Bonanos et al. 2023: Introducing the ASSESS project: Episodic Mass Loss in Evolved Massive Stars – Key to Understanding the Explosive Early Universe

Introducing the ASSESS project: Episodic Mass Loss in Evolved Massive Stars – Key to Understanding the Explosive Early Universe

A.Z. Bonanos, G. Maravelias, M. Yang, F. Tramper, S. de Wit, E. Zapartas, K. Antoniadis, E. Christodoulou, G. Munoz-Sanchez

Episodic mass loss is not understood theoretically, neither accounted for in state-of-the-art models of stellar evolution, which has far-reaching consequences for many areas of astronomy. We introduce the ERC-funded ASSESS project (2018-2024), which aims to determine whether episodic mass loss is a dominant process in the evolution of the most massive stars, by conducting the first extensive, multi-wavelength survey of evolved massive stars in the nearby Universe. It hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to derive physical parameters of ∼1000 dusty, evolved massive stars in ∼25 nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models, and quantify the duration and frequency of episodic mass loss as a function of metallicity. The approach involves applying machine-learning algorithms to select dusty, luminous targets from existing multi-band photometry of nearby galaxies. We present the first results of the project, including the machine-learning methodology for target selection and results from our spectroscopic observations so far. The emerging trend for the ubiquity of episodic mass loss, if confirmed, will be key to understanding the explosive early Universe and will have profound consequences for low-metallicity stars, reionization, and the chemical evolution of galaxies.

Figure 1. Newly identified spectra of evolved massive stars in our FORS2 data from the VLT, including a candidate LBV and RSG in NGC 55 (top row), a sgB[e] in NGC 300 and a RSG in NGC 7793 (bottom row).

arXiv.org: 2308.04891

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