Abstract
We report on the discovery of correlations between dust and CO gas tracers of the 0.1–10 au region in planet-forming disks around young intermediate-mass stars. The abundance of refractory elements on stellar photospheres decreases as the location of hot CO gas emission recedes to larger disk radii, and as the near-infrared excess emission from hot dust in the inner disk decreases. The linked behavior between these observables demonstrates that the recession of infrared CO emission to larger disk radii traces an inner disk region where dust is being depleted. We also find that Herbig disk cavities have either low (~5–10%) or high (~20–35%) near-infrared excess, a dichotomy that has not been captured by the classic definition of “pre-transitional” disks.
Highlights
The vast majority of exoplanets discovered so far lies at less than 3 au from the central star, with super-Earths abundant well inside 1 au (e.g., Petigura et al 2013)
First measurements on the evolution of inner disks came from spatially unresolved observations of spectral energy distributions (SEDs), which in some disks show a lower infrared (IR) flux that was attributed to a deficit of hot inner material (Strom et al 1989)
The Fe/H measurements in Herbig Ae/Be stars have recently been found to correlate with the presence or absence of dust cavities detected by millimeter interferometry imaging, suggesting that the stellar photospheres keep an imprint of the dust/gas ratio of their inner disks through the accreted material (Kama et al 2015)
Summary
The vast majority of exoplanets discovered so far lies at less than 3 au from the central star, with super-Earths abundant well inside 1 au (e.g., Petigura et al 2013). First measurements on the evolution of inner disks came from spatially unresolved observations of spectral energy distributions (SEDs), which in some disks show a lower infrared (IR) flux that was attributed to a deficit of hot inner material (Strom et al 1989) This deficit has been interpreted as due to inner holes (“transitional” disks) or gaps (“pre-transitional” disks), depending on the level of near-IR flux related to an inner dust belt inside the cavity (Espaillat et al 2007). Independent studies have found new evidence in dust and gas tracers pointing in the direction of depletion processes in these inner disk regions (Kama et al 2015; Banzatti & Pontoppidan 2015). This behavior demonstrates a strong link between molecular gas and dust, providing an important framework for better understanding the evolving structure of planet-forming regions at 10 au (Sect. 4)
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