[Paper Review] Properties of detached shells around carbon stars: Evidence of interacting winds

F. L. Schoeier, M. Lindqvist+1 more|ArXiv.org|Mar 10, 2005

Astrophysics and Star Formation Studies50 citations

TL;DR

This study presents radiative transfer modeling of CO line emission and dust continuum from detached molecular shells around carbon stars, providing strong evidence for interacting winds. A brief, high-velocity mass-loss episode (~10⁻⁵ M⊙ yr⁻¹ for ~300 years) followed by a return to low mass-loss rates (~10⁻⁸ M⊙ yr⁻¹) explains the observed shell kinematics and mass distribution, with DR~Ser identified as a new example of such a young shell system.

ABSTRACT

The nature of the mechanism responsible for producing the spectacular, geometrically thin, spherical shells found around some carbon stars has been an enigma for some time. Based on extensive radiative transfer modelling of both CO line emission and dust continuum radiation for all objects with known detached molecular shells, we present compelling evidence that these shells show clear signs of interaction with a surrounding medium. The derived masses of the shells increase with radial distance from the central star while their velocities decrease. A simple model for interacting winds indicate that the mass-loss rate producing the faster moving wind has to be almost two orders of magnitudes higher (~10^-5 solar masses per year) than the slower AGB wind (a few 10^-7 solar masses per year) preceding this violent event. At the same time, the present-day mass-loss rates are very low indicating that the epoch of high mass-loss rate was relatively short, on the order of a few hundred years. This, together with the number of sources exhibiting this phenomenon, suggests a connection with He-shell flashes (thermal pulses). We report the detection of a detached molecular shell around the carbon star DR Ser, as revealed from new single-dish CO (sub-)millimetre line observations. The properties of the shell are similar to those characterising the young shell around U Cam.

Motivation & Objective

• To investigate the physical origin of geometrically thin, detached molecular shells observed around carbon stars.
• To determine whether these shells result from a transient, high-velocity mass-loss event interacting with a pre-existing slow wind.
• To constrain the mass-loss history of carbon stars using multi-line CO and dust continuum observations.
• To identify new detached shell sources and assess the prevalence of such phenomena among AGB stars.
• To evaluate the role of He-shell flashes (thermal pulses) in driving the observed mass-loss modulations.

Proposed method

• Conducted single-dish CO (sub-)millimeter line observations of R~Scl, U~Cam, V644~Sco, and DR~Ser to detect and characterize detached shells.
• Perfomed radiative transfer modeling of both CO line emission and dust continuum emission to derive physical parameters of the shells.
• Used a two-wind interacting model: a fast, high-mass-loss-rate wind (~10⁻⁵ M⊙ yr⁻¹) interacting with a prior, slow AGB wind (~10⁻⁷ M⊙ yr⁻¹).
• Modeled shell kinematics and intensity profiles across multiple CO rotational transitions (J=1→0 to J=8→7) to constrain temperature and column density.
• Compared model predictions with observed line intensities and spatial morphology to assess consistency.
• Incorporated constraints from dust scattered light and infrared data to cross-validate shell location and mass.

Experimental results

Research questions

• RQ1 What causes the formation of geometrically thin, detached molecular shells around carbon stars?
• RQ2 Can the observed shell kinematics and mass distribution be explained by an interacting wind scenario?
• RQ3 What are the timescales and mass-loss rates associated with the high-velocity ejection event?
• RQ4 How do the physical properties of the shells (mass, velocity, temperature) vary with radial distance from the central star?
• RQ5 Is there a connection between the shell formation and He-shell flashes (thermal pulses) in AGB evolution?

Key findings

• A new detached molecular shell was detected around the carbon star DR~Ser, bringing the total number of known such systems to seven.
• The shell masses of all observed sources increase with radial distance from the central star, while their expansion velocities decrease.
• The data are best explained by a brief, high-velocity mass-loss episode (~10⁻⁵ M⊙ yr⁻¹) lasting ~300 years, followed by a return to low mass-loss rates (~10⁻⁸ M⊙ yr⁻¹).
• The interacting wind model successfully reproduces the observed shell morphology, kinematics, and line intensities across multiple CO transitions.
• The gas kinetic temperature remains the most uncertain parameter, with high- $J$ CO transitions offering the best potential for future constraint.
• R~Scl exhibits a discrepancy between dust-scattered light and CO modeling, suggesting a possibly distinct mass-loss history requiring further study via interferometry.