ALMA observations of the nearby AGB star L₂ Puppis

II. Gas disk properties derived from ¹²CO and ¹³CO J = 3–2 emission

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
e-mail: ward.homan@ster.kuleuven.be
² Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, 1058 Santiago, Chile
³ LESIA (UMR 8109), Observatoire de Paris, PSL Research University, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁴ JBCA, Department Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁵ Sterrenkundig Instituut “Anton Pannekoek”, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶ Universidad Catolica del Norte, Instituto de Astronomia, Avenida Angamos 0610, Antofagasta, Chile

Received: 23 December 2016
Accepted: 28 February 2017

Abstract

The circumstellar environment of the AGB star L₂ Puppis was observed with ALMA in cycle 3, with a resolution of 15 × 18 mas. The molecular emission shows a differentially rotating disk, inclined to a nearly edge-on position. In the first paper in this series (Paper I) the molecular emission was analysed to accurately deduce the motion of the gas in the equatorial regions of the disk. In this work we model the optically thick ¹²CO J = 3−2 and the optically thin ¹³CO J = 3−2 rotational transition to constrain the physical conditions in the disk. To realise this effort we make use of the 3D NLTE radiative transfer code LIME. The temperature structure and velocity structure show a high degree of complexity, both radially and vertically. The radial H₂ density profile in the disk plane is characterised by a power law with a slope of −3.1. We find a ¹²CO over ¹³CO abundance ratio of 10 inside the disk. Finally, estimations of the angular momentum in the disk surpass the expected available angular momentum of the star, strongly supporting the indirect detection of a compact binary companion reported in Paper I. We estimate the mass of the companion to be around 1 Jupiter mass.