On the radiation driven alignment of dust grains: Detection of the polarization hole in a starless core

Aims. We aim to investigate the polarization properties of a starl ess core in a very early evolutionary stage. Linear polariza tion data reveal the properties of the dust grains in the distinct phas es of the interstellar medium. Our goal is to investigate how the polarization degree and angle correlate with the cloud and core gas. Methods. We use optical, near infrared and submillimeter polarization observations toward the starless object Pipe-109 in the Pipe nebula. Our data cover a physical scale range of 0.08 to 0.4 pc, comprising the dense gas, envelope and the surrounding cloud. Results. The cloud polarization is well traced by the optical data. The near infrared polarization is produced by a mixed population of grains from the core border and the cloud gas. The optical and near infrared polarization toward the cloud reach the maximum possible value and saturate with respect to the visual extinction. Th e core polarization is predominantly traced by the submillimeter data and have a steep decrease with respect to the visual extinction. Modeling of the submillimeter polarization indicates a magnetic field main direction projected onto the plane-of-sky and loss of grain alignment for densities higher than 6× 10 4 cm −3 (or AV> 30 mag). Conclusions. Pipe-109 is immersed in a magnetized medium, with a very ordered magnetic field. The absence of internal source of radiation significantly a ffects the polarization effi ciencies in the core, creating a polarization hole at the cen ter of the starless core. This result supports the theory of dust grain alignment via r adiative torques