31 have confirmed the presence of a strong magnetic field with an hourglass-shaped morphology. This result suggests that the magnetic field could have a relevant role in regulating the star-forming process of massive stars at all scales, although it cannot prevent the collapse. However, it cannot be ruled out that the large opacity of the central region of the core may hinder the study of the magnetic field at circumstellar scales. Therefore, high-angular resolution observations at longer wavelengths, tracing optically thinner emission, are needed to confirm this self-similarity.
polarization
ISM: individual objects: G31.41+0.31
techniques: interferometric
ISM: magnetic fields
stars: formation
Author
M. T. Beltran
Istituto nazionale di astrofisica (INAF)
M. Padovani
Istituto nazionale di astrofisica (INAF)
D. Galli
Istituto nazionale di astrofisica (INAF)
N. Anez-Lopez
Paris Cité University
J. M. Girart
Institute of Space Studies of Catalonia (IEEC)
Spanish National Research Council (CSIC)
R. Cesaroni
Istituto nazionale di astrofisica (INAF)
Daria Dall` Olio
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
G. Anglada
Spanish National Research Council (CSIC)
Chi Yan Law
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
A. Lorenzani
Istituto nazionale di astrofisica (INAF)
L. Moscadelli
Istituto nazionale di astrofisica (INAF)
A. Sanchez-Monge
Spanish National Research Council (CSIC)
Institute of Space Studies of Catalonia (IEEC)
M. Osorio
Spanish National Research Council (CSIC)
Q. Zhang
Harvard-Smithsonian Center for Astrophysics
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 686 A281Subject Categories
Astronomy, Astrophysics and Cosmology
Geophysics
Fusion, Plasma and Space Physics
DOI
10.1051/0004-6361/202348927