An article published in “The Astrophysical Journal” reports a study on the orbits of protoplanetary disks in binary systems, where different geometries were found. A team of astronomers led by Ian Czekala of the University of California at Berkeley used the ALMA radio telescope to study star systems in formation around two stars, which in jargon are called circumbinary disks. The conclusion is that the disks that orbit the more compact binary systems share almost the same plane while the disks that orbit the wider binary systems have significantly tilted orbital planes.
The discovery of some thousand exoplanets in recent years includes several in binary systems. They were dubbed Tatooine, like Luke Skywalker’s planet in the Star Wars saga. Many studies concern the formation of exoplanets and are useful to understand the processes that lead to the birth of star systems. The ALMA radio telescope (Atacama Large Millimeter/submillimeter Array), inaugurated in March 2013, is an extraordinary tool for these studies. It too has made it possible to observe mainly protoplanetary disks that orbit a single star, but in some cases they’re instead circumbinary disks, which orbit a pair of stars. This allowed to discover cases of deformation and tilt of that type of disk, which leads to a misalignment with respect to the stars’ orbital plane. Ian Czekala’s team studied the data collected on 19 circumbinary disks to better understand those geometries.
This study was also based on the comparison of data collected with the ALMA radio telescope about circumbinary disks with data collected on a dozen Tatooine exoplanets discovered by NASA’s Kepler space telescope, the extraordinary planet hunter that ended its mission on October 30, 2018.
The analysis of the orbital geometries of these binary systems showed a correlation between the misalignment of the stars with respect to their circumbinary disks with the orbital period of the stars. Basically, the faster the two stars orbit each other, the more likely the circumbinary disk is to orbit in the same plane. The Tatooines discovered by the Kepler space telescope are in binary systems with pairs of stars that orbit each other in less than 40 days and are aligned with their orbits.
The image (ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello) shows two examples of binary systems with the two possibilities. On the left, the HD 98800 B system, where the circumbinary disk isn’t aligned with the orbits of the internal binary system. The two stars orbit each other (from our perspective, towards us and away from us) in 315 Earth days. On the right, AK Sco, where the circumbinary disk is aligned with the orbits of its binary stars. The two stars orbit each other in 13.6 Earth days.
The next step for Ian Czekala’s team is to look for the reason for the correlation between the alignment of the circumbinary disk and the orbital period of the pair of stars. The ALMA radio telescope will continue to be important, and in the next few years new generation instruments will begin to enter service, useful also to better understand the formation processes of star systems.
