Free Astronomy Magazine November-December 2020

18 NOVEMBER-DECEMBER 2020 SPACE CHRONICLES To reach these conclusions, the team observed GW Orionis for over 11 years. Starting in 2008, they used the AMBER and later the GRAVITY in- struments on ESO’s VLT Interferom- eter in Chile, which combines the light from different VLT telescopes, to study the gravitational dance of the three stars in the system and map their orbits. “We found that the three stars do not orbit in the same plane, but their orbits are mis- aligned with respect to each other and with respect to the disc,” says Alison Young of the Universities of Exeter and Leicester and a member of the team. They also observed the system with the SPHERE instrument on ESO’s VLT and with ALMA, in which ESO is a partner, and were able to image the inner ring and confirm its misalign- ment. ESO’s SPHERE also allowed them to see, for the first time, the shadow that this ring casts on the rest of the disc. This helped them figure out the 3D shape of the ring and the overall disc. The international team, which in- cludes researchers from the UK, Bel- gium, Chile, France and the US, then combined their ex- haustive observa- tions with com- puter simulations to understand what had hap- pened to the sys- tem. For the first time, they were able to clearly link the observed mis- alignments to the theoretical “disc- tearing effect”, which suggests that the conflict- ing gravitational pull of stars in dif- ferent planes can warp and break their discs. Their simulations showed that the misalignment in the orbits of the three stars could cause the disc around them to break into distinct rings, which is exactly what they see in their observations. The observed shape of the inner ring also matches predictions from numerical simula- tions on how the disc would tear. Interestingly, another team who studied the same system using ALMA believe another in- gredient is needed to under- stand the system. “We think that the presence of a planet between these rings is needed to explain why the disc tore apart,” says Jiaqing Bi of the University of Victoria in Canada who led a study of GW Orionis published in The Astrophysi- cal Journal Letters in May this year. His team identified three dust rings in the ALMA obser- vations, with the outermost ring being the largest ever ob- served in planet-forming discs. Future observations with ESO’s ELT and other telescopes may help astronomers fully unravel the nature of GW Orionis and reveal young planets forming around its three stars. T his ALMA image shows the disc’s ringed structure, with the innermost ring (part of which is visible as an ob- long dot at the very centre of the image) separated from the rest of the disc. The SPHERE observations al- lowed astronomers to see for the first time the shadow of this innermost ring on the rest of the disc, which made it possible for them to reconstruct its warped shape. [ESO/Exeter/ Kraus et al., ALMA (ESO/NAOJ/NRAO)] T his ‘fly-through’ animation allows the viewer to see the three stars at the very centre of GW Orionis, as well as its warped disc and the tilted ring that was torned apart from it. The animation is based on a computer model of the inner region of GW Orionis, provided by the team; they were able to reconstruct the 3D orbits of the stars and the 3D shape of the disc from the observational data. [ESO/Exeter/ Kraus et al., ALMA (ESO/NAOJ/NRAO)] !