[Paper Review] Mass-loss rates and luminosity functions of dust-enshrouded AGB stars and red supergiants in the LMC

JT van Loon, M. A. T. Groenewegen+6 more|arXiv (Cornell University)|Sep 24, 1999

Stellar, planetary, and galactic studies1 references19 citations

TL;DR

This study uses radiative transfer modeling of ISO and ground-based infrared data to derive mass-loss rates and bolometric luminosities for 57 dust-enshrouded AGB stars and red supergiants in the Large Magellanic Cloud. It finds a luminosity gap at M_bol ≈ -7.5 mag separating AGB stars from RSGs, with mass-loss rates spanning 10⁻⁷ to 10⁻³ M⊙ yr⁻¹, and shows that intense mass loss occurs in short, recurrent phases where rates exceed radiation momentum limits due to multiple scattering, particularly in RSGs and evolved AGB stars.

ABSTRACT

A radiative transfer code is used to model the spectral energy distributions of 57 mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC) for which ISO spectroscopic and photometric data are available. As a result we derive mass-loss rates and bolometric luminosities. A gap in the luminosity distribution around M_bol = -7.5 mag separates AGB stars from RSGs. The luminosity distributions of optically bright carbon stars, dust-enshrouded carbon stars and dust-enshrouded M-type stars have only little overlap, suggesting that the dust-enshrouded AGB stars are at the very tip of the AGB and will not evolve significantly in luminosity before mass loss ends their AGB evolution. Derived mass-loss rates span a range from Mdot about 10^-7 to 10^-3 M_sun/yr. More luminous and cooler stars are found to reach higher mass-loss rates. The highest mass-loss rates exceed the classical limit set by the momentum of the stellar radiation field, L/c, by a factor of a few due to multiple scattering of photons in the circumstellar dust envelope. Mass-loss rates are lower than the mass consumption rate by nuclear burning, Mdot_nuc, for most of the RSGs. Two RSGs have Mdot >> Mdot_nuc, however, suggesting that RSGs shed most of their stellar mantles in short phases of intense mass loss. Stars on the thermal pulsing AGB may also experience episodes of intensified mass loss, but their quiescent mass-loss rates are usually already higher than Mdot_nuc.

Motivation & Objective

• Quantify mass-loss rates and bolometric luminosities for dust-enshrouded AGB stars and red supergiants in the Large Magellanic Cloud (LMC) using infrared SEDs.
• Identify evolutionary distinctions between AGB stars and RSGs through luminosity function analysis.
• Determine whether mass-loss rates exceed the radiation momentum limit (L/c) and assess the role of multiple scattering in driving winds.
• Investigate the relationship between mass-loss rates, luminosity, effective temperature, and nuclear burning mass consumption rates.
• Clarify the evolutionary status of carbon-rich and oxygen-rich AGB stars by analyzing their luminosity distributions and mass-loss behavior.

Proposed method

• Applied a spherically symmetric radiative transfer code to model the spectral energy distributions (SEDs) of 57 AGB stars and RSGs in the LMC using ISO spectroscopic and photometric data.
• Used input parameters including the stellar SED, dust grain optical properties, and CSE geometry to fit observed IR SEDs and derive physical parameters.
• Calibrated bolometric luminosities using the known distance to the LMC and derived effective temperatures from SED fitting.
• Compared derived mass-loss rates with the radiation momentum limit (L/c) and the nuclear mass consumption rate (Ṁ_nuc) to assess driving mechanisms and evolutionary implications.
• Classified stars by chemical composition (oxygen-rich vs. carbon-rich) and spectral type to analyze luminosity and mass-loss trends.
• Assessed the role of multiple scattering in enhancing momentum transfer by comparing observed mass-loss rates with single-scattering limits.

Experimental results

Research questions

• RQ1 What is the distribution of bolometric luminosities for dust-enshrouded AGB stars and red supergiants in the LMC, and is there a clear separation between the two populations?
• RQ2 How do mass-loss rates vary with luminosity and effective temperature for AGB stars and RSGs, and what drives the observed trends?
• RQ3 Are observed mass-loss rates consistent with the radiation momentum limit (L/c), or do they require multiple scattering to be explained?
• RQ4 What is the relationship between mass-loss rates and the rate of mass consumption by nuclear burning (Ṁ_nuc), and what does this imply for the evolutionary state of RSGs?
• RQ5 Can the existence of luminous carbon stars be explained by a final thermal pulse and HBB switching off near the tip of the AGB?

Key findings

• The luminosity distribution of dust-enshrouded AGB stars and RSGs in the LMC shows a clear gap at M_bol ≈ -7.5 mag, separating the AGB and RSG populations.
• Mass-loss rates span a wide range from ~10⁻⁷ to 10⁻³ M⊙ yr⁻¹, with higher rates found in more luminous and cooler stars.
• Mass-loss rates in some cases exceed the radiation momentum limit (L/c) by a factor of a few, providing evidence for multiple scattering of photons in the dusty circumstellar envelope.
• RSGs with mass-loss rates significantly exceeding their nuclear mass consumption rate (Ṁ_nuc) are rare but exist, indicating short-lived, intense mass-loss phases.
• Mass-loss rates for dust-enshrouded AGB stars consistently exceed Ṁ_nuc, suggesting that mass loss is a dominant evolutionary driver even in quiescent phases.
• Stars with later spectral subclasses (cooler temperatures) and higher luminosities exhibit larger mass-loss rates, indicating a strong dependence on effective temperature and luminosity.