Structure and dynamics of the molecular gas in M 2-9: a follow-up study with ALMA
Sánchez Contreras, C.; Alcolea, J.; Santander-García, M.; Bujarrabal, V.; Neri, R.; Castro-Carrizo, A.; Nyman, L. -A.
France, Spain, Chile
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: 12CO and 13CO 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 12CO 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 × 10-3M⊙ was derived for the detected molecular component, the larger and older equatorial ring hosting most ( 90%) of this gas.