Formation of Massive Star Systems by Accretion Krumholz, Mark R; Klein, Richard I; McKee, Christopher F ...
Science (American Association for the Advancement of Science),
02/2009, Letnik:
323, Številka:
5915
Journal Article
Recenzirano
Odprti dostop
Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to ...prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through nonaxisymmetric disks and filaments that self-shield against radiation while allowing radiation to escape through optically thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.
The star-forming efficiency of dense gas is thought to be set within cores by outflow and radiative feedback. We use magnetohydrodynamic simulations to investigate the relation between protostellar ...outflow evolution, turbulence, and star formation efficiency. We model the collapse and evolution of isolated dense cores for 0.5 Myr including the effects of turbulence, radiation transfer, and both radiation and outflow feedback from forming protostars. We show that outflows drive and maintain turbulence in the core environment even with strong initial fields. The star formation efficiency decreases with increasing field strength, and the final efficiencies are 15%-40%. The Stage 0 lifetime, during which the protostellar mass is lower than that of the dense envelope, increases proportionally with the initial magnetic field strength and ranges from . The average accretion rate is well represented by a tapered turbulent core model, which is a function of the final protostellar mass and is independent of the magnetic field strength. By tagging material launched in the outflow, we demonstrate that the outflow entrains about three times the actual launched gas mass, a ratio that remains roughly constant in time regardless of the initial magnetic field strength. However, turbulent driving increases for stronger fields since momentum is more efficiently imparted to non-outflow material. The protostellar outflow momentum is highest during the first 0.1 Myr and declines thereafter by a factor of as the accretion rate diminishes.
The initial multiplicity of stellar systems is highly uncertain. A number of mechanisms have been proposed to explain the origin of binary and multiple star systems, including core fragmentation, ...disk fragmentation and stellar capture. Observations show that protostellar and pre-main-sequence multiplicity is higher than the multiplicity found in field stars, which suggests that dynamical interactions occur early, splitting up multiple systems and modifying the initial stellar separations. Without direct, high-resolution observations of forming systems, however, it is difficult to determine the true initial multiplicity and the dominant binary formation mechanism. Here we report observations of a wide-separation (greater than 1,000 astronomical units) quadruple system composed of a young protostar and three gravitationally bound dense gas condensations. These condensations are the result of fragmentation of dense gas filaments, and each condensation is expected to form a star on a timescale of 40,000 years. We determine that the closest pair will form a bound binary, while the quadruple stellar system itself is bound but unstable on timescales of 500,000 years (comparable to the lifetime of the embedded protostellar phase). These observations suggest that filament fragmentation on length scales of about 5,000 astronomical units offers a viable pathway to the formation of multiple systems.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
The initial mass function (IMF) of stars is a key quantity affecting almost every field of astrophysics, yet it remains unclear what physical mechanisms determine it. We present the first ...runs of the STAR FORmation in Gaseous Environments project, using a new numerical framework to follow the formation of individual stars in giant molecular clouds (GMCs) using the gizmo code. Our suite includes runs with increasingly complex physics, starting with isothermal ideal magnetohydrodynamics (MHD) and then adding non-isothermal thermodynamics and protostellar outflows. We show that without protostellar outflows the resulting stellar masses are an order of magnitude too high, similar to the result in the base isothermal MHD run. Outflows disrupt the accretion flow around the protostar, allowing gas to fragment and additional stars to form, thereby lowering the mean stellar mass to a value similar to that observed. The effect of jets upon global cloud evolution is most pronounced for lower mass GMCs and dense clumps, so while jets can disrupt low-mass clouds, they are unable to regulate star formation in massive GMCs, as they would turn an order unity fraction of the mass into stars before unbinding the cloud. Jets are also unable to stop the runaway accretion of massive stars, which could ultimately lead to the formation of stars with masses ${\gt}500\, \mathrm{M}_{\rm \odot }$. Although we find that the mass scale set by jets is insensitive to most cloud parameters (i.e. surface density, virial parameter), it is strongly dependent on the momentum loading of the jets (which is poorly constrained by observations) as well as the temperature of the parent cloud, which predicts slightly larger IMF variations than observed. We conclude that protostellar jets play a vital role in setting the mass scale of stars, but additional physics are necessary to reproduce the observed IMF.
Forming stars emit a substantial amount of radiation into their natal environment. We use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodyanics code, to simulate ...low-mass star formation in a turbulent molecular cloud. We compare the distributions of stellar masses, accretion rates, and temperatures in the cases with and without radiative transfer, and we demonstrate that radiative feedback has a profound effect on accretion, multiplicity, and mass by reducing the number of stars formed and the total rate at which gas turns into stars. We also show that once the star formation reaches a steady state, protostellar radiation is by far the dominant source of energy in the simulation, exceeding viscous dissipation and compressional heating by at least an order of magnitude. Calculations that omit radiative feedback from protstars significantly underestimate the gas temperature and the strength of this effect. Although heating from protostars is mainly confined to the protostellar cores, we find that it is sufficient to suppress disk fragmentation that would otherwise result in very low-mass companions or brown dwarfs. We demonstrate that the mean protostellar accretion rate increases with the final stellar mass so that the star formation time is only a weak function of mass.
We characterize the infall rate onto protostellar systems forming in self-gravitating radiation-hydrodynamics simulations. Using two dimensionless parameters to determine the disks' susceptibility to ...gravitational fragmentation, we infer limits on protostellar system multiplicity and the mechanism of binary formation. We show that these parameters give robust predictions even in the case of marginally resolved protostellar disks. We find that protostellar systems with radiation feedback predominately form binaries via turbulent fragmentation, not disk instability, and predict that turbulent fragmentation is the dominant channel for binary formation for low-mass stars. We clearly demonstrate that systems forming in simulations including radiative feedback have fundamentally different parameters than those in purely hydrodynamics simulations.
We present astrochemical photodissociation region models in which cosmic-ray (CR) attenuation has been fully coupled to the chemical evolution of the gas. We model the astrochemical impact of CRs, ...including those accelerated by protostellar accretion shocks, on molecular clouds hosting protoclusters. Our models with embedded protostars reproduce observed ionization rates. We study the imprint of CR attenuation on ions for models with different surface CR spectra and different star formation efficiencies. We find that abundances, particularly ions, are sensitive to the treatment of CRs. We show the column densities of ions are underpredicted by the "classic" treatment of CRs by an order of magnitude. We also test two common chemistry approximations used to infer ionization rates. We conclude that the approximation based on the abundance underpredicts the ionization rate, except in regions where the CRs dominate the chemistry. Our models suggest the chemistry in dense gas will be significantly impacted by the increased ionization rates, leading to a reduction in molecules such as NH3 and causing H2-rich gas to become C ii bright.
Abstract
We define a sample of 200 protostellar outflows showing blue- and redshifted CO emission in the nearby molecular clouds Ophiuchus, Taurus, Perseus, and Orion, to investigate the correlation ...between outflow orientations and local, but relatively large-scale, magnetic field directions traced by Planck 353 GHz dust polarization. At high significance (
p
∼ 10
−4
), we exclude a random distribution of relative orientations and find that there is a preference for alignment of projected plane of sky outflow axes with magnetic field directions. The distribution of relative position angles peaks at ∼30° and exhibits a broad dispersion of ∼50°. These results indicate that magnetic fields have dynamical influence in regulating the launching and/or propagation directions of outflows. However, the significant dispersion around perfect alignment orientation implies that there are large measurement uncertainties and/or a high degree of intrinsic variation caused by other physical processes, such as turbulence or strong stellar dynamical interactions. Outflow to magnetic field alignment is expected to lead to a correlation in the directions of nearby outflow pairs, depending on the degree of order of the field. Analyzing this effect, we find limited correlation, except on relatively small scales ≲0.5 pc. Furthermore, we train a convolutional neural network to infer the inclination angle of outflows with respect to the line of sight and apply it to our outflow sample to estimate their full 3D orientations. We find that the angles between outflow pairs in 3D space also show evidence of small-scale alignment.
Stars rarely form in isolation. Nearly half of the stars in the Milky Way have a companion, and this fraction increases in star-forming regions. However, why some dense cores and filaments form bound ...pairs while others form single stars remains unclear. We present a set of three-dimensional, gravo-magnetohydrodynamic simulations of turbulent star-forming clouds, aimed at understanding the formation and evolution of multiple-star systems formed through large-scale ( 103 au) turbulent fragmentation. We investigate three global magnetic field strengths, with global mass-to-flux ratios of φ = 2, 8, and 32. The initial separations of protostars in multiples depend on the global magnetic field strength, with stronger magnetic fields (e.g., φ = 2) suppressing fragmentation on smaller scales. The overall multiplicity fraction (MF) is between 0.4 and 0.6 for our strong and intermediate magnetic field strengths, which is in agreement with observations. The weak field case has a lower fraction. The MF is relatively constant throughout the simulations, even though stellar densities increase as collapse continues. While the MF rarely exceeds 60% in all three simulations, over 80% of all protostars are part of a binary system at some point. We additionally find that the distribution of binary spin misalignment angles is consistent with a randomized distribution. In all three simulations, several binaries originate with wide separations and dynamically evolve to 102 au separations. We show that a simple model of mass accretion and dynamical friction with the gas can explain this orbital evolution.
Abstract
Young stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To ...identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars and use it to identify ∼3 × 10
4
candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis of all groups with substructure, using age estimates to reveal each region’s star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ∼4 km s
−1
(∼4 pc Myr
−1
). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales.
PDF
