Transitional DiskEdit

Transitional disks are a distinctive class of circumstellar disks around young stars that exhibit a deficit of material in their inner regions while retaining substantial material at larger radii. This configuration produces a characteristic signature in their emission: diminished near-infrared flux from the inner disk, contrasted with strong mid- to far-infrared and millimeter emission from the outer disk. The term reflects the idea that these systems are in a transitional phase between the dense, actively accreting protoplanetary disks and more evolved, debris-like disks. For researchers, transitional disks offer a crucial window into the early steps of planet formation and disk evolution, as the inner cavities may be carved by forming planets, by disk physics, or by a combination of processes. See protoplanetary disk and planet formation for broader context.

These disks are typically found around young stars in nearby star-forming regions and associations, commonly several million years old. The architectural differences among transitional disks—ranging from sharp, well-defined inner holes to more diffuse inner gaps—reflect a diversity of evolutionary pathways. Observations across wavelengths, including spectral energy distributions (SEDs) and resolved imaging, are key to diagnosing their structure. See Spectral energy distribution and ALMA for examples of the diagnostic tools used to study these systems.

Definition and context

A transitional disk is a subset of the broader category of protoplanetary disk systems characterized by a depleted inner region. The inner cavity, cleared of dust and sometimes gas, spans a range of sizes from a few astronomical units (AU) up to tens of AU in several well-studied cases. The outer disk remains optically thick and continues to feed material outward, maintaining substantial infrared and submillimeter emission. The classification sometimes distinguishes between transition disks with a complete inner hole and “pre-transitional” disks that retain a narrow, optically thick inner ring with an optically thinner gap separating it from the outer disk. See pre-transitional disk for related terminology.

Advances in high-resolution instrumentation, including millimeter interferometry with ALMA and scattered-light or thermal emission imaging with large ground-based observatories, have enabled direct images of cavities and gaps in some transitional disks. Notable examples of systems that have been studied extensively in this context include TW Hydrae, LkCa 15, and HD 100546 among others. The spatial scales of detected cavities vary, illustrating that the clearing mechanisms—whatever their combination—act over a range of disk environments and stellar properties. See TW Hydrae and LkCa 15 for case studies.

Observational signatures

Spectral energy distribution: The near-infrared portion of the SED is reduced relative to full disks, indicating a drop in hot dust emission from the innermost disk, while mid- to far-infrared and millimeter wavelengths remain prominent due to the cooler material in the outer disk. See Spectral energy distribution.
Spatially resolved cavities: Interferometric imaging in the millimeter or submillimeter bands can directly reveal inner holes or gaps. Resolved imaging confirms the presence of cleared zones and outlines the outer disk geometry. See ALMA for the instrument behind many of these discoveries and HD 100546 as an example of a well-studied system with spatially resolved structure.
Gas and dust distribution: In some transitional disks, gas may persist in the inner regions even as dust is depleted, while others show little to no inner disk gas. Observations of molecular lines (for example, CO) help map gas distributions and accretion activity. See gas in disks and CO (carbon monoxide) lines for related topics.

Physical mechanisms and debates

Several physical processes can lead to inner clearing, and many disks probably involve more than one mechanism over their lifetimes. The main proposed pathways include:

Planet formation and disk-planet interactions: The gravitational influence of forming planets can carve gaps and cavities in the disk. A planet can open a clean, sharp inner edge and repel dust while allowing some gas to flow through, potentially sustaining continued accretion onto the star. Direct imaging and indirect dynamical inferences have linked some cavities to planetary companions, and notable systems such as PDS 70 showcase protoplanets within disk cavities. See planet formation and PDS 70 for related discussions.
Photoevaporation: Stellar radiation (extreme ultraviolet, far ultraviolet, and X-ray) heats the disk gas, driving a photoevaporative wind that can remove material from the inner disk once the accretion rate drops below the photoevaporation rate. This process can create or enlarge cavities and is a major competing explanation for some transition disks, especially those with little ongoing accretion. See photoevaporation for the mechanism and the observational implications.
Dust evolution and grain growth: As dust grains collide and grow, they can settle toward the midplane and drift, reducing the opacity of the inner regions. This dust evolution can mimic or contribute to inner holes, even when gas is still present. See grain growth and dust in disks for context.
Disk dynamics and winds: Magnetic fields, disk winds, and complex dynamical interactions can contribute to clearing or reshaping inner disk regions, sometimes in concert with the other processes above. See magnetohydrodynamics in disks for a broader framework.

Because transitional disks can present a mix of indicators, current research often emphasizes a multi-faceted approach: combining SED analysis with high-resolution imaging and molecular-line spectroscopy to distinguish whether a given cavity is planet-induced, photoevaporative, or the result of dust physics. See ALMA observations of transitional disks and gas in disks for methodological context.

Evolution and demographics

Transitional disks occupy a transitional niche in the broader story of disk evolution from dense, actively accreting protoplanetary disks to more tenuous debris-dominated systems. Their incidence among young stellar objects informs theories of planet formation timescales and disk clearing mechanisms. Studies suggest that transitional disks may represent a substantial fraction of disks at ages of a few million years in some star-forming regions, though observational biases and selection effects influence reported frequencies. The relative importance of the proposed clearing mechanisms likely varies with stellar mass, radiation field, and environmental history. See star formation and disk evolution for wider context.

Notable case studies—such as the well-studied TW Hya system and other nearby young stars—provide laboratory conditions to test theories of cavity formation and evolution. See TW Hydrae and HD 100546 for detailed examples and ongoing debates about their inner disk structures and possible planetary companions.

Notable transitional disks

TW Hydrae: One of the closest and best-studied transitional disks, offering high-quality imaging and spectroscopy across wavelengths. See TW Hydrae.
LkCa 15: A system with evidence for an inner cavity and candidate planetary companions inferred from imaging and interferometry. See LkCa 15.
HD 100546: A young star with a prominent outer disk and a well-defined inner region that has been the subject of extensive imaging studies. See HD 100546.
PDS 70: Notable for hosting confirmed protoplanets within its cavity, providing a direct observational link between planet formation and disk clearing. See PDS 70.

These examples illustrate the diversity of transitional disks, from those where clear planetary companions are inferred or detected to others where disk physics alone account for the observed inner clearing.