The formation of massive stars, with masses in excess of 10 M0, is problematic due to the high radiation pressure on dust grains, which may actually halt the infall of gas and thus appears to limit stellar masses. Simulations to date suggest that this sets an upper mass limit to accretion somewhere in the (10-40)MQ range (Wolfire & Casinelli 1986; Yorke and Sonnhalter 2002; Edgar and Clarke, 2004). Clearly, there needs to be a mechanism for circumventing this problem because stars as massive as (80-150)Mq exist (Figer 2005). Possible solutions include disc accretion and radiation beaming (Yorke & Sonnhalter 2002), Rayleigh Taylor instabilities in the infalling gas (Krumholz et al. 2005a), stellar collisions between single stars (Bonnell et al. 1998; Bonnell and Bate 2002; Bally & Zinnecker 2005) and the collisional merger of close binary systems (Bonnell & Bate 2005) (Figure 39.8).
If radiation pressure does prove to be a significant impediment to the formation of massive stars, then a metal-rich environment should prove more difficult for the formation of these objects. The expectation is then that there should be fewer massive stars formed in such environments and especially that a signature in terms of the IMFs should be apparent as a function of metallicity.
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