[Paper Review] The circumstellar envelope around the S-type AGB star W Aql

S. Ramstedt, S. Mohamed+10 more|arXiv (Cornell University)|Sep 21, 2017

Stellar, planetary, and galactic studiesPhysics and Astronomy43 references21 citations

TL;DR

This study investigates the circumstellar envelope (CSE) of the S-type AGB star W Aquilae using high-resolution ALMA observations of CO(J=3→2) emission. It combines 3D smoothed particle hydrodynamics (SPH) binary interaction models with radiative transfer modeling to show that the dominant large-scale arc structures (~10′′ separation) are consistent with gravitational shaping by the known, distant companion (180 AU), while smaller-scale arcs (2–3′′) near the star are asymmetric and likely linked to unresolved inner structures or episodic mass loss, though their origin remains uncertain.

ABSTRACT

Context. Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars. Aims. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3 → 2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. Methods. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. Results. The estimated average mass-loss rate of W Aql is Ṁ = 3.0 × 10-6M⊙ yr-1 and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10′′ of the CSE is asymmetric with arc-like structures at separations of 2−3′′ scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations. Conclusions. The CO(J = 3 → 2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of ~10′′ and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yr) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

Motivation & Objective

• To understand how a binary companion shapes the circumstellar envelope (CSE) of an S-type AGB star, using W Aquilae as a case study.
• To determine the average mass-loss rate of W Aql using ALMA CO(J=3→2) data and single-dish constraints.
• To investigate whether observed arc-like structures in the CSE are caused by the known companion or other processes.
• To assess the detectability limits of binary-induced asymmetries in CSEs based on ALMA sensitivity and resolution.
• To explore the role of orbital eccentricity and mass-loss variability in shaping asymmetric CSE features.

Proposed method

• Acquired high-resolution ALMA subarcsecond observations of CO(J=3→2) emission toward W Aql.
• Constrained the average mass-loss rate using 1D radiative transfer modeling of the CO line emission, informed by ALMA brightness distribution and prior single-dish measurements.
• Constructed 3D SPH binary interaction models with the same orbital parameters as W Aql (separation ~180 AU, companion mass ~1.04–1.09 M⊙), testing two orbital eccentricities (e=0.2 and e=0.6).
• Performed 3D radiative transfer modeling on the SPH simulations to synthesize observable intensity maps and compare them with ALMA data.
• Simulated interferometric response to assess the detectability of density contrasts in the CSE under different orbital and mass-loss conditions.
• Used polarimetric dust-scattered light images (from PolCor) to compare dust and gas morphology.

Experimental results

Research questions

• RQ1 What is the average mass-loss rate of W Aql, and how does it compare to previous single-dish estimates?
• RQ2 Are the observed arc-like structures in the CO(J=3→2) emission caused by the known binary companion, and what orbital parameters are consistent with the data?
• RQ3 Why are there two distinct arc patterns at different separations (2–3′′ and ~10′′), and what physical processes could explain the smaller-scale asymmetry?
• RQ4 How does the orbital eccentricity of the known companion affect the detectability and morphology of CSE features in ALMA observations?
• RQ5 To what extent can ALMA resolve or detect density contrasts induced by a binary companion at separations larger than 180 AU?

Key findings

• The average mass-loss rate of W Aql is estimated at 3.0×10⁻⁶ M⊙ yr⁻¹, consistent with previous single-dish CO line observations.
• The CO(J=3→2) emission is dominated by a smooth, symmetric component, with superimposed weak arc-like structures at separations of ~10′′ and 2–3′′.
• The larger-scale arc pattern (~10′′ separation) is consistent with a binary interaction model assuming a low-eccentricity orbit (e≈0.2), indicating it is likely caused by the known companion at 180 AU.
• The smaller-scale arc pattern (2–3′′ separation) is asymmetric and coincides spatially with the previously mapped dust enhancement on the southwest side of the star, but its formation timescale (~200 years) does not match any known process in the system.
• The known companion’s separation is too large to produce strong density contrasts, and even larger separations would be undetectable with ALMA unless the orbit is highly eccentric or the mass-loss rate is significantly higher.
• The inner CSE shows signs of asymmetry and possible disruption, potentially hinting at a third, closer companion, though this remains speculative and requires deeper monitoring.