MILLIMETER-WAVE OBSERVATIONS OF CN AND HNC AND THEIR¹⁵N ISOTOPOLOGUES: A NEW EVALUATION OF THE¹⁴N/¹⁵N RATIO ACROSS THE GALAXY

Abstract

The N = 1 → 0 transitions of CN and C 15 N (X 2 Σ +), as well as the J = 1 → 0 lines of HN 13 C and H 15 NC, have been observed toward 11 molecular clouds using the new 3 mm ALMA-type receiver of the 12 m telescope of the Arizona Radio Observatory. These sources span a wide range of distances from the Galactic center and are all regions of star formation. From these observations, 14 N/ 15 N ratios have been determined using two independent methods. First, the measurements of C 14 N and C 15 N were directly compared to establish this ratio, correcting for high opacities when needed, as indicated by the nitrogen hyperfine intensities. Second, the ratio was calculated from the quantity [HN 13 C]/[H 15 NC], determined from the HNC data, and then scaled by 12 C/ 13 C ratios previously established, i.e., the so-called double isotope method. Values from both methods are in reasonable agreement, and fall in the range ∼120-400, somewhat lower than previous 14 N/ 15 N ratios derived from HCN. The ratios exhibit a distinct positive gradient with distance from the Galactic center, following the relationship 14 N/ 15 N = 21.1 (5.2) kpc −1 D GC + 123.8 (37.1). This gradient is consistent with predictions of Galactic chemical evolution models in which 15 N has a secondary origin in novae, while primary and secondary sources exist for 14 N. The local interstellar medium value was found to be 4 N/ 15 N = 290 ± 40, in agreement with the ratio found in nearby diffuse clouds and close to the value of 272 found in Earth's atmosphere.

Doubly15N-substituted diazenylium: THz laboratory spectra and fractionation models

“…The 14 N/ 15 N isotopic ratio for atmospheric N 2 is 273 ± 1 (Nier 1950), a value comparable to that of the local interstellar medium (ISM) obtained from observations in objects where nitrogen fractionation should be negligible: 274 ± 18 estimated from the C 14 N/C 15 N ratio in diffuse molecular clouds (Ritchey et al 2015), and 290 ± 40 derived from the gradient of the 14 N/ 15 N ratio in the Galaxy observed towards warm molecular clouds (Adande & Ziurys 2012).…”

Nitrogen hydrides in interstellar gas

“…Lower ratios have been found, for instance, in low mass cores, 334 ± 50 (Lis et al 2010), and on Earth, ∼270. Adande & Ziurys (2011) find values of ∼100-350 using CN and HNC in ten dense molecular clouds located at various galactic distances from the Galactic Centre, but this is still several times higher than inferred from a possible o-15 NH 3 absorption. The 14 N/ 15 N ratio is also difficult to determine and the results using cyanides may differ from the ratio inferred from other species such as N 2 H + , and may not reflect the true 14 N/ 15 N abundance ratio.…”

THE C 14 N/C 15 N RATIO IN DIFFUSE MOLECULAR CLOUDS

“…Various sources of 14 N and 15 N have been incorporated into models of Galactic chemical evolution (GCE; e.g., Tosi 1982;Romano & Matteucci 2003;Kobayashi et al 2011), which commonly predict that the 14 N/ 15 N ratio should increase with Galactocentric radius. Millimeter-wave emission studies of nitrogen-bearing molecules and their isotopologues in warm dense clouds of the Galactic disk have indeed revealed such a trend (e.g., Dahmen et al 1995;Wilson 1999;Adande & Ziurys 2012). Adande & Ziurys (2012), who examined millimeter observations of CN and HNC in 11 molecular clouds, and reevaluated previous observations of HCN, inferred a nitrogen isotopic ratio for the solar neighborhood of 290 ± 40 from the gradient seen in their data.…”

“…Millimeter-wave emission studies of nitrogen-bearing molecules and their isotopologues in warm dense clouds of the Galactic disk have indeed revealed such a trend (e.g., Dahmen et al 1995;Wilson 1999;Adande & Ziurys 2012). Adande & Ziurys (2012), who examined millimeter observations of CN and HNC in 11 molecular clouds, and reevaluated previous observations of HCN, inferred a nitrogen isotopic ratio for the solar neighborhood of 290 ± 40 from the gradient seen in their data.…”