This Joint Discussion contains 4 Invited Reviews by Leaders in the Field. Young and established scientists are encouraged to submit abstracts for contributed talks.

1. How massive are the most massive stars?

(A) The various pieces of evidence for VMS

Crowther et al. (2010) recently re-analyzed the most massive hydrogen- and nitrogen-rich Wolf-Rayet (WNh) stars in the center of R136. The conclusion was that stars usually assumed to be below the Figer (2005) stellar upper-mass limit of 150 solar masses, were actually found to be much more luminous -- almost 10 million times the solar luminosity -- corresponding to present-day masses of up to 265 solar masses, implying initial masses up to 320 solar masses. 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. Therefore, before discussing the implications, we shall first scrutinize the sources of uncertainty and additional aspects of their empirical properties (variability, multiplicity, environment).

(A) The formation mechanisms of VMS

For decades it was a struggle to form stars over 10 solar masses. The reason is that the standard "disk accretion" scenario commonly applied to solar-mass stars may not operate for high-mass stars, as radiation pressure on dust grains might halt and reverse the accretion flow onto the central object. Because of this problem, astrophysicist have been creative in forming massive stars via competitive accretion and merging in dense cluster environments (e.g., Bonnell et al. 1998). In more recent times several multi-D simulations have shown that massive stars might form via disk accretion after all (e.g., Krumholz et al. 2009; Kuiper et al. 2010). In the light of recent claims for the existence of 300+ solar-mass objects in dense clusters, however, we set aside some fraction of the time to redress the issue of forming such very massive stars in extreme environments.

2. How much mass do these extreme objects lose?

(A) What are their mass-loss rates? Do they lose additional mass in eruptions?

(A) How do these extreme objects evolve and die?

IAU XXVIII General Assembly: VMS in local Universe, Sep 2011.