Revisited nitrogen isotopic ratio in molecular clouds

Details Revisited nitrogen isotopic ratio in molecular clouds Revisited nitrogen isotopic ratio in molecular clouds

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p..76a&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

Astronomy and Astrophysics

Astronomy

Scientific paper

Fundamental for the understanding of nucleosynthesis processes as well as galactic and solar system evolution, the nitrogen isotope ratio in the galaxy has long been a puzzle. Particularly, meteorites, comets and IDPs show a strong enhancement of the 15N isotope compared to values estimated from molecular clouds, which casts doubts on the pristine nature of the cometary and meteoritic material. Due to high optical depths of the main nitrogen carriers, direct measurement of [14N/15N] is difficult. To date only a few global studies of the nitrogen isotope ratio in dense molecular clouds exist, which employ indirect methods to estimate this ratio. We revisit the nitrogen isotope ratio in dense molecular clouds using two nitrogen carriers, HNC and CN, and several methods. The J=1-0 and 3-2 transitions of HNC at 90 and 271 GHz, and H15NC at 88 and 266 GHz were observed at the Arizona Radio Observatory (ARO) 12m and SMT telescopes, toward the clouds SgrB2, W31, G34.3, W51M, M17-SW, DR-21, Orion A, W3(OH), NGC7538 and S156, located at various galactic distances from the Galactic Center. HNC being optically thick, its abundance was estimated using known [12C/13C] ratios, as well as radiative transfer modeling using the freely available code RADEX. Finally, the N=1-0 transition of CN and C15N at 113 and 110 GHz were also recorded at the 12m telescope. The last and most direct estimation method makes use of the intensities of CN hyperfines to estimate an optical depth and abundance for the main isotope. Results seem to indicate a [14N/15N] ratio of about ˜ 100-350, lower than previously reported, and more in line with cometary or meteoritic values.

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