ALMA–IRDC – II. First high-angular resolution measurements of the 14N/15N ratio in a large sample of infrared-dark cloud cores

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% of the cores, depending on the host cloud. The 14N/15N values measured towards the millimeter 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 singledish 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.