An Observational Perspective of Transitional Disks

Abstract

Transitional disks are objects whose inner disk regions have undergone substantial clearing. The Spitzer Space Telescope produced detailed spectral energy distributions (SEDs) of transitional disks that allowed us to infer their radial dust disk structure in some detail, revealing the diversity of this class of disks. The growing sample of transitional disks also opened up the possibility of demographic studies, which provided unique insights. There now exist (sub)millimeter and infrared images that confirm the presence of large clearings of dust in transitional disks. In addition, protoplanet candidates have been detected within some of these clearings. Transitional disks are thought to be a strong link to planet formation around young stars and are a key area to study if further progress is to be made on understanding the initial stages of planet formation. Here we provide a review and synthesis of transitional disk observations to date with the aim of providing timely direction to the field, which is about to undergo its next burst of growth as ALMA reaches its full potential. We discuss what we have learned about transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm and infrared. We note the limitations of these techniques, particularly with respect to the sizes of the clearings currently detectable, and highlight the need for pairing broad-band SEDs with multi-wavelength images to paint a more detailed picture of transitional disk structure. We review the gas in transitional disks, keeping in mind that future observations with ALMA will give us unprecedented access to gas in disks, and also observed infrared variability pointing to variable transitional disk structure, which may have implications for disks in general. We then distill the observations into constraints for the main disk clearing mechanisms proposed to date (i.e., photoevaporation, grain growth, and companions) and explore how the expected observational signatures from these mechanisms, particularly planet-induced disk clearing, compare to actual observations. Lastly, we discuss future avenues of inquiry to be pursued with ALMA, JWST, and next generation of ground-based telescopes.

Near-infrared imaging polarimetry of LkCa 15: A possible warped inner disk

“…Among some mechanisms (e.g., grain growth, photoevaporation, disk-planet interaction; see Espaillat et al 2014) that have been proposed to explain the clearing of the gaps in transitional disks, only gravitational interaction between disks and orbiting multiple planets can clear a large inner gap of > ∼ 15 AU or more (Zhu et al 2011) and preserve optically thick inner disk. Furthermore, de Juan Ovelar et al (2013) suggested a 1 MJup planet would create a similar size of outer gap edge at NIR and (sub-)mm wavelengths; conversely planets more massive than 1 MJup make different radial grain-size distribution in the dusty disk, and observations at different wavelengths capture different parts of grain-size distribution.…”

“…Analysis of their spectral energy distribution (SED) and the results of interferometry at infrared to millimeter wavelengths reveal the evidence of gap and cavity structures in many circumstellar disks. Such disks have been called transitional disks, and are thought to be an intermediate phase between gas-rich primordial disks and gas-poor debris disks (e.g., Espaillat et al 2014). When newly formed planet(s) are embedded in the disks, a gap structure (i.e., op- † Based on IRCS and HiCIAO data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.…”

Infrared study of transitional disks in Ophiuchus withHerschel

“…We consider objects SR 42 S (#5), WSB 46 (#7), and SR 20 W (#16) as tentative candidates despite they do not fulfill the criteria for their nominal values, but we are considering a high error in PACS photometry. In particular, SR 42 S (#5) has already been classified as transitional disk, and appears in Espaillat et al (2014) as so. For WSB 46 (#7) and SR 20 W (#16), further study is needed to determine their nature.…”

“…Transitional disks are key in this study since they appear to have an unusual radial structure. They have been proposed as the environment for planet formation (Marsh & Mahoney 1992) and have other proposed formation mechanisms, such as photo-evaporation by ultraviolet light emitted by the central star (Clarke et al 2001), grain growth (Dullemond & Dominik 2005), and gravitational instabilities (see Espaillat et al 2014, for a recent review on transitional disks). Recently, Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.…”

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…Disks that show these properties have been proposed to be in a transition phase ("transitional disks") toward their dispersal (e.g., Strom et al 1989;Skrutskie et al 1990;Takeuchi & Artymowicz 2001). The concept of disks in "transition" has been developing ever since its early definition, under new evidence provided by larger samples, more detailed observations, and improved models (e.g., Espaillat et al 2014;Owen 2016). Understanding the properties, origin, and evolution of inner disk dispersal and of gaps in protoplanetary disks bears the potential to indicate when, where, and in what physical and chemical environments planets form.…”