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 mm and 2.0 mm imaging capability of the NIKA camera on the IRAM 30 m telescope to construct maps of \beta for both clouds , and by incorporating Herschel observations , we created H _ { 2 } column density maps with 13 ^ { \prime \prime } angular resolution .

Results : While we find no significant systematic radial variations around the most massive clumps in either cloud on \gtrsim 0.1 pc scales , their mean \beta values are significantly different , with \bar { \beta } = 2.07 \pm 0.09 ( random ) \pm 0.25 ( systematic ) for SDC18.888-0.476 and \bar { \beta } = 1.71 \pm 0.09 ( random ) \pm 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 \sim 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 .