First Measurement of the 14N/15N Ratio in the Analog of the Sun Progenitor OMC-2 FIR4

Abstract

We present a complete census of the <SUP>14</SUP>N/<SUP>15</SUP>N isotopic ratio in the most abundant N-bearing molecules toward the cold envelope of the protocluster OMC-2 FIR4, the best known Sun progenitor. To this scope, we analyzed the unbiased spectral survey obtained with the IRAM 30 m telescope at 3, 2, and 1 mm. We detected several lines of CN, HCN, HNC, HC<SUB>3</SUB>N, N<SUB>2</SUB>H<SUP>+</SUP>, and their respective <SUP>13</SUP>C and <SUP>15</SUP>N isotopologues. The lines’ relative fluxes are compatible with LTE conditions, and moderate line opacities have been corrected via a population diagram method or theoretical relative intensity ratios of the hyperfine structures. The five species lead to very similar <SUP>14</SUP>N/<SUP>15</SUP>N isotopic ratios, without any systematic difference between amine- and nitrile-bearing species as previously found in other protostellar sources. The weighted average of the <SUP>14</SUP>N/<SUP>15</SUP>N isotopic ratio is 270 ± 30. This <SUP>14</SUP>N/<SUP>15</SUP>N value is remarkably consistent with the [250-350] range measured for the local galactic ratio but significantly differs from the ratio measured in comets (around 140). High-angular resolution observations are needed to examine whether this discrepancy is maintained at smaller scales. In addition, using the CN, HCN, and HC<SUB>3</SUB>N lines, we derived a <SUP>12</SUP>C/<SUP>13</SUP>C isotopic ratio of 50 ± 5.