Thermal and Fragmentation Properties of Star-forming Clouds in Low-Metallicity Environments

By K. Omukai, T. Tsuribe, R. Schneider, A. Ferrara. 2005

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The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai, updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z ~ 10-5 to 10-3 Z☉ and density ≈105 cm-3. Early on, CMB radiation prevents the gas temperature from falling below TCMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation ℰ ≡ (b - a)/a > ℰNL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at ℰ = ℰNL. Given these assumptions and the initial (Gaussian) distribution of ℰ, we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, ≲103 M☉, consistent with previous studies; (2) for Z > 10-6 Z☉ a few clumps go through an additional high-density (≳1010 cm-3) fragmentation phase driven by dust cooling, leading to low-mass fragments; (3) the mass fraction in low-mass fragments is initially very small, but at Z ~ 10-5 Z☉ it becomes dominant and continues to grow as Z is increased; (4) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z☉ < 0.1; and (5) for ≳0.1 Z☉, the two peaks merge into a single-peaked mass function, which might be regarded as the precursor of the ordinary Salpeter-like initial mass function.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/6a8dc15933d3d75146b39763581b2ee1784be0c7

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Citations in the corpus

THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, background): “This is a reasonable approximation for dense, molecular gas of solar metallicity, over a wide range of densities (Wolfire et al. 1995; Omukai et al. 2005; Pavlovski et al. 2006; Glover & Mac Low 2007a,b; Glover et al. 2010; Hill et al. 2011; Hennemann et al. 2012).”
THE GENERATION OF STRONG MAGNETIC FIELDS DURING THE FORMATION OF THE FIRST STARS (2010, methodology): “Consistent with previous works of Omukai et al. (2005) and Glover & Savin (2009), which follow the thermodynamics during the collapse, we adopt an effective equation of state with Γ = d logT/d log ρ + 1 = 1.1 for number densities in the range n = 105 − 1010 cm−3.”
GRACKLE: a chemistry and cooling library for astrophysics (2016, methodology): “The evolve_freefall function closely follows the one-zone freefall collapse model introduced by Omukai (2000) and modified by Omukai et al. (2005) to include the effects of thermal pressure support.”
The Formation of the First Stars. II. Radiative Feedback Processes and Implications for the Initial Mass Function (2007, background): “…is uncertain, with estimates ranging from 10 6 Z if the cooling is dominated by small dust grains that contain a significant fraction of the metals (Omukai et al. 2005) to 10 3.5 Z if the cooling is dominated by the finestructure lines of C and O and there is negligible H2 (Bromm & Loeb 2003); if…”
Inefficient star formation through turbulence, magnetic fields and feedback (2015, background): “…including turbulence have a velocity dispersion σv = 1kms −1 and an rms Mach number of M = 5, given the sound speed cs = 0.2 kms −1, appropriate for molecular gas with temperature T = 10K over the wide range of densities that lead to dense core and eventually star formation (Omukai et al. 2005).”
Fragmentation of star-forming clouds enriched with the first dust (2006, methodology): “Following Omukai et al. (2005), H2 collisional transition rates and HD parameters are taken from Galli & Palla (1998).”
A NEW JEANS RESOLUTION CRITERION FOR (M)HD SIMULATIONS OF SELF-GRAVITATING GAS: APPLICATION TO MAGNETIC FIELD AMPLIFICATION BY GRAVITY-DRIVEN TURBULENCE (2011, methodology): “Consistent with previous works of Omukai et al. (2005) and Glover & Savin (2009), we adopt an effective equation of state with Γ = 1.1 for number densities in the range n = 105–1010 cm−3.”
ENZO: AN ADAPTIVE MESH REFINEMENT CODE FOR ASTROPHYSICS (2013, methodology): “For the value of f in this test problem, we use the piecewise function of Omukai et al. (2005), given by f =”
A Comprehensive Study of Galaxies at z ∼ 9–16 Found in the Early JWST Data: Ultraviolet Luminosity Functions and Cosmic Star Formation History at the Pre-reionization Epoch (2022, background): “Indeed in the early universe, the IMF is expected to be more top-heavy because of a lower metallicity or a higher CMB temperature (e.g., Omukai et al. 2005; Chon et al. 2022), resulting in a higher Jeans mass, especially for Population III stellar populations (e.g., Hirano et al. 2014, 2015).”
Population III stars: hidden or disappeared? (2007, background): “Within the critical metallicity range, low-mass gas clouds can form if a sufficient amount of metals are depleted onto dust grains, which provide an additional efficient cooling channel at high density (Schneider et al. 2003; 2006a; Omukai et al. 2005; Omukai & Tsuribe 2006).”
The evolution of the dust and gas content in galaxies (2013, background): “…are available in electronic form at http://www.aanda.org the shape of the initial mass function (IMF), through favouring the formation of low-mass stars by fostering cloud fragmentation in low-metallicity environments and inhibiting the formation of massive stars (Omukai et al. 2005).”
Chemical Evolution in Hierarchical Models of Cosmic Structure. I. Constraints on the Early Stellar Initial Mass Function (2005, background): “…efforts have identified the gas metallicity as the critical factor influencing the cooling, collapse, and fragmentation properties of low- or zero-metallicity gas (Bromm et al. 2002; Omukai et al. 2005), it is sensible to speak of a single probability distribution for zero-metallicity gas.”
The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization (2015, background): “With the increase of the metallicity of the star-forming gas, the IMF switches rapidly from the top-heavy Pop III IMF to a much more bottom-heavy IMF, characteristic of present-day star formation (Bromm et al. 2001; Schneider et al. 2003; Omukai et al. 2005; Dopcke et al. 2013).”
Stellar sources of dust in the high-redshift Universe (2009, background): “Thus, variations of the stellar characteristic mass over cosmic history due to the smaller gas metallicity and/or to the larger cosmic microwave background temperature (see, e.g., Omukai et al. 2005) might significantly affect the origin and properties of dust in the high redshift Universe.”
The First Stellar Cluster (2007, methodology): “On the other hand, if the collapse is faster than Omukai et al. (2005) assume or if the gas opacity is greater, then 8 more heating would occur.”