The Initial Conditions of Star Formation: Cosmic Rays as the Fundamental Regulators
Cosmic rays (CRs) control the thermal, ionization and chemical state of the dense H2 gas regions that otherwise remain shielded from far-UV and optical stellar radiation propagating through the dusty ISM of galaxies. It is in such CR-dominated regions (CRDRs) rather than Photon-dominated regions (PDRs) of H2 clouds where the star formation initial conditions are set, making CRs the ultimate star-formation feedback factor in galaxies, able to operate even in their most deeply dust-enshrouded environments. CR-controlled star formation initial conditions naturally set the stage for a near-invariant stellar Initial Mass Function (IMF) in galaxies as long as their average CR energy density UCR permeating their molecular ISM remains within a factor of ∼ 10 of its Galactic value. Nevertheless, in the extreme environments of the compact starbursts found in merging galaxies, where UCR ∼ (few) × 103 UCR,Gal, CRs dramatically alter the initial conditions of star formation. In the resulting extreme CRDRs H2 cloud fragmentation will produce far fewer low mass ( < 8 M⊙) stars, yielding a top-heavy stellar IMF. This will be a generic feature of CR-controlled star-formation initial conditions, lending a physical base for a bimodal IMF during galaxy formation, with a top-heavy one for compact merger-induced starbursts, and an ordinary IMF preserved for star formation in isolated gas-rich disks. In this scheme the integrated galactic IMFs (IGIMF) are expected to be strong functions of the star formation history of galaxies. Finally the large, CR-induced, ionization fractions expected for (far-UV)-shielded H2 gas in the CRDRs of compact starbursts will lengthen the ambipolar diffusion (AD) timescales so much as to render the alternative AD-regulated rather (Jeans mass)-driven star formation scenario as utterly unrealistic for the ISM in such galaxies.
KeywordsStar Formation Star Formation Rate Ambipolar Diffusion Initial Mass Function Reconnection Diffusion
Padelis P. Papadopoulos would like to warmly thank the organizers of the conference in Sant Cugat for an exciting week where much was learned and new vistas for both high energy and the so-called low energy Astrophysics came into view. This project was also funded by the John Latsis Public Benefit Foundation. The sole responsibility for the content lies with its authors.
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