Mid-infrared JWST spectra of carbon stars in the Large Magellanic Cloud

TL;DR

This paper presents high-resolution mid-infrared spectra of carbon stars in the Large Magellanic Cloud from JWST, revealing molecular features, spectral variability, and questioning previous dust emission detections.

Contribution

First high-resolution JWST spectra of LMC carbon stars, showing molecular chemistry, spectral variability, and implications for dust feature identification.

Findings

C3 molecule causes a strong absorption at 5.2 um.

CS and HCN are detected in some stars, especially less dusty ones.

Spectral changes over 15-20 years are mainly due to stellar pulsation.

Abstract

Mid-infrared spectra from the Medium Resolution Spectrometer on the James Webb Space Telescope have revealed the molecular chemistry of carbon stars in the Large Magellanic Cloud with better resolution and sensitivity than previously possible. Our sample spans a range of dust-production rates and includes three relatively dust-free semiregular variables and six dustier Mira variables. All were observed 15-20 yr earlier with the Infrared Spectrograph on the Spitzer Space Telescope at lower spectral resolution. The new spectra show that the C3 molecule is responsible for a strong absorption band centered at 5.2 um. CS is clearly present in some of the sample, especially the stars with less dust. HCN also appears to be present. Some of the spectra have changed significantly between the Spitzer epoch and the MRS observations in 2023 and 2024, and in most cases these changes can be attributed to the stellar pulsation cycle. One exception is the disappearance of a dust emission feature at ~18 um in one of the Miras. The new spectra reveal a dip centered at ~10 um, which could arise either from an unknown carrier or from variable molecular emission to the red and blue. The presence of this spectral structure on the short-wavelength side of the SiC dust emission feature at ~11.3 um along with the broad C2H2 band centered at 14 um raise the possibility that some previously reported detections of weak SiC dust emission in other carbon stars may not be real.