The onset of energetic particle irradiation in Class 0 protostars

Context. The early stages of low-mass star formation are likely to besubject to intense ionization by protostellar energetic MeV particles.As a result, the surrounding gas is enriched in molecular ions, such asHCO+ and N2H+. Nonetheless, thisphenomenon remains poorly understood for Class 0 objects. Recently,based on Herschel observations taken as part of the key programmeChemical HErschel Surveys of Star forming regions (CHESS), a very lowHCO+/N2H+ abundance ratio of aboutthree to four, has been reported towards the protocluster OMC-2 FIR4.This finding suggests a cosmic-ray ionization rate in excess of10-14 s-1, much higher than the canonical value ofζ = 3 × 10-17 s-1 (value expected inquiescent dense clouds).
Aims: We aim to assess the specificityof OMC-2 FIR4, we have extended this study to a sample of sources inlow- and intermediate mass. More specifically, we seek to measure theHCO+/N2H+ abundance ratio from highenergy lines (J ≥ 6) towards this source sample in order to infer theflux of energetic particles in the warm and dense gas surrounding theprotostars.
Methods: We have used observations performed with theHeterodyne Instrument for the Far-Infrared spectrometer on board theHerschel Space Observatory towards a sample of nine protostars.
Results: We report HCO+/N2H+ abundanceratios in the range of five up to 73 towards our source sample. Thelarge error bars do not allow us to conclude whether OMC-2 FIR4 is apeculiar source. Nonetheless, an important result is that the measuredHCO+/N2H+ ratio does not vary with thesource luminosity. At the present time, OMC-2 FIR4 remains the onlysource where a high flux of energetic particles is clearly evident. Moresensitive and higher angular resolution observations are required tofurther investigate this process.Herschel is an ESA space observatory with science instruments providedby European-led principal investigator consortia and with importantparticipation from NASA.