The onset of energetic particle irradiation in Class 0 protostars^⋆

¹ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
e-mail: cfavre@arcetri.astro.it
² CNRS, IPAG, 38000 Grenoble, France
³ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’ Hères, France
⁴ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁵ Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁶ Université de Toulouse, UPS-OMP, IRAP, 31013 Toulouse, France
⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ Observatorio Astronómico Nacional (OAN, IGN), Apdo 112, 28803 Alcalá de Henares, Spain
⁹ Leiden Observatory, PO Box 9513, 2300 RA, Leiden, The Netherlands
¹⁰ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
¹¹ Cahill Center for Astronomy and Astrophysics 301-17, California Institute of Technology, Pasadena, CA 91125, USA
¹² Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
¹³ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy

Received: 1 December 2016
Accepted: 21 August 2017

Abstract

Context. The early stages of low-mass star formation are likely to be subject to intense ionization by protostellar energetic MeV particles. As a result, the surrounding gas is enriched in molecular ions, such as HCO⁺ and N₂H⁺. Nonetheless, this phenomenon remains poorly understood for Class 0 objects. Recently, based on Herschel observations taken as part of the key programme Chemical HErschel Surveys of Star forming regions (CHESS), a very low HCO⁺/N₂H⁺ abundance ratio of about three to four, has been reported towards the protocluster OMC-2 FIR4. This finding suggests a cosmic-ray ionization rate in excess of 10^-14 s^-1, much higher than the canonical value of ζ = 3 × 10^-17 s^-1 (value expected in quiescent dense clouds).

Aims. We aim to assess the specificity of OMC-2 FIR4, we have extended this study to a sample of sources in low- and intermediate mass. More specifically, we seek to measure the HCO⁺/N₂H⁺ abundance ratio from high energy lines (J ≥ 6) towards this source sample in order to infer the flux of energetic particles in the warm and dense gas surrounding the protostars.

Methods. We have used observations performed with the Heterodyne Instrument for the Far-Infrared spectrometer on board the Herschel Space Observatory towards a sample of nine protostars.

Results. We report HCO⁺/N₂H⁺ abundance ratios in the range of five up to 73 towards our source sample. The large error bars do not allow us to conclude whether OMC-2 FIR4 is a peculiar source. Nonetheless, an important result is that the measured HCO⁺/N₂H⁺ ratio does not vary with the source luminosity. At the present time, OMC-2 FIR4 remains the only source where a high flux of energetic particles is clearly evident. More sensitive and higher angular resolution observations are required to further investigate this process.