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

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.

Properties of Anomalous and Type II Cepheids in the Small and Large Magellanic Clouds

“…We have found infrared excess in 10% of the W Vir stars and 60% of the RV Tau stars. The findings by [9] of the existence of (pulsating) stars that have a lower luminosity value than the luminosity predicted if they would have been evolving as a single star, and that they show IR excess connected to binarity, are also confirmed by us.…”

“…What was surprising is the detection of IR excess among the WVir stars below luminosities predicted by single star evolutionary tracks. This IR excess was independently found by [7][8][9] to be dusty post-RGB stars and possibly related to binary evolution.…”

Infrared excess in semi-regular variables in the Magellanic Clouds

“…However, for ∼ 60% of the RVT and ∼ 10% of the (p)W Vir objects an infrared excess is detected from the SED fitting. The results of [4] are confirmed that there exist (pulsating) stars with luminosities below that predicted from single-star evolution that show a clear infrared excess, likely related to binarity.…”

Constraining Roche-Lobe Overflow Models Using the Hot-Subdwarf Wide Binary Population

“…Their existence however is supported by the existence of CB disks around AGB stars (e.g. Hillen et al 2017) and by the discovery of dusty post RGB binaries (Kamath et al 2016). Direct observations of dust around sdB binaries, with for example ALMA, could be used to further constrain the properties of CB disks.…”