title: ALMA high spatial resolution observations of the dense molecular region of NGC 6302.
abstract: BACKGROUND
  The mechanism behind the shaping of bipolar planetary nebulae is still poorly understood. It is becoming increasingly clear that the main agents must operate at their innermost regions, where a significant equatorial density enhancement should be present and related to the collimation of light and jet launching from the central star preferentially towards the polar directions. Most of the material in this equatorial condensation must be lost during the asymptotic giant branch as stellar wind and later released from the surface of dust grains to the gas phase in molecular form. Accurately tracing the molecule-rich regions of these objects can give valuable insight into the ejection mechanisms themselves.
  
  
  OBJECTIVE
  We investigate the physical conditions, structure and velocity field of the dense molecular region of the planetary nebula NGC 6302 by means of ALMA band 7 interferometric maps.
  
  
  METHODS
  The high spatial resolution of the12CO and13COJ=3-2 ALMA data allows for an analysis of the geometry of the ejecta in unprecedented detail. We built a spatio-kinematical model of the molecular region with the software SHAPE and performed detailed non-LTE calculations of excitation and radiative transfer with the shapemol plug-in.
  
  
  RESULTS
  We find that the molecular region consists of a massive ring out of which a system of fragments of lobe walls emerge and enclose the base of the lobes visible in the optical. The general properties of this region are in agreement with previous works, although the much greater spatial resolution of the data allows for a very detailed description. We confirm that the mass of the molecular region is 0.1 M⊙. Additionally, we report a previously undetected component at the nebular equator, an inner, younger ring inclined ~60° with respect to the main ring, showing a characteristic radius of 7.5×1016cm, a mass of 2.7×10-3M⊙, and a counterpart in optical images of the nebula. This inner ring has the same kinematical age as the northwest optical lobes, implying it was ejected approximately at the same time, hundreds of years after the ejection of the bulk of the molecular ring-like region. We discuss a sequence of events leading to the formation of the molecular and optical nebulae, and briefly speculate on the origin of this intriguing inner ring.
authors.affiliation: Instituto de Ciencia de Materiales de Madrid (CSIC), E-28049, Madrid, Spain; Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, E-28014, Madrid, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Santander-García
authors.affiliation: Observatorio Astronómico Nacional (IGN), Ap. de Correos 112, E-28803, Alcalá de Henares, Madrid, Spain.
authors.forename: V
authors.initials: V
authors.lastname: Bujarrabal
authors.affiliation: Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, E-28014, Madrid, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Alcolea
authors.affiliation: Institut de Radioastronomie Millimétrique, 38406, Saint Martin d'Hères, France.
authors.forename: A
authors.initials: A
authors.lastname: Castro-Carrizo
authors.affiliation: Centro de Astrobiología, CSIC-INTA, E-28692 Villanueva de la Cañada, Madrid, Spain.
authors.forename: C
authors.initials: C
authors.lastname: Sánchez Contreras
authors.affiliation: Instituto de Ciencia de Materiales de Madrid (CSIC), E-28049, Madrid, Spain.
authors.forename: G
authors.initials: G
authors.lastname: Quintana-Lacaci
authors.affiliation: Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain.
authors.forename: R L M
authors.initials: RL
authors.lastname: Corradi
authors.affiliation: Institut de Radioastronomie Millimétrique, 38406, Saint Martin d'Hères, France.
authors.forename: R
authors.initials: R
authors.lastname: Neri
keywords: Interstellar medium; NGC 6302, Physical data and processes; general – planetary nebulae; individual; kinematics and dynamics – planetary nebulae; radiative transfer