Seeds of Life in Space (SOLIS)

I. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4^⋆,⋆⋆

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: fontani@arcetri.astro.it
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Max-Planck-Institut für extraterrestrische Physik (MPE), 85748 Garching, Germany
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
⁵ Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, 263 avenue du Général Leclerc, 35042 Rennes Cedex, France
⁶ Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
⁷ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, 50125 Firenze, Italy
⁸ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
⁹ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁰ University of AL-Muthanna, College of Science, Physics Department, AL-Muthanna, Iraq
¹¹ Observatorio Astronómico Nacional (OAN, IGN), Calle Alfonso XII, 3, 28014 Madrid, Spain
¹² Department of Physics and Astronomy, University College London, Gower St., London, WC1E 6BT, UK
¹³ LERMA, Université de Cergy-Pontoise, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 95000 Cergy Pontoise, France
¹⁴ Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁵ Astronomy Unit, School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK
¹⁶ Department of Physics, The University of Tokyo, Bunkyo-ku, 113-0033 Tokyo, Japan
¹⁷ Department of Physics and Astronomy, The University of Western Ontario, 1151 Richmond Street, London, N6A 3K7, Canada
¹⁸ The Institute of Physical and Chemical Research (RIKEN), 2-1, Hirosawa, Wako-shi, 351-0198 Saitama, Japan
¹⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300- RA Leiden, The Netherlands
²⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²¹ Aix-Marseille Université, PIIM UMR-CNRS 7345, 13397 Marseille, France
²² Dipartimento di Chimica and NIS Centre, Università degli Studi di Torino, via P. Giuria 7, 10125 Torino, Italy
²³ Ural Federal University, 620002 Ekaterinburg, Russia

Received: 30 January 2017
Accepted: 6 July 2017

Abstract

The interstellar delivery of carbon atoms locked into molecules might be one of the key ingredients for the emergence of life. Cyanopolyynes are carbon chains delimited at their two extremities by an atom ofhydrogen and a cyano group, meaning that they could be excellent reservoirs of carbon. The simplest member, HC₃N, is ubiquitous in the galactic interstellar medium and found also in external galaxies. Thus, understanding the growth of cyanopolyynes in regions forming stars similar to our Sun, and what affects them, is particularly relevant. In the framework of the IRAM/NOEMA Large Program SOLIS (Seeds Of Life In Space), we have obtained a map of two cyanopolyynes, HC₃N and HC₅N,  in the protocluster OMC-2 FIR4. Because our Sun is thought to be born in a rich cluster, OMC-2 FIR4 is one of the closest and best known representatives of the environment in which the Sun may have been born. We find a HC₃N/HC₅N abundance ratio across the source in the range ~1−30, with the smallest values (≤10) in FIR5 and in the eastern region of FIR4. The ratios ≤10 can be reproduced by chemical models only if: (1) the cosmic-ray ionisation rate ζ is ~ 4 × 10^-14 s^-1; (2) the gaseous elemental ratio C/O is close to unity; and (3) oxygen and carbon are largely depleted. The large ζ is comparable to that measured in FIR4 by previous works and was interpreted as due to a flux of energetic (≥10 MeV) particles from embedded sources. We suggest that these sources could lie east of FIR4 and FIR5. A temperature gradient across FIR4, with T decreasing from east to west by about 10 K, could also explain the observed change in the HC₃N/HC₅N line ratio, without the need of a cosmic ray ionisation rate gradient. However, even in this case, a high constant cosmic-ray ionisation rate (of the order of 10^-14 s^-1) is necessary to reproduce the observations.