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.

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Créditos: Carol Rojas, Núcleo de Formación Planetaria (NPF)

Clues for planet formation?

Protoplanetary disks are the places where planets form. In many of them, ring-like structures, such as bright rings, or central cavities in the dust distribution have been observed, structures that are commonly interpreted as indirect clues to the presence of planets.

An international team of astrophysicists led by Karina Maucó, a postdoctoral researcher at Millennium Nucleus for Planet Formation (NPF), observed with ALMA radio facility a young star surrounded by a dusty disk with a huge central cavity, a system called Sz91. The star is located in the Lupus molecular cloud. The disk is formed by a thin ring of dust and the main conclusion of the study is that planetesimal formation, a key step in planet formation, may be taking place in this thin ring.

Also participating in this research were Matthias R. Schreiber, deputy director of the NPF, Johan Olofsson, research associate, Amelia Bayo, director of the center, and Claudio Caceres, associate researcher. The paper was published in the prestigious Astrophysical Journal.

The scientists knew that showed clear signs of the presence of submillimeter dust grains in the ring, with the presence of smaller grains (on the order of microns) in the innermost regions. It therefore represents an excellent opportunity to test models of grain growth in disks around young stars. “This type of feature is normally attributed to the interaction of forming planets shaping the disk. This is why we decided to observe this source at 2.1 mm ALMA data at an unprecedented resolution for this source. The idea is to characterize in detail the dust grains in the ring to try to understand how planets form”, explains Maucó.

By observing the disk in different ALMA bands, scientists can study the brightness distribution, i.e., whether the object is brighter at higher or lower frequencies. This “brightness ratio” is called the spectral index, and is directly related to the dominant size of the dust grains responsible for the detected emission. The researchers compared this spectral index of Sz91 with the rest of the disk population in Lupus, as well as with other disks in different star-forming regions, to understand its evolutionary state.

“We obtained that the spectral index in the ring is almost constant throughout the ring and implies maximum grain sizes of about 0.61 mm, so we show that there is indeed grain growth in the dust ring. By comparing this spectral index with the population of disks in Lupus, we observed that Sz91 stands out as the star with the highest spectral index”, says Maucó. The astrophysicist explains that in disks like Sz91 we see outer parts of the “original” disk, more separated from the star, where the density of gas and dust is lower and we can see the full emission of the disk. In more compact disks, the emission produced in the inner parts of the disk is not able to “pass through” all the remaining disk material because it is much denser (a phenomenon known as being “optically thick”), so the emission we see is that produced by the material at the surface of the disk, which need not be representative of the emission from the disk as a whole.

On the other hand, the emission coming from the Sz91 dust ring is not optically thick, at ALMA wavelengths, so these data can be used to correctly characterize the dust in the system.

“Comparing these observational results with theoretical models of grain growth in dust rings, which include the processes of fragmentation and formation of planetesimals (km-sized objects that are the main “seeds” for planet formation), we find that our results are in very good agreement with the predictions of these models, concluding that planetesimal formation is most likely occurring in the dust ring around Sz91,” says Karina Maucó.

Future Work

Scientists will obtain new ALMA data at higher resolution and longer wavelengths, along with data taken with the VLA, to obtain information about the larger grains and corroborate the existence of centimeter-sized grains.

“The idea is to resolve the ring in the radial direction – studying the disk sections themselves – to be able to investigate in detail the properties of the dust at different radial distances as well as inhomogeneities in the disk. With these data the spectral index (and hence grain size) can be studied over a wider range of wavelengths and along the radial direction of the ring. This will allow us to study azimuthal accumulations or “clumps” – inhomogeneities along the ring, not in its section but in its length – observed in the ring (these may be regions where planets may form in the future)”, concludes Maucó.

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Credits: Carol Rojas, NPF