A SPHERE survey of self-shadowed planet-forming disks

To date, nearly two hundred planet-forming disks have been imaged at high resolution. Our propensity to study bright and extended objects does, however, bias our view of the disk demography. In this work, we aim to help alleviate this bias by analyzing fifteen disks targeted with VLT/SPHERE that look faint in scattered light. Sources were selected based on a low far-infrared excess from the spectral energy distribution. The comparison with the ALMA images available for a few sources shows that the scattered light surveyed by these datasets is only detected from a small portion of the disk extent. The mild anticorrelation between the disk brightness and the near-infrared excess demonstrates that these disks are self-shadowed: the inner disk rim intercepts much starlight and leaves the outer disk in penumbra. Based on the uniform distribution of the disk brightness in scattered light across all spectral types, self-shadowing would act similarly for inner rims at a different distance from the star. We discuss how the illumination pattern of the outer disk may evolve with time. Some objects in the sample are proposed to be at an intermediate stage toward bright disks from the literature, with either no shadow or with signs of azimuthally confined shadows.