Structure and dynamics of the molecular gas in M 2–9: a follow-up study with ALMA

Abstract

Context. M 2–9 is a young planetary nebula (PN) that shows the characteristics of its last ejections in unprecedented detail. These last ejections are thought to trigger the post−asymptotic giant branch evolution.Aims. To assemble an overall picture of how M 2–9 was shaped, we analyzed the characteristics of the different molecular gas components and their relation with the warmer parts of the nebula that are visible in the optical domain.Methods. 12 CO and 13 CO J = 3−2 line emission maps were obtained with the Atacama Large Millimeter/submillimeter Array with high angular-resolution and sensitivity. Results. Two equatorial rings are found to host most of the cold molecular gas in M 2–9, as has been described for previous 12 CO J = 2−1 emission observations. In addition, we have detected a double crown-shaped structure that is symmetric with respect to the main nebular axis, which is located 1.5′′ away from both sides of the equatorial plane. Their distribution and kinematics show a very close relationship with the inner molecular ring: both are part of the same small hourglass structure formed ~900 yr ago. Two clearly distinct ejections with a remarkable axial symmetry are found to have shaped the molecular gas distribution in M 2–9, in agreement with the ejection processes that were probably responsible for the optical lobes. For the first time, the physical conditions of the different molecular components in M 2–9 are comprehensively analyzed with a radiative transfer model. They are found to follow standard laws, like those obtained in other young PN, with densities and temperatures decreasing with radius and ballistic expansion. A total mass of ~5 × 10-3 M ⊙ was derived for the detected molecular component, the larger and older equatorial ring hosting most (~90%) of this gas.