The GRAVITY instrument detects the accretion flows that feed young stars

Research
On  August 26, 2020
Artist's view of hot gas streams on a star. The material of the surrounding protoplanetary disc, in which planets are born, is hurled onto the star's surface by the magnetic field at supersonic velocities. © A. Mark Garlick
Artist's view of hot gas streams on a star. The material of the surrounding protoplanetary disc, in which planets are born, is hurled onto the star's surface by the magnetic field at supersonic velocities. © A. Mark Garlick
Using the GRAVITY instrument of the European Southern Observatory (ESO), an international team of French, Irish, German and Portuguese astronomers observed for the first time the columns of matter that feed young stars. The material comes from the disks surrounding these stars, the same disks that give birth to planets. These results, published on August 26 in Nature, echo recent work carried out by a team from the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG - CNRS/UGA [1]) on the star-disk interaction region of the star DoAr44. These two results thus show that the strong magnetic field of the star perturbs the internal disk and controls the accretion flux on the star.
The disk surrounding a young star is known as the protoplanetary disk. These regions are the cradle of planets and are formed when matter is still being accumulated by the young star. Theoretical models suggest that young stars acquire matter via their magnetic fields and that this material falls to the surface at supersonic speeds. Earth-like planets would thus form in the inner regions of these disks where huge amounts of energy are released by the accretion process. Therefore, understanding how these processes occur is crucial to our understanding of the formation of planets, including Earth.

Although it occurs at scales equivalent to a few solar rays [2], this accretion process has never been directly observed before. Indeed, the nearest young star is so far away from us that it requires some of the world's largest telescopes and very sophisticated instrumentation to observe it. To study hot gas emissions in the regions closest to these young stars, the teams therefore used the GRAVITY instrument, which combines the light from the four 8-metre telescopes of ESO's Very Large Telescope into a super-telescope with a resolution equivalent to that of a 130-meter diameter telescope.

Work carried out by the international team of astronomers, involving several Grenoble IPAG scientists, has focused on one of the young stars closest to us, TW Hya in the constellation of Hydra, the water snake. The star is "only" a few million years old. It is located very close to Earth, 196 light years away, and the disc of matter surrounding it is directly in front of us. This makes it an ideal candidate for probing how the matter in a planet-forming disc is channeled to the stellar surface. The results show that, given the size of the hot gas emission region and the measured velocities, the hot gas does not seem to come from the expulsion of matter from the disk or the stellar surface (i.e. from a blast), but must come from matter accretion fluxes on the star.

These results echo previous work published in the Astronomy & Astrophysics journal last April, carried out on the star DoAr44 located in the r Ophiuchus cloud 476 light-years away. These studies were conducted with the GRAVITY instrument by a Grenoble team led by Jérôme Bouvier, CNRS research director at IPAG, in the framework of the SPIDI project, winner of a grant from the European Commission (ERC). By a direct measurement of the scale of the star-disk interaction region in a young system, this work shows that interactions occur very close to the star, over a distance of less than 5 stellar rays. Moreover, they show for the first time that the interaction region is not exactly centered on the star but slightly offset from it, by less than one stellar ray.

Both results thus confirm the theoretical models that predicted a star-disk’s interaction region is governed by the stellar magnetic field. Just as the Earth's magnetic field channels high-energy particles from the solar wind to give rise to auroras, the strong magnetic field of the star perturbs the internal disk and controls the accretion flux on the star. Such direct evidence of magnetically controlled accretion on the star has remained rare until now and provides new insights into the environmental conditions that prevailed at the time of planet formation in protoplanetary disks.
 
Notes
[1] IPAG is a member of the OSUG - Observatoire des Sciences de l'Univers de Grenoble.
[2] The solar radius measures 695,700 km.
Published on  September 3, 2020
Updated on  October 7, 2020