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

¹ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères France
e-mail: ccarrizo@iram.fr
² Observatorio Astronómico Nacional, Ap 112, 28803 Alcalá de Henares, Spain
e-mail: v.bujarrabal@oan.es
³ Observatorio Astronómico Nacional, Alfonso XII No. 3, 28014 Madrid, Spain
⁴ Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
⁵ Instituto de Ciencia de Materiales de Madrid (CSIC), 28049 Madrid, Spain
⁶ Joint ALMA Observatory (JAO) and European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile

Received: 21 November 2016
Accepted: 13 January 2017

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.