Extreme Cosmic-Ray-Dominated-Regions: a new paradigm for high star formation density events in the Universe

TL;DR

This paper proposes that extreme cosmic-ray environments in galaxies significantly alter the initial conditions of star formation, leading to higher temperatures and Jeans masses, which could redefine our understanding of high-density star formation in the universe.

Contribution

It introduces a new paradigm where cosmic rays dominate the ISM conditions during intense star formation events, fundamentally changing star formation initial conditions.

Findings

Cosmic rays raise the minimum temperature of dense gas to 50-100K.

Gas temperature remains decoupled from dust temperature at high densities.

Jeans mass increases by factors of 10-100 in extreme CRDRs.

Abstract

We examine in detail the recent proposal that extreme Cosmic-Ray-Dominated-Regions (CRDRs) characterize the ISM of galaxies during events of high-density star formation, fundamentally altering its initial conditions (Papadopoulos 2010). Solving the coupled chemical and thermal state equations for dense UV-shielded gas reveals that the large cosmic ray energy densities in such systems (U_{CR} (few)x(10^3-10^4) U_{CR,Gal}) will indeed raise the minimum temperature of this phase (where the initial conditions of star formation are set) from ~10K (as in the Milky Way) to (50-100)K. Moreover in such extreme CRDRs the gas temperature remains fully decoupled from that of the dust, with T_{kin} >> T_{dust}, even at high densities (n(H_2)~10^5--10^6 cm^{-3}), quite unlike CRDRs in the Milky Way where T_k T_{dust} when n(H_2) >= 10^5 cm^{-3}. These dramatically different star formation initial conditions will: a) boost the Jeans mass of UV-shielded gas regions by factors of ~10--100 with respect to those in quiescent or less extreme star forming systems, and b) "erase" the so-called inflection point of the effective equation of state (EOS) of molecular gas. Both these effects occur across the entire density range of typical molecular clouds, and may represent {\it a new paradigm for all high-density star formation in the Universe}, with cosmic rays as the key driving mechanism, operating efficiently even in the high dust extinction environments of extreme starbursts...