SIMBAD references

Context. The gas and dust in circumstellar disks provide the raw materials to form planets. The study of organic molecules and their building blocks in such disks offers insight into the origin of the prebiotic environment of terrestrial planets.
Aims. We aim to determine the distribution of formaldehyde, H₂CO, in the disk around HD 163296 to assess the contribution of gas- and solid-phase formation routes of this simple organic.
Methods. Three formaldehyde lines were observed (H₂CO 3₀₃-2₀₂, H₂CO 3₂₂-2₂₁, and H₂CO 3₂₁-2₂₀) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at ∼0.5'' (60AU) spatial resolution. Different parameterizations of the H₂CO abundance were compared to the observed visibilities, using either a characteristic temperature, a characteristic radius or a radial power law index to describe the H₂CO chemistry. Similar models were applied to ALMA Science Verification data of C¹⁸O. In each scenario, χ² minimization on the visibilities was used to determine the best-fit model in each scenario.
Results. H₂CO 3₀₃-2₀₂ was readily detected via imaging, while the weaker H₂CO 3₂₂-2₂₁ and H₂CO 3₂₁-2₂₀ lines required matched filter analysis to detect. H₂CO is present throughout most of the gaseous disk, extending out to ∼550AU. An apparent 50AU inner radius of the H₂CO emission is likely caused by an optically thick dust continuum. The H₂CO radial intensity profile shows a peak at ∼100AU and a secondary bump at ∼300AU, suggesting increased production in the outer disk. In all modeling scenarios, fits to the H₂CO data show an increased abundance in the outer disk. The overall best-fit H₂CO model shows a factor of two enhancement beyond a radius of 270±20AU, with an inner abundance (relative to H₂) of 2-5x10^–12. The H₂CO emitting region has a lower limit on the kinetic temperature of T>20K. The C¹⁸O modeling suggests an order of magnitude depletion of C¹⁸O in the outer disk and an abundance of 4-12x10^–8 in the inner disk.
Conclusions. There is a desorption front seen in the H₂CO emission that roughly coincides with the outer edge of the 1.3 millimeter continuum. The increase in H₂CO outer disk emission could be a result of hydrogenation of CO ices on dust grains that are then sublimated via thermal desorption or UV photodesorption. Alternatively, there could be more efficient gas-phase production of H₂CO beyond ∼300AU if CO is photodisocciated in this region.