Spitzer SAGE-Spec: Near Infrared Spectroscopy, Dust Shells, and Cool Envelopes in Extreme Large Magellanic Cloud AGB Stars

TL;DR

This study presents near-infrared spectra of 39 AGB stars in the Large Magellanic Cloud, revealing their dust and molecular features, and identifying a post-AGB star with a circumstellar disk, enhancing understanding of stellar evolution.

Contribution

It provides new near-infrared spectral data for LMC AGB stars, linking spectral features with infrared colors and identifying a post-AGB star with a circumstellar disk.

Findings

Spectra match their infrared color-magnitude positions.

Identification of a post-AGB star with CO emission and a circumstellar disk.

Dusty envelopes and cool cores in extreme AGB stars.

Abstract

K-band spectra are presented for a sample of 39 Spitzer IRS SAGE-Spec sources in the Large Magellanic Cloud. The spectra exhibit characteristics in very good agreement with their positions in the near infrared - Spitzer color-magnitude diagrams and their properties as deduced from the Spitzer IRS spectra. Specifically, the near infrared spectra show strong atomic and molecular features representative of oxygen-rich and carbon-rich asymptotic giant branch stars, respectively. A small subset of stars were chosen from the luminous and red extreme "tip" of the color magnitude diagram. These objects have properties consistent with dusty envelopes but also cool, carbon-rich "stellar" cores. Modest amounts of dust mass loss combine with the stellar spectral energy distribution to make these objects appear extreme in their near infrared and mid infrared colors. One object in our sample, HV 915, a known post asymptotic giant branch star of the RV Tau type exhibits CO 2.3 micron band head emission consistent with previous work that demonstrates the object has a circumstellar disk.