2024A&A...682A..63R

Context. Nearby associations are ideal regions to study coeval samples of protoplanetary and debris disks down to late M-type stars. Those aged 5-10 Myr, where most of the disk should have already dissipated forming planets, are of particular interest.
Aims. We present the first complete study of both protoplanetary and debris disks in a young region, using the η Chamaeleontis (η Cha) association as a test bench to study the cold disk content. We obtained submillimeter data for the entire core population down to late M-type stars, plus a few halo members.
Methods. We performed a continuum submillimeter survey with APEX/LABOCA of all the core populations of the η Cha association. These data were combined with archival multiwavelength photometry to compile a complete spectral energy distribution. The disk properties were derived by modeling protoplanetary and debris disks using RADMC 2D and DMS, respectively. We compute a lower limit of the disk millimeter fraction, which is then compared to the corresponding disk fraction in the infrared for η Cha. We also revisit and refine the age estimate for the region, using the Gaia eDR3 astrometry and photometry for the core sources.
Results. We find that protoplanetary disks in η Cha typically have holes with radii on the order of 0.01-0.03 AU, while ring-like emission from the debris disks is located between 20 and 650 au from the central star. The parallaxes and Gaia eDR3 photometry, in combination with the PARSEC and COLIBRI isochrones, enable us to confirm an age of η Cha between 7 and 9 Myr. In general, the disk mass seems insufficient to support accretion over a long time, even for the lowest mass accretors, a clear difference with other regions and also a sign that the mass budget is further underestimated. We do not find a correlation between the stellar masses, accretion rates, and disk masses, although this could be due to sample issues (very few, mostly low-mass objects). We confirm that the presence of inner holes is not enough to stop accretion unless accompanied by dramatic changes to the total disk mass content. Comparing η Cha with other regions at different ages, we find that the physical processes responsible for debris disks (e.g., dust growth, dust trapping) efficiently act in less than 5 Myr.