Modeling the physical and excitation conditions of the molecular envelope of NGC 7027^⋆

¹ Observatorio Astronómico Nacional, Ap. de Correos 112, 28803, Alcalá de Henares, Madrid, Spain
e-mail: m.santander@oan.es
² Centro de Astrobiología, CSIC-INTA, Ctra de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain

Received: 12 March 2012
Accepted: 2 July 2012

Abstract

Context. The link between the shaping of bipolar planetary nebulae and the mass ejection activity of their central stars is still poorly understood. Appropriately characterizing the evolution of shells ejected during the late stages of stellar evolution and the interaction between these shells is fundamental to gain insight into the mechanism of nebular shaping. It must include the study of the molecular emission, which tracks the mass-loss history during the late asymptotic giant branch (AGB) and post-AGB stages, when the nebula is being actively shaped.

Aims. The Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel is an invaluable tool because it opens a new window (most of the sub-mm and far-infrared range is only accessible from space) from which to probe warm molecular gas (~50–1000 K). This paper presents a radiative transfer, spatio-kinematic modeling of the molecular envelope of the young planetary nebula NGC 7027 in several high- and low-J¹²CO and ¹³CO transitions observed by Herschel/HIFI and the IRAM 30-m radio telescope, and discusses the structure and dynamics of the molecular envelope.

Methods. We developed a code that, used along with the SHAPE software, implements spatio-kinematic modeling with accurate non-LTE calculations of line excitation and radiative transfer in molecular species. We used this code to build a relatively simple “Russian doll” model to account for the physical and excitation conditions of the molecular envelope of NGC 7027.

Results. The model nebula consists of four nested, mildly bipolar shells plus a pair of high-velocity blobs. The innermost shell is the thinnest and shows a significant increase in physical conditions (temperature, density, abundance and velocity) compared to the adjacent shell. This is a clear indication of a shock front in the system, which may have played a role in shaping the nebula. Each of the high-velocity blobs is divided into two sections with considerably different physical conditions. The striking presence of H₂O in NGC 7027, a C-rich nebula, is likely due to photo-induced chemistry from the hot central star, although formation of water by shocks cannot be ruled out. The computed molecular mass of the nebula is 1.3 M_⊙, compatible with that derived from previous works.