Protostars: Forges of cosmic rays?

Abstract

Context. Galactic cosmic rays are particles presumably accelerated in supernova remnant shocks that propagate in the interstellar medium up to the densest parts of molecular clouds, losing energy and their ionisation efficiency because of the presence of magnetic fields and collisions with molecular hydrogen. Recent observations hint at high levels of ionisation and at the presence of synchrotron emission in protostellar systems, which leads to an apparent contradiction. Aims. We want to explain the origin of these cosmic rays accelerated within young protostars as suggested by observations. Methods. Our modelling consists of a set of conditions that has to be satisfied in order to have an efficient cosmic-ray acceleration through diffusive shock acceleration. We analyse three main acceleration sites (shocks in accretion flows, along the jets, and on protostellar surfaces), then we follow the propagation of these particles through the protostellar system up to the hot spot region. Results. We find that jet shocks can be strong accelerators of cosmic-ray protons, which can be boosted up to relativistic energies. Other promising acceleration sites are protostellar surfaces, where shocks caused by impacting material during the collapse phase are strong enough to accelerate cosmic-ray protons. In contrast, accretion flow shocks are too weak to efficiently accelerate cosmic rays. Though cosmic-ray electrons are weakly accelerated, they can gain a strong boost to relativistic energies through re-acceleration in successive shocks. Conclusions. We suggest a mechanism able to accelerate both cosmic-ray protons and electrons through the diffusive shock acceleration mechanism, which can be used to explain the high ionisation rate and the synchrotron emission observed towards protostellar sources. The existence of an internal source of energetic particles can have a strong and unforeseen impact on the ionisation of the protostellar disc, on the star and planet formation processes, and on the formation of pre-biotic molecules.

Stellar energetic particle ionization in protoplanetary disks around T Tauri stars

“…We extrapolate their results up to energies typical for Galactic cosmic rays of ≈ 10 GeV. This is consistent with the maximum energy E max ≈ 30 GeV derived by Padovani et al (2015Padovani et al ( , 2016 for particles accelerated on protostellar surfaces. As seen from Fig.…”

“…X-ray flares and particles can be produced close to the inner disk in the so called reconnection ring where the stellar and disk magnetic field interact (X-wind model Shu et al 1994Shu et al , 1997. More recently Padovani et al (2015Padovani et al ( , 2016 proposed jet shocks as alternative acceleration sites for high energy particles.…”

Radio jets from young stellar objects

“…Their non-thermal nature was interpreted as synchrotron emission from a small population of relativistic particles that would be accelerated in the ensuing strong shocks. This scenario is supported by several theoretical studies, showing the feasibility of strong shocks in protostellar jets as efficient particle accelerators (e.g., Araudo et al 2007;Bosch-Ramon et al 2010;Padovani et al 2015Padovani et al , 2016Cécere et al 2016).…”

The Highly Collimated Radio Jet of HH 80–81: Structure and Nonthermal Emission

“…This seems not to be the case in other YSOs with slower jets and denser media. In particular, if the particle density of not-fully ionized atoms is large, ionization losses can quench the acceleration (Bosch-Ramon et al 2010, Padovani et al 2015, Padovani et al 2016. The crucial point is that if the shock is radiative, instead of adiabatic, a lot of heat will be generated in the shocked region and this will dramatically increase the entropy.…”