A Massive Star is Born: How Feedback from Stellar Winds, Radiation Pressure, and Collimated Outflows Limits Accretion onto Massive Stars

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 ($v_{\rm w} \gtrsim 10^3$ km/s) 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 series of 3D radiation-magnetohydrodynamic simulations of the collapse of massive prestellar cores and include radiative feedback from the direct stellar and dust-reprocessed radiation fields, collimated outflows, and, for the first time, isotropic stellar winds to model how these processes affect the formation of massive (proto)stars. We find that winds are initially launched when the massive protostar is still accreting and the wind properties evolve as the star contracts to the main sequence. Wind feedback drives asymmetric adiabatic wind bubbles that have a bipolar morphology because the dense circumstellar material pinches the expansion of the hot shock-heated gas, which preferentially expands along low-density channels. We term this the "wind tunnel effect." For unmagnetized cores, we find that wind feedback eventually quenches accretion onto massive stars. For magnetized cores, we find that wind feedback is less efficient at halting the accretion flow initially because magnetic tension delays the growth of the wind-driven bubbles. Once winds become strong enough, wind feedback launches adiabatic wind bubbles that eventually reduce accretion. Additionally, we discuss the implications of observing adiabatic wind bubbles with Chandra while the massive protostars are still highly embedded.Comment: 24 pages, 17 figures, 2 tables. Submitted to Ap

An unstable truth: How massive stars get their mass

The pressure exerted by massive stars' radiation fields is an important mechanism regulating their formation. Detailed simulation of massive star formation therefore requires an accurate treatment of radiation. However, all published simulations have either used a diffusion approximation of limited validity; have only been able to simulate a single star fixed in space, thereby suppressing potentially important instabilities; or did not provide adequate resolution at locations where instabilities may develop. To remedy this, we have developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. We use our new tool to perform 3D radiation-hydrodynamic simulations of the collapse of massive pre-stellar cores with laminar and turbulent initial conditions and properly resolve regions where we expect instabilities to grow. We find that mass is channelled to the stellar system via gravitational and Rayleigh-Taylor (RT) instabilities, in agreement with previous results using stars capable of moving, but in disagreement with methods where the star is held fixed or with simulations that do not adequately resolve the development of RT instabilities. For laminar initial conditions, proper treatment of the direct radiation field produces later onset of instability, but does not suppress it entirely provided the edges of radiation-dominated bubbles are adequately resolved. Instabilities arise immediately for turbulent pre-stellar cores because the initial turbulence seeds the instabilities. Our results suggest that RT features should be present around accreting massive stars throughout their formatio

The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation

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 prestellar cores. Here we present a series of 3D adaptive mesh refinement radiation-magnetohydrodynamic simulations of the collapse of initially turbulent, massive prestellar 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 punch holes in the dusty circumstellar gas along the star's polar directions, thereby increasing the size of optically thin regions through which radiation can escape. Precession of the outflows as the star's 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 ~3 larger than the injected mass and the momentum and energy contained in the entrained material are ~25% and ~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.A.L.R. acknowledges support from NASA through Einstein Postdoctoral Fellowship grant No. PF7-180166 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8- 0306

What Sets the Star Formation Rate of Molecular Clouds? The Density Distribution as a Fingerprint of Compression and Expansion Rates

We use a suite of 3D simulations of star-forming molecular clouds, with and without stellar feedback, magnetic fields, and driven turbulence, to study the compression and expansion rates of the gas as functions of density. We show that, around the mean density, supersonic turbulence promotes rough equilibrium between the amounts of compressing and expanding gas, consistent with continuous gas cycling between high- and low-density states. We find that the inclusion of protostellar jets produces rapidly expanding and compressing low-density gas. We find that the gas mass flux peaks at the transition between the lognormal and power-law forms of the density probability distribution function (PDF). This is consistent with the transition density tracking the post-shock density, which promotes an enhancement of mass at this density (i.e., shock compression and filament formation). At high densities, the gas dynamics are dominated by self-gravity: the compression rate in all of our runs matches the rate of the run with only gravity, suggesting that processes other than self-gravity have little effect at these densities. The net gas mass flux becomes constant at a density below the sink formation threshold, where it equals the star formation rate. The density at which the net gas mass flux equals the star formation rate is one order of magnitude lower than our sink threshold density, corresponds to the formation of the second power-law tail in the density PDF, and sets the overall star formation rates of these simulations