Red Rectangle ALMA observations confirm rotating disk and outflow

High-resolution ALMA data and detailed 2D modeling confirm a rotating equatorial disk and a smaller CO-rich outflow around the Red Rectangle; the disk mass is reported at about 0.01 solar masses and its lifetime is estimated near 10,000 years.

The Red Rectangle is a bipolar nebula around a double stellar system whose primary is a post-asymptotic giant branch star. New ALMA observations in CO J = 6-5 and HCN J = 4-3, together with a re-reduction of CO J = 3-2, were analyzed with a detailed 2D nonlocal radiative transfer model. The study aims to characterize both rotating and expanding gas components and their relation. Rotating disks are uncommon in post-AGB nebulae, where gas is more often found in expansion. Key findings: - The data and modeling confirm a rotating equatorial disk plus an outflow that is mainly formed of gas leaving the disk. - The disk mass is reported at about 0.01 solar masses, and the CO-rich outflow mass is around ten times smaller. - Temperatures of roughly ≳100 K are derived for most nebular components. - The disk lifetime is roughly estimated at about 10,000 years; lifetimes for disks in a few other post-AGB composite nebulae are reported to range from about 5,000 to more than 20,000 years. - The disk angular momentum is reported as high, near ~9 M AU km s (as given in the study). - HCN observations show a wide velocity dispersion and indicate HCN is abundant only in the inner Keplerian disk, at ≲60 AU; the authors suggest HCN is formed in a dense photodissociation region driven by a UV excess, and they further suggest this UV excess could encourage formation and excitation of PAHs and other carbon-bearing macromolecules. Summary: The observations and modeling strengthen evidence for a long-lived, massive rotating disk that feeds a smaller CO-rich outflow and provide mass, temperature, and angular momentum constraints. The study links inner-disk chemistry, including HCN and likely PAH activity, to UV-driven photodissociation region processes around the central stellar system. Undetermined at this time.