title: Further ALMA observations and detailed modeling of the Red Rectangle.
abstract: OBJECTIVE
  We aim to study the rotating and expanding gas in the Red Rectangle, which is a well known object that recently left the asymptotic giant branch (AGB) phase. We analyze the properties of both components and the relation between them. Rotating disks have been very elusive in post-AGB nebulae, in which gas is almost always found to be in expansion.
  
  
  METHODS
  We present new high-quality ALMA observations of C17OJ=6-5 and H13CNJ=4-3 line emission and results from a new reduction of already published13COJ=3-2 data. A detailed model fitting of all the molecular line data, including previous maps and single-dish observations of lines of CO, CII, and CI, was performed using a sophisticated code that includes an accurate nonlocal treatment of radiative transfer in 2D. These observations (of low- and high-opacity lines requiring various degrees of excitation) and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties, particularly in the determination of the boundaries of the CO-rich gas and some properties of the outflow.
  
  
  RESULTS
  We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01M⊙, and that of the CO-rich outflow is around ten times smaller. High temperatures of ≳ 100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between 5000 and more than 20000 yr. The angular momentum of the disk is found to be high, ~ 9M⊙AU km s-1, which is comparable to that of the stellar system at present. Our observations of H13CN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ≲ 60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to an efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.
authors.affiliation: Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, E-28803 Alcalá de Henares, Spain.
authors.forename: V
authors.initials: V
authors.lastname: Bujarrabal
authors.affiliation: Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406, Saint Martin d'Hères, France.
authors.forename: A
authors.initials: A
authors.lastname: Castro-Carrizo
authors.affiliation: Observatorio Astronómico Nacional (OAN-IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Alcolea
authors.affiliation: Observatorio Astronómico Nacional (OAN-IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain; Instituto de Ciencia de Materiales de Madrid (CSIC). Calle Sor Juana Inés de la Cruz 3, E-28049 Cantoblanco, Madrid, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Santander-García
authors.affiliation: Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium.
authors.forename: H
authors.initials: H
authors.lastname: Van Winckel
authors.affiliation: Centro de Astrobiología (CSIC-INTA), ESAC Campus, E-28691 Villanueva de la Cañada, Madrid, Spain.
authors.forename: C
authors.initials: C
authors.lastname: Sánchez Contreras
keywords: circumstellar matter; planetary nebulae: individual: Red Rectangle; radio-lines: stars; stars: AGB and post-AGB