Properties of Carbon-rich Asymptotic Giant Branch Stars in the LMC and the Milky Way

TL;DR

This study compares carbon-rich AGB stars in the LMC and the Milky Way by compiling large, cross-matched catalogs, constructing spectral energy distributions, and fitting them with radiative-transfer dust-shell models calibrated on the LMC. Distances for LMC stars are anchored by its known distance, enabling robust scaling to Galactic CAGB stars via SED fitting; Mira variables additionally leverage a period–magnitude relation derived from LMC Miras. The results show broadly similar infrared properties in both galaxies, but the LMC lacks extremely dust-enshrouded CAGB objects, likely due to lower metallicity and star-formation activity. Overall, SED-based distances are found to be reliable and practical for large Galactic samples, and the authors provide an updated Galactic CAGB catalog with multiple distance estimates for broad use in future work.

Abstract

We present a comparative study of carbon-rich asymptotic giant branch (CAGB) stars in the Large Magellanic Cloud (LMC; 7347 stars) and the Milky Way (7163 stars) using infrared color-magnitude diagrams, spectral energy distributions (SEDs), two-color diagrams, and variability data. Observed SEDs are compared with theoretical models to characterize the central stars and their circumstellar dust envelopes and to estimate distances. For the LMC, a set of best-fitting CAGB models is derived by fitting observed SEDs with radiative transfer models, utilizing the galaxy's well-established distance. For Galactic CAGB stars, where Gaia DR3 parallaxes are uncertain, we estimate distances by fitting observed SEDs with the CAGB models validated against LMC stars, and for Mira variables, from the period-magnitude relation calibrated with LMC Miras. A comparison of these approaches demonstrates that the SED-based distances are both reliable and practical for a large sample of Galactic CAGB stars. We find that CAGB stars in both galaxies show broadly similar infrared properties, although the LMC sample lacks stars with extremely thick dust envelopes.

Figures (11)

• Figure 1: The top panel shows CAGB model SEDs for seven representative models (see Table \ref{['tab:tab3']}). For LMC-CAGB stars, the figure shows the distance distribution (middle) and the relations between dust optical depth and luminosity (bottom) derived from CAGB model SEDs (Table \ref{['tab:tab3']}). See Section \ref{['sec:lum-lcagb']} for details.
• Figure 2: For LMC-CAGB stars, the top panels show the relations between optical depth and luminosity derived using two different methods based on CAGB models. In the top panels, the areas of the filled circular markers are proportional to the square root of the number of objects at each point. The bottom panels present IR CMDs for LMC-CAGB stars, with the brown arrow indicating an object likely to be an extrinsic carbon star (see Section \ref{['sec:sedlmc']}). For CAGB models (AMC, $T_c$ = 1000 K): $\tau_{10}$ = 0.0001, 0.01, 0.1, 0.5, 1, 2, and 4 from left to right (see Table \ref{['tab:tab3']}). The total number of objects in each subgroup is given, with the values in parentheses denoting the number of plotted sources for which data are available. See Section \ref{['sec:lum-lcagb']} for details.
• Figure 3: The top panels show IR CMDs for Galactic CAGB stars. For Galactic CAGB stars, distances were derived from CAGB model SEDs, with Gaussian Monte Carlo scatter (GS) in $\log_{10} d$ ($\sigma$=$\pm$0.0798 dex; obtained from LMC-CAGB stars; see Figure \ref{['f1']}). For CAGB stars in the LMC and the Milky Way, the middle panels show the relations between optical depth and luminosity, and the bottom panels show the luminosity functions. In the middle panels, the areas of the filled circular markers are proportional to the square root of the number of objects at each point. The total number of objects is given, with the values in parentheses denoting the number of plotted sources for which data are available. See Section \ref{['sec:lum-gcagb']} for details.
• Figure 4: Observed and model SEDs for CAGB stars. The upper four panels show LMC stars, and the lower four show Milky Way stars. Each panel indicates the $\tau_{10}$ value and the corresponding scaling distance (bottom right). A suffix “s2” on $\tau_{10}$ denotes AMC+SiC dust opacity (20 % SiC). See Section \ref{['sec:seds']} for details.
• Figure 5: Observed and model SEDs for two LMC-CAGB stars with thick dust envelopes. The object in the right panel is likely an extrinsic carbon star (see Section \ref{['sec:sedlmc']}).