SHAPEMOL: a 3D code for calculating CO line emission in planetary and protoplanetary nebulae

Detailed model-fitting of the complex nebula NGC 6302^⋆

¹ Observatorio Astronómico Nacional, Ap. de Correos 112, 28803 Alcalá de Henares, Madrid, Spain
e-mail: m.santander@oan.es
² Instituto de Ciencia de Materiales de Madrid (CSIC), 28049 Madrid, Spain
³ Department of Physics & Astronomy, University of Calgary, Calgary, Alberta, Canada
⁴ Instituto de Astronomía Universidad Nacional Autónoma de México, C.P. 22860, Ensenada, Mexico

Received: 23 July 2013
Accepted: 21 October 2014

Abstract

Context. Modern instrumentation in radioastronomy constitutes a valuable tool for studying the Universe: ALMA has reached unprecedented sensitivities and spatial resolution, while Herschel/HIFI has opened a new window (most of the sub-mm and far-infrared ranges are only accessible from space) for probing molecular warm gas (~50−1000 K). On the other hand, the software SHAPE has emerged in the past few years as a standard tool for determining the morphology and velocity field of different kinds of gaseous emission nebulae via spatio-kinematical modelling. Standard SHAPE implements radiative transfer solving, but it is only available for atomic species and not for molecules.

Aims. Being aware of the growing importance of the development of tools for simplifying the analyses of molecular data from new-era observatories, we introduce the computer code shapemol, a complement to SHAPE, with which we intend to fill the so-far under-developed molecular niche.

Methods. shapemol enables user-friendly, spatio-kinematic modelling with accurate non-LTE calculations of excitation and radiative transfer in CO lines. Currently, it allows radiative transfer solving in the ¹²CO and ¹³CO J = 1−0 to J = 17−16 lines, but its implementation permits easily extending the code to different transitions and other molecular species, either by the code developers or by the user. Used along SHAPE, shapemol allows easily generating synthetic maps to test against interferometric observations, as well as synthetic line profiles to match single-dish observations.

Results. We give a full description of how shapemol works, and we discuss its limitations and the sources of uncertainty to be expected in the final synthetic profiles or maps. As an example of the power and versatility of shapemol, we build a model of the molecular envelope of the planetary nebula NGC 6302 and compare it with ¹²CO and ¹³CO J = 2−1 interferometric maps from SMA and high-J transitions from Herschel/HIFI. We find the molecular envelope to have a complex, broken ring-like structure with an inner, hotter region and several “fingers” and high-velocity blobs, emerging outwards from the plane of the ring. We derive a mass of 0.11 M_⊙ for the molecular envelope.