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Register a new userThe Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation
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Stellar feedback in the form of radiation pressure and magnetically-driven collimated outflows may limit the maximum mass that a star can achieve and affect the star-formation efficiency of massive pre-stellar cores. Here we present a series of 3D adaptive mesh refinement radiation-magnetohydrodynamic simulations of the collapse of initially turbulent, massive pre-stellar cores. Our simulations include radiative feedback from both the direct stellar and dust-reprocessed radiation fields, and collimated outflow feedback from the accreting stars. We find that protostellar outflows punches holes in the dusty circumstellar gas along the stars polar directions, thereby increasing the size of optically thin regions through which radiation can escape. Precession of the outflows as the stars spin axis changes due to the turbulent accretion flow further broadens the outflow, and causes more material to be entrained. Additionally, the presence of magnetic fields in the entrained material leads to broader entrained outflows that escape the core. We compare the injected and entrained outflow properties and find that the entrained outflow mass is a factor of $sim$3 larger than the injected mass and the momentum and energy contained in the entrained material are $sim$25% and $sim$5% of the injected momentum and energy, respectively. As a result, we find that, when one includes both outflows and radiation pressure, the former are a much more effective and important feedback mechanism, even for massive stars with significant radiative outputs.
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Truncated abstract: The formation of a protostellar disc is a natural outcome during the star formation process. As gas in a molecular cloud core collapses under self-gravity, the angular momentum of the gas will slow its collapse on small scales and promote the formation of a protostellar disc. Although the angular momenta of dense star-forming cores remain to be fully characterized observationally, existing data indicates that typical cores have enough angular momenta to form relatively large, rotationally supported discs. However, molecular clouds are observed to be permeated by magnetic fields, which can strongly affect the evolution of angular momentum through magnetic braking. Indeed, in the ideal MHD limit, magnetic braking has been shown to be so efficient as to remove essentially all of the angular momentum of the material close to the forming star such that disc formation is suppressed. This is known as the magnetic braking catastrophe. The catastrophe must be averted in order for the all-important rotationally supported discs to appear, but when and how this happens remains debated. We review the resolutions proposed to date, with emphasis on misalignment, turbulence and especially non-ideal effects. The dissipative non-ideal effects weaken the magnetic field, and the dispersive term redirects it to promote or hinder disc formation. When self-consistently applying non-ideal processes, rotationally supported discs of at least tens of au form, thus preventing the magnetic braking catastrophe. The non-ideal processes are sensitive to the magnetic field strength, cosmic ray ionization rate, and gas and dust grain properties, thus a complete understanding of the host molecular cloud is required. Therefore, the properties of the host molecular cloud -- and especially its magnetic field -- cannot be ignored when numerically modelling the formation and evolution of protostellar discs.
We use three dimensional radiation magneto-hydrodynamic simulations to study the effects of magnetic fields on the energy transport and structure of radiation pressure dominated main sequence massive star envelopes at the region of the iron opacity peak. We focus on the regime where the local thermal timescale is shorter than the dynamical timescale, corresponding to inefficient convective energy transport. We begin with initially weak magnetic fields relative to the thermal pressure, from 100-1000G in differing geometries. The unstable density inversion amplifies the magnetic field, increasing the magnetic energy density to values close to equipartition with the turbulent kinetic energy density. By providing pressure support, the magnetic fields presence significantly increases the density fluctuations in the turbulent envelope, thereby enhancing the radiative energy transport by allowing photons to diffuse out through low density regions. Magnetic buoyancy brings small scale magnetic fields to the photosphere and increases the vertical energy transport with the energy advection velocity proportional to the Alfven velocity, although in all cases we study photon diffusion still dominates the energy transport. The increased radiative and advective energy transport causes the stellar envelope to shrink by several scale heights. We also find larger turbulent velocity fluctuations compared to the purely hydrodynamic case, reaching $approx$ 100 km/s at the stellar photosphere. The photosphere also shows vertical oscillations with similar averaged velocities and periods of a few hours. The increased turbulent velocity and oscillations will have strong impacts on the line broadening and periodic signals in massive stars.
Stars with masses of >~ 20 solar masses have short Kelvin times that enable them to reach the main sequence while still accreting from their natal clouds. The resulting nuclear burning produces a huge luminosity and a correspondingly large radiation pressure force on dust grains in the accreting gas. This effect may limit the upper mass of stars that can form by accretion. Indeed, simulations and analytic calculations to date have been unable to resolve the mystery of how stars of 50 solar masses and up form. We present two new ideas to solve the radiation pressure problem. First, we use three-dimensional radiation hydrodynamic adaptive mesh refinement simulations to study the collapse of massive cores. We find that in three dimensions a configuration in which radiation holds up an infalling envelope is Rayleigh-Taylor unstable, leading radiation driven bubbles to collapse and accretion to continue. We also present Monte Carlo radiative transfer calculations showing that the cavities created by protostellar winds provides a valve that allow radiation to escape the accreting envelope, further reducing the ability of radiation pressure to inhibit accretion.
We present results of 1.3 mm dust polarization observations toward 16 nearby, low-mass protostars, mapped with ~2.5 resolution at CARMA. The results show that magnetic fields in protostellar cores on scales of ~1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they formed.
The Galactic transient V1309 Sco was the result of a merger in a low-mass star system, while V838 Mon was thought to be a similar merger event from a more massive B-type progenitor. In this paper we study an optical/IR transient discovered in the nearby galaxy NGC4490, which appeared similar to these merger events (unobscured progenitor, irregular multi-peaked light curve, increasingly red color, similar optical spectrum, IR excess at late times), but which had a higher peak luminosity and longer duration in outburst. NGC4490-OT has less in common with the class of SN~2008S-like transients. A progenitor detected in pre-eruption HST images, combined with upper limits in the IR, requires a luminous and blue progenitor that has faded in late-time HST images. The same source was detected by Spitzer and ground-based data as a luminous IR transient, indicating a transition to a self-obscured state qualitatively similar to the evolution seen in other stellar mergers and in LBVs. The post-outburst dust-obscured source is too luminous and too warm at late times to be explained with an IR echo, suggesting that the object survived the event. The luminosity of the enshrouded IR source is similar to that of the progenitor. Compared to proposed merger events, the more massive progenitor of NGC4490-OT seems to extend a correlation between stellar mass and peak luminosity, and may suggest that both of these correlate with duration. We show that spectra of NGC4490-OT and V838 Mon also resemble light-echo spectra of eta Car, prompting us to speculate that eta Car may be an extreme extension of this phenomenon.
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