Mid-IR period-magnitude relations for AGB stars

Asymptotic Giant Branch variables are found to obey period-luminosity relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24 microns where emission from circumstellar dust is expected to be dominant. Their loci in the M, logP diagrams are essentially the same for the LMC and for NGC6522 in spite of different ages and metallicities. There is no systematic trend of slope with wavelength. The offsets of the apparent magnitude vs. logP relations imply a difference between the two fields of 3.8 in distance modulus. The colours of the variables confirm that a principal period with log P > 1.75 is a necessary condition for detectable mass-loss. At the longest observed wavelength, 24 microns, many semi-regular variables have dust shells comparable in luminosity to those around Miras. There is a clear bifurcation in LMC colour-magnitude diagrams involving 24 micron magnitudes.

💡 Research Summary

The paper investigates the period‑luminosity (P‑L) relations of Asymptotic Giant Branch (AGB) variables across a wide range of mid‑infrared wavelengths, extending the well‑known relations at the near‑infrared Kₛ band (2.14 µm) out to 24 µm where thermal emission from circumstellar dust dominates. Two distinct stellar populations are examined: the Large Magellanic Cloud (LMC) and the Galactic bulge field NGC 6522. Both fields contain a mixture of Mira‑type long‑period variables (LPVs) and semi‑regular variables (SRVs) with periods derived from extensive time‑series photometry (OGLE‑III for the LMC, VVV/2MASS for NGC 6522). Mid‑infrared photometry from the Spitzer IRAC (3.6, 4.5, 5.8, 8.0 µm) and MIPS (24 µm) instruments provides mean magnitudes for each star, which are then plotted against log P.

Key findings are:

1. Wavelength‑independent slopes – Linear fits of absolute magnitude versus log P at 3.6, 4.5, 5.8, 8.0 and 24 µm all yield essentially the same slope (≈ –3.2 mag dex⁻¹). This indicates that the underlying pulsation physics, which drives the luminosity changes, is not significantly altered by the increasing contribution of dust emission at longer wavelengths.

2. Parallel P‑L sequences for LMC and NGC 6522 – The two fields share virtually identical P‑L tracks, offset in magnitude by 3.8 mag. This offset corresponds to the known distance‑modulus difference (LMC ≈ 18.5 mag, NGC 6522 ≈ 14.7 mag), confirming that the relations can be used as reliable distance indicators even when dust emission is strong.

3. Period threshold for detectable mass loss – Colour indices involving the 24 µm band, particularly (Kₛ–