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Register a new userOutflows from Massive Stars
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Authors
John Bally
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The properties of outflows powered by massive stars are reviewed with an emphasis on the nearest examples, Orion and Cepheus-A. The Orion OMC1 outflow may have been powered by the dynamical decay of a non-hierarchical massive star system that resulted in the ejection of the BN object, and poossibly radio soruces I and n from the OMC1 core. This interaction must have produced at least one massive binary whose gravitational binding energy ejected the stars and powered the outflow. A specific model for the coupling of this energy to the gas is proposed. The radio source HW2 in the Cep-A region appears to drive a pulsed, precessing jet that may be powered by a moderate-mass companion in an eccentric and inclined orbit. This configuration may be the result of binary formation by capture. These outflows demonstrate that dynamical interactions among massive stars are an important feature of massive star formation.
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We consider the effects of an outflow on radiation escaping from the infalling envelope around a massive protostar. Using numerical radiative transfer calculations, we show that outflows with properties comparable to those observed around massive stars lead to significant anisotropy in the stellar radiation field, which greatly reduces the radiation pressure experienced by gas in the infalling envelope. This means that radiation pressure is a much less significant barrier to massive star formation than has previously been thought.
A set of 66 3D hydrodynamical simulations explores how galactic stellar mass affects three-phase, starburst-driven outflows. Simulated velocities are compared to two basic analytic models: with (Johnson & Axford 1971) and without (Chevalier & Clegg 1985) a gravitational potential. For stellar mass $<10^{10}$ solar masses, simulated velocities match those of both analytical models and are unaffected by the potential; above they reduce significantly as expected from the analytic model with gravity. Gravity also affects total outflow mass and each of the three phases differently. Outflow mass in the hot, warm, and cold phases each scale with stellar mass as $log M_*=$ -0.25, -0.97, and -1.70, respectively. Thus, the commonly used Chevalier & Clegg analytic model should be modified to include gravity when applied to higher mass galaxies. In particular, using M82 as the canonical galaxy to interpret hydrodynamical simulations of starburst-driven outflows from higher mass galaxies will underestimate the retarding effect of gravity. Using the analytic model of Johnson & Axford with realistic thermalization efficiency and mass loading I find that only galaxy masses that are less than $sim10^{11.5}$ solar masses can outflow.
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 study of feedback from massive star.
It is typically assumed that radiation pressure driven winds are accelerated to an asymptotic velocity of V ~ v_esc, where v_esc is the escape velocity from the central source. We note that this is not the case for dusty shells and clouds. Instead, if the shell or cloud is initially optically-thick to the UV emission from the source of luminosity L, then there is a significant boost in V that reflects the integral of the momentum absorbed as it is accelerated. For shells reaching a generalized Eddington limit, we show that V ~ (4R_UV L/M_sh c)^1/2, in both point-mass and isothermal-sphere potentials, where R_UV is the radius where the shell becomes optically-thin to UV photons, and M_sh is the mass of the shell. The asymptotic velocity significantly exceeds v_esc for typical parameters, and can explain the ~1000-2000km/s outflows observed from rapidly star-forming galaxies and active galactic nuclei if the surrounding halo has low gas density. Similarly fast outflows from massive stars can be accelerated on few - 10^3 yr timescales. These results carry over to clouds that subtend only a small fraction of the solid angle from the source of radiation and that expand as a consequence of their internal sound speed. We further consider the dynamics of shells that sweep up a dense circumstellar or circumgalactic medium. We calculate the momentum ratio Mdot v/(L/c) in the shell limit and show that it can only significantly exceed ~2 if the effective optical depth of the shell to re-radiated FIR photons is much larger than unity. We discuss simple prescriptions for the properties of galactic outflows for use in large-scale cosmological simulations. We also briefly discuss applications to the dusty ejection episodes of massive stars, the disruption of giant molecular clouds, and AGN.
We have undertaken the largest survey for outflows within the Galactic Plane using simultaneously observed 13CO and C18O data. 325 out of a total of 919 ATLASGAL clumps have data suitable to identify outflows, and 225 (69+-3%) of them show high velocity outflows. The clumps with detected outflows show significantly higher clump masses (M_{clump}), bolometric luminosities (L_{bol}), luminosity-to-mass ratios (L_{bol}/M_{clump}) and peak H_2 column densities (N_{H_2}) compared to those without outflows. Outflow activity has been detected within the youngest quiescent clump (i.e.,70um weak) in this sample and we find that the outflow detection rate increases with M_{clump},L_{bol},L_{bol}/M_{clump} and N_{H_2},approaching 90% in some cases(uchii regions=93+-3%;masers=86+-4%;hchii regions=100%). This high detection rate suggests that outflows are ubiquitous phenomena of massive star formation. The mean outflow mass entrainment rate implies a mean accretion rate of ~10^{-4}M_odot,yr^{-1}, in full agreement with the accretion rate predicted by theoretical models of massive star formation. Outflow properties are tightly correlated with M_{clump},L_{bol} and L_{bol}/M_{clump},and show the strongest relation with the bolometric clump luminosity. This suggests that outflows might be driven by the most massive and luminous source within the clump. The correlations are similar for both low-mass and high-mass outflows over 7 orders of magnitude, indicating that they may share a similar outflow mechanism. Outflow energy is comparable to the turbulent energy within the clump, however, we find no evidence that outflows increase the level of clump turbulence as the clumps evolve. This implies that the origin of turbulence within clumps is fixed before the onset of star formation.
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