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

¹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: falves@astro.uni-bonn.de
² Instituto de Ciencia de Materiales de Madrid (CSIC), , Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain
³ Observatorio Astronómico Nacional , Alfonso XII 3, 28014 Madrid, Spain
⁴ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències , C5 par 2 a, 08193 Bellaterra, Catalunya, Spain
⁵ Departamento de Física – ICEx – UFMG , Caixa Postal 702, 30.123-970 Belo Horizonte, Brazil
⁶ Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
⁷ Max-Planck-Institut für Radioastronomie , Auf dem Hügel 69, 53121 Bonn, Germany

Received: 25 July 2014
Accepted: 21 August 2014

Abstract

Aims. We aim to investigate the polarization properties of a starless core in an early evolutionary stage. Linear polarization data reveal the properties of the dust grains in the distinct phases 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 on 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 reaches the maximum possible value and saturates with respect to the visual extinction. The core polarization is predominantly traced by the submillimeter data and has 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 × 10⁴ cm^-3 (or A_V> 30 mag).

Conclusions. The object is immersed in a magnetized medium with a very ordered magnetic field. The absence of internal source of radiation significantly affects the polarization efficiencies in the core, creating a polarization hole at the center of the starless core. This result supports the theory of dust grain alignment via radiative torques