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

Marco Padovani (1) , Alexandre Marcowith (2) , Daniele Galli (1) , Leslie K. Hunt (1) , Francesco Fontani (1)

1 OAA - INAF - Osservatorio Astrofisico di Arcetri
2 LUPM - Laboratoire Univers et Particules de Montpellier

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.Key words: stars: formation / cosmic rays / ISM: jets and outflows / radio continuum: ISM / acceleration of particles

Mots clés: surface; Bayesian; spectral; ultraviolet; radio wave; signature; turbulence; ionization; magnetic field; field strength; photon; synchrotron; star: formation; particle: relativistic; jet: velocity; cosmic radiation: acceleration; acceleration of particles; radio continuum: ISM; cosmic rays; ISM: jets and outflows; stars: formation; cosmic radiation: flux; TeV

Domaines: Astrophysique [astro-ph]

Marco Padovani, Alexandre Marcowith, Daniele Galli, Leslie K. Hunt, Francesco Fontani. The double signature of local cosmic-ray acceleration in star-forming regions. Astronomy and Astrophysics - A&A, 2021, 649, pp.A149. ⟨10.1051/0004-6361/202039918⟩. ⟨hal-03186219⟩