  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$_{2}$H$^{+}$. Nonetheless, this phenomenon remains poorly understood for
Class 0 objects. Recently, based on Herschel observations taken as part of the
key program Chemical HErschel Surveys of Star forming regions (CHESS), a very
low HCO$^{+}$/N$_{2}$H$^{+}$ abundance ratio of about 3-4, has been reported
toward 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 $\zeta$ = 3$\times$10$^{-17}$ s$^{-1}$ (value expected in
quiescent dense clouds). 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$_2$H$^{+}$ abundance ratio
from high energy lines (J $\ge$ 6) toward this source sample in order to infer
the flux of energetic particles in the warm and dense gas surrounding the
protostars. We use observations performed with the Heterodyne Instrument for
the FarInfrared spectrometer on board the Herschel Space Observatory toward a
sample of 9 protostars. We report HCO$^{+}$/N$_2$H$^{+}$ abundance ratios in
the range of 5 up to 73 toward 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$_2$H$^{+}$ 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.
