Star Clusters Across Cosmic Time Krumholz, Mark R; McKee, Christopher F; Bland-Hawthorn, Joss
Annual review of astronomy and astrophysics,
08/2019, Letnik:
57, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Star clusters stand at the intersection of much of modern astrophysics: the ISM, gravitational dynamics, stellar evolution, and cosmology. Here, we review observations and theoretical models for the ...formation, evolution, and eventual disruption of star clusters. Current literature suggests a picture of this life cycle including the following several phases:
Clusters form in hierarchically structured, accreting molecular clouds that convert gas into stars at a low rate per dynamical time until feedback disperses the gas.
The densest parts of the hierarchy resist gas removal long enough to reach high star-formation efficiency, becoming dynamically relaxed and well mixed. These remain bound after gas removal.
In the first ∼100 Myr after gas removal, clusters disperse moderately fast, through a combination of mass loss and tidal shocks by dense molecular structures in the star-forming environment.
After ∼100 Myr, clusters lose mass via two-body relaxation and shocks by giant molecular clouds, processes that preferentially affect low-mass clusters and cause a turnover in the cluster mass function to appear on ∼1-10-Gyr timescales.
Even after dispersal, some clusters remain coherent and thus detectable in chemical or action space for multiple galactic orbits.
In the next decade, a new generation of space- and adaptive optics-assisted ground-based telescopes will enable us to test and refine this picture.
ABSTRACT We construct an analytic phenomenological model for extended warm/hot gaseous coronae of L* galaxies. We consider UV O vi Cosmic Origins Spectrograph (COS)-Halos absorption line data in ...combination with Milky Way (MW) X-ray O vii and O viii absorption and emission. We fit these data with a single model representing the COS-Halos galaxies and a Galactic corona. Our model is multi-phased, with hot and warm gas components, each with a (turbulent) log-normal distribution of temperatures and densities. The hot gas, traced by the X-ray absorption and emission, is in hydrostatic equilibrium in an MW gravitational potential. The median temperature of the hot gas is 1.5 × 10 6 K and the mean hydrogen density is ∼ 5 × 10 − 5 cm − 3 . The warm component as traced by the O vi, is gas that has cooled out of the high density tail of the hot component. The total warm/hot gas mass is high and is 1.2 × 10 11 M . The gas metallicity we require to reproduce the oxygen ion column densities is 0.5 solar. The warm O vi component has a short cooling time ( ∼ 2 × 10 8 years), as hinted by observations. The hot component, however, is ∼ 80 % of the total gas mass and is relatively long-lived, with t cool ∼ 7 × 10 9 years. Our model supports suggestions that hot galactic coronae can contain significant amounts of gas. These reservoirs may enable galaxies to continue forming stars steadily for long periods of time and account for "missing baryons" in galaxies in the local universe.
We construct a new analytic phenomenological model for the extended circumgalactic material (CGM) of L* galaxies. Our model reproduces the O vii/O viii absorption observations of the Milky Way (MW) ...and the O vi measurements reported by the COS-Halos and eCGM surveys. The warm/hot gas is in hydrostatic equilibrium in an MW gravitational potential, and we adopt a barotropic equation of state, resulting in a temperature variation as a function of radius. A pressure component with an adiabatic index of is included to approximate the effects of a magnetic field and cosmic rays. We introduce a metallicity gradient motivated by the enrichment of the inner CGM by the Galaxy. We then present our fiducial model for the corona, tuned to reproduce the observed O vi-O viii column densities and with a total mass of inside . The gas densities in the CGM are low ( cm−3), and its collisional ionization state is modified by the metagalactic radiation field. We show that for O vi-bearing warm/hot gas with typical observed column densities cm−2 at large ( kpc) impact parameters from the central galaxies, the ratio of the cooling to dynamical times, / , has a model-independent upper limit of . In our model, / at large radii is . We present predictions for a wide range of future observations of the warm/hot CGM, from UV/X-ray absorption and emission spectroscopy to dispersion measure and Sunyaev-Zel'dovich cosmic microwave background measurements. We provide the model outputs in machine-readable data files for easy comparison and analysis.
How do bound star clusters form? Krumholz, Mark R; McKee, Christopher F
Monthly notices of the Royal Astronomical Society,
05/2020, Letnik:
494, Številka:
1
Journal Article
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ABSTRACT
Gravitationally bound clusters that survive gas removal represent an unusual mode of star formation in the Milky Way and similar spiral galaxies. While forming, they can be distinguished ...observationally from unbound star formation by their high densities, virialized velocity structures, and star formation histories that accelerate towards the present, but extend multiple free-fall times into the past. In this paper, we examine several proposed scenarios for how such structures might form and evolve, and carry out a Bayesian analysis to test these models against observed distributions of protostellar age, counts of young stellar objects relative to gas, and the overall star formation rate of the Milky Way. We show that models in which the acceleration of star formation is due either to a large-scale collapse or a time-dependent increase in star formation efficiency are unable to satisfy the combined set of observational constraints. In contrast, models in which clusters form in a ‘conveyor belt’ mode where gas accretion and star formation occur simultaneously, but the star formation rate per free-fall time is low, can match the observations.
ABSTRACT We reanalyze data on the surface densities and vertical distribution of baryonic matter in the solar neighborhood and tabulate the results. We find a local total surface density of M dwarfs ...of , whcih is significantly higher than previous values. Our result for the total local surface density of visible stars (main-sequence stars and giants), , is close to previous estimates due to a cancellation of opposing effects: more mass in M dwarfs, less mass in the others. The total local surface density in white dwarfs is ; in brown dwarfs, it is , but with considerable uncertainty. We find that the total local surface density of stars and stellar remnants is , which is somewhat less than previous estimates but within the errors of many of them. We analyze data on 21 cm emission and absorption and obtain good agreement with recent results on the local amount of neutral atomic hydrogen obtained with the Planck satellite. The local surface density of gas is . The total baryonic mass surface density that we derive for the solar neighborhood is ( within 1.1 kpc of the midplane). Combining these results with others' measurements of the total surface density of matter within 1-1.1 kpc of the plane, we find that the local density of dark matter is GeV cm . The local density of all matter is . We discuss limitations on the properties of a possible thin disk of dark matter.
ABSTRACT
While magnetic fields are important in contemporary star formation, their role in primordial star formation is unknown. Magnetic fields of the order of 10−16 G are produced by the Biermann ...battery due to the curved shocks and turbulence associated with the infall of gas into the dark matter minihaloes that are the sites of formation of the first stars. These fields are rapidly amplified by a small-scale dynamo until they saturate at or near equipartition with the turbulence in the central region of the gas. Analytical results are given for the outcome of the dynamo, including the effect of compression in the collapsing gas. The mass-to-flux ratio in this gas is two to three times the critical value, comparable to that in contemporary star formation. Predictions of the outcomes of simulations using smooth particle hydrodynamics (SPH) and grid-based adaptive mesh refinement are given. Because the numerical viscosity and resistivity for the standard resolution of 64 cells per Jeans length are several orders of magnitude greater than the physical values, dynamically significant magnetic fields affect a much smaller fraction of the mass in simulations than in reality. An appendix gives an analytical treatment of free-fall collapse, including that in a constant-density background. Another appendix presents a new method of estimating the numerical viscosity; results are given for both SPH and grid-based codes.
Star formation in our Galaxy occurs in molecular clouds that are self-gravitating, highly turbulent, and magnetized. We study the conditions under which cloud cores inherit large-scale magnetic field ...morphologies and how the field is governed by cloud turbulence. We present four moving-mesh simulations of supersonic, turbulent, isothermal, self-gravitating gas with a range of magnetic mean-field strengths characterized by the Alfvénic Mach number , resolving prestellar core formation from parsec to a few astronomical unit scales. In our simulations with the turbulent kinetic energy density dominating over magnetic pressure ( ), we find that the collapse is approximately isotropic with B ∝ 2/3, core properties are similar regardless of initial mean-field strength, and the field direction on 100 au scales is uncorrelated with the mean field. However, in the case of a dominant large-scale magnetic field ( ), the collapse is anisotropic with B ∝ 1/2. This transition at is not expected to be sharp, but clearly signifies two different paths for magnetic field evolution in star formation. Based on observations of different star-forming regions, we conclude that star formation in the interstellar medium may occur in both regimes. Magnetic field correlation with the mean field extends to smaller scales as decreases, making future Atacama Large Millimeter Array observations useful for constraining of the interstellar medium.
Theory of Star Formation McKee, Christopher F; Ostriker, Eve C
Annual review of astronomy and astrophysics,
01/2007, Letnik:
45, Številka:
1
Journal Article
Recenzirano
We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence ...plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity—are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.
Abstract
We present a large suite of simulations of the formation of low-mass star clusters. Our simulations include an extensive set of physical processes – magnetohydrodynamics, radiative transfer, ...and protostellar outflows – and span a wide range of virial parameters and magnetic field strengths. Comparing the outcomes of our simulations to observations, we find that simulations remaining close to virial balance throughout their history produce star formation efficiencies and initial mass function (IMF) peaks that are stable in time and in reasonable agreement with observations. Our results indicate that small-scale dissipation effects near the protostellar surface provide a feedback loop for stabilizing the star formation efficiency. This is true regardless of whether the balance is maintained by input of energy from large-scale forcing or by strong magnetic fields that inhibit collapse. In contrast, simulations that leave virial balance and undergo runaway collapse form stars too efficiently and produce an IMF that becomes increasingly top heavy with time. In all cases, we find that the competition between magnetic flux advection towards the protostar and outward advection due to magnetic interchange instabilities, and the competition between turbulent amplification and reconnection close to newly formed protostars renders the local magnetic field structure insensitive to the strength of the large-scale field, ensuring that radiation is always more important than magnetic support in setting the fragmentation scale and thus the IMF peak mass. The statistics of multiple stellar systems are similarly insensitive to variations in the initial conditions and generally agree with observations within the range of statistical uncertainty.
Abstract
Star formation in a filamentary infrared dark cloud (IRDC) is simulated over the dynamic range of 4.2 pc to 28 au for a period of 3.5 × 105 yr, including magnetic fields and both radiative ...and outflow feedback from the protostars. At the end of the simulation, the star formation efficiency is 4.3 per cent and the star formation rate per free-fall time is εff ≃ 0.04, within the range of observed values. The total stellar mass increases as ∼t2, whereas the number of protostars increases as ∼t1.5. We find that the density profile around most of the simulated protostars is ∼ρ ∝ r−1.5. At the end of the simulation, the protostellar mass function approaches the Chabrier stellar initial mass function. We infer that the time to form a star of median mass 0.2 M⊙ is about 1.4 × 105 yr from the median mass accretion rate. We find good agreement among the protostellar luminosities observed in the large sample of Dunham et al., our simulation and a theoretical estimate, and we conclude that the classical protostellar luminosity problem is resolved. The multiplicity of the stellar systems in the simulation agrees, to within a factor of 2, with observations of Class I young stellar objects; most of the simulated multiple systems are unbound. Bipolar protostellar outflows are launched using a subgrid model, and extend up to 1 pc from their host star. The mass–velocity relation of the simulated outflows is consistent with both observation and theory.
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