A rotational line CO cooling rate prescription for AGB outflows

Abstract

Asymptotic Giant Branch (AGB) stars significantly contribute to the chemical composition of the universe. In their outflows, complex chemistry takes place, which critically depends on the local temperature. Therefore, if we want to accurately model the AGB environment, we need accurate cooling rates. The CO molecule is abundant in AGB outflows, and has a dipole moment, which enables it to cool through emission from its rotational transitions. We therefore expect it to significantly contribute to cooling in this environment, even at low temperatures ($10$ K $⩽T⩽3000$ K). Currently, CO cooling rates are available for ISM-like conditions, which encompasses a different parameter regime, with generally lower densities and velocity gradients, compared to AGB winds. Therefore, these ISM cooling rates might not be applicable to the AGB regime. In this paper, we compute CO cooling rates for hydrodynamics simulations of AGB outflows. To evaluate the net cooling rate, we calculate the energy level distribution of CO self-consistently, using the non Local Thermodynamical Equilibrium (NLTE) line radiative transfer code Magritte. We verify whether already existing CO cooling rate prescriptions for the interstellar medium (ISM) are applicable for this regime. We noticed minor differences between these prescriptions and our calculated cooling rates in general. However, when used far outside their originally intended parameter regimes, significant differences occur. Therefore, we propose a new CO cooling rate prescription for the AGB environment and we study how the computed cooling rate varies depending on input parameters.

arXiv comment15 pages, 33 figures; corrected typos in equations 12 and 14

References in corpus (16)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• An atomic and molecular database for analysis of submillimetre line observations
• Phantom: A smoothed particle hydrodynamics and magnetohydrodynamics code for astrophysics
• Rotational quenching of CO due to H${}_2$ collisions
• Protostellar Collapse with Various Metallicities
• Approximations for radiative cooling and heating in the solar chromosphere
• Low-Metallicity Star Formation : Prestellar Collapse and Protostellar Accretion in the Spherical Symmetry
• Evolution and Mass Loss of Cool Ageing Stars: a Daedalean Story
• The wind of W Hya as seen by Herschel - I. The CO envelope
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Magritte, a modern software library for 3D radiative transfer: I. Non-LTE atomic and molecular line modelling
• 3D simulations of AGB stellar winds -- I. Steady winds and dust formation
• Magritte, a modern software library for 3D radiative transfer: II. Adaptive ray-tracing, mesh construction and reduction
• A simple approach to CO cooling in molecular clouds
• Sensitivity study of chemistry in AGB outflows using chemical kinetics
• SPH modelling of AGB wind morphology in hierarchical triple systems \& comparison to observation of R Aql