[Paper Review] ALMA observations of the nearby AGB star L$_{ m 2}$ Puppis. II. Gas disk properties derived from $^{ m 12}$CO and $^{ m13}$CO $J=$3$-$2 emission

W. Homan, A. M. S. Richards+5 more|arXiv (Cornell University)|Mar 14, 2018

Astrophysics and Star Formation Studies73 references20 citations

TL;DR

This study uses ALMA high-resolution 12CO and 13CO J=3-2 observations to model the 3D non-LTE radiative transfer of the gas disk around the nearby AGB star L2 Puppis. It reveals a differentially rotating, edge-on disk with a radial H2 density profile steepened at -3.1, a 12CO/13CO abundance ratio of 10, and a disk mass of ~2×10⁻⁴ M☉. The derived angular momentum greatly exceeds that of the star, strongly supporting a ~1 Jupiter-mass companion as the source of disk formation and dynamics.

ABSTRACT

The circumstellar environment of the AGB star L$_{ m 2}$ Puppis was observed with ALMA in cycle 3, with a resolution of $15 imes 18 m\ mas$. 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 $^{ m 12}$CO $J=$3$-$2 and the optically thin $^{ m 13}$CO $J=$3$-$2 rotational transition to constrain the physical conditions in the disk. To realise this effort we make use of the 3D NLTE radiative transfer code { t LIME}. The temperature structure and velocity structure show a high degree of complexity, both radially and vertically. The radial H$_{ m 2}$ density profile in the disk plane is characterised by a power law with a slope of $-3.1$. We find a $^{ m 12}$CO over $^{ m 13}$CO 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.

Motivation & Objective

• To constrain the physical structure of the circumstellar gas disk around the AGB star L2 Puppis using high-resolution ALMA data.
• To model the 3D non-LTE radiative transfer of 12CO and 13CO J=3-2 emission to derive temperature, density, velocity, and abundance profiles.
• To investigate the origin and dynamics of the disk, particularly the role of a putative binary companion in angular momentum transfer and disk formation.
• To determine the molecular abundance ratio of 12CO to 13CO and assess the disk's mass and evolutionary state.
• To test whether the observed disk angular momentum can be explained by the star's intrinsic angular momentum or requires a companion.

Proposed method

• 3D non-LTE radiative transfer modeling using the LIME code to simulate the observed 12CO and 13CO J=3-2 emission.
• Use of CASA tools for data reduction and post-processing of ALMA visibility data.
• Assumption of a thin, vertically isothermal, non-self-gravitating disk in hydrostatic equilibrium with a power-law radial density profile.
• Inversion of observed channel maps to derive the 3D velocity field, temperature structure, and density distribution.
• Fitting of radial and vertical temperature profiles using a piecewise function with distinct regimes: inner disk, outer disk, and cold outer edge.
• Estimation of disk mass and angular momentum by integrating the derived H2 density and velocity structure.

Experimental results

Research questions

• RQ1 What is the 3D temperature and density structure of the gas disk around L2 Puppis, and how does it vary radially and vertically?
• RQ2 What is the radial velocity profile of the gas, and does it show Keplerian or sub-Keplerian motion, particularly in the outer disk?
• RQ3 What is the 12CO to 13CO abundance ratio in the disk, and what does it imply about the origin of the gas?
• RQ4 How does the observed angular momentum in the disk compare to the intrinsic angular momentum of the central star?
• RQ5 Can the observed disk structure and dynamics be explained by a single star, or is a binary companion required?

Key findings

• The radial H2 density profile in the disk plane follows a power law with a slope of -3.1, indicating strong radial concentration.
• The 12CO to 13CO abundance ratio is 10, consistent with values in other oxygen-rich AGB stars, suggesting the gas originates from the star itself.
• The disk mass is estimated at approximately 2×10⁻⁴ M☉, derived from the constrained density structure.
• The disk's angular momentum exceeds the star's intrinsic angular momentum by a factor of over 30, indicating an external source of angular momentum.
• A companion of approximately 1 Jupiter mass is required to explain the observed angular momentum, supporting the indirect detection reported in the prior paper.
• The temperature structure shows a steep radial drop in the inner disk (1000 K to 500 K within 6 AU), followed by a plateau and a sharp drop to below 100 K at the outer edge (23 AU), with strong vertical stratification in the inner disk.