Exploration of Cosmic-ray Acceleration in Protostellar Accretion Shocks and a Model for Ionization Rates in Embedded Protoclusters

Abstract

We construct a model for cosmic ray acceleration from protostellar accretion shocks and calculate the resulting cosmic ray ionization rate within star-forming molecular clouds. We couple a protostar cluster model with an analytic accretion shock model to calculate the cosmic ray acceleration from protostellar surfaces. We present the cosmic ray flux spectrum from keV to GeV energies for a typical low-mass protostar. We find that at the shock surface the spectrum follows a power-law trend across 6 orders of magnitude in energy. After attenuation, the spectrum at high energies steepens, while at low energies it is relatively flat. We calculate the cosmic ray pressure and cosmic ray ionization rate from relativistic protons at the protostellar surface and at the edge of the core. We present the cosmic ray ionization rate for individual protostars as a function of their instantaneous mass and final mass. The protostellar cosmic ray ionization rate is ζ ≈ 0.01 − 1 s −1 at the accretion shock surface. However, at the edge of the core, the cosmic ray ionization rate drops substantially to between ζ ≈ 10 −20 to 10 −17 s −1 . There is a large spatial gradient in the cosmic ray ionization rate, such that inner regions may experience cosmic ray ionization rates larger than the often assumed fiducial rate, ζ = 3 × 10 −17 s −1 . Finally, we calculate the cosmic ray ionization rate for protostellar clusters over 5 orders of magnitude of cluster size. We find that clusters with more than approximately 200 protostars produce a higher cosmic ray ionization rate within their natal cloud than the fiducial galactic value.

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. I. Identifying and Characterizing the Protostellar Content of the OMC-2 FIR4 and OMC-2 FIR3 Regions

“…Once a protostar is much more massive than ∼3 M , its luminosity becomes dominated by the stellar photosphere rather than accretion (Palla & Stahler 1993;Offner & McKee 2011). The inferred accretion rates are sufficiently high to produce significant cosmic-ray ionization, as predicted by Padovani et al (2016) and Gaches & Offner (2018), even if the current luminosity of HOPS-108 is as low as 37 L . Thus, it remains possible that the protostellar accretion, even though not from a high-mass star, could be driving the chemistry through local production of energetic particles or photons as suggested by Ceccarelli et al (2014).…”

“…In addition to the photometric, radio, and [OI] studies, FIR4 presents a diverse array of line emission from molecules that may be indicative of chemical processes driven by a source of locally generated energetic particles (i.e., cosmic rays) or photons that are catalysts for chemistry (Ceccarelli et al 2014;Gaches & Offner 2018). Most studies of this region, however, have been conducted at resolutions ≥3 , which are insufficient to resolve the protostars completely from their environment (Favre et al 2018).…”

C-type shock modelling – the effect of new H2–H collisional rate coefficients

“…They estimated the initial H 2 OPR in the preshock gas being close to 1. Note, accreting protostars can accelerate cosmic rays, and the ionization rate in the surrounding cloud may be comparable or even higher than due to Galactic cosmic rays (Podio et al 2014;Padovani et al 2016;Gaches & Offner 2018). At high cosmic ray ionization rate, ζ 10 −14 s −1 , the steady-state H 2 OPR in the preshock gas is close to 1.…”

Impact of Low-Energy Cosmic Rays on Star Formation

“…Clouds with significant CR ionisation are also correspondingly warmer (Bisbas et al, 2017;Gaches and Offner, 2018). The CR pressure is dynamically negligible (Gaches and Offner, 2018); however, the resulting elevated temperatures of 30 − 50 K increase the thermal pressure, which in turn may make it more difficult to form stars. However, it is worth noting that molecular gas is not necessarily a prerequisite for star formation (Krumholz, 2012).…”