The double signature of local cosmic-ray acceleration in star-forming regions

Abstract

Context. Recently, there has been an increased interest in the study of the generation of low-energy cosmic rays (< 1 TeV) in shocks situated on the surface of a protostar or along protostellar jets. These locally accelerated cosmic rays offer an attractive explanation for the high levels of non-thermal emission and ionisation rates observed close to these sources. Aims. The high ionisation rate observed in some protostellar sources is generally attributed to shock-generated UV photons. The aim of this article is to show that when synchrotron emission and a high ionisation rate are measured in the same spatial region, a locally shock-accelerated cosmic-ray flux is sufficient to explain both phenomena. Methods. We assume that relativistic protons and electrons are accelerated according to the first-order Fermi acceleration mechanism, and we calculate their emerging fluxes at the shock surface. These fluxes are used to compute the ionisation rate and the non-thermal emission at centimetre wavelengths. We then apply our model to the star-forming region OMC-2 FIR 3/FIR 4. Using a Bayesian analysis, we constrain the parameters of the model and estimate the spectral indices of the non-thermal radio emission, the intensity of the magnetic field, and its degree of turbulence. Results. We demonstrate that the local cosmic-ray acceleration model makes it possible to simultaneously explain the synchrotron emission along the HOPS 370 jet within the FIR 3 region and the ionisation rate observed near the FIR 4 protocluster. In particular, our model constrains the magnetic field strength (∼250−450 μG), its turbulent component (∼20−40 μG), and the jet velocity in the shock reference frame for the three non-thermal sources of the HOPS 370 jet (between 350 km s−1 and 1000 km s−1). Conclusions. Beyond the modelling of the OMC-2 FIR 3/FIR 4 system, we show how the combination of continuum observations at centimetre wavelengths and molecular transitions is a powerful new tool for the analysis of star-forming regions: These two types of observations can be simultaneously interpreted by invoking only the presence of locally accelerated cosmic rays, without having to resort to shock-generated UV photons.

Cosmic rays in molecular clouds probed by H2 rovibrational lines

“…The spread of ζ ion in dense cores (Caselli et al 1998) is supposed to be related to uncertainties in the chemical network, in the depletion process of elements such as carbon and oxygen, as well as because of the presence of tangled magnetic fields (Padovani & Galli 2011;Padovani et al 2013;Silsbee et al 2018). We note that the models presented here only account for the propagation of interstellar CRs, but in more evolved sources, such as in high-mass star-forming regions and hot cores, there could be a substantial contribution from locally accelerated charged particles (Padovani et al 2015(Padovani et al , 2016Gaches & Offner 2018;Padovani et al 2021b). (Caselli et al 1998), empty hexagons (Bialy et al 2022), an empty circle (Maret & Bergin 2007), and an empty pentagon (Fuente et al 2016).…”

“…Hollenbach & McKee 1989;Karska et al 2018). However, in the same shocks, for example along a protostellar jet or on the surface of a protostar, it is also possible to locally accelerate CRs (Padovani et al 2015(Padovani et al , 2016(Padovani et al , 2021bGaches & Offner 2018), and therefore even more intense H 2 lines should be observed. Consequently, this technique could also be used to further confirm the enhanced ionisation triggered by local CRs expected in star-forming regions.…”

Cosmic rays in molecular clouds probed by H$_{2}$ rovibrational lines -- Perspectives for the James Webb Space Telescope

“…The spread of ζ ion in dense cores (Caselli et al 1998) is supposed to be related to uncertainties in the chemical network, in the depletion process of elements such as carbon and oxygen, as well as because of the presence of tangled magnetic fields (Padovani & Galli 2011;Padovani et al 2013;Silsbee et al 2018). We note that the models presented here only account for the propagation of interstellar CRs, but in more evolved sources, such as in high-mass star-forming regions and hot cores, there could be a substantial contribution from locally accelerated charged particles (Padovani et al 2015(Padovani et al , 2016Gaches & Offner 2018;Padovani et al 2021b).…”

“…However, in the same shocks, e.g. along a protostellar jet or on the surface of a protostar, it is also possible to locally accelerate CRs (Padovani et al 2015(Padovani et al , 2016Gaches & Offner 2018;Padovani et al 2021b), and therefore even more intense H 2 lines should be observed. Consequently, this technique could also be used to further confirm the enhanced ionisation triggered by local CRs expected in star-forming regions.…”

Recent progress with observations and models to characterize the magnetic fields from star-forming cores to protostellar disks

“…The observations of Cabedo et al (2022) also show the CRs ionization rate is increasing at small envelope radii, toward the central protostellar embryo. Several theoretical works have investigated the role of shocks at the protostellar surface and magnetic mirroring within the jets as efficient forges to accelerate locally low-energy cosmic rays, and their role in increasing the ionization rate of the shielded protostellar material (Padovani et al, 2015;Silsbee et al, 2018;Fitz Axen et al, 2021;Padovani et al, 2021). In B335, it seems that local acceleration of CRs, and not the penetration of interstellar CRs, may be responsible for the gas ionization at small envelope radii.…”