Joint Discussion on Very Massive Stars in the Local Universe

  • - Weighing the most massive stars from their binary motions
  • - Stellar spectra of O and Wolf-Rayet stars
  • - Mass determinations from stellar spectroscopy and model atmosphere analysis
  • - Formation of the most massive stars
  • - Mass loss mechanisms, incl. eruptions of Luminous Blue Variables
  • - Stellar structure and evolution modeling
  • - The fate of the most massive stars (over cosmological time)
  • - Mass and energy return to the interstellar medium (ISM)
  • Abstract

    Recent studies have claimed the existence of very massive stars (VMS) up to ~300 solar masses in the local Universe. As this finding seems to represent a paradigm shift for the stellar upper-mass limit, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects.

    Rationale

    We hold a 1.5 Day (spread out over 3 Days) during the first week of the GA (20-24 August 2012) -- the same week as the Special Session "Infrared view of massive stars". Whereas very massive stars (VMS) with over 100 solar masses have been claimed to exist in the early Universe, recent studies such as those of Crowther et al. (2010) and Gal-Yam et al. (2009) -- discussing the existence and deaths of stars up to ~300 solar masses in the local Universe -- came as a surprise to many workers both inside and outside the field. Before the full implications of these findings can be explored, it is imperative and timely to discuss the various lines of evidence for VMS in relatively nearby massive clusters. We discuss the determination of both the current and final masses of the most massive stars. The aim is to reach broad consensus between observers and theorists on how to identify and quantify the importance of the dominant physical processes. The objects may evolve almost chemically homogeneously, implying that the detailed mixing processes (e.g., rotation, magnetic fields) could be less relevant compared to ~10-50 solar-mass stars. Instead, the evolution and death of VMS is likely dominated by mass loss.
  • The main topics:
  • 1) How massive are the most massive stars?
  • (A) the various pieces of evidence for VMS, including discovery techniques, status of data, their reliability and uncertainties; and
  • (B) the Formation of VMS (e.g., via disk accretion and/or mergers in dense cluster environments)
  • 2. How much mass do these extreme objects lose?
  • (A) What are their mass-loss rates, or do they lose additional mass in Luminous Blue Variable--LBV-type eruptions? (or through interactions with companions?)
  • (B) How do these extreme objects evolve and die?
  • Specifically, do these VMS have a sufficiently extreme mass loss to end their lives as normal Wolf-Rayet stars (possibly giving rise to Type Ibc supernovae; SNe). Or do they explode prematurely during the LBV phase? Or might they even give rise to pair-instability SNe?
  • The answers have strong implications for the chemical enrichment of the interstellar medium and galaxies, as well as for their injected energy via ionizing radiation, stellar winds, and subsequent SN explosions.
  • IAU XXVIII General Assembly: VMS in local Universe, Sep 2011.