Abstract
Abstract Magnetic fields are believed to play an important role in controlling the stability and contraction of the dense condensations of gas and dust that lead to the formation of stars and planetary systems. In the present study, the magnetic field of FeSt 1–457, a cold starless molecular cloud core, was mapped on the basis of the polarized near-infrared light from 185 background stars after being dichroically absorbed by dust aligned with the magnetic field in the core. A distinct “hourglass-shaped” magnetic field was identified in the region of the core, and was interpreted as the first evidence of a magnetic field structure distorted by mass condensation in a starless core. The steep curvature of the magnetic field lines obtained in the present study indicates that the distortion was mainly created during the formation phase of the dense core. The derived mass-to-magnetic flux ratio indicates that the core is in a magnetically supercritical state. However, the stability of the core can be considered to be in a nearly critical state if the additional contributions from the thermal and turbulent support are included. Further diffusion of the magnetic field and/or turbulent dissipation would cause the onset of the dynamical collapse of the core. The geometrical relationship between the direction of the magnetic field lines and the elongation of the core was found to be in good agreement with theoretical predictions for the formation of Sun-like stars under the influence of a magnetic field.
Highlights
The characteristics of newborn stars are thought to be determined by the physical properties of the nursing molecular cloud core prior to the onset of gravitational collapse, and the magnetic field pervading the core is believed to play an important role in controlling the stability and contraction of the core (e.g., Shu et al 1987)
If the magnetic field plays a dominant role in supporting a core, the core will evolve quasistatically through the gravitationally induced drift of neutral particles with respect to ions and magnetic fields in a process known as ambipolar diffusion (Mestel & Spitzer 1956)
As neutral particles slowly migrate toward the center of the cloud, the mass-to-magnetic flux ratio gradually increases, leading up to the onset of dynamical collapse
Summary
The characteristics of newborn stars are thought to be determined by the physical properties of the nursing molecular cloud core prior to the onset of gravitational collapse, and the magnetic field pervading the core is believed to play an important role in controlling the stability and contraction of the core (e.g., Shu et al 1987). It is important to investigate the magnetic field structure affecting the cores of molecular clouds in the starless phase in order to clarify the initial conditions of star formation. Polarized dust emission from cold low-mass starless cores is too weak to be detected by presently available instruments For this reason, magnetic field structures in starless cores have only been mapped toward dense central regions with a small number of data points (e.g., Ward-Thompson et al 2000; Crutcher et al 2004). The local background was subtracted from the original image using the mean of a circular annulus around the source on the image This process was carried out for each position angle image (I0, I45, I22.5, and I67.5).
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