Circumstellar disc

Lore & Background

According to the widely accepted model of star formation, a young star (protostar) forms by gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses angular momentum, resulting in a gaseous protoplanetary disc around the young, rotating star. This disc continues to feed the central star, containing a few percent of the star's mass, mainly hydrogen gas. The main accretion phase lasts a few million years, with accretion rates typically between 10⁻⁷ and 10⁻⁹ solar masses per year. The disc gradually cools during the T Tauri star stage, allowing small dust grains to coagulate into planetesimals, and if sufficiently massive, runaway accretions lead to planetary embryos. A Sun-like star usually takes around 10 million years to form.

Reader's Guide

Circumstellar discs are fundamental to understanding planetary system formation and evolution. Around young stars, protoplanetary discs provide the raw material for planet formation, with dust grains coagulating into planetesimals and eventually planetary embryos. Around mature stars, debris discs indicate that planetesimal formation has occurred, while discs around white dwarfs show that planetary material survived stellar evolution. In binary systems, the infall of gas allows formation of circumprimary, circumsecondary, and circumbinary discs, with the latter creating an inner cavity due to spiral density waves at Lindblad resonances. Accretion variability in circumbinary discs occurs over short timescales (binary orbital period) and long timescales (apsidal precession), affecting orbital evolution and planet formation. Misaligned discs can arise from processes such as the Bardeen-Petterson effect, and evidence is seen in systems like Her X-1 and SS 433. Studies using ALMA data show that circumbinary discs around short-period binaries are often aligned with the binary orbit, while those around longer-period binaries are typically misaligned.

Did You Know?

• A circumstellar disc can be torus-, pancake- or ring-shaped.
• The main accretion phase of a protoplanetary disc lasts a few million years, with accretion rates between 10⁻⁷ and 10⁻⁹ solar masses per year.
• In binary systems, a circumbinary disc may form, though its upper mass limit is not a widely established canonical fact.
• Strong evidence of tilted disks is seen in the systems Her X-1, SMC X-1, and SS 433, where periodic X-ray blockages occur on the order of 50–200 days.

Birth from Collapsing Clouds

A circumstellar disc begins its existence as a direct consequence of gravity drawing matter together. When a dense pocket within a giant molecular cloud collapses under its own weight, the infalling material carries angular momentum that prevents it from falling straight inward. Instead, it flattens into a rotating, pancake-shaped disc of dense gas and dust surrounding the newborn protostar. This protoplanetary disc can hold a few percent of the central star's mass, predominantly in the form of hydrogen gas. The main accretion phase, during which the disc feeds the growing star, persists for a few million years, with mass transfer rates typically ranging from 10⁻⁷ to 10⁻⁹ solar masses per year. As the disc cools into what astronomers call the T Tauri stage, tiny grains of rock and ice begin to coagulate into planetesimals. In sufficiently massive discs, runaway accretion produces planetary embryos, making planet formation a natural byproduct of star birth. A Sun-like star typically requires around 100 million years to complete this entire process.

Traces of Planetary History in Mature Systems

Long after a star has settled into maturity, circumstellar discs continue to tell a story about planetary formation. In our own Solar System, the asteroid belt between Mars and Jupiter serves as a reservoir of small bodies and a source of interplanetary dust, while the Edgeworth-Kuiper belt and the scattered disc extend beyond Neptune's orbit. Even the inner Oort cloud retains a toroid-like shape, though the outer Oort cloud is more spherical. Around white dwarfs, the final remnants of stellar evolution, the presence of circumstellar material is particularly striking because it demonstrates that planetary bodies survived the entire lifetime of their host star. In mature systems, these discs are evidence that planetesimal formation once occurred, preserving a record of the system's early architecture. The diversity of shapes and compositions in these remnants, from rocky asteroid belts to icy outer reservoirs, reflects the varied conditions under which solid bodies originally condensed from the primordial gas and dust.

Discs in Binary Star Systems

When two stars form together from a shared cloud of infalling gas, the resulting disc architecture depends on how much angular momentum the gas carries. The simplest configuration, a circumprimary disc, orbits the more massive star and forms whenever any angular momentum is present in the infalling material. A circumsecondary disc, orbiting the less massive companion, requires a higher threshold of angular momentum, with the exact amount depending on the mass ratio between the two stars. At the highest angular momentum levels, a circumbinary disc eventually forms, encircling both stars with an inner radius far exceeding the binary's orbital separation. Such a disc can reach an upper mass limit of roughly 0.005 solar masses before the binary can no longer perturb it strongly enough to drive further accretion. The star system GG Tauri provides a known example. Once a circumbinary disc is established, spiral density waves at the outer Lindblad resonances inevitably carve out an inner cavity around the binary pair, with the cavity size scaling in proportion to the binary separation.

Rhythms of Accretion and Orbital Evolution

The flow of gas from a circumbinary disc into the inner cavity is far from steady. For non-eccentric binaries, accretion variability follows a period approximately five times the binary's orbital period, driven by lumps in the inner gas corresponding to m = 1 outer Lindblad resonances. In eccentric binaries, the pattern shifts: each component scoops matter from the disc as it reaches apocenter, making the variability period match the binary orbital period itself. Over much longer secular timescales, hundreds of times the binary period, eccentric binaries experience additional variability tied to the apsidal precession of the inner cavity edge. This edge develops its own eccentricity and influences a significant region of the inner disc extending out to roughly ten times the binary separation. These precession-driven changes ripple outward, potentially affecting circumbinary planet formation and migration. The interplay between the binary's eccentricity, the cavity geometry, and resonant wave patterns thus creates a complex, multi-timescale accretion environment that shapes the long-term orbital evolution of the system.