A newly discovered stellar type: dusty post-red giant branch stars in the Magellanic Clouds

TL;DR

This paper identifies a new class of dusty, low-luminosity evolved stars in the Magellanic Clouds, likely representing post-RGB binaries with circumstellar dust, and establishes their evolutionary connection to RGB binaries.

Contribution

It introduces and characterizes a new stellar class, linking dusty post-RGB stars to binary evolution and providing observational evidence for this phase.

Findings

Order-of-magnitude agreement between observed and predicted birthrates.

Most dusty post-RGB stars likely result from binary interactions.

Mergers are common among low-RGB stars with IR excesses.

Abstract

Context: We present a newly discovered class of low-luminosity, dusty, evolved objects in the Magellanic Clouds. These objects have dust excesses, stellar parameters, and spectral energy distributions similar to those of dusty post-asymptotic giant branch (post-AGB) stars. However, they have lower luminosities and hence lower masses. We suggest that they have evolved off the red giant branch (RGB) instead of the AGB as a result of binary interaction. Aims: In this study we aim to place these objects in an evolutionary context and establish an evolutionary connection between RGB binaries (such as the sequence E variables) and our new sample of objects. Methods: We compared the theoretically predicted birthrates of the progeny of RGB binaries to the observational birthrates of the new sample of objects. Results: We find that there is order-of-magnitude agreement between the observed and predicted birthrates of post-RGB stars. The sources of uncertainty in the birthrates are discussed; the most important sources are probably the observational incompleteness factor and the post-RGB evolution rates. We also note that mergers are relatively common low on the RGB and that stars low on the RGB with mid-IR excesses may recently have undergone a merger. Conclusions: Our sample of dusty post-RGB stars most likely provides the first observational evidence for a newly discovered phase in binary evolution: post-RGB binaries with circumstellar dust.