Variability in Proto-Planetary Nebulae: VII. Light Curve Studies of Five Medium-bright, Oxygen-rich or Mixed-chemistry Post-AGB/Post-RGB Objects

TL;DR

This study monitors light variations over ten years in five evolved stars with infrared excesses, identifying pulsation periods and suggesting some are post-AGB while others may be post-RGB stars due to their low luminosity.

Contribution

First long-term light curve analysis of five evolved, oxygen-rich or mixed-chemistry stars, revealing pulsation periods and proposing a new identification of dusty post-RGB objects in the Milky Way.

Findings

Three stars are confirmed proto-planetary nebulae with pulsation periods.

Two stars likely represent dusty post-RGB objects with lower luminosity.

Periodic pulsations are detected in the cooler stars, with some showing longer-term variations.

Abstract

We have monitored over a ten-year interval the light variations of five evolved stars with very large mid-infrared excesses. All five objects appear to have oxygen-rich or mixed oxygen-rich and carbon-rich chemistries. They all vary in light: four over a small range of $\sim$0.2 mag and the fifth over a larger range of $\sim$0.7 mag. Spectral types range from G2 to B0. Periodic pulsations are found for the first time in the three cooler ones, IRAS 18075$-$0924 (123 days), 19207$+$2023 (96 days), and 20136$+$1309 (142 days). No significant periodicity is found in the hotter ones, but they appear to vary on a shorter time scale of a few days or less. Two also show some evidence of longer-term periodic variations ($\sim$4 yrs). Three appear to be proto-planetary nebulae, in the post-asymptotic giant branch (post-AGB) phase of stellar evolution. Their light variations are in general agreement with the relationships between temperature, pulsation period, and pulsation amplitude found in previously studied PPNe. The other two, however, appear to have too low a luminosity (1000$-$1500 L$_{sun}$), based on Gaia distances, to be in the post-AGB phase. Instead, they appear to be Milky Way analogues of the recently identified class of dusty post-red giant branch stars found in the Magellanic Clouds, which likely had their evolution interrupted by interaction with a binary companion. If this is the case, then these would be among the first dusty post-RGB objects identified in the Milky Way Galaxy.