A dichotomy in group II Herbig disks

Abstract

Context. Herbig stars can be classified as group I or group II depending on the shape of the far-infrared excess from the spectral energy distribution. This distinction may be evolutionary and related to the vertical structure of the Herbig disks. Aims. Our aim is to determine the emission height of Herbig disks and compare the resulting vertical extent of both groups. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of 12CO J = 2−1 emission lines at sufficient velocity (~0.3 km s−1) and spatial resolution (~30 au) of eight Herbig disks (four group I and four group II sources) to determine the emission heights from channel maps generated via geometrical methods previously developed in other works. Results. We find that all group I disks are vertically extended with a height to radius ratio of at least 0.25 and that for three of the disks, the gas emission profile can be traced out to 200–500 au. The group II disks are divided between MWC 480 and HD 163296, which have emission height profiles similar to the group I disks, and AK Sco and HD 142666, which are very flat (not exceeding a height of 10 au over the full extent traced) and more compact (<200 au in size). The brightness temperatures show no differences between the disks when the luminosity of the host star is accounted for. Conclusions. Our findings agree with previous work that suggests group I disks are vertically extended and that group II disks are either large and self-shadowed or compact. Both MWC 480 and HD 163296 could be precursors of group I disks that have not yet formed a cavity that would allow for irradiation of the outer parts of the disk. The very flat disks, AK Sco and HD 142666, could have been caused by significant settling due to the advanced age of the disks (~20 instead of <10 Myr). The large differences in vertical structure are not reflected in the spectral energy distributions of these disks. More and deeper observations at higher spatial and velocity resolution are necessary to further characterize the Herbig subgroups.