According to the most widely accepted theory of star formation (the nebular hypothesis), stars and planets form from huge clouds of dust and gas. These clouds undergo gravitational collapse at their center, resulting in the birth of new stars, while the rest of the matter forms disks around it. Over time, these disks become ring structures that accumulate to form systems of planets, planetoids, asteroid belts, and Kuiper belts. For some time, astronomers have wondered how interactions between early stellar environments might affect their formation and evolution.
For example, it has been theorized that gravitational interactions with a passing star or shock waves from a supernova may have triggered the core collapse that led to our Sun. To investigate this possibility, an international team of astronomers observed three interacting twin disk systems using the Spectro-Polarimetric High Contrast Exoplanet Search (SPHERE) on the Very Large Telescope (VLT) of the ESO. Their findings show that due to their dense stellar environments, gravitational encounters between star systems at an early stage play an important role in their evolution.
The research team was made up of astronomers from the European Southern Observatory (ESO), the Space Telescope Science Institute (STScI), the Millennium Nucleus on Young Exoplanets and their Moons (YEMS), the Center for Interdisciplinary Research in Astrophysics and space exploration (CIRAS), the Institute for Particle Physics and Astrophysics at ETH Zurich, the Max-Planck Institute for Astronomy (MPIA), the Mullard Space Science Laboratory, the Kavli Institute for Astrophysics and space research and several universities.
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The classic idea of star formation states that stars form individually from an isolated, spherically symmetrical prestellar core. This has been questioned in recent years as astronomers have made more observations that don’t fit this pattern. On the one hand, astronomers have observed newborn stars embedded in thin filaments within molecular clouds, suggesting that large-scale processes are at work in these dynamic environments. Additionally, investigations of star-forming regions have shown that a statistically significant fraction of prestellar cores lead to the formation of multiple systems rather than just one.
This is thought to have been the case with our Sun, which formed from the same nebula as several “solar siblings” that were later scattered throughout the Milky Way. These pieces of evidence all point to interactions between early systems, and the effect this has on their evolution is not yet well-defined. But by examining early star systems and the protoplanetary disks that orbit them, astronomers can observe the gravitational disturbances these interactions would cause. As they indicate in their article, the stellar interaction can occur in three ways.
These include non-recurring, hyperbolic or parabolic passes (aka “flyovers”), through the co-evolution of binary stars, or through one star capturing another (aka binary capture). To test their predictions, the team analyzed three interacting twin-disc systems (AS 205, EM* SR 24 and FU Orionis) using dual infrared imaging and spectrograph (IRDIS) on the SPHERE instrument of the VLT. This cryogenic camera allows SPHERE to perform dual polarization light observations of AS 205, EM* SR 24 and FU Orionis in the near infrared (NIR) band from 0.95 to 2.32 ?m.
All have been previously observed by instruments such as the Atacama Large Millimeter-submillimeter Array (ALMA) and the NaCo Nasmyth Adaptive Optics System Near-Infrared Imager and Spectrograph (NAOS-CONICA, or NACO) instrument on the VLT. These observations confirmed that these binary stars are twin disk systems, where the primary and companion stars have confirmed protoplanetary disks. They also compared their observations of polarized light with data sets of similar resolution (ALMA and gas emission data) to impose tighter constraints on the geometry and content of each system.
By examining how the light from these stars was scattered by their disks, the team discerned spiral patterns that were likely caused by gravitational interaction and connecting filaments between the stars. From this, they were able to predict what kinds of interactions took place between the binary companions and their respective disks. As they stated:
“The overall structure observed in AS 205 is consistent with a hyperbolic stellar flyby as [the] dynamic origin. The clockwise direction of the spirals S1 and S2 around AS 205N suggests a counter-clockwise flyby and that the orbit periapsis (closest approach location) has already been crossed.
“The north disk constitutes a circumbinary disk around the SR 24Na and SR 24Nb components. In addition, the north disk displays [a] strong asymmetric structure with the presence of an extended scattering to the northwest of the stars (?1), tracing a spiral arm which opens into [a] counterclockwise and is opposed by a spiral arm southwest of SR 24N (?2). The south spiral arm smoothly merges into the bridge to SR 24S.
“The Scattered Light [of FU Orionis] shows strongly disturbed structures such as the bright extended arm east of North Source, possibly related to gravitational interaction and subsequent stellar outburst. The arm(?) kinematics connects it to both the systematic speed of the north and south source, further promoting the idea that it resulted from a recent close stellar encounter.
The authors also acknowledge that possible dynamical scenarios could explain the spiral perturbations they observed, such as “coupled binary formation from a common molecular cloud of high angular momentum.” They also note that their analysis has uncertainties because it does not account for systemic error and their results should not be considered definitive. However, this presents an opportunity for follow-up observations and future investigations to further constrain the geometry of these systems and test these results.
These surveys will benefit from new generation instruments such as the James Webb Space Telescope (JWST) optimized for the study of objects in the near and mid-infrared spectrum. Ground-based observatories like the Extremely Large Telescope (ELT) will be able to directly image these systems using a combination of coronagraphs, spectrometers and adaptive optics. These studies will reveal much about young star systems and the formation and evolution of planetary systems.
Further reading: arXiv
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