By the time young stellar objects become optically visible T Tauri stars, the star-building process is mostly over, and the mass remaining in the circumstel- lar disks represents <1% of the mass of the stars (Andrews & Williams, 2007; Andrews, 2015). Primordial disks typically last 2-3 Myrs before dissipating through different processes and becoming disk-less (Class III) stars. The tran- sitional stage between Class II and III objects is characterized by decreased levels of NIR and/or MIR excess (NIR: 1-5µm, MIR: 5-20µm), and an excess emission at wavelengths ≥ 20µm similar to that of a primordial disk. This can be explained by an inner disk region devoid of hot dust grains. The first
objects showing such observational signatures were identified by Strom et al. (1989) and are currently known as transitional disks. In general, the term “transitional disk” (TD) is used to describe protoplanetary disk which is op- tically thick and gas-rich with central clearings due to a non-continuos dust distribution. Observations of transitional disks have shown dust-poor cavities that contain significant gas material (P´erez et al., 2014). Although the com- position of the inner disk is only ∼ 1% dust, it dominates the opacity of the disk. The lack of near-IR excesses implies that the small particles in the inner disk region should be depleted by around 10−4 from standard full disk values.
Therefore, the transitional disk classification is based on the evolution of dust particles instead of gas, which represents 99% of the mass and drives the dy- namics in the disk. Understanding the evolution of the dust particles is key because they are the “raw” material for the formation ofterrestrial planets and the cores of thegiants planets.
TDs are a diverse group of objects. Their central cavities have a wide range of radii (∼1-100 au); however, more massive stars tend to have TDs with larger cavities (Mer´ın et al., 2010). Owen & Clarke (2012a) showed observational evidence for two “different” types of transitional disks based on their millimeter flux. The first type is characterized by low millimeter flux, inner holes of less than 10 au and low accretion rates of around<10−9M/yr. This type of disk
is consistent with the last stages of disk dissipation. The second type presents high millimeter fluxes, inner holes larger than 10 au and accretion rates of around 10−9-10−8 M/yr. Different physical mechanisms are proposed in the literature to explain inner holes of transition disks: 1) planetary formation, 2) photoevaporation, 3) grain growth/coagulation/settling, 4) magnetorotational
instability, and 5) dynamical effects of the presence of a stellar companion, as discussed below.
Planet Formation
In general, there are two main models proposed for the formation of gas gi- ant planets: core accretion and gravitational instability. In the core accretion model, micron-sized dust grains stick together to build up mm-to-cm-sized ag- gregates. Although important barriers exist, these aggregates eventually form 10-100 km planetesimals (e.g., as a product of a gravitationally unstable dust layer; Furuya & Nakagawa, 2001). Gravitational interactions allow the oli- garchic growth of some of these objects to create planetary embryos. Even- tually, these embryos collide and grow into a core with a critical mass of ∼10 M⊕, starting a phase of rapid gas accretion. This gas accretion phase continues until the planet opens a gap in the protoplanetary disk (Chambers, 2011). The pressure gradient at the edge of the gaps filters the dust being transported inward from the outer disk (Pinilla et al. 2015) and the forming planets divert most of the material onto itself. The combination of both effects results in the dust poor inner disks characteristic of transition objects.
Photoevaporation
Another probable mechanism responsible for opening gaps in the inner regions of disks is photoevaporation by its central star. It has been shown that at the surface layers of the inner disk, the temperature of the gas can be higher than the dust temperature as a result of the radiation from the central star. This
gas flows off the disk and escapes from the gravitational field of the host star. The main sources of heating are the strong EUV and FUV acting at different radii in the disk and penetration depth. The evaporative mass depends on the penetration depth of the FUV and EUV photons and the temperature of the heated gas (Dullemond et al., 2007). FUV dominates at r > 50 au and EUV affects mostly the planet-forming region, r<<50 au. While the mass transport across the disk is high, material moving in from the outer disks compensates any loses due to photoevaporation. As both the disk mass and the accretion rate decrease, the mass-loss rate becomes important and a gap is formed in the disk at a critical radius. Eventually, the inner disk dissipates in dynamical timescales, leaving an empty, photo evaporating cavity that expands from the inside-out.
Binary Stars
Dynamical interactions in binary systems can also produce inner holes in pro- toplanetary disks and transition disk SEDs (Ireland & Kraus, 2008). Most stars are formed in binary or multiple systems; therefore, understanding the effects of this multiplicity on disk evolution is important for planet forma- tion theory. Observationally, planets in binary systems are less common than planetary systems in single stars, which is reflected with only∼60 planets de- tected that reside around binary systems. To date, most of these circumbinary planets reside in binary systems with a relatively large separation, often with separations of > 500 au (Roell et al., 2012), while only ∼10 of these binaries have a separation of less than 100 au, and 5 exoplanets reside in close bina- ries with separations of ∼20 au. Those planets orbiting only one of the stars,
with the second star acting as a perturber, are known as S-type configuration planets(Dvorak, Froeschle & Froeschle, 1986). Planets orbiting both stars in a binary system are known as P-Type or circumbinary planets (e.g. Beuermann et al., 2011).
From theoretical and observational studies, it is established that a single star is more suitable for the formation of a planet. In binary stars, especially those or- biting closer than∼ 100 au, the protoplanetary disk is hotter and dynamically more excited impacting the initial steps for planet formation, such as the co- agulation and growth of planetesimals or the gravitational instability process. However, planets in S-Type configuration can still form at small distances from the star (Nelson et al., 2000). Close-binary stars with separations < 1 au are more suitable for the formation of a P-type of circumbinary planets.
The second part of my thesis assesses these transitional disks in the context of their impact on disk evolution, and addresses questions such as:
1. How many disks classified as transitional disks are actually cir-
cumbinary disks?
2. How long does the true pre-transitional and transitional disk