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Published December 2017 | Published + Submitted
Journal Article Open

The onset of energetic particle irradiation in Class 0 protostars

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 N_2H^+. 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^+/N_2H^+ 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^+/N_2H^+ 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^+/N_2H^+ 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^+/N_2H^+ 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.

Additional Information

© ESO, 2017. Received: 1 December 2016. Accepted: 21 August 2017. Herschel is an ESA space observatory with science instruments provided by European-led principal investigator consortia and with important participation from NASA. Support for this work was provided by the French space agency CNES. M.P. acknowledges funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 664931. Support for this work was also provided by NASA (Herschel OT funding) through an award issued by JPL/Caltech. This paper makes use of Herschel/HIFI data. Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France, and the U.S. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA); Sweden: Chalmers University of Technology-MC2, RSS and GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Finally, we thank the anonymous referee for helpful comments and suggestions.

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Created:
August 19, 2023
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October 18, 2023