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Register a new userAccretion outbursts in massive star formation
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D. M.-A. Meyer
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Using the HPC ressources of the state of Baden-Wurttemberg, 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.
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We report the discovery of a massive protostar M17 MIR embedded in a hot molecular core in M17. The multi-wavelength data of M17 MIR during 1993 to 2019 show significant mid-IR (MIR) variations, which can be split into three stages, the decreasing phase during 1993.03 to mid 2004, the quiescent phase during mid 2004 to mid 2010, and the re-brightening phase since mid 2010 untill now. The H2O maser emission variation toward M17 MIR, together with the MIR variation, indicate an enhanced disk accretion rate onto M17 MIR during the decreasing and re-brightening phase. According to the kinematics of H2O maser spots, accretion rate ~7x10^-4 Msun/yr is estimated in the initial stage of the re-brightening phase, and a higher rate ~2x10^-3 Msun/yr is obtained in later stage, given by the MIR flux increased by a factor of 3. Radiative transfer modeling of SEDs of M17~MIR in the 2005 (quiescent) and 2017 epoch (accretion outburst) constrains the basic stellar parameters of M17 MIR, which is an intermediate-mass protostar (M~5.4 Msun) with lower accretion rate ~1.1x10^-5 Msun in quiescent and two orders of magnitude higher rate ~1.7x10^-3 Msun/yr in outburst. The enhanced accretion rate during outburst induces the luminosity outburst $Delta Lapprox7600 $Lsun, and a larger stellar radius is required to produce accretion rate consistent with observations. The decreasing and re-brightening phase reflect two accretion bursts ($Delta tsim 9-20$ yr) with burst magnitudes of 2 mag, separated by a quiescent phase lasting $sim6$ yr. The fraction time in accretion ourbusrt is about 83% over 26 yr. M17 MIR is the youngest one among the six confirmed sources with accretion burst. The extreme youth of M17 MIR suggests that minor accretion bursts are frequent at the earliest stages of massive star formation.
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. Consequently, the process of accretion-induced luminous flares is also conceivable in the high-mass regime of star formation and we propose to regard this phenomenon as a general mechanism that can affect protostars regardless of their mass and/or the chemical properties of the parent environment in which they form. In addition to the commonness of accretion-driven outbursts in the star formation machinery, we conjecture that luminous flares from regions hosting forming high-mass star may be an observational implication of the fragmentation of their accretion disks.
It is now a widely held view that, in their formation and early evolution, stars build up mass in bursts. The burst mode of star formation scenario proposes that the stars grow in mass via episodic accretion of fragments migrating from their gravitationally-unstable circumstellar discs and it naturally explains the existence of observed pre-main-sequence bursts from high mass protostars. We present a parameter study of hydrodynamical models of massive young stellar objects (MYSOs) that explores the initial masses of the collapsing clouds (Mc = 60-200Mo) and ratio of rotational-to-gravitational energies (beta = 0:005-0:33). An increase in Mc and/or beta produces protostellar accretion discs that are more prone to develop gravitational instability and to experience bursts. We find that all MYSOs have bursts even if their pre-stellar core is such that beta <= 0.01. Within our assumptions, the lack of stable discs is therefore a major difference between low- and high-mass star formation mechanisms. All our disc masses and disk-to-star mass ratios Md=M* > 1 scale as a power-law with the stellar mass. Our results confirm that massive protostars accrete about 40-60% of their mass in the burst mode. The distribution of time periods between two consecutive bursts is bimodal: there is a short duration (~ 1-10 yr) peak corresponding to the short, faintest bursts and a long duration peak (at ~ 10^3-10^4 yr) corresponding to the long, FU-Orionis-type bursts appearing in later disc evolution, i.e., around 30 kyr after disc formation. We discuss this bimodality in the context of the structure of massive protostellar jets as potential signatures of accretion burst history.
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 exceeds a critical radius. Trapped Hii regions in the torus plane allow accretion to progress, while expanding Hii regions disrupt the accretion torus preventing the central star from aggregating more mass, thereby setting the stars final mass. We obtain constraints for the luminosities and torus densities that lead to both scenarios.
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.
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