Context.
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 R
AMSES
with R
AMSES
-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
≃ 0.7
τ
ff
, 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.
Context.
Massive star formation remains one of the most challenging problems in astrophysics, as illustrated by the fundamental issues of the radiative pressure barrier and the initial fragmentation. ...The wide variety of physical processes involved, in particular the protostellar radiative feedback, increase the complexity of massive star formation in comparison with its low-mass counterpart.
Aims.
We aim to study the details of mass accretion and ejection in the vicinity of massive star forming cores using high-resolution (5 au) three-dimensional numerical simulations. We investigated the mechanisms at the origin of outflows (radiative force versus magnetic acceleration). We characterised the properties of the disc forming around massive protostars depending on the physics included: hydrodynamics, magnetic fields, and ambipolar diffusion.
Methods.
We used state-of-the-art three-dimensional adaptive-mesh-refinement models of massive dense core collapse, which integrate the equations of (resistive) grey radiation magnetohydrodynamics, and include sink particle evolution. For the first time, we include both protostellar radiative feedback via pre-main-sequence evolutionary tracks and magnetic ambipolar diffusion. To determine the role of magnetic fields and ambipolar diffusion play in the formation of outflows and discs, we studied three different cases: a purely hydrodynamical run, a magnetised simulation under the ideal approximation (perfect coupling), and a calculation with ambipolar diffusion (resistive case). In the most micro-physically complex model (resistive MHD), we also investigated the effect the initial amplitude of both magnetic field and solid body rotation have on the final properties of the massive protostellar system. We used simple criteria to identify the outflow and disc material and follow their evolution as the central star accretes mass up to 20
M
⊙
in most of our models. The radiative, magnetic, and hydrodynamical properties of the outflows and discs are quantitatively measured and cross-compared between models.
Results.
Massive stars form in all our models, together with outflows and discs. The outflow is completely different when magnetic fields are introduced, so magneto-centrifugal processes are the main driver of the outflow up to stellar masses of 20
M
⊙
. Then, the disc properties heavily depend on the physics included. In particular, the disc formed in the ideal and resistive runs show opposite properties in terms of plasma beta; that is, the ratio of thermal-to-magnetic pressures and of magnetic field topology. While the disc in the ideal case is dominated by the magnetic pressure and the toroidal magnetic fields, the one formed in the resistive runs is dominated by the thermal pressure and essentially has a vertical magnetic field in the inner regions (
R
< 100−200 au).
Conclusions.
We find that magnetic processes dominate the early evolution of massive protostellar systems (
M
⋆
< 20
M
⊙
) and shapes the accretion and ejection as well as the disc formation. Ambipolar diffusion is mainly at work at disc scales and regulates its properties. We predict magnetic field’s topology within the disc and outflows, as well as disc masses and radii to be compared with observations. Lastly, our finding for the outflow and disc properties are reminiscent of the low-mass star formation framework, suggesting that accretion and ejection in young massive and low-mass protostars are regulated by the same physical processes in the early stages.
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 that promotes multiple stellar system formation in order to investigate the influence of magnetic fields on the multiple system’s properties.
Results.
In the purely hydrodynamical models, we always obtain (at least) binary systems following the fragmentation of an axisymmetric density bump in a Toomre-unstable disk around the primary sink. This result sets the frame for further study of stellar multiplicity. When more than two stars are present in these early phases, their gravitational interaction triggers mergers until there are only two stars left. The following gas accretion increases their orbital separation, and hierarchical fragmentation occurs so that both stars host a comparable disk as well as a stellar system that then also forms a similar disk. Disk-related fragmenting structures are qualitatively resolved when the finest resolution is approximately 1/20 of the disk radius. 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 is initially elongated into a bar; it produces less fragments; disk formation and arm-arm collision are captured at comparatively higher resolution; and 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.
Context.
Recent observational progress has challenged the dust grain-alignment theories used to explain the polarized dust emission routinely observed in star-forming cores.
Aims.
In an effort to ...improve our understanding of the dust grain alignment mechanism(s), we have gathered a dozen ALMA maps of (sub)millimeter-wavelength polarized dust emission from Class 0 protostars and carried out a comprehensive statistical analysis of dust polarization quantities.
Methods.
We analyze the statistical properties of the polarization fraction
P
frac
and the dispersion of polarization position angles
S
. More specifically, we investigate the relationship between
S
and
P
frac
as well as the evolution of the product
S
×
P
frac
as a function of the column density of the gas in the protostellar envelopes. We compare the observed trends with those found in polarization observations of dust in the interstellar medium and in synthetic observations of non-ideal magneto-hydrodynamic (MHD) simulations of protostellar cores.
Results.
We find a significant
S
∝
P
frac
−0.79
correlation in the polarized dust emission from protostellar envelopes seen with ALMA; the power-law index significantly differs from the one observed by
Planck
in star-forming clouds. The product
S
×
P
frac
, which is sensitive to the dust grain alignment efficiency, is approximately constant across three orders of magnitude in envelope column density (from
N
H
2
= 10
22
cm
−2
to
N
H
2
= 10
25
cm
−2
), with a mean value of 0.36
−0.17
+0.10
. This suggests that the grain alignment mechanism producing the bulk of the polarized dust emission in star-forming cores may not systematically depend on the local conditions such as the local gas density. However, in the lowest-luminosity sources in our sample, we find a hint of less efficient dust grain alignment with increasing column density. Our observations and their comparison with synthetic observations of MHD models suggest that the total intensity versus the polarized dust are distributed at different intrinsic spatial scales, which can affect the statistics from the ALMA observations, for example, by producing artificially high
P
frac
. Finally, synthetic observations of MHD models implementing radiative alignment torques (RATs) show that the statistical estimator
S
×
P
frac
is sensitive to the strength of the radiation field in the core. Moreover, we find that the simulations with a uniform perfect alignment (PA) of dust grains yield, on average, much higher
S
×
P
frac
values than those implementing RATs; the ALMA values lie among those predicted by PA, and they are significantly higher than the ones obtained with RATs, especially at large column densities.
Conclusions.
Ultimately, our results suggest that dust alignment mechanism(s) are efficient at producing dust polarized emission in the various local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. Further studies will be needed to understand how more efficient grain alignment via, for example, different irradiation conditions, dust grain characteristics, or additional grain alignment mechanisms can reproduce the observations.
Context. While nobody would deny the presence of quasi-periodic oscillations in the power density spectrum of black hole binaries nor their importance in the understanding of the mechanisms powering ...the X-ray emissions, the possible impact on the time-averaged disk energy spectrum from the phenomenon responsible for quasi-periodic oscillations is largely ignored in models of sources emission. Aims. Here we investigate the potential impact of such a structure on the resultant energy spectrum. Methods. Using data from the well-documented outbursts of XTE J1550-564, we looked at possible hints that the presence of quasi-periodic oscillations actually impacts the energy spectrum emitted by the source. In particular, we look at the evolution of the relation between the inner disk radius and the inner disk temperature obtained from fits to the spectral data. We then test this further by developing a simple model to simulate the spectrum of a disk with a structure mimicking quasi-periodic oscillations that are increasing in strength simulated results to those obtained from real data. Results. We detect a similar departure in the inner radius – inner temperature curve coming from the standard fit of our simulated observations as is seen in XTE J1550-564 data. We interpret our results as evidence that the structure at the origin of the quasi-periodic oscillations impacts the energy spectrum. Conclusions. Furthermore, in states with significant disk emission the inaccuracy of the determination of the disk parameters increases with the strength of quasi-periodic oscillations, an increase that then renders the value given by the fit unreliable for strong quasi-periodic oscillations.
Context.
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
~ 10
5
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.
Context. 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.
Context.
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 R
AMSES
, 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.
Context. 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.
Context
. Most massive stars are located in multiple stellar systems. The modeling of disk fragmentation, a mechanism that may plausibly lead to stellar multiplicity, relies on parallel 3D simulation ...codes whose agreement remains to be evaluated.
Aims
. Cartesian adaptive-mesh refinement (AMR) and spherical codes have frequently been used in the past decade to study massive star formation. We aim to study how the details of collapse and disk fragmentation depend on these codes.
Methods
. Using the Cartesian AMR code
RAMSES
within its self-gravity radiation-hydrodynamical framework, we compared disk fragmentation in a centrally condensed protostellar system to the findings of earlier studies performed on a grid in spherical coordinates using
PLUTO
.
Results
. To perform the code comparison, two
RAMSES
runs were considered, effectively giving qualitatively distinct pictures. On the one hand, when allowing for unlimited sink particle creation with no initial sink, Toomre instability and subsequent gas fragmentation leads to a multiple stellar system whose multiplicity is affected by the grid when triggering fragmentation and via numerically assisted mergers. On the other hand, using a unique, central, fixed-sink particle, a centrally-condensed system forms that is similar to that reported by
PLUTO
. Hence, the
RAMSES-PLUTO
comparison was performed with the latter and an agreement between the two codes is found as to the first rotationally supported disk formation, the presence of an accretion shock onto it, and the first fragmentation phase. Gaseous fragments form. The properties of the fragments (i.e., number, mass, and temperature) are dictated by local thermodynamics and are in agreement between the two codes given that the system has entered a highly nonlinear phase. Over the simulations, the stellar accretion rate is made of accretion bursts and continuous accretion on the same order of magnitude. As a minor difference between both codes, the dynamics of the fragments causes the disk structure to be sub-Keplerian in
RAMSES
, whereas it is found to be Keplerian, thus reaching 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 use - rather than the potential grid effects.
Conclusions
. In a centrally condensed system, the agreement between
RAMSES
and
PLUTO
regarding many of the collapse properties and fragmentation process is good. In contrast, fragmentation occurring in the innermost region and given specific numerical choices (use of sink particles, grid, etc.) have a crucial impact when similar but smooth initial conditions are employed. These aspects prove more crucial than the choice of code, with regard to the system being multiple or centrally condensed.
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