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Resolving the Birth of High-Mass Binary Stars

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Circumstellar disk
Artist’s impression of a young star surrounded by a circumstellar disk. Scientists have recently resolved for the first time two young, high-mass stars in a binary system, as well as their circumstellar and circumbinary disks. [ESO/L. Calçada]

New observations may help us to learn more about the birth of high-mass star systems. For the first time, scientists have imaged a very young, high-mass binary system and resolved the individual disks that surround each star and the binary.

Massive Multiples

It’s unusually common for high-mass stars to be discovered in multiple-star systems. More than 80% of all O-type stars — which have masses greater than 16 times that of the Sun — are in close multiple systems, compared with a multiplicity fraction of only ~20% for stars of ~3 solar masses, for instance.

IRAS17216-3801

Reconstructed VLTI observations of the two components of the high-mass binary IRAS17216-3801. [Adapted from Kraus et al. 2017]

Why do more massive stars preferentially form in multiple-star systems? Many different models of high-mass star formation have been invoked to explain this observation, but before we can better understand the process, we need better observations. In particular, past observations have placed few constraints on the architecture and disk structure of early high-mass stars.

Conveniently, a team of scientists led by Stefan Kraus (University of Exeter) may have found exactly what we need: a high-mass “protobinary” that is still in the process of forming. Using ESO’s Very Large Telescope Interferometer (VLTI), Kraus and collaborators have captured the first observations of a very young, high-mass binary system in which the circumbinary disk and the two circumstellar dust disks could all be spatially resolved.

Clues from Resolved Disks

The VLTI near-infrared observations reveal that IRAS17216-3801, originally thought to be a single high-mass star, is instead a close binary separated by only ~170 AU. Its two components are both surrounded by disks from which the protostars are actively accreting mass, and both of these circumstellar disks are strongly misaligned with respect to the separation vector of the binary. This confirms that the system is very young, as tidal forces haven’t yet had time to align the disks.

binary model

The authors’ model of the geometry of the binary system, including the orientations of the two circumstellar disks (sketch not to scale). [Adapted from Kraus et al. 2017]

Fitting models to the observations, Kraus and collaborators find the best-fitting description of the system’s geometry and the masses of the components — roughly 18 and 20 solar masses, they estimate.

By tracing the hot gas in their observations of the system, the authors also determine that the secondary, smaller component is accreting at a higher rate than the larger star. This suggests that the secondary disrupts the accretion stream onto the primary star, channeling the infalling material onto its own disk instead — an observation that confirms the prediction of hydrodynamic simulations.

IRAS17216-3801 is roughly three times more massive and five times more compact than other high-mass multiple star systems imaged in infrared, and it is the first system in which resolution of its component disks has been possible. These images present an exciting laboratory for studying star–disk interactions and the formation of high-mass multiple systems.

Citation

S. Kraus et al 2017 ApJL 835 L5. doi:10.3847/2041-8213/835/1/L5