Dust spectrum and polarisation at 850 μm in the massive IRDC G035.39-00.33

¹ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
e-mail: mika.juvela@helsinki.fi
² Institut d’Astrophysique Spatiale, CNRS, Universitris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay Cedex, France
³ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
⁴ Korea Astronomy and Space Science Institute, 776 Daedeokdaero, Yuseong-gu, Daejeon 34055, Republic of Korea
⁵ East Asian Observatory, 660 N. A’ohōkū Place, Hilo, HI 96720-2700, USA
⁶ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France
⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ Department of Physics, School of Science and Technology, Nazarbayev University, Astana 010000, Kazakhstan
⁹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁰ Astrophysics Research Institute, Liverpool John Moores University, Ic2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
¹¹ Academia Sinica, Institute of Astronomy and Astrophysics, Taipei, Taiwan
¹² Korea University of Science and Technology, 217 Gajang-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
¹³ Institute of Physics I, University of Cologne, Cologne, Germany
¹⁴ Institut UTINAM, CNRS UMR 6213, OSU THETA, Université de Franche-Comté, 41bis avenue de l’Observatoire, 25000 Besançon, France
¹⁵ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁶ SOFIA Science Center, Universities Space Research Association, NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁷ IAPS – INAF, via Fosso del Cavaliere, 100, 00133 Roma, Italy
¹⁸ National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
¹⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 17 April 2018
Accepted: 3 September 2018

Abstract

Context. The sub-millimetre polarisation of dust emission from star-forming clouds carries information on grain properties and on the effects that magnetic fields have on cloud evolution.

Aims. Using observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction.

Methods. JCMT SCUBA-2/POL-2 observations at 850 μm were combined with Planck 850 μm(353 GHz) data to map polarisation fraction at small and large scales. With previous total intensity SCUBA-2 observations (450 and 850 μm) and Herschel data, the column densities were determined via modified black-body fits and via radiative transfer modelling. Models were constructed to examine how the observed polarisation angles and fractions depend on potential magnetic field geometries and grain alignment processes.

Results. POL-2 data show clear changes in the magnetic field orientation. These are not in contradiction with the uniform orientation and almost constant polarisation fraction seen by Planck, because of the difference in the beam sizes and the POL-2 data being affected by spatial filtering. The filament has a peak column density of N(H₂) ~ 7 × 10²² cm⁻², a minimum dust temperature of T ~ 12 K, and a mass of ~4300 M_⊙ for the area N(H₂) > 5 × 10²¹ cm⁻². The estimated average value of the dust opacity spectral index is β ~ 1.9. The ratio of sub-millimetre and J-band optical depths is τ (250 μm)∕τ (J) ~ 2.5 × 10⁻³, more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p ~ 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H₂) > 2 × 10²² cm⁻². The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above ~10⁴ cm⁻³ or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions.

Conclusions. The G035.39-00.33 filament shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.