Numerous stars are in binary systems, sets of stars gravitationally bound to one another, often with the two components having similar masses. However, it remains indistinct how these systems assemble and accrete material. And what happens when two stars form in a disk of matter?
For the first time, observations from the high-resolution images of a young stellar binary system will reveal a complex network of accretion filaments nurturing two protostars at the center of the circumbinary disk. These observations enable astronomers to identify a two-level accretion process, circumbinary disk to circumstellar disk to stars, constraining the conditions leading to the formation and evolution of binary star systems.
An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has observed the formation of such a binary star system with high-resolution ALMA images. They mainly targeted the system called [BHB2007] 11, the youngest member of a small cluster of young stellar objects in the Barnard 59 core in the Pipe nebula molecular cloud.
Previous observations of the same system revealed an accretion envelope surrounding a circumbinary disk. This time, new observations revealed its inner structure.
Felipe Alves from MPE, who led the study, said, “We see two compact sources, that we interpret as circumstellar disks around the two young stars. The size of each of these disks is similar to the asteroid belt in our Solar System, and their mutual distance is about 28 times the distance between the Earth and the Sun.”
“Both protostars are surrounded by a circumbinary disk with a total mass of about 80 Jupiter masses, which shows a complex network of dust structures distributed in spiral shapes. The shape of the filaments suggest streamers of in-falling material, which is confirmed by the observation of molecular emission lines.”
Paola Caselli, director, and MPE and head of the center of Astrochemical Studies said, “This is a significant result. We have finally imaged the complex structure of young binary stars, with their “feeding filaments” connecting them to the circumbinary disk. This provides essential constraints for current models of star formation.”
The astronomers interpret the filaments as inflow streamers from the extended circumbinary disk, where the circumstellar disk around the less massive of the two protostars receives more input, steady with hypothetical forecasts.
The estimated accretion rate is only about 0.01 Jupiter masses per year, which agrees with rates estimated for other protostellar systems. Similarly, as the circumbinary disk feeds the circumstellar discs, each circumstellar disk feeds the protostar in its center.
At the disk-star level, though, the accretion rate inferred from the observations is higher for the more massive object. The observation of emission from an extended radio jet for the northern object confirms this result, which is an independent indication that this protostar is indeed accreting more material.
Alves said, “We expect this two-level accretion process to drive the dynamics of the binary system during its mass accretion phase. While the good agreement of these observations with the theory is already very promising, we will need to study more young binary systems in detail to further constrain the conditions that lead to stellar multiplicity.”