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

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, beta, 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 3 '' m telescope to construct maps of beta for both clouds, and by incorporating Herschel observations, we created H-2 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 greater than or similar to 0.1 pc scales, their mean beta values are significantly different, with (beta) over bar = 2.07 +/- 0.09 (random) +/- 0.25 (systematic) for SDC18.888-0.476 and (beta) over bar= 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 beta on scales of similar to 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 beta 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.