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Register a new userThe relation between accretion rates and the initial mass function in hydrodynamical simulations of star formation
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We analyse a hydrodynamical simulation of star formation. Sink particles in the simulations which represent stars show episodic growth, which is presumably accretion from a core that can be regularly replenished in response to the fluctuating conditions in the local environment. The accretion rates follow $dot{m} propto m^{2/3}$, as expected from accretion in a gas-dominated potential, but with substantial variations over-laid on this. The growth times follow an exponential distribution which is tapered at long times due to the finite length of the simulation. The initial collapse masses have an approximately lognormal distribution with already an onset of a power-law at large masses. The sink particle mass function can be reproduced with a non-linear stochastic process, with fluctuating accretion rates $propto m^{2/3}$, a distribution of seed masses and a distribution of growth times. All three factors contribute equally to the form of the final sink mass function. We find that the upper power law tail of the IMF is unrelated to Bondi-Hoyle accretion.
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Recent analyses of mass segregation diagnostics in star forming regions invite a comparison with the output of hydrodynamic simulations of star formation. In this work we investigate the state of mass segregation of stars (i.e. sink particles in the simulations) in the case of hydrodynamical simulations which omit feedback. We first discuss methods to quantify mass segregation in substructured regions, either based on the minimum spanning tree (Allisons Lambda), or through analysis of correlations between stellar mass and local stellar surface number densities. We find that the presence of even a single outlier (i.e. a massive object far from other stars) can cause the Allison Lambda method to describe the system as inversely mass segregated, even where in reality the most massive sink particles are overwhelmingly in the centres of the subclusters. We demonstrate that a variant of the Lambda method is less susceptible to this tendency but also argue for an alternative representation of the data in the plane of stellar mass versus local surface number density. The hydrodynamical simulations show global mass segregation from very early times which continues throughout the simulation, being only mildly influenced during sub-cluster merging. We find that up to approx. 2-3% of the massive sink particles (m > 2.5 Msun) are in relative isolation because they have formed there, although other sink particles can form later in their vicinity. Ejections of massive sinks from subclusters do not contribute to the number of isolated massive sink particles, as the gravitational softening in the calculation suppresses this process.
The stellar initial mass function (IMF) is a fundamental property of star formation, offering key insight into the physics driving the process as well as informing our understanding of stellar populations, their by-products, and their impact on the surrounding medium. While the IMF appears to be fairly uniform in the Milky Way disk, it is not yet known how the IMF might behave across a wide range of environments, such as those with extreme gas temperatures and densities, high pressures, and low metallicities. We discuss new opportunities for measuring the IMF in such environments in the coming decade with JWST, WFIRST, and thirty-meter class telescopes. For the first time, we will be able to measure the high-mass slope and peak of the IMF via direct star counts for massive star clusters across the Milky Way and Local Group, providing stringent constraints for star formation theory and laying the groundwork for understanding distant and unresolved stellar systems.
While the stellar Initial Mass Function (IMF) appears to be close to universal within the Milky Way galaxy, it is strongly suspected to be different in the primordial Universe, where molecular hydrogen cooling is less efficient and the gas temperature can be higher by a factor of 30. In between these extreme cases, the gas temperature varies depending on the environment, metallicity and radiation background. In this paper we explore if changes of the gas temperature affect the IMF of the stars considering fragmentation and accretion. The fragmentation behavior depends mostly on the Jeans mass at the turning point in the equation of state where a transition occurs from an approximately isothermal to an adiabatic regime due to dust opacities. The Jeans mass at this transition in the equation of state is always very similar, independent of the initial temperature, and therefore the initial mass of the fragments is very similar. Accretion on the other hand is strongly temperature dependent. We argue that the latter becomes the dominant process for star formation efficiencies above 5 - 7 %, increasing the average mass of the stars.
We discuss the possibility that gravitational focusing, is responsible for the power-law mass function of star clusters $N(log M) propto M^{-1}$. This power law can be produced asymptotically when the mass accretion rate of an object depends upon the mass of the accreting body as $dot{M} propto M^2$. While Bondi-Hoyle-Littleton accretion formally produces this dependence on mass in a uniform medium, realistic environments are much more complicated. However, numerical simulations in SPH allowing for sink formation yield such an asymptotic power-law mass function. We perform pure N-body simulations to isolate the effects of gravity from those of gas physics and to show that clusters naturally result with the power-law mass distribution. We also consider the physical conditions necessary to produce clusters on appropriate timescales. Our results help support the idea that gravitationally-dominated accretion is the most likely mechanism for producing the cluster mass function.
We present radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between $m_{rm gas}=10^3$~M$_odot$ to $3 times 10^5$~M$_odot$ and gas densities typical of clouds in the local universe ($overline n_{rm gas} sim 1.8times 10^2$~cm$^{-3}$) and 10$times$ and 100$times$ denser, expected to exist in high-redshift galaxies. The main results are: {it i}) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about $40%$ of the mass of the sink particle, while the remaining $60%$ is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope $0.8$, satisfy this empirical prescription. {it ii}) The star formation law that best describes our set of simulation is $drho_*/dt propto rho_{gas}^{1.5}$ if $overline n_{gas}<n_{cri}approx 10^3$~cm$^{-3}$, and $drho_*/dt propto rho_{rm gas}^{2.5}$ otherwise. The duration of the star formation episode is roughly $6$ clouds sound crossing times (with $c_s=10$~km/s). {it iii}) The total star formation efficiency in the cloud is $f_*=2% (m_{rm gas}/10^4~M_odot)^{0.4}(1+overline n_{rm gas}/n_{rm cri})^{0.91}$, for gas at solar metallicity, while for metallicity $Z<0.1$~Z$_odot$, based on our limited sample, $f_*$ is reduced by a factor of $sim 5$. {it iv)} The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.
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