UCD Research proves theory on star growth

Image Credit: ESA/Hubble & NASA Acknowledgement: R. Buta (University of Alabama)

Dr Rebeca García López, an Ad Astra Fellow of the UCD School of Physics, and her team have recently proven a 30-year-old theory of how stars use their magnetic field lines to regulate their size and growth. Jade Norton reports.

From birth to death, a star is a massive body made of a variety of gases, but the binding force that allows the star to grow whilst also keeping it from disintegrating out into space has been under consideration for many years. It is not easy for a massive, gravitating body to collect dust and gas, nor is it easy to keep, as the star must find a way around the concept of conservation of angular momentum. This means that for the star to grow the gases surrounding it must keep an orbital distance rather than falling onto its surface. To maintain the stereotypical spherical shape commonly attributed to stars, these gases interact with the star’s magnetic field which is similar to that of Earth’s. It is usual for stars to have magnetic fields and it is possible to see the magnetic field of our closest star - the Sun - from the Northern or Southern lights, which visualise the particles that have been ejected from the Sun and are directed to the poles that protect the Earth. Research undertaken by Dr Rebeca García López and her team aimed to understand a theory postulated 30 years ago and, due to improving technology, they were able to discern the exact physical concept that is used to the binding force holding stars together.

The research, published in the science journal Nature, was led by Dr Rebeca García López, an Ad Astra Fellow in the UCD School of Physics, in collaboration with the Dublin Institute for Advanced Studies (DIAS), and the Max Planck Institute for Astronomy, Germany. They used observations from the GRAVITY telescope, based at the European Southern Observatory in the Andes Mountains of Chile, to measure the near-infrared radiation that was being emitted from the TW Hydrae, a young T Tauri star found in the constellation of Hydra. The GRAVITY telescope links the four 8-meter-telescopes of the VLT and uses a technique called interferometry to visualise stellar details that are so small they can not be seen by a single telescope.

From their observations they were able to conclude that the process the star was going through was called magnetospheric accretion. This is a process that uses the magnetic fields of the stars to guide gases that are falling centrally toward the star from the inner circumstellar disk to the surface. This means that stars gather material from around them using a disk-shaped magnetic field and it allows them to grow in mass. The magnetic fields guide the gases for the inner layer of the star to a surrounding disk in column-like flows, which essentially created a funnel guiding the particles that are similar to that of the poles of the Earth. GRAVITY was able to visualise the inner part of the gas disk surrounding TW Hydrae which showed that the light emitted from the star was located within three and a half times the radius of the star. This is significant as the small distance of the circumstellar disk is in conflict with the standard physical models and leads to the only plausible explanation of a physical model to be a magnetospheric accretion model. 

The original theory was conceived by Max Camenzind, a German astrophysicist. He postulated an answer as to how matter manages to reach the accretion disk and overcome the conservation of angular momentum. His theory was magnetospheric accretion, but he was not able to prove it due to limitations in technology. It was from his work and others that Dr García López developed on and used the improvements in technology to understand where the extra energy created by the rotational momentum went to. This energy should have thrown all the gathered material back into space in a cycle of accumulation and simultaneous disintegration. However, observations from GRAVITY show a hydrogen emission across the radius which could be attributed to a stellar magnetosphere but due to their small size it makes them difficult to resolve and make an interpretation of the observation. This led to the idea that the hydrogen emission was due to accretion columns which are funnel flows of matter being added to the star. This is to be expected from magnetospheric accretion models rather than from wind emitted at a much larger distance.

Improvements in technology have allowed for many physical models to be proven that were discovered many years ago. Future observations will allow researchers to get a more detailed reconstruction of the processes that are being undertaken close to the star. A further avenue of research may be to find if the star's axis of rotation is similar to or in the same location as the magnetic north and south poles, but due to the complex and diverse nature of magnetic fields this answer could also be 30 years in the future. No matter what the case is though, this research will only further drive progression in the field of astronomy and space science.