A NIKA view of two star-forming infrared dark clouds: Dust emissivity variations and mass concentration^★

¹ School of Physics & Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
e-mail: RigbyA@cardiff.ac.uk
² Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble Alpes, CNRS/IN2P3, 53 avenue des Martyrs, Grenoble, France
³ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Blvd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
⁴ Laboratoire AIM, CEA/IRFU, CNRS/INSU, Université Paris Diderot, CEA-Saclay, 91191 Gif-Sur-Yvette, France
⁵ Institut d’Astrophysique Spatiale (IAS), CNRS and Université Paris Sud, Orsay, France
⁶ Institut Néel, CNRS and Université Grenoble Alpes, Grenoble, France
⁷ Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁸ Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Institut de RadioAstronomie Millimétrique (IRAM), Grenoble, France
¹⁰ Institut de RadioAstronomie Millimétrique (IRAM), Granada, Spain
¹¹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹² LERMA, CNRS, Observatoire de Paris, 61 avenue de l’Observatoire, Paris, France
¹³ School of Earth and Space Exploration and Department of Physics, Arizona State University, Tempe, AZ 85287, USA
¹⁴ Institut d’Astrophysique de Paris, CNRS (UMR7095), 98 bis boulevard Arago, 75014 Paris, France

Received: 8 November 2017
Accepted: 29 January 2018

Abstract

Context. The thermal emission of dust grains is a powerful tool for probing cold, dense regions of molecular gas in the interstellar medium, and so constraining dust properties is key to obtaining accurate measurements of dust mass and temperature.

Aims. By placing constraints on the dust emissivity spectral index, β, towards two star-forming infrared dark clouds – SDC18.888–0.476 and SDC24.489–0.689 – we aim to evaluate the role of mass concentration in the associated star-formation activity.

Methods. We exploited the simultaneous 1.2 and 2.0 mm imaging capability of the NIKA camera on the IRAM 30 m telescope to construct maps of β for both clouds, and by incorporating Herschel observations, we created H₂ column density maps with 13′′ angular resolution.

Results. While we find no significant systematic radial variations around the most massive clumps in either cloud on ≳0.1 pc scales, their mean β values are significantly different, with β̅ = 2.07 ± 0.09 (random) ± 0.25 (systematic) for SDC18.888–0.476 and β̅ = 1.71 ± 0.09 (random) ± 0.25 (systematic) for SDC24.489–0.689. These differences could be a consequence of the very different environments in which both clouds lie, and we suggest that the proximity of SDC18.888–0.476 to the W39 H II region may raise β on scales of ~1 pc. We also find that the mass in SDC24.489–0.689 is more centrally concentrated and circularly symmetric than in SDC18.888–0.476, and is consistent with a scenario in which spherical globally-collapsing clouds concentrate a higher fraction of their mass into a single core than elongated clouds that will more easily fragment, distributing their mass into many cores.

Conclusions. We demonstrate that β variations towards interstellar clouds can be robustly constrained with high signal-to-noise ratio (S/N) NIKA observations, providing more accurate estimates of their masses. The methods presented here will be applied to the Galactic Star Formation with NIKA2 (GASTON) guaranteed time large programme, extending our analysis to a statistically significant sample of star-forming clouds.