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Register a new userGravity and rotation drag the magnetic field in high-mass star formation
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The formation of hot stars out of the cold interstellar medium lies at the heart of astrophysical research. Understanding the importance of magnetic fields during star formation remains a major challenge. With the advent of the Atacama Large Millimeter Array, the potential to study magnetic fields by polarization observations has tremendously progressed. However, the major question remains how much magnetic fields shape the star formation process or whether gravity is largely dominating. Here, we show that for the high-mass star-forming region G327.3 the magnetic field morphology appears to be dominantly shaped by the gravitational contraction of the central massive gas core where the star formation proceeds. We find that in the outer parts of the region, the magnetic field is directed toward the gravitational center of the region. Filamentary structures feeding the central core exhibit U-shaped magnetic field morphologies directed toward the gravitational center as well, again showing the gravitational drag toward the center. The inner part then shows rotational signatures, potentially associated with an embedded disk, and there the magnetic field morphology appears to be rotationally dominated. Hence, our results demonstrate that for this region gravity and rotation are dominating the dynamics and shaping the magnetic field morphology.
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A full understanding of high-mass star formation requires the study of one of the most elusive components of the energy balance in the interstellar medium: magnetic fields. We report ALMA 1.2 mm, high-resolution (700 au) dust polarization and molecular line observations of the rotating hot molecular core embedded in the high-mass star-forming region IRAS 18089-1732. The dust continuum emission and magnetic field morphology present spiral-like features resembling a whirlpool. The velocity field traced by the H13CO+ (J=3-2) transition line reveals a complex structure with spiral filaments that are likely infalling and rotating, dragging the field with them. We have modeled the magnetic field and find that the best model corresponds to a weakly magnetized core with a mass-to-magnetic-flux ratio (lambda) of 8.38. The modeled magnetic field is dominated by a poloidal component, but with an important contribution from the toroidal component that has a magnitude of 30% of the poloidal component. Using the Davis-Chandrasekhar-Fermi method, we estimate a magnetic field strength of 3.5 mG. At the spatial scales accessible to ALMA, an analysis of the energy balance of the system indicates that gravity overwhelms turbulence, rotation, and the magnetic field. We show that high-mass star formation can occur in weakly magnetized environments, with gravity taking the dominant role.
I review theoretical models of star formation and how they apply across the stellar mass spectrum. Several distinct theories are under active study for massive star formation, especially Turbulent Core Accretion, Competitive Accretion and Protostellar Mergers, leading to distinct observational predictions. These include the types of initial conditions, the structure of infall envelopes, disks and outflows, and the relation of massive star formation to star cluster formation. Even for Core Accretion models, there are several major uncertainties related to the timescale of collapse, the relative importance of different processes for preventing fragmentation in massive cores, and the nature of disks and outflows. I end by discussing some recent observational results that are helping to improve our understanding of these processes.
Context. The role of magnetic fields during the formation of high-mass stars is not yet fully understood, and the processes related to the early fragmentation and collapse are largely unexplored today. The high-mass star forming region G9.62+0.19 is a well known source, presenting several cores at different evolutionary stages. Aims. We determine the magnetic field morphology and strength in the high-mass star forming region G9.62+0.19, to investigate its relation to the evolutionary sequence of the cores. Methods. We use Band 7 ALMA observations in full polarisation mode and we analyse the polarised dust emission. We estimate the magnetic field strength via the Davis-Chandrasekhar-Fermi and the Structure Function methods. Results. We resolve several protostellar cores embedded in a bright and dusty filamentary structure. The polarised emission is clearly detected in six regions. Moreover the magnetic field is oriented along the filament and appears perpendicular to the direction of the outflows. We suggest an evolutionary sequence of the magnetic field, and the less evolved hot core exhibits a magnetic field stronger than the more evolved one. We detect linear polarisation from thermal line emission and we tentatively compared linear polarisation vectors from our observations with previous linearly polarised OH masers observations. We also compute the spectral index, the column density and the mass for some of the cores. Conclusions. The high magnetic field strength and the smooth polarised emission indicate that the magnetic field could play an important role for the fragmentation and the collapse process in the star forming region G9.62+019 and that the evolution of the cores can be magnetically regulated. On average, the magnetic field derived by the linear polarised emission from dust, thermal lines and masers is pointing in the same direction and has consistent strength.
Star formation is a multi-scale, multi-physics problem ranging from the size scale of molecular clouds ($sim$10s pc) down to the size scales of dense prestellar cores ($sim$0.1 pc) that are the birth sites of stars. Several physical processes like turbulence, magnetic fields and stellar feedback, such as radiation pressure and outflows, are more or less important for different stellar masses and size scales. During the last decade a variety of technological and computing advances have transformed our understanding of star formation through the use of multi-wavelength observations, large scale observational surveys, and multi-physics multi-dimensional numerical simulations. Additionally, the use of synthetic observations of simulations have provided a useful tool to interpret observational data and evaluate the importance of various physical processes on different scales in star formation. Here, we review these recent advancements in both high- ($M gtrsim 8 , M_{rm odot}$) and low-mass star formation.
Methods: We observed the high-mass hot core region G351.77-0.54 with ALMA and more than 16km baselines. Results: At a spatial resolution of 18/40au (depending on the distance), we identify twelve sub-structures within the inner few thousand au of the region. The brightness temperatures are high, reaching values greater 1000K, signposting high optical depth toward the peak positions. Core separations vary between sub-100au to several 100 and 1000au. The core separations and approximate masses are largely consistent with thermal Jeans fragmentation of a dense gas core. Due to the high continuum optical depth, most spectral lines are seen in absorption. However, a few exceptional emission lines are found that most likely stem from transitions with excitation conditions above1000K. Toward the main continuum source, these emission lines exhibit a velocity gradient across scales of 100-200au aligned with the molecular outflow and perpendicular to the previously inferred disk orientation. While we cannot exclude that these observational features stem from an inner hot accretion disk, the alignment with the outflow rather suggests that it stems from the inner jet and outflow region. The highest-velocity features are found toward the peak position, and no Hubble-like velocity structure can be identified. Therefore, these data are consistent with steady-state turbulent entrainment of the hot molecular gas via Kelvin-Helmholtz instabilities at the interface between the jet and the outflow. Conclusions: Resolving this high-mass star-forming region at sub-50au scales indicates that the hierarchical fragmentation process in the framework of thermal Jeans fragmentation can continue down to the smallest accessible spatial scales. Velocity gradients on these small scales have to be treated cautiously and do not necessarily stem from disks, but may be better explained with outflow emission.
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