Toward a Model of the Stellar Initial Mass Function from the Density Distribution of Molecular Cloud Clumps (original) (raw)
Related papers
2010
In this work, we derive the stellar initial mass function (IMF) from the superposition of mass distributions of dense cores, generated through gravoturbulent fragmentation of unstable clumps in molecular clouds (MCs) and growing through competitive accretion. MCs are formed by the turbulent cascade in the interstellar medium at scales L from 100 down to ~0.1 pc. Their internal turbulence is essentially supersonic and creates clumps with a lognormal distribution of densities n. Our model is based on the assumption of a power-law relationship between clump mass and clump density: n~m^x, where x is a scale-free parameter. Gravitationally unstable clumps are assumed to undergo isothermal fragmentation and produce protostellar cores with a lognormal mass distribution, centred around the clump Jeans mass. Masses of individual cores are then assumed to grow further through competitive accretion until the rest of the gas within the clump is being exhausted. The observed IMF is best reproduced for a choice of x=0.25, for a characteristic star formation timescale of ~5 Myr, and for a low star formation efficiency of ~10 %.
On the Origins of the Stellar Initial Mass Function
2015
In this reading, a new theoretical model of star and cluster formation is posited. This model seeks to set a mathematical framework to understand the origins of the stellar Initial Mass Function and within this framework, explain star and cluster formation from a unified perspective by tieing together into a single garment three important observational facts: (1) that the most massive stars of most observed clusters of stars are preferentially found in their centers; (2) Larson's 1982 empirical observation that the maximum stellar mass is related to the total mass of the parent cloud; (3) that clump masses in giant molecular clouds exhibit a power mass spectrum law akin to that found in star clusters and this behavior is also true for molecular clouds as well. Key to this model is the way the cloud fragments to form cores from which the new stars are born. We show that the recently proposed azimuthally symmetric theory of gravitation has two scale of fragmentation where one is the scale that leads to cloud collapse and the other is the scale on which the cloud fragments. The collapse and fragmentation takes place simultaneously. If the proposed model is anything to go by, then, one can safely posit that the slope of the IMF can be explained from two things: the star formation rate of the cores from which these stars form and the density index describing the density profile. Additionally and more importantly, if the present is anything to by, then, fragmentation of molecular clouds is posited as being a result of them possessing some spin angular momentum.
Fragmentation of Molecular Clouds: The Initial Phase of a Stellar Cluster
The Astrophysical Journal, 1998
The isothermal gravitational collapse and fragmentation of a region within a molecular cloud and the subsequent formation of a protostellar cluster is investigated numerically. The clump mass spectrum which forms during the fragmentation phase can be well approximated by a power law distribution dN/dM ∝ M −1.5. In contrast, the mass spectrum of protostellar cores that form in the centers of Jeans-unstable clumps and evolve through accretion and N-body interactions is described by a log-normal distribution with a width that is in excellent agreement with observations of multiple stellar systems.
Clump mass function at an early stage of molecular cloud evolution – II. Galactic cloud complexes
The statistical approach for derivation of the clump mass function (ClMF) developed by Donkov, Veltchev & Klessen is put to observational test through comparison with mass distributions of clumps from molecular emission and dust continuum maps of Galactic cloud complexes, obtained by various authors. The results indicate gravitational boundedness of the dominant clump population, with or without taking into account the contribution of their thermal and magnetic energy. The ClMF can be presented by combination of two-power-law functions separated by a characteristic mass from about ten to hundreds of solar masses. The slope of the intermediate-mass ClMF is shallow and nearly constant (−0.25 IM −0.55) while the high-mass part is fitted by models that imply gravitationally unstable clumps and exhibit slopes in a broader range (−0.9 IM −1.6), centred at the value of the stellar initial mass function ( HM −1.3).
Clump mass function at an early stage of molecular cloud evolution: I. A statistical approach
2016
The statistical approach for derivation of the clump mass function (ClMF) developed by Donkov, Veltchev & Klessen is put to observational test through comparison with mass distributions of clumps from molecular emission and dust continuum maps of Galactic cloud complexes, obtained by various authors. The results indicate gravitational boundedness of the dominant clump population, with or without taking into account the contribution of their thermal and magnetic energy. The ClMF can be presented by combination of two power-law functions separated by a characteristic mass from about ten to hundreds solar masses. The slope of the intermediate-mass ClMF is shallow and nearly constant (−0.25 Γ IM −0.55) while the high-mass part is fitted by models that imply gravitationally unstable clumps and exhibit slopes in a broader range (−0.9 Γ IM −1.6), centered at the value of the stellar initial mass function (Γ HM −1.3).
Statistical mass function of prestellar cores from the density distribution of their natal clouds
Astronomy & Astrophysics, 2020
The mass function of clumps observed in molecular clouds raises interesting theoretical issues, especially in its relation to the stellar initial mass function (IMF). We propose a statistical model of the mass function of prestellar cores (CMF), formed in self-gravitating isothermal clouds at a given stage of their evolution. The latter is characterized by the mass-density probability distribution function (ρ-PDF), which is a power-law with slope q. The different molecular clouds are divided into ensembles according to the PDF slope and each ensemble is represented by a single spherical cloud. The cores are considered as elements of self-similar structure typical for fractal clouds and are modeled by spherical objects populating each cloud shell. Our model assumes relations between size, mass, and density of the statistical cores. Out of these, a core mass-density relationship ρ ∝ mx is derived where x = 1∕(1 + q). We find that q determines the existence or nonexistence of a thresho...
Observation and theory of the initial stellar mass function
2000
Observations of normal galactic star-forming regions suggest there is widespread near-uniformity in the initial stellar mass function (IMF) in spite of diverse physical conditions. Fluctuations may come largely from statistical effects and observational selection. There are also tantalizing, but uncertain reports that the IMF shifts systematically in peculiar regions, giving a low mass bias in quiescent gas, and a high mass bias in active starbursts. Theoretical proposals for the origin of the IMF are reviewed. The theories generally focus on a combination of four physical effects: wind-limited accretion of stellar mass, coalescence of protostellar gas clumps, mass limitations at the thermal Jeans mass, and power-law cloud structure. Hybrid theories combining the best of each may be preferred.
Modeling mass functions of clumps formed during the early MC evolution
The statistical approach for description of molecular cloud substructure, proposed by Donkov, Veltchev and Klessen (2011, 2012), allows for alternative models, operating with different type of objects: an ensemble of clumps or a larger cloudlet. We demonstrate briefly the predictive power of both models, applied to molecular emission and dust extinction studies of Galactic clouds.
On the Initial Conditions for Star Formation and the Initial Mass Function
The Astrophysical Journal, 2011
Density probability distribution functions (PDFs) for turbulent self-gravitating clouds should be convolutions of the local log-normal PDF, which depends on the local average density ρ ave and Mach number M, and the PDFs for ρ ave and M, which depend on the overall cloud structure. When self-gravity drives a cloud to increased central density, the total PDF develops an extended tail. If there is a critical density or column density for star formation, then the fraction of the local mass exceeding this threshold becomes higher near the cloud center. These elements of cloud structure should be in place before significant star formation begins. Then the efficiency is high so that bound clusters form rapidly, and the stellar initial mass function (IMF) has an imprint in the gas before destructive radiation from young stars can erase it. The IMF could arise from a power-law distribution of mass for cloud structure. These structures should form stars down to the thermal Jeans mass M J at each density in excess of a threshold. The high-density tail of the PDF, combined with additional fragmentation in each star-forming core, extends the IMF into the brown dwarf regime. The core fragmentation process is distinct from the cloud structuring process and introduces an independent core fragmentation mass function (CFMF). The CFMF would show up primarily below the IMF peak.