Structure and dynamics of the molecular gas in M2-9: a follow-up study with ALMA

Castro Carrizo, A., Bujarrabal, V., Neri, R., Alcolea, J., Contreras, C. S., Santander García, M., Nyman, L. A. 2017. Structure and dynamics of the molecular gas in M2-9: a follow-up study with ALMA. Astronomy and Astrophysics 600, DOI: 10.1051/0004-6361/201630101

M2-9 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 M2-9 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. (CO)-C-12 and (CO)-C-13 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 M2-9, as has been described for previous (CO)-C-12 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 similar to 900 yr ago. Two clearly distinct ejections with a remarkable axial symmetry are found to have shaped the molecular gas distribution in M2-9, 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 M2-9 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 similar to 5 x 10 (3) M-circle dot was derived for the detected molecular component, the larger and older equatorial ring hosting most (similar to 90%) of this gas.

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