The role of magnetic fields in the formation of multiple massive stars

International audience(Abridged) Context. Most massive stars are located in multiple stellar systems. Magnetic fields are believed to be essential in the accretion and ejection processes around single massive protostars. Aims. Our aim is to unveil the influence of magnetic fields in the formation of multiple massive stars, in particular on the fragmentation modes and properties of the multiple protostellar system. Methods. Using RAMSES, we follow the collapse of a massive pre-stellar core with (non-ideal) radiation-(magneto-)hydrodynamics. We choose a setup which promotes multiple stellar system formation. Results. In the purely hydrodynamical models, we always obtain (at least) binary systems. When more than two stars are present, their gravitational interaction triggers mergers until there are two stars left. The following gas accretion increases their orbital separation and hierarchical fragmentation occurs so that both stars host a comparable disk and stellar system which then form similar disks as well. We identify several modes of fragmentation: Toomre-unstable disk fragmentation, arm-arm collision and arm-filament collision. Disks grow in size until they fragment and become truncated as the newly-formed companion gains mass. When including magnetic fields, the picture evolves: the primary disk produces less fragments, arm-filament collision is absent. Magnetic fields reduce the initial orbital separation but do not affect its further evolution, which is mainly driven by gas accretion. With magnetic fields, the growth of individual disks is regulated even in the absence of fragmentation or truncation. Conclusions. Hierarchical fragmentation is seen in unmagnetized and magnetized models. Magnetic fields, including non-ideal effects, are important because they remove certain fragmentation modes and limit the growth of disks, which is otherwise only limited through fragmentation

Collapse of turbulent massive cores with ambipolar diffusion and hybrid radiative transfer: II. Outflows

International audienceContext. Most massive protostars exhibit bipolar outflows. Nonetheless, there is no consensus regarding the mechanism at the origin of these outflows, nor on the cause of the less-frequently observed monopolar outflows.Aims. We aim to identify the origin of early massive protostellar outflows, focusing on the combined effects of radiative transfer and magnetic fields in a turbulent medium.Methods. We use four state-of-the-art radiation-magnetohydrodynamical simulations following the collapse of massive 100 M⊙ pre-stellar cores with the RAMSES code. Turbulence is taken into account via initial velocity dispersion. We use a hybrid radiative transfer method and include ambipolar diffusion.Results. Turbulence delays the launching of outflows, which appear to be mainly driven by magnetohydrodynamical processes. We study both the magnetic tower flow and the magneto-centrifugal acceleration as possible origins. Both contribute to the acceleration and the former operates on larger volumes than the latter. Our finest resolution, 5 AU, does not allow us to get converged results on magneto-centrifugally accelerated outflows. Radiative acceleration takes place as well, dominates in the star vicinity, enlarges the outflow extent, and has no negative impact on the launching of magnetic outflows (up to M ~17 M⊙, L ~ 105 L⊙). We observe mass outflow rates of 10−5−10−4 M⊙ yr−1 and momentum rates of the order ~10−4 M⊙ km s−1 yr−1. The associated opening angles (20−30deg when magnetic fields dominate) are in a range between observed values for wide-angle outflows and collimated outflows. If confirmed with a finer numerical resolution at the outflow interface, this suggests additional (de-)collimating effects. Outflows are launched nearly perpendicular to the disk and are misaligned with the initial core-scale magnetic fields, in agreement with several observational studies. In the most turbulent run, the outflow is monopolar.Conclusions. Magnetic processes are dominant over radiative ones in the acceleration of massive protostellar outflows of up to ~17 M⊙. Turbulence perturbs the outflow launching and is a possible explanation for monopolar outflows

The role of magnetic fields in the formation of multiple massive stars

International audience(Abridged) Context. Most massive stars are located in multiple stellar systems. Magnetic fields are believed to be essential in the accretion and ejection processes around single massive protostars. Aims. Our aim is to unveil the influence of magnetic fields in the formation of multiple massive stars, in particular on the fragmentation modes and properties of the multiple protostellar system. Methods. Using RAMSES, we follow the collapse of a massive pre-stellar core with (non-ideal) radiation-(magneto-)hydrodynamics. We choose a setup which promotes multiple stellar system formation. Results. In the purely hydrodynamical models, we always obtain (at least) binary systems. When more than two stars are present, their gravitational interaction triggers mergers until there are two stars left. The following gas accretion increases their orbital separation and hierarchical fragmentation occurs so that both stars host a comparable disk and stellar system which then form similar disks as well. We identify several modes of fragmentation: Toomre-unstable disk fragmentation, arm-arm collision and arm-filament collision. Disks grow in size until they fragment and become truncated as the newly-formed companion gains mass. When including magnetic fields, the picture evolves: the primary disk produces less fragments, arm-filament collision is absent. Magnetic fields reduce the initial orbital separation but do not affect its further evolution, which is mainly driven by gas accretion. With magnetic fields, the growth of individual disks is regulated even in the absence of fragmentation or truncation. Conclusions. Hierarchical fragmentation is seen in unmagnetized and magnetized models. Magnetic fields, including non-ideal effects, are important because they remove certain fragmentation modes and limit the growth of disks, which is otherwise only limited through fragmentation

The role of magnetic fields in the formation of multiple massive stars

International audience(Abridged) Context. Most massive stars are located in multiple stellar systems. Magnetic fields are believed to be essential in the accretion and ejection processes around single massive protostars. Aims. Our aim is to unveil the influence of magnetic fields in the formation of multiple massive stars, in particular on the fragmentation modes and properties of the multiple protostellar system. Methods. Using RAMSES, we follow the collapse of a massive pre-stellar core with (non-ideal) radiation-(magneto-)hydrodynamics. We choose a setup which promotes multiple stellar system formation. Results. In the purely hydrodynamical models, we always obtain (at least) binary systems. When more than two stars are present, their gravitational interaction triggers mergers until there are two stars left. The following gas accretion increases their orbital separation and hierarchical fragmentation occurs so that both stars host a comparable disk and stellar system which then form similar disks as well. We identify several modes of fragmentation: Toomre-unstable disk fragmentation, arm-arm collision and arm-filament collision. Disks grow in size until they fragment and become truncated as the newly-formed companion gains mass. When including magnetic fields, the picture evolves: the primary disk produces less fragments, arm-filament collision is absent. Magnetic fields reduce the initial orbital separation but do not affect its further evolution, which is mainly driven by gas accretion. With magnetic fields, the growth of individual disks is regulated even in the absence of fragmentation or truncation. Conclusions. Hierarchical fragmentation is seen in unmagnetized and magnetized models. Magnetic fields, including non-ideal effects, are important because they remove certain fragmentation modes and limit the growth of disks, which is otherwise only limited through fragmentation

The role of magnetic fields in the formation of multiple massive stars

International audience(Abridged) Context. Most massive stars are located in multiple stellar systems. Magnetic fields are believed to be essential in the accretion and ejection processes around single massive protostars. Aims. Our aim is to unveil the influence of magnetic fields in the formation of multiple massive stars, in particular on the fragmentation modes and properties of the multiple protostellar system. Methods. Using RAMSES, we follow the collapse of a massive pre-stellar core with (non-ideal) radiation-(magneto-)hydrodynamics. We choose a setup which promotes multiple stellar system formation. Results. In the purely hydrodynamical models, we always obtain (at least) binary systems. When more than two stars are present, their gravitational interaction triggers mergers until there are two stars left. The following gas accretion increases their orbital separation and hierarchical fragmentation occurs so that both stars host a comparable disk and stellar system which then form similar disks as well. We identify several modes of fragmentation: Toomre-unstable disk fragmentation, arm-arm collision and arm-filament collision. Disks grow in size until they fragment and become truncated as the newly-formed companion gains mass. When including magnetic fields, the picture evolves: the primary disk produces less fragments, arm-filament collision is absent. Magnetic fields reduce the initial orbital separation but do not affect its further evolution, which is mainly driven by gas accretion. With magnetic fields, the growth of individual disks is regulated even in the absence of fragmentation or truncation. Conclusions. Hierarchical fragmentation is seen in unmagnetized and magnetized models. Magnetic fields, including non-ideal effects, are important because they remove certain fragmentation modes and limit the growth of disks, which is otherwise only limited through fragmentation

A new hybrid radiative transfer method for massive star formation

International audienceContext. Frequency-dependent and hybrid approaches for the treatment of stellar irradiation are of primary importance in numerical simulations of massive star formation.Aims. We seek to compare outflow and accretion mechanisms in star formation simulations. We investigate the accuracy of a hybrid radiative transfer method using the gray M1 closure relation for proto-stellar irradiation and gray flux-limited diffusion (FLD) for photons emitted everywhere else.Methods. We have coupled the FLD module of the adaptive-mesh refinement code RAMSES with RAMSES-RT, which is based on the M1 closure relation and the reduced speed-of-light-approximation. Our hybrid (M1+FLD) method takes an average opacity at the stellar temperature for the M1 module, instead of the local environmental radiation field. Due to their construction, the opacities are consistent with the photon origin. We have tested this approach in radiative transfer tests of disks irradiated by a star for three levels of optical thickness and compared the temperature structure with the radiative transfer codes RADMC-3D and MCFOST. We applied it to a radiation-hydrodynamical simulation of massive star formation.Results. Our tests validate our hybrid approach for determining the temperature structure of an irradiated disk in the optically-thin (2% maximal error) and moderately optically-thick (error smaller than 25%) regimes. The most optically-thick test shows the limitation of our hybrid approach with a maximal error of 65% in the disk mid-plane against 94% with the FLD method. The optically-thick setups highlight the ability of the hybrid method to partially capture the self-shielding in the disk while the FLD alone cannot. The radiative acceleration is ≈100 times greater with the hybrid method than with the FLD. The hybrid method consistently leads to about + 50% more extended and wider-angle radiative outflows in the massive star formation simulation. We obtain a 17.6 M⊙ star at t ≃ 0.7τff, while the accretion phase is still ongoing, with a mean accretion rate of ≃7 × 10−4 M⊙ yr−1. Finally, despite the use of refinement to resolve the radiative cavities, no Rayleigh–Taylor instability appears in our simulations, and we justify their absence by physical arguments based on the entropy gradient

Collapse of turbulent massive cores with ambipolar diffusion and hybrid radiative transfer: I. Accretion and multiplicity

International audienceContext. Massive stars form in magnetized and turbulent environments and are often located in stellar clusters. The accretion and outflows mechanisms associated with forming massive stars and the origin of the stellar multiplicity of their system are poorly understood.Aims. We study the effect of magnetic fields and turbulence on the accretion mechanism of massive protostars and their multiplicity. We also focus on disk formation as a prerequisite for outflow launching.Methods. We present a series of four radiation-magnetohydrodynamical simulations of the collapse of a massive magnetized, turbulent core of 100 M⊙ with the adaptive-mesh-refinement code RAMSES, including a hybrid radiative transfer method for stellar irradiation and ambipolar diffusion. We varied the Mach and Alfvénic Mach numbers to probe sub- and super-Alfvénic turbulence and sub- and supersonic turbulence regimes.Results. Sub-Alfvénic turbulence leads to single stellar systems, and super-Alfvénic turbulence leads to binary formation from disk fragmentation following the collision of spiral arms, with mass ratios of 1.1–1.6 and a separation of several hundred AU that increases with initial turbulent support and with time. In these runs, infalling gas reaches the individual disks through a transient circumbinary structure. Magnetically regulated, thermally dominated (plasma beta β > 1) Keplerian disks form in all runs, with sizes 100–200 AU and masses 1–8 M⊙. The disks around primary and secondary sink particles have similar properties. We obtain mass accretion rates of ~10−4 M⊙ yr−1 onto the protostars and observe higher accretion rates onto the secondary stars than onto their primary star companion. The primary disk orientation is found to be set by the initial angular momentum carried by turbulence rather than by magnetic fields. Even without turbulence, axisymmetry and north–south symmetry with respect to the disk plane are broken by the interchange instability and thermally dominated streamers, respectively.Conclusions. Small (≲300 AU) massive protostellar disks such as those that are frequently observed today can so far only be reproduced in the presence of (moderate) magnetic fields with ambipolar diffusion, even in a turbulent medium. The interplay between magnetic fields and turbulence sets the multiplicity of stellar clusters. A plasma beta β > 1 is a good indicator for distinguishing streamers and individual disks from their surroundings

Disk fragmentation around a massive protostar: a comparison of two three-dimensional codes

International audience(Abridged) Most massive stars are located in multiple systems. The modeling of disk fragmentation, a possible mechanism leading to stellar multiplicity, relies on parallel 3D simulation codes whose agreement remains to be evaluated. Using the Cartesian AMR code RAMSES, we compare disk fragmentation in a centrally-condensed protostellar system to the study of Oliva & Kuiper (2020) performed on a grid in spherical coordinates using PLUTO. Two RAMSES runs are considered and give qualitatively distinct pictures. When allowing for unlimited sink particle creation, gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and by numerically-assisted mergers. On the other hand, using a unique, central, fixed sink particle, a centrally-condensed system forms, similar to that reported in PLUTO. The RAMSES-PLUTO comparison is performed with the latter: agreement between the two codes is found regarding the first rotationally-supported disk formation, the presence of an accretion shock onto it, the first fragmentation phase. Gaseous fragments form and their properties are in agreement between the two codes. As a minor difference, fragments dynamics causes the disk structure to be sub-Keplerian in RAMSES whereas it is found to be Keplerian and reaches quiescence in PLUTO. We attribute this discrepancy to the central star being twice less massive in RAMSES because of the different stellar accretion subgrid models. In a centrally-condensed system, the agreement between RAMSES and PLUTO regarding many of the collapse properties and fragmentation process is good. Fragmentation occurring in the innermost region and numerical choices (use of sink particles, grid) have a crucial impact when similar but smooth initial conditions are employed - more crucial than the code's choice - on the system's outcome, multiple or centrally-condensed

Disk fragmentation around a massive protostar: a comparison of two three-dimensional codes

International audience(Abridged) Most massive stars are located in multiple systems. The modeling of disk fragmentation, a possible mechanism leading to stellar multiplicity, relies on parallel 3D simulation codes whose agreement remains to be evaluated. Using the Cartesian AMR code RAMSES, we compare disk fragmentation in a centrally-condensed protostellar system to the study of Oliva & Kuiper (2020) performed on a grid in spherical coordinates using PLUTO. Two RAMSES runs are considered and give qualitatively distinct pictures. When allowing for unlimited sink particle creation, gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and by numerically-assisted mergers. On the other hand, using a unique, central, fixed sink particle, a centrally-condensed system forms, similar to that reported in PLUTO. The RAMSES-PLUTO comparison is performed with the latter: agreement between the two codes is found regarding the first rotationally-supported disk formation, the presence of an accretion shock onto it, the first fragmentation phase. Gaseous fragments form and their properties are in agreement between the two codes. As a minor difference, fragments dynamics causes the disk structure to be sub-Keplerian in RAMSES whereas it is found to be Keplerian and reaches quiescence in PLUTO. We attribute this discrepancy to the central star being twice less massive in RAMSES because of the different stellar accretion subgrid models. In a centrally-condensed system, the agreement between RAMSES and PLUTO regarding many of the collapse properties and fragmentation process is good. Fragmentation occurring in the innermost region and numerical choices (use of sink particles, grid) have a crucial impact when similar but smooth initial conditions are employed - more crucial than the code's choice - on the system's outcome, multiple or centrally-condensed

Disk fragmentation around a massive protostar: a comparison of two three-dimensional codes

International audience(Abridged) Most massive stars are located in multiple systems. The modeling of disk fragmentation, a possible mechanism leading to stellar multiplicity, relies on parallel 3D simulation codes whose agreement remains to be evaluated. Using the Cartesian AMR code RAMSES, we compare disk fragmentation in a centrally-condensed protostellar system to the study of Oliva & Kuiper (2020) performed on a grid in spherical coordinates using PLUTO. Two RAMSES runs are considered and give qualitatively distinct pictures. When allowing for unlimited sink particle creation, gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and by numerically-assisted mergers. On the other hand, using a unique, central, fixed sink particle, a centrally-condensed system forms, similar to that reported in PLUTO. The RAMSES-PLUTO comparison is performed with the latter: agreement between the two codes is found regarding the first rotationally-supported disk formation, the presence of an accretion shock onto it, the first fragmentation phase. Gaseous fragments form and their properties are in agreement between the two codes. As a minor difference, fragments dynamics causes the disk structure to be sub-Keplerian in RAMSES whereas it is found to be Keplerian and reaches quiescence in PLUTO. We attribute this discrepancy to the central star being twice less massive in RAMSES because of the different stellar accretion subgrid models. In a centrally-condensed system, the agreement between RAMSES and PLUTO regarding many of the collapse properties and fragmentation process is good. Fragmentation occurring in the innermost region and numerical choices (use of sink particles, grid) have a crucial impact when similar but smooth initial conditions are employed - more crucial than the code's choice - on the system's outcome, multiple or centrally-condensed