Large-scale coherent behaviour of star forming systems with feedbacks (original) (raw)

Feedback Processes in Massive Star Formation

2020

We propose a programme of e-MERLIN observations on a large and well-selected sample of massive young stars that spans the full range of evolutionary stages to address key questions on feedback processes. Deep continuum imaging will determine when MHD driven jets turn on and when they give way to radiatively driven disc winds and over what mass range. When the ionizing radiation first breaks out into a hyper-compact H II region we will determine the density distribution and the physical processes that have shaped it. Simultaneously, we will map the 3D magnetic field structure in a number of sources through methanol and excited OH maser polarization. Together these data will test both the physics of the feedback mechanisms and evolutionary models of massive star formation. Name Institution

Stellar feedback and triggered star formation

Proceedings of the International Astronomical Union, 2009

Young, massive stars influence their ambient medium through winds and radiation. The outcome of this feedback depends on the number of massive stars in a star cluster and on the density of the ambient medium. This contribution is based on a comparison of observations to the results of numerical simulations. We discuss the gravitational fragmentation of feedback-driven shells expanding from young stellar clusters. The thin-shell approximation is compared to 3D hydrodynamical simulations with smoothed-particle hydrodynamics and adaptive-mesh refinement codes. We explore the influence of external pressure and propose a thick-shell dispersion relation, where the pressure of the external medium is included. The mass spectrum of the shell fragments is constructed and we speculate about the origin of the deficit of low-mass objects.

Simulating the Formation of Massive Protostars. I. Radiative Feedback and Accretion Disks

The Astrophysical Journal, 2016

We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M ⊙. We follow their gravitational collapse and the formation of a massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the accretion rate by a factor of 2-10. Although the disk becomes unstable, no other stars are formed. In the case of our 100 and 200 M ⊙ simulation, the star becomes highly super-Eddington and begins to drive bipolar outflow cavities that expand outwards. These radiatively-driven bubbles appear stable, and appear to be channeling gas back onto the protostellar accretion disk. Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our 30 M ⊙ simulation shows a star with a mass of 5.48 M ⊙ and a disk of mass 3.3 M ⊙ , while our 100 M ⊙ simulation forms a 28.8 M ⊙ mass star with a 15.8 M ⊙ disk over the course of 41.6 kyr, and our 200 M ⊙ simulation forms a 43.7 M ⊙ star with an 18 M ⊙ disk in 21.9 kyr. In the absence of magnetic fields or other forms of feedback, the masses of the stars in our simulation do not appear limited by their own luminosities.

Self-Organized Large-Scale Coherence in Simulations of Galactic Star Formation

1997

It is often assumed that galaxies cannot generate large-scale coherent star- forming activity without some organizing agent, such as spiral density waves, bars, large-scale instabilities, or external perturbations due to encounters with other galaxies. We present simulations of a simple model of star formation in which local spatial couplings lead to large-scale coherent, and even synchronized, patterns of star formation without any explicit propagation or any separate organizing agent. At a given location, star formation is assumed to occur when the gas velocity dispersion falls below a critical value dependent on the density. Young stars inject energy into the gas in their neighborhood, increasing the velocity dispersion and inhibiting the instability. A dissipation function continually "cools" the gas. The stability of this local inhibitory feedback model is examined both analytically and numerically. A large number of two-dimensional simulations are used to examine the effect of spatial couplings due to energy injection into neighboring regions. We find that several distinct types of behavior can be demarcated in a phase diagram whose parameter axes are the density (assumed constant in most models) and spatial coupling strength. These "phases" include, with decreasing density, a spatially homogeneous steady state, oscillatory "islands," traveling waves of star formation or global synchronization, and scattered "patches" of star formation activity. The coherence effects are explained in terms of the ability of the energy injected near a star formation site to introduce phase correlations in the subsequent cooling curves of neighboring regions.

A detailed study of feedback from a massive star

Monthly Notices of the Royal Astronomical Society, 2015

We present numerical simulations of a 15 M ⊙ star in a suite of idealised environments in order to quantify the amount of energy transmitted to the interstellar medium (ISM). We include models of stellar winds, UV photoionisation and the subsequent supernova based on theoretical models and observations of stellar evolution. The system is simulated in 3D using RAMSES-RT, an Adaptive Mesh Refinement Radiation Hydrodynamics code. We find that stellar winds have a negligible impact on the system owing to their relatively low luminosity compared to the other processes. The main impact of photoionisation is to reduce the density of the medium into which the supernova explodes, reducing the rate of radiative cooling of the subsequent supernova. Finally, we present a grid of models quantifying the energy and momentum of the system that can be used to motivate simulations of feedback in the ISM unable to fully resolve the processes discussed in this work.

Star Cluster Formation and Feedback

Protostars and Planets VI, 2014

Stars do not generally form in isolation. Instead, they form in clusters, and in these clustered environments newborn stars can have profound effects on one another and on their parent gas clouds. Feedback from clustered stars is almost certainly responsible for a number of otherwise puzzling facts about star formation: that it is an inefficient process that proceeds slowly when averaged over galactic scales; that most stars disperse from their birth sites and dissolve into the galactic field over timescales 1 Gyr; and that newborn stars follow an initial mass function (IMF) with a distinct peak in the range 0.1 − 1 M , rather than an IMF dominated by brown dwarfs. In this review we summarize current observational constraints and theoretical models for the complex interplay between clustered star formation and feedback.

Large Scale Star Formation: Density Waves, Superassociations and Propagation

Star Forming Regions, 1986

The hypothesis that density waves trigger star formation is critically exam ined. Much of the former evidence in favor of the hypothesis is shown to be inconsistent with modern observations. A comparison between galaxies with and without density waves reveals no significant difference in their star formation rates. A new role for density waves in the context of star formation might be based on four principles: 1. density waves are in trinsically strong, 2. the gas is compressed more than the stars in the wave, 3. star formation follows the gas, with no preferential trigger related to the wave itself, and 4. regions of star formation are larger in the spiral arms than they are between the arms. This new role for density waves is primarily one of organization: the waves place most of the gas in the arms, so most of the star formation is in the arms too. The waves also promote the coagulation of small clouds into large cloud complexes, or superclouds, by what appears to be a combination of collisional agglomeration and large-scale gravitational instabilities. Special regions where density waves do trigger a true excess of star formation are discussed, and possible reasons for the difference between these triggering waves and the more common, organizing, waves are mentioned. Other aspects of large-scale star formation, such as the occurrence of kiloparsec-size regions of activity and kiloparsec-range propagation, are illustrated with nu merous examples. The importance of these largest scales to the overall mechanism of star formation in galaxies is emphasized. 1. INTRODUCTION Three aspects of star formation on a galactic scale are addressed here: 1. the relationship between galactic spiral arms and star formation; 2. the apparent coherence of some star formation on a kiloparsec scale, and 3. the propagation of star formation over kiloparsec distances. The correspondence between star formation and galactic spiral arms was first noted for M31 by Baade and Mayall (1951), and then used by Morgan et al. (1952, 1953) to de lineate the local spiral arms. Baade (1963) later commented that HII regions in galaxies of ten line up "like pearls on a necklace." One explanation given for this phenomenon is that spiral arms are density waves (Lin and Shu 1964) and density waves trigger star formation (Roberts 1969). An alternative explanation is that swing amplification in galactic disks trig gers the formation of both stellar and gaseous arms, and that the compression and shocks which develop in the gaseous arms lead to star formation (Goldreich and Lynden-Bell 1965; Lynden-Bell 1966; Toomre 1981). The most recent interpretation is that propagating star 457

A prescription for star formation feedback: the importance of multiple shell interactions

Monthly Notices of the Royal Astronomical Society, 1999

The relation between the star formation rate and the kinetic energy increase in a region containing a large number of stellar sources is investigated as a possible prescription for star formation feedback in larger scale galaxy evolution simulations, and in connection with observed scaling relations for molecular clouds, extragalactic giant H II regions, and starburst galaxies. The kinetic energy increase is not simply proportional to the source input rate, but depends on the competition between stellar power input and dissipation due to interactions between structures formed and driven by the star formation. A simple one-zone model is used to show that, in a steady-state, the energy increase should be proportional to the two-thirds power of the stellar energy injection rate, with additional factors depending on the mean density of the region and mean column density of fragments. The scaling relation is tested using two-dimensional pressureless hydrodynamic simulations of wind-driven star formation, in which star formation occurs according to a threshold condition on the column density through a shell, and a large number of shells are present at any one time. The morphology of the simulations resembles an irregular network or web of dynamically-interacting filaments. A set of 16 simulations in which different parameters were varied agree remarkably with the simple analytical prescription for the scaling relation. Converting from wind power of massive stars to Lyman continuum luminosity shows that the cluster wind model for giant H II regions may still be viable.

Star Formation: Theory and Modelling

Springer proceedings in physics, 1997

Star formation (SF) is a complex combination of various processes that include the effects of self-gravity, magnetic field, radiation, chemical reactions, etc. Theoretical modelling of SF requires division of the processes into separate pieces. In terms of basic physics, we review our current theoretical understanding of those processes: thermal evolution of gas, gravitational fragmentation to form molecular cloud cores with and without magnetic field, radiation hydrodynamical evolution of a low-mass protostar, and the evolution of a circumstellar disc.

Observations and Theory of Dynamical Triggers for Star Formation

Origins, 1998

Star formation triggering mechanisms are reviewed, including the direct compression of clouds and globules, the compression and collapse of molecular clouds at the edges of HII regions and supernovae, the expansion and collapse of giant rings and shells in galaxy disks, and the collision and collapse between two clouds. Collapse criteria are given. A comprehensive tabulation of regions where these four types of triggering have been found suggests that dynamical processes sustain and amplify a high fraction of all star formation that begins spontaneously in normal galaxy disks.