Episodic Mass Loss in Evolved Massive Stars (ASSESS)

ASSESS (Episodic MAss LoSS in Evolved MaSsive Stars) is an ERC-funded project (2018-2023, PI Bonanos) investigating the role of episodic mass loss in the evolution of massive stars and its role in the early Universe. We will conduct a systematic multi-wavelength survey of evolved massive stars in the local Universe, covering a large range of metallicities (1/15 ∼ 3 z). The project hinges on the fact that mass-losing stars form dust and are thus bright sources in the mid-IR. It targets 27 nearby galaxies for which Spitzer point-source catalogues are available. The mid-IR photometry is supplemented by data from Pan-STARRS, the UKIRT Hemisphere Survey, the VISTA Hemisphere Survey, and Gaia.

Spectra of a large number (~1000) of sources selected from the photometry will be obtained using multi-object spectroscopy.
Key scientific questions we aim to address are:

  • What is the frequency of major mass ejection events – like the giant eruption of η Car – and how does this frequency depend on metallicity?
  • Is the presence of significant mid-IR excess a hallmark of LBVs, and is mass loss from LBVs mostly episodic or in the form of a continuous wind?
  • Do all massive stars undergo one or more mass ejection episodes in the months/years before they explode as supernova?
  • What is the frequency and evolutionary status of B supergiant stars with circumstellar discs (sgB[e] stars), and is there a connection with LBVs?
  • How long are the cool and hot hypergiant phases, and which parameters determine whether a massive star will go through these post-RSG stages?

The VLT/Flames Tarantula Survey (VFTS)

The VFTS is a European Southern Observatory (ESO) Large Programme which has obtained multi-epoch FLAMES spectroscopy of over 900 stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). 30 Dor is our closest view of a starburst-like region in the local universe, giving us a unique laboratory in which to study stellar and cluster evolution. It’s a rich stellar nursery, with many examples of the rare, short-lived evolutionary phases of the most massive stars.

The VFTS has an unprecedented dataset of multi-epoch observations of the stellar content of 30 Dor. We are now analysing these data to provide answers to fundamental questions such as the effects that stellar rotation has on the evolution of stars, the binary fraction of massive stars, how binarity affects stellar evolution, and to also study the gas and stellar dynamics in this intricate and beautiful cluster, to provide input for models of star and cluster formation.

The VFTS has resulted in more than 35 publications with a current total of over 1700 citations. A list of publications can be found here.

The Tarantula Massive Binary Monitoring (TMBM)

TMBM is a follow-up of the VFTS project, and has obtained 32 additional epochs of FLAMES spectra for 102 potential binary systems in 30 Doradus. This data has been used to obtain orbital solutions for 82 of these systems. One of the key results so far is the discovery of VFTS 352 as being the most massive over-contact binary known to date.

Southern Massive Stars at High Angular Resolution (SMASH+)

SMASH+ is a large VLTi programme designed to detect companion stars in the separation range between those resolved by spectroscopy and direct imaging, making use of interferometric observations. The programme observed all (~180) nearby Southern O-type stars brighter then H = 7 mag, and detected a total of almost 200 previously unknown companions in the separation range of 1 mas to 8″. Including known spectroscopic binaries, this implies a multiplicity fraction for Galactic O stars of 90% for separations smaller than 8″.