Tracing the orbits of young stars with interferometry

Stars are usually thought of as isolated objects, but most of them form in multiple systems. This can be very helpful to better characterize them, since by following their relative motions astronomers constrain some crucial parameters, such as their masses. Unfortunately, studying their movement is very complicated, because if the objects are too far away from each other, we will need to observe them for decades to see their motions. But if they are too close to each other, it may be impossible to distinguish the light coming from each individual object, obtaining a blurred image of the system.

Fortunately, technological advances in astronomical instrumentation are reducing these limitations. In a recent investigation led by Sebastián Zúñiga-Fernández, a PhD student of the Millennium Nucleus for Planet Formation (NPF), it was possible to separate for the first time the light from each component of HD98800, a quadruple system formed by two similar stars very close to each other, orbiting another pair of similar stars.

To achieve this, the researchers used the PIONIER instrument that combines the light from the VLTI (Very Large Telescope Interferometer) telescopes of the European Southern Observatory (ESO) in Chile. In this case, the telescopes used are the so-called “Auxiliary Telescope” (AT), which were recently improved by incorporating an adaptive optics system that compensates for the effect of the atmosphere on the observations.  Thanks to this, it was possible to distinguish the individual components of each pair of stars (one of them for the first time) and to follow the motion of the four stars in different time bands.

Johan Olofsson, NPF research associate, Amelia Bayo, director of the center, and María Paula Ronco, postdoctoral researcher, also participated in this research, published in the scientific journal Astronomy & Astrophysics. 

This research is a major step towards the complete characterization of this quadruple system. A quadruple system can be arranged in different configurations. For example, in this case, the scientists explain, there is a pair of stars very close to each other -called subsystem A and composed of the objects Aa and Ab-, and, at a greater distance, another pair of stars close to each other -subsystem B, with the individual objects Ba and Ba-. Then, we have a hierarchical configuration A(AaAb) + B (BaBb), although other combinations could also occur.

It is thought that the formation of multiple systems is due to a combination of fundamental mechanisms of star formation and interaction between their components, throughout their first millions of years of life. The new results obtained by the researchers provide more information about how HD98800 formed and about interesting future events. “Knowing the relative motions of the system with great precision allows us to predict phenomena on intermediate time scales, such as when the BaBb subsystem – in which both stars are surrounded by a somewhat peculiar protoplanetary disk – will transit in front of the AaAb subsystem and allow us to study this disk in a perfect natural survey light,” explains Zúñiga-Fernández, who is a PhD student at the University of Valparaíso and also a student at ESO.

Determining the parameters that describe the orbit of a binary system, such as the ones that compose this quadruple system, besides giving us clues about how these systems are formed, allows us to calculate empirically the masses of its components (dynamical masses) and the distance to the system. Mass is a fundamental stellar parameter of great importance for testing and improving models of stellar evolution. In this work, we updated these values for the BaBb system and obtained, for the first time, the masses of the components of the AaAb system.

To resolve the relative position of the components of both subsystems, the high resolution of the VLTI, which combines four telescopes to achieve a resolution equivalent to a telescope of approximately 100 meters in diameter, was paramount.

“The observation with the VLTI opens the possibility of taking a big step towards estimating all the parameters that describe the orbit of the subsystems that compose it, i.e., the two “inner” binaries (AaAa and BaBb), and the outer orbit (AB). The formation mechanism of the 2+2 hierarchical system (i.e., pairs of binary stars) remains an open question, and the orbital parameters of this hierarchical system can inform us about its formation history,” says Zúñiga-Fernández.

The scientist adds that one of the binary stars in the HD98800 system hosts a protoplanetary disk in polar configuration, i.e., perpendicular to the orbital plane of the host binary, so a better orbital characterization of this system allows predicting and preparing the next transit behind the disk, as suggested in a previous study of the system by Grant Kennedy. 

“We have performed simulations to predict when this event will take place, and it should start in 2026 or 2027. But there are still some uncertainties about the exact date, so with our collaborators we will soon start observing the system very frequently,” notes Johan Olofsson, second author of the paper.

Future work

There is still work to be done on this quadruple system. 

To reduce possible biases in the estimation of the orbital parameters, the scientists need to collect new radial velocity measurements of both systems. “For this we will propose for high-resolution spectroscopy observations. A part of our observing campaign with PIONIER was left pending by the pandemic and could be observed in March next year, and this new observation of the BaBb subsystem in conjunction with the spectroscopic observations could be the next step to finally be able to fully characterize both binary systems. We estimate the transit of the disk in front of AaAb close to 2026, so we are already starting to plan the observations that will allow us to learn as much as possible from this event”, concludes Zúñiga-Fernández.

Scientific publication

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