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MAGnetic fIeld and Kinematic coupling for massive STAR formation

Periodic Reporting for period 1 - Magik Star (MAGnetic fIeld and Kinematic coupling for massive STAR formation)

Okres sprawozdawczy: 2019-09-02 do 2021-09-01

Star formation is a fundamental process in astrophysics, the physical mechanisms of which have been studied for decades (see e.g. reviews by André et al. 2014, Krumholz et al. 2015). On large scales, it regulates the evolution of galaxies, while on small scales it determines the initial conditions for the formation of planetary systems. As of now, most of our knowledge is concentrated on the formation of stars of a few solar masses. If galaxies’ total stellar mass is dominated by low-mass stars, their energy budget is exclusively controlled by the enormous luminosity and powerful feedback of massive stars (Mstar > 8 Msun). Another interest in studying massive star-formation relates to the formation of atoms. Massive stars forge all the atoms listed in the Mendeleïev table with an atomic number exceeding eight (oxygen). This includes elements such as Sodium, Aluminium, Silicon, Sulfur, Potassium, Calcium, Iron, etc… These elements are fundamental to the chemical reactions on which life depends. Therefore, the understanding of Earth formation and the understanding of Life pass by the understanding of star formation — all types of stars. Despite their importance for forging most of the elements present on Earth, and for the life cycle of galaxies, the mechanisms leading to the formation of high-mass stars remain a mystery in many aspects. For instance, we do not know how were formed to most massive stars we know, a binary system of 300 and 150 Msun in the Tarantula nebula, located in the Large Magellanic Cloud. We do not know, in a star-formation event, if massive stars form first or last. We do not know what processes permit to counterbalance the pressure engendered by the luminosity of a massive protostar. We do not know how the massive dense cores (MDCs) form; MDCs are 2000-3000 au cloud structures denser than ten million of particles per cubic centimetres in which high-mass star forms. We do not know if a massive protostar accretes gas only from the MDC, or at (much) larger scale. The Magik-Star project focuses on the formation of these so-called massive dense cores, with two objectives: i) question the observational robustness of the massive dense core, and ii) quantify the impact of kinematics and magnetic fields onto massive dense cores.
The Magik-Star project had many objectives, related to observations on the one hand, and numerical simulations with Ramses on the other hand. Many of these objectives have been completed — some were even extended. We reduced all the ALMA data in our hands, thanks to the clusters located at CEA. This encompasses molecular line data, that permit quantifying the kinematics, and dust polarisation maps, that permit to map the magnetic field morphology. We also managed to extract from the archive of numerical simulations the runs matching best the observational fields; and ran numerical simulations dedicated to some observational fields — those that were so extreme in terms of density or dynamic that had not been explored through numerical simulations.
More importantly, we manage to set up a procedure permitting comparing directly numerical simulation outputs with observations. This was compulsory for the success of the Magik-Star project, and of paramount importance to compare numerical simulations and observations in a broader context. As planned, we studied the interplay of magnetic field and gas dynamic in the context of high-mass star formation in both observations and numerical simulations. We did not find any coupling between the kinematics and the magnetic fields from one parsec to a tenth of a parsec, which is puzzling for the field. This result, if confirmed, will impact strongly our community.
Another item of the Magik-Star project was to question the robustness of the size of cores. We found that the size of the cores — and their mean mass — were strongly affected by the angular resolution. This work was published in 2021 in Astronomy & Astrophysics, and presented in various conferences.
The Magik-Star project aimed at pushing further our understanding of the coupling between the gas dynamics and the magnetic fields. In this respect, we obtained an unexpected result: at the physical scales we focus, from ≈100.000 down to ≈3000 au, it seems that there is no correlation between the two.
In a sense, more than progressing beyond the state of art, the Magik-Star project questioned and partially rewrote this state to the art. One may see that as a drawback, but we see it as a huge progress. There is nothing more damageable than continuing to build a wall when the very foundation is unstable. Instead, we deconstructed the wall and established new foundations on which we can safely, we hope, re-build our knowledge. 
In detail, the Magik-Star project permitted to firmly established that the `cores’ colleagues (and myself!) described as fixed objects with a given size and a given mass are biased by observations. Of course, these cores exist. But their size and flux depend strongly on the angular resolution at which one makes the measure. This result alone impact the community in several ways: i) it is impossible to compare two cores, or two set of cores, if the data have not been acquired with similar physical resolution (the angular resolution expressed in astronomical units at the distance of the target). This is why an incoming publication by the consortium of the ALMA-IMF large program (60 people from 15 countries) decided to smooth their sample to have the same physical scale on target. The Magik-Star project permitted this change of paradigm in the community. ii) The use of `deconvolution’ to account for the beam spreading of the source is inadequate. iii) The peak of the initial function of cores — a paramount indicator to understand the origin of the initial mass function of stars (the so-called IMF) — is very likely inaccurate in all past studies. This aspect is critical and will impact galactic and extra-galactic communities for years.
The Magik-Star project permitted us to establish that previous measurements of the width of filaments were biased by the observational angular resolution. Filamentary structures have been known for more than 50 years but appeared universal in molecular clouds thanks to the observations with the Herschel far-IR satellite around 2010. Since most of the protostars form on the crest of these filaments, they started to play a central role to explain star-formation. Here, several teams used Herschel data and showed that the width of filaments seemed universal at ≈0.1 pc. Theorists tried and explained this typical width from then. More importantly, many observational experiments have been prepared, or proposed, to investigate this width. Magik-Star showed that there is no typical width for filaments, neither in numerical simulations that include all the known physics (including the non-ideal MHD effects) nor in the observations. Or, if a typical width exists in observations, it must be at lower scales than 0.1 pc. Indeed, we observe a linear dependence of the width of the filaments on angular resolution with no sign of convergence in up to date observations.

These results started to diffuse in the community, and we are confident they will close dead ends and open new paths toward a more robust understanding of star-formation. 
Evolution of core features with angular resolution