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Abstract

The ¹⁴N/¹⁵N ratio in molecules exhibits a large variation in star-forming regions, especially when measured from N²H⁺ isotopologues. However, there are only a few studies performed at high-angular resolution. We present the first interferometric survey of the ¹⁴N/¹⁵N ratio in N²H⁺ obtained with Atacama Large Millimeter Array observations towards four infrared-dark clouds harbouring 3 mm continuum cores associated with different physical properties. We detect N¹⁵NH⁺ (1–0) in ∼20−40 per cent of the cores, depending on the host cloud. The ¹⁴N/¹⁵N values measured towards the millimetre continuum cores range from a minimum of ∼80 up to a maximum of ∼400. The spread of values is narrower than that found in any previous single-dish survey of high-mass star-forming regions and than that obtained using the total power data only. This suggests that the ¹⁴N/¹⁵N ratio is on average higher in the diffuse gaseous envelope of the cores and stresses the need for high-angular resolution maps to measure correctly the ¹⁴N/¹⁵N ratio in dense cores embedded in IRDCs. The average ¹⁴N/¹⁵N ratio of ∼210 is also lower than the interstellar value at the Galactocentric distance of the clouds (∼300–330), although the sensitivity of our observations does not allow us to unveil ¹⁴N/¹⁵N ratios higher than ∼400. No clear trend is found between the ¹⁴N/¹⁵N ratio and the core physical properties. We find only a tentative positive trend between ¹⁴N/¹⁵N and H₂ column density. However, firmer conclusions can be drawn only with higher sensitivity measurements.