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

TL;DR

This study presents the first high-angular resolution measurements of the 14N/15N ratio in N2H+ molecules within infrared-dark cloud cores, revealing a narrower ratio spread and emphasizing the importance of high-resolution mapping for accurate core analysis.

Contribution

It provides the first interferometric survey of 14N/15N ratios in IRDC cores, highlighting the differences from previous single-dish observations and the significance of high-angular resolution data.

Findings

14N/15N ratios range from 80 to 400 in cores.

The ratio spread is narrower than in previous surveys.

Average ratio of 210 is lower than the interstellar value.

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 the Atacama Large Millimeter Array towards four infrared-dark clouds harbouring 3~mm continuum cores associated with different physical properties. We detect N15NH+ (1-0) in about 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 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 the H2 column density. However, firmer conclusions can be drawn only with higher sensitivity measurements.