What determines the inner sizes of protoplanetary disks?

Gas and dust “protoplanetary disks” that surround young stars are the sites where planets form. It is known that dust particles do not survive temperatures above 1000-2000 K, for which all disks should be cleared of dust close to the central stars. The size of such inner dust holes should be larger for more luminous (hotter) sources, ranging between ~ 0.01 and 20 au in optically-visible young stars. This range is critical for our understanding of planet formation, as it corresponds to the region where most planets’ orbits lie. In turn, those inner disk sizes translate to angular scales of milli-arcsecs even for the closest young stars, for which they can be resolved only from near-infrared interferometry.

This poster summarises our work in Marcos-Arenal et al. (2021, A&A 652, A68), where we have inferred inner disk sizes of a sample of young stars based on observations taken with GRAVITY at the ESO/Paranal Interferometer. Our interferometric measurements were complemented with all similar ones from the literature, which were updated by using Gaia EDR3 distances. As a result, we present the most complete “size-luminosity diagram” to date of optically-visible young stars. Although the overall trend relating both parameters is confirmed, there is significant scatter and stars with similar luminosities show widely different inner dust sizes. We have tested the three main hypotheses aiming to account for the observed size-luminosity relation and its scatter: the presence or absence of large amounts of gas close to the central star, alternative disk-to-star accretion mechanisms, and the different dust disk properties as inferred from the spectral energy distributions. None of these scenarios serve as a general explanation, and the origin of the size-luminosity relation and its scatter remains an open question. Future observations avoiding underlying trends with the distance are proposed, which may help to better understand what determines the inner sizes of protoplanetary disks.