A WISE view on extreme AGB stars

Abstract

The IntroductionAt the end of their lives almost all low-and intermediate mass stars (with initial masses from ∼ 0.9 to ∼ 10 M ) will go through the (super)-asymptotic giant branch ((S-)AGB) phase. They end up as ∼ 0.55 − 1.4 M white dwarfs which implies that a large fraction of the initial mass of the star is returned to the interstellar medium (ISM). Pulsation is an important characteristic of AGB stars, and they are typically divided into stars with small amplitudes (the semi-regular variables, SRVs) and the large-amplitude Mira variables. The term long-period variable (LPV) is now commonly used for a pulsating AGB star, regardless of pulsation amplitude. The most promising mechanism to explain wind driving are pulsation-induced shock waves and radiation pressure on dust, especially regarding the more evolved AGB stars with low effective temperatures, large pulsation amplitudes, and high mass-loss rates (MLRs; see the review by Höfner & Olofsson (2018). Analysis of the MLRs of essentially complete samples of AGB stars in the Magellanic Clouds (MCs) has shown that the gas and dust return to the ISM is dominated by a small percentage of stars with the highest MLRs (e.g. Matsuura et al. (2009), Boyer et al. (2012), Nanni et al. (2019, and references therein). These stars are often characterised by the longest pulsation periods. Current surveys in the optical domain (including OGLE and Gaia) will however miss the reddest, most obscured AGB stars. At the very end of the AGB the (dust) MLR may become so high that the object becomes very faint, beyond the OGLE I-band detection limit of about 21 mag or the Gaia G-band limit of about 21.5 mag. The dust grains in the circumstellar envelope (CSE) scatter and absorb the emission in the optical to re-emit it in the NIR and mid-infrared (MIR), where they become bright sources. These stars are known to exist in the MCs. They were initially selected and identified as having Infrared Astronomical Satellite (IRAS) colours similar to obscured AGB stars in our Galaxy, and later on photometric and spectroscopic observations with the Spitzer Space Telescope (SST) confirmed this and added additional examples of this class of extreme mass-losing objects, mostly carbonrich AGB stars (Gruendl et al. 2008, Sloan et al. 2016, Groenewegen & Sloan 2018. The Wide-field Infrared Survey Explorer (WISE) (Wright et. al. 2010) and the Near-Earth Object WISE (NEOWISE) and NEO-WISE Reactivation mission (Mainzer et al. 2011(Mainzer et al. , 2014 are ideal surveys to study Long Period Variables (LPVs). The total times pan covered is about nine years which covers two or more pulsation cycles even for extreme long periods. Other advantages are that they survey at wavelengths where the reddest objects are brightest, and that they survey the entire sky. This poster describes the selection of candidate extreme AGB stars based on AllWISE mean colours and the fitting of the light curves (LCs) in the W1 and W2 bands. Based on these results several scientific topics are briefly discussed: Discovery of new st...

Infrared Properties of Carbon Stars in Our Galaxy

“…From the sample of 4909 CAGB stars with IRAS counterparts (referred to as CAGB_IC objects; see Table 1), we have selected a subset comprising 152 relatively well-known CAGB objects (e.g., Suh 2000;Groenewegen et al 2002;Chen & Yang 2012;Groenewegen 2022). Within this subset, encompassing 51 visual-CAGB_IC objects characterized by thin dust shells (τ 10 < 0.5) and 101 IR-CAGB_IC objects featuring thick dust shells (τ 10 0.5), we have meticulously computed detailed theoretical model SEDs (see Section 4) that demonstrate a commendable ability to replicate the observations.…”

“…The consensus in the scientific community holds that more evolved (or more massive) AGB stars tend to display larger pulsation amplitudes, longer pulsation periods, and higher mass-loss rates (De Beck et al 2010;Suh & Kwon 2013b;Groenewegen 2022).…”

unTimely: a Full-sky, Time-domain unWISE Catalog

“…3. Mira variables and other long-period variables (e.g., Chen et al 2018;Groenewegen 2022;Sanders et al 2022). 4.…”

CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB

“…The K s -band light curves have been analysed using the codes and methodology outlined in Groenewegen et al (2020) assuming a single period. For the WISE data a very similar approach was adopted to analyse the light curves in the W1 (3.4 µm) and W2 (4.6 µm) bands, as outlined in Groenewegen (2022). The results, including the adopted period are collected in Table 3 that also cites periods found in the literature.…”