We study feedback during massive star formation using semi-analytic methods, considering the effects of disk winds, radiation pressure, photoevaporation and stellar winds, while following protostellar evolution in collapsing massive gas cores. We find that disk winds are the dominant feedback mechanism setting star formation efficiencies (SFEs) from initial cores of ~0.3-0.5. However, radiation pressure is also significant to widen the outflow cavity causing reductions of SFE compared to the disk-wind only case, especially for >100Msun star formation at clump mass surface densities Sigma<0.3g/cm2. Photoevaporation is of relatively minor importance due to dust attenuation of ionizing photons. Stellar winds have even smaller effects during the accretion stage. For core masses Mc~10-1000Msun and Sigma~0.1-3g/cm2, we find the overall SFE to be 0.31(Rc/0.1pc)^{-0.39}, potentially a useful sub-grid star-formation model in simulations that can resolve pre-stellar core radii, Rc=0.057(Mc/60Msun)^{1/2}(Sigma/g/cm2)^{-1/2}pc. The decline of SFE with Mc is gradual with no evidence for a maximum stellar-mass set by feedback processes up to stellar masses of ~300Msun. We thus conclude that the observed truncation of the high-mass end of the IMF is shaped mostly by the pre-stellar core mass function or internal stellar processes. To form massive stars with the observed maximum masses of ~150-300Msun, initial core masses need to be >500-1000Msun. We also apply our feedback model to zero-metallicity primordial star formation, showing that, in the absence of dust, photoevaporation staunches accretion at ~50Msun. Our model implies radiative feedback is most significant at metallicities ~10^{-2}Zsun, since both radiation pressure and photoevaporation are effective in this regime.
The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that…
We perform two-dimensional axially symmetric radiation-hydrodynamic simulations to assess the impact of outflows and radiative force feedback from massive protostars by varying when the protostellar outflow starts, the ratio of ejection to accretion rates, and the strength of the wide angle disk wind component. The star formation efficiency, i.e. the ratio of final stellar mass to initial core mass, is dominated by radiative forces and the ratio of outflow to accretion rates. Increasing this ratio has three effects: First, the protostar grows slower with a lower luminosity at any given time, l…
We use the gas-grain chemistry code UCLCHEM to explore the impact of cosmic-ray feedback on the chemistry of circumstellar disks. We model the attenuation and energy losses of the cosmic-rays as they propagate outwards from the star and also consider ionization due to stellar radiation and radionuclides. For accretion rates typical of young stars, $\dot M_* \sim 10^{-9}-10^{-6}$ M_\odot yr$^{-1}$, we show that cosmic rays accelerated by the stellar accretion shock produce a cosmic-ray ionization rate at the disk surface $ζ\gtrsim 10^{-15}$ s$^{-1}$, at least an order of magnitude higher than t…
The relation between the mass accretion rate onto the circumstellar disc and the rate of mass ejection by magnetically driven winds is investigated using three-dimensional magnetohydrodynamics simulations. Using a spherical cloud core with a varying ratio of thermal to gravitational energy, which determines the mass accretion rate onto the disc, to define the initial conditions, the outflow propagation for approximately 10^4 yr after protostar formation is then calculated for several cloud cores. The mass ejection rate and accretion rate are comparable only when the magnetic energy of the init…
We present the first simulations of the formation and feedback of massive stars which account for radiation forces as well as photoionization feedback (along with protostellar outflows). In two different accretion scenarios modeled, we determine the relative strength of these feedback components and derive the size of the reservoir from which the forming stars gained their masses. We find that protostellar outflows alone limit the stellar mass growth only in an accretion scenario with a finite mass reservoir; when including accretion and ram pressure from large scales (> 0.1 pc), protostellar …
The formation of massive stars is a long standing problem. Although a number of theories of massive star formation exist, ideas appear to converge to a disk-mediated accretion scenario. Here we present radiative hydrodynamic simulations of a star accreting mass via a disk embedded in a torus. We use a Monte Carlo based radiation hydrodynamics code to investigate the impact that ionizing radiation has on the torus. Ionized regions in the torus midplane are found to be either gravitationally trapped or in pressure driven expansion depending on whether or not the size of the ionized region exceed…
We investigate the upper stellar mass limit set by radiative feedback by the forming star with various accretion rates and metallicities. To this end, we numerically solve the structures of both a protostar and its surrounding accretion envelope assuming a spherical symmetric and steady flow. The optical depth of the dust cocoon, a dusty part of the accretion envelope, differs among the direct light from the stellar photosphere and the diffuse light re-emitted as dust thermal emission. As a result, varying the metallicity qualitatively changes the way that the radiative feedback suppresses the…
Using the HPC ressources of the state of Baden-Württemberg, we modelled for the first time the luminous burst from a young massive star by accretion of material from its close environment. We found that the surroundings of young massive stars are shaped as a clumpy disk whose fragments provoke outbursts once they fall onto the protostar and concluded that similar strong luminous events observed in high-mass star forming regions may be a signature of the presence of such disks.
We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD) simulations of the evolution of primordial protostars, their immediate environment, and the dynamic accretion history under the influence of stellar ionizing and dissociating UV feedback. Our coupled SE-RHD calculations result in a wide diversity of final stellar masses covering 10 Msun $\lesssim M_* \lesssim$ 1000 Msun. The formation of very massive ($\gtrsim$ 250 Msun) stars is possible under weak UV feedback, whereas ordinary massive (a few x 10 Msun) stars form when UV feedback can efficiently halt the accretion.…
Accretion-driven luminosity outbursts are a vivid manifestation of variable mass accretion onto protostars. They are known as the so-called FU Orionis phenomenon in the context of low-mass protostars. More recently, this process has been found in models of primordial star formation. Using numerical radiation hydrodynamics simulations, we stress that present-day forming massive stars also experience variable accretion and show that this process is accompanied by luminous outbursts induced by the episodic accretion of gaseous clumps falling from the circumstellar disk onto the protostar. Consequ…
The bright maser emission produced by several molecular species at centimeter to long millimeter wavelengths provides an essential tool for understanding the process of massive star formation. Unimpeded by the high dust optical depths that affect shorter wavelength observations, the high brightness temperature of these emission lines offers a way to resolve accretion and outflow motions down to scales below $\sim$1 au in deeply embedded Galactic star-forming regions at kiloparsec distances. The recent identification of extraordinary accretion outbursts in two high-mass protostars, both of whic…
A model for massive stars is constructed by piecing together evolutionary algorithms for the protostellar structure, the environment, the inflow and the radiation feedback. We investigate specified accretion histories of constant, decelerating and accelerating forms and consider both hot and cold accretion, identified with spherical free-fall and disk accretion, respectively. Diagnostic tools for the interpretation of the phases of massive star formation and testing the evolutionary models are then developed. Evolutionary tracks able to fit Herschel Space Telescope data require the generated s…
Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through non-axisymmetric disks and filaments that self-shield against radiation, while allowing radiatio…
We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M$_{\odot}$. We follow their gravitational collapse and the formation of a massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the acc…
In this review we discuss the population of stellar-mass black holes in our galaxy and beyond, which are the extreme endpoints of massive star evolution. In particular we focus on how we can attempt to balance the available accretion energy with feedback to the environment via radiation, jets and winds, considering also possible contributions to the energy balance from black hole spin and advection. We review quantitatively the methods which are used to estimate these quantities, regardless of the details of the astrophysics close to the black hole. Once these methods have been outlined, we wo…
We report the discovery of a massive protostar M17~MIR embedded in a hot molecular core in M17. The multiwavelength data obtained during 1993--2019 show significant mid-IR (MIR) variations, which can be split into three stages: the decreasing phase during 1993.03--mid-2004, the quiescent phase from mid-2004 to mid-2010, and the rebrightening phase from mid-2010 until now. The variation of the 22\,GHz H$_2$O maser emission, together with the MIR variation, indicates an enhanced disk accretion rate onto M17~MIR during the decreasing and rebrightening phases. Radiative transfer modeling of the sp…
Massive protostars attain high luminosities as they are actively accreting and the radiation pressure exerted on the gas in the star's atmosphere may launch isotropic high-velocity winds. These winds will collide with the surrounding gas producing shock-heated ($T\sim 10^7$ K) tenuous gas that adiabatically expands and pushes on the dense gas that may otherwise be accreted. We present a suite of 3D radiation-magnetohydrodynamic simulations of the collapse of massive prestellar cores and include radiative feedback from the stellar and dust-reprocessed radiation fields, collimated outflows, and,…
The burst mode of accretion in massive star formation is a scenario linking the initial gravitational collapse of parent pre-stellar cores to the properties of their gravitationally unstable discs and of their accretion-driven bursts. In this study, we present a series of high-resolution 3D radiation-hydrodynamics numerical simulations for young massive stars formed out of collapsing 100 Mo molecular cores spinning with several values of the ratio of rotational-to-gravitational energies beta=5%-9%. The models include the indirect gravitational potential caused by disc asymmetries. We find that…
Ejection activities in S255IR-SMA1 and AFGL 5142 were investigated by multi-epoch VLBI observations of 22 GHz water masers, tracing bowshocks leading collimated jets. The history of ejections, revealed by the 3D maser motions and supplemented by the literature, suggests that these massive stars formed by episodic accretion, inferred via the accretion-ejection connection. This contribution centers on the role of episodic accretion in overcoming the radiation pressure problem of massive star formation - with maser VLBI and single-dish observations providing essential observational tools.
Massive stars influence their surroundings through radiation, winds, and supernova explosions far out of proportion to their small numbers. However, the physical processes that initiate and govern the birth of massive stars remain poorly understood. Two widely discussed models are monolithic collapse of molecular cloud cores and competitive accretion. To learn more about massive star formation, we perform simulations of the collapse of rotating, massive, cloud cores including radiative heating by both non-ionizing and ionizing radiation using the FLASH adaptive mesh refinement code. These simu…
This paper investigates the gravitational trapping of HII regions predicted by steady-state analysis using radiation hydrodynamical simulations. We present idealised spherically symmetric radiation hydrodynamical simulations of the early evolution of HII regions including the gravity of the central source. As with analytic steady state solutions of spherically symmetric ionised Bondi accretion flows, we find gravitationally trapped HII regions with accretion through the ionisation front onto the source. We found that, for a constant ionising luminosity, fluctuations in the ionisation front are…
We use the stellar evolution code MESA to study the negative jet feedback mechanism in common envelope jets supernovae (CEJSNe) where a neutron star (NS) launches jets in the envelope of a red supergiant (RSG), and find that the feedback reduces the mass accretion rate to be $χ_{\rm j} \simeq 0.04-0.3$ times the mass accretion rate without the operation of jets. We mimic the effect of the jets on the RSG envelope by depositing the energy that the jets carry into the envelope zones outside the NS orbit. The energy deposition inflates the envelope, therefore reducing the density in the NS vicini…
The most massive stars can form via standard disk accretion - despite of the radiation pressure generated - due to the fact that the massive accretion disk yields a strong anisotropy in the radiation field, releasing most of the radiation pressure perpendicular to the disk accretion flow. Here, we analyze the self-gravity of the forming circumstellar disk as the potential major driver of the angular momentum transport in such massive disks responsible for the high accretion rates needed for the formation of massive stars. For this purpose, we perform self-gravity radiation hydrodynamics simula…
Mass accretion onto (proto-)stars at high accretion rates > 10^-4 M_sun/yr is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper we examine the evolution via disk accretion. We consider a limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated …
Rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (PMS) objects. Understanding the formation of massive stars requires taking into account the accretion of angular momentum during their PMS phase. We study the PMS evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. In particular, we look at the physical conditions that allow the production of massive stars by accretion. We present PMS models computed with a new…
We present radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. We treat frequency dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, it is possible for the first time to simulate the whole accretion phase (up to the end of the accretion disk epoch) for the forming massive star and to perform a broad scan of the parameter space. Our simulation series show evidently the necessity to incorporate the dust sublimation front to preserve the high…
During a common envelope episode in a binary system, the engulfed companion spirals to tighter orbital separations under the influence of drag from the surrounding envelope material. As this object sweeps through material with a steep radial gradient of density, net angular momentum is introduced into the flow, potentially leading to the formation of an accretion disk. The presence of a disk would have dramatic consequences for the outcome of the interaction because accretion might be accompanied by strong, polar outflows with enough energy to unbind the entire envelope. Without a detailed und…
Massive star formation requires the accretion of gas at high rate while the star is already bright. Its actual luminosity depends sensitively on the stellar structure. We compute pre-main-sequence tracks for massive and intermediate-mass stars with variable accretion rates and study the evolution of stellar radius, effective temperature and ionizing luminosity, starting at $2\,M_\odot$ with convective or radiative structures. The radiative case shows a much stronger swelling of the protostar for high accretion rates than the convective case. For radiative structures, the star is very sensitive…
Massive stars are powerful sources of radiation, stellar winds, and supernova explosions. The radiative and mechanical energies injected by massive stars into the interstellar medium (ISM) profoundly alter the structure and evolution of the ISM, which subsequently influences the star formation and chemical evolution of the host galaxy. In this review, we will use the Large Magellanic Cloud (LMC) as a laboratory to showcase effects of energy feedback from massive young stellar objects (YSOs) and mature stars. We will also use the Carina Nebula in the Galaxy to illustrate a multi-wavelength stud…
We present radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. We treat frequency dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, it is possible for the first time to simulate the whole accretion phase of several 10^5 yr for the forming massive star and to perform a comprehensive scan of the parameter space. Our simulation series show evidently the necessity to incorporate the dust sublimation front to preserve the high shielding proper…
We study the similarities of jet-medium interactions in several quite different astrophysical systems using 2D and 3D hydrodynamical numerical simulations, and find many similarities. The systems include cooling flow (CF) clusters of galaxies, core-collapse supernovae (CCSNe), planetary nebulae (PNe), and common envelope (CE) evolution. The similarities include hot bubbles inflated by jets in a bipolar structure, vortices on the sides of the jets, vortices inside the inflated bubbles, fragmentation of bubbles to two and more bubbles, and buoyancy of bubbles. The activity in many cases is regul…
We follow the premise that most intermediate luminosity optical transients (ILOTs) are powered by rapid mass accretion onto a main sequence star, and study the effects of jets launched by an accretion disk. The disk is formed due to large specific angular momentum of the accreted mass. The two opposite jets might expel some of the mass from the reservoir of gas that feeds the disk, and therefore reduces and shortens the mass accretion process. We argue that by this process ILOTs limit their luminosity and might even shut themselves off in this negative jet feedback mechanism (JFM). The group o…
Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the HR diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the Hertzsprung-Russell (HR) diagram. Finally, we identify th…
Massive protostars grow and evolve under the effect of rapid accretion of circumstellar gas and dust, falling at high rates ($\ge 10^{-4}$-$10^{-3}\, \rm M_{\odot}\, \rm yr^{-1}$). This mass infall has been shown, both numerically and observationally, to be episodically interspersed by accretion of dense gaseous clumps migrating through the circumstellar disc to the protostellar surface, causing sudden accretion and luminous bursts. Using numerical gravito-radiation-hydrodynamics and stellar evolution calculations, we demonstrate that, in addition to the known bloating of massive protostars, v…
We briefly overview our newly developed radiation transport module for MHD simulations and two actual applications. The method combines the advantage of the speed of the Flux-Limited Diffusion approximation and the high accuracy obtained in ray-tracing methods.
The first stars in the universe ionized the ambient primordial gas through various feedback processes. "Second-generation" primordial stars potentially form from this disturbed gas after its recombination. In this Letter, we study the late formation stage of such second-generation stars, where a large amount of gas accretes onto the protostar and the final stellar mass is determined when the accretion terminates. We directly compute the complex interplay between the accretion flow and stellar ultraviolet (UV) radiation, performing radiation-hydrodynamic simulations coupled with stellar evoluti…
We carry out a numerical experiment of ejecting winds in a massive colliding wind binary system, and quantifying the accretion onto the secondary star under different primary mass loss rates. We set a binary system comprising a Luminous Blue Variable (LBV) as the primary and a Wolf-Rayet (WR) star as the secondary, and vary the mass loss rate of the LBV to obtain different values of wind momentum ratio $η$. Our simulations include two sets of cases: one where the stars are stationary, and one that includes the orbital motion. As $η$ decreases the colliding wind structure moves closer to the se…
We study the flow structure in the jittering-jets explosion model of core-collapse supernovae (CCSNe) using 2.5D hydrodynamical simulations and find that some basic requirements for explosion are met by the flow. In the jittering-jets model jets are launched by intermittent accretion disk around the newly born neutron star and in stochastic directions. They deposit their kinetic energy inside the collapsing core and induce explosion by ejecting the outer core. The accretion and launching of jets is operated by a feedback mechanism: when the jets manage to eject the core, the accretion stops. W…
