What determines the inner sizes of protoplanetary disks? I. Mendigutía1,*, P. Marcos-Arenal1,2, E. Koumpia3, R.D. Oudmaijer4, M. Vioque5,6, J. Guzmán-Díaz1, C. Wichittanakom7, W.J. de Wit3, B. Montesinos1, J.D. Ilee4 1 Centro de Astrobiología (CAB), CSIC-INTA, Madrid, Spain 2 European Space Agency, ESA-ESAC, Madrid, Spain 3 European Southern Observatory, Santiago, Chile 4 University of Leeds, UK 5Joint ALMA Observatory, Santiago, Chile 6National Radio Astronomy Observatory, Charlottesville, USA 7Thammasat University, Pathum Thani, Thailand * imendigutia@cab.inta-csic.es Background Planets form in gas and dust disks that surround young stars. Dust particles do not survive temperatures above 1000-2000 K, for which protoplanetary disks are 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 exoplanets’ orbits lie1. In turn, those inner disk sizes translate to angular scales of milli-arcsecs even for the closest sources, and can be resolved mainly for intermediate-mass “Herbig Ae/Be” stars only by using near-infrared interferometry. Although previous interferometric studies have shown that there is a correlation linking the stellar luminosity of Herbig Ae/Be stars and the size of their inner dust disks, this “size-luminosity correlation” may change for early type Herbig Bes and shows considerable scatter2,3. The main physical scenarios proposed to explain the observed size-luminosity distribution are related to the presence/absence of innermost gas, different disk-to-star accretion mechanisms, or different dust disk properties as inferred from spectral energy distributions (SEDs)4,5,6. However, no general observational confirmation has been provided to date. Our goal is to identify the physical mechanism that drives the observed distribution in the size-luminosity correlation. References (see the associated paper Marcos-Arenal et al. 2021, A&A, 652, A68) Fig. 1. Example: the young star V590 Mon. (Left) GRAVITY/VLTI interferometric observables -flux (red), squared visibility (blue) and differential phase (green)- vs wavelength around the Brɣ emission line for two different VLT baselines. (Right) Continuum squared visibilities vs spatial frequency (solid dots with errorbars). The best fit is indicated with a solid line and corresponds to a Gaussian disk on top of the central star, leading to an inner dust disk radius ~ 2 au. Alternative fits corresponding to ring and uniform disk models are plotted with dashed and dot-dashed lines. XV Reunión Científica Sociedad Española de Astronomía. Tenerife. September 2022 1: http://exoplanet.eu/ 2: Monnier & Millan-Gabet. 2002, ApJ, 579, 694 Fig. 2. (Left) Size-luminosity relation where each panel emphasizes a different aspect. Top left: Herbig Ae and Herbig Be stars; the five with our new GRAVITY data are highlighted in red. Top right: the sources with strong (blue) and weak (red) accretion rates. Bottom left: the sources with spectro-interferometric measurements indicating the presence of atomic gas inside the inner dust disk (blue) and the ones without such evidence (red). Bottom right: Group I and Group II sources based on the SED shape. For all panels, triangles indicate upper and lower limits and the solid and dashed lines indicate the expected boundaries of the inner dust disk radius based on the two main accretion scenarios and innermost gas densities. (Right) Size-luminosity relation where the stars have been color-coded to indicate different distance ranges, the rest of the symbols and lines as in the left panel. disk Results 1) We have updated the size-luminosity diagram, almost doubling the number of optically visible young stars considered in previous works. 2) We find no general trend linking the presence/absence of innermost gas,