Planets Have Just Started to Form in This Binary System

Astronomers have watched the young binary star system SVS 13 for decades. Astronomers don’t know much about how planets form around proto-binary stars like SVS 13, and the earliest stages are especially mysterious. A new study based on three decades of research reveals three potentially planet-forming disks around the binary star.

Planets form as they accrete material from the disk of dust and gas around a young star called a protoplanetary disk. Most scientific models of planetary accretion are based on single stars like our Sun. But most stars form in binary systems, where the gravitational interactions could have an enormous effect on planet formation. Astronomers aren’t as knowledgeable about planet formation around binary stars as they’d like to be.

That’s what makes SVS 13 important. The young star system is still embryonic, and astronomers are learning a lot from observing it. The stars are both stellar embryos, and their combined mass is approximately equal to the Sun’s. SVS 13 is a close binary system, and about 90 astronomical units separate them. Their orbital period is about 850 years. The system is only about 980 light-years away, close enough for detailed study.

The new research is based on extensive observations of SVS 13 with the Very Large Array (VLA) and the Atacama Large Millimeter/Submillimeter Array (ALMA.) The study’s title is “The Physical Properties of the SVS 13 Protobinary System: Two Circumstellar Disks and a Spiraling Circumbinary Disk in the Making.” The lead author is Ana Karla Díaz-Rodríguez from the University of Manchester, and the paper is published in The Astronomical Journal.

“Our results have revealed that each star has a disk of gas and dust around it and that, in addition, a larger disk is forming around both stars,” said Díaz-Rodríguez. “This outer disk shows a spiral structure that is feeding matter into the individual disks, and in all of them, planetary systems could form in the future. This is clear evidence for the presence of disks around both stars and the existence of a common disk in a binary system.”

Cartoon model of the system. The red-blue colours indicate the motion of the gas. (Red – away from us, blue – towards us.) Image Credit: University of Manchester.

VLA observations of SVS 13 span more than three decades. The research team behind this study used those observations combined with new ALMA observations. The long-baseline data allowed astronomers to trace the stars’ orbits around each other and ascertain their geometry and orientation. More detailed information from the observations also uncovered the protostars’ masses, the disk masses, and the disk temperatures. These observations have created a detailed scientific portrait of the young stars and their system.

“This work shows how careful, systematic studies of young stars can provide a remarkably detailed view of their structure and properties,” said collaborator Gary Fuller of The University of Manchester.

When astronomers started observing SVS 13 years ago, optical observations showed a single star. But when radio telescopes observed the system, it became clear it was a close binary.

“At the IAA, we began studying this system twenty-five years ago. We were surprised when we discovered that SVS 13 was a radio binary because only one star is seen in the optical,” said study co-author Guillem Anglada, a researcher at the Instituto de Astrofísica de Andalucía.

Young stars can be challenging to see in optical light because the protoplanetary disk shrouds them. Radio telescopes like ALMA and VLA are the tool of choice for watching young systems because radio emissions are visible through gas and dust. Even then, astronomers see them more clearly the further along they are in the formation process. ALMA and the VLA also have high angular resolutions, and observers can see features down to about 10 to 20 AU in size.

The SVS 13 system has three disks. A disk surrounds each protostar, and a third, larger disk surrounds the pair. The larger disk is a spiral and appears to be in the earliest stages of formation. It’s also feeding material into the two smaller disks.

“Normally, stellar embryos are detected in radio, but they only become visible at the end of the gestation process. It was very strange to discover a pair of twin stars where one of them seemed to have evolved much faster than the other,” said Anglada. Not only is VLA 4B more massive than its partner, but its disk appears to be thicker too.

This image from the study shows 0.9 mm radio emissions from the young protobinary SVS 13. The two stellar components are named VLA 4B and VLA4A. The pair are about 90 astronomical units apart. Image Credit: Díaz-Rodríguez et al. 2022.

One of the most exciting features of SVS 13 is the spiral arms.

“Spirals have been observed in a number of protoplanetary disks, and various mechanisms, which can possibly act together, have been suggested to explain their origin, including planet-disk interactions, gravitational instabilities, or shadows,” the authors write in their paper.

Other researchers have modelled the spiral arms and disks in systems like SVS 13. A 2019 study used numerical simulations to investigate systems like this and found that “Each model exhibits circumstellar disks, spiral arms, and a circumbinary disk with an inner gap or cavity.” That study also found that the circumbinary disk is asymmetric.

The authors of this new research referred to the 2019 paper when they wrote, “This result from the numerical simulations appears to be broadly consistent with our observations of SVS 13 if the secondary were VLA 4A (the component with the more prominent spiral arm, S1) and the primary were VLA 4B (with the S2 spiral surrounding this protostar and being connected to some undetermined point between VLA 4A and VLA 4B).”

Unfortunately, the researchers only have rough mass estimates showing that VLA 4B is the primary star in the binary pair. The team says they need more accurate mass measurements for VLA 4A and VLA 4B to take their work even further. Future observations “… should be able to shed more light on the masses of the SVS 13 binary and to reliably identify the primary and secondary components. This will make it possible to test the models of the evolution of a binary system which is undergoing the development of a circumbinary disk and spiral arms, as seems to be the case for SVS 13.”

This figure from the study highlights what appear to be the spiral arms of a disk surrounding SVS 13 and feeding material into the two smaller disks. The image on the right is the same as on the left but with different intensities to highlight the arms. S1 and S2 are pretty clearly defined spirals, but for S3, it is unclear whether it is a spiral arm or a feature of the northern part of the circumbinary disk. Image Credit: Díaz-Rodríguez et al. 2022.

More accurate measurements are always desirable in astronomy, and sometimes decades pass before astronomers can get those measurements. But even with the mass estimates they have, the team of researchers say their modelling shows that both of the stars could form planets.

“We designed several experiments to get more details and to find out if in such a case, either of the stars could form planets. Now we have seen that both stars are very young and that both can form planets,” says Guillem Anglada, a researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC) who is coordinating the studies of SVS 13.

The observations of the system also revealed something else. There appear to be several complex chemical building blocks related to life in the disks around the system. The team identified 30 different molecules around both protostars, including thirteen complex organic molecules (COMs). COMs are molecules with six or more atoms with at least one carbon atom. They detected seven of them in the SVS 13 system for the first time. “This means that when planets begin to form around these two suns, the building blocks of life will be there,” said Díaz-Rodríguez.

The study has some uncertainties, and the authors make those clear. They say they can confirm that SVS 13 is a “… multiple protostellar system with at least two embedded protostars, separated by 90 au…” Unfortunately, they can’t rule out other protostars since their resolution is limited to about 10 to 20 AU. “However,” they write, “we favour the interpretation in terms of a binary system.”

Modern astronomy involves a lot of simulations. Those simulations go hand in hand with observations. But the simulations are only as good as the tests you can put them through. The authors say our increasingly detailed understanding of the SVS 13 system will only improve simulations of young binary systems as a group.

“In conclusion, SVS 13 appears to be an excellent testbed for testing numerical simulations of the earliest stages in the formation of binary and multiple stellar systems.”

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Evan Gough

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