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The Initial Conditions of Star Formation: Cosmic Rays as the Fundamental Regulators

Abstract : 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.
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Submitted on : Wednesday, March 23, 2022 - 3:12:08 PM
Last modification on : Thursday, March 24, 2022 - 8:03:40 AM

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Padelis P. Papadopoulos, Wing-Fai Thi. The Initial Conditions of Star Formation: Cosmic Rays as the Fundamental Regulators. Cosmic Rays in Star-Forming Environments, Astrophysics and Space Science Proceedings, 2013, 34, pp.41. ⟨10.1007/978-3-642-35410-6_5⟩. ⟨insu-03617531⟩

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