Variability in Proto-Planetary Nebulae: VI. Multi-Telescope Light Curves Studies of Several Medium-Bright (V=13-15), Carbon-Rich Objects

TL;DR

This study presents a decade of photometric data on five evolved stars, including three carbon-rich proto-planetary nebulae and two giants, revealing their pulsation periods, variability, and dust-related brightness changes, enhancing understanding of their late stellar evolution.

Contribution

It provides refined pulsation periods for three PPNe, identifies two giant stars with unique variability patterns, and links dust obscuration phenomena to stellar evolution in these objects.

Findings

Refined pulsation periods for three PPNe (71, 80, 84 days)

Discovery of giant stars with unique dust-related brightness variations

Identification of non-pulsational variability similar to R Corona Borealis stars

Abstract

We present ten years of new photometric monitoring of the light variability of five evolved stars with strong mid-infrared emission from surrounding dust. Three are known carbon-rich proto-planetary nebulae (PPNe) with F$-$G spectral types; the nature of the other two was previously unknown. For the three PPNe, we determine or refine the pulsation periods of IRAS 04296+3429 (71 days), 06530$-$0213 (80 days), and 23304+6147 (84 days). A secondary period was found for each, with a period ratio P$_2$/P$_1$ of 0.9. The light variations are small, 0.1-0.2 mag. These are similar to values found in other PPNe. The other two are found to be giant stars. IRAS 09296+1159 pulsates with a period of only 47 days but reaches pulsational light variations of 0.5 mag. Supplemental spectroscopy reveals the spectrum of a CH carbon star. IRAS 08359$-$1644 is a G1III star that does not display pulsational variability; rather, it shows non-periodic decreases of brightness of up to 0.5 mag over this ten-year interval. These drops in brightness are reminiscent of the light curves of R Corona Borealis variables, but with much smaller decreases in brightness, and are likely due to transient dust obscuration. Its SED is very similar to that of the unusual oxygen-rich giant star HDE 233517, which possesses mid-infrared hydrocarbon emission features. These two non-PPNe turn out to members of the rare group of giant stars with large mid-infrared excesses due to dust, objects which presumably have interesting evolutionary histories.