Massive Star Formation at Supersolar Metallicities: Constraints on the Initial Mass Function

Abstract

Metals enhance the cooling efficiency of molecular clouds, promoting fragmentation. Consequently, increasing the metallicity may boost the formation of low-mass stars. Within the integrated galaxy initial mass function (IGIMF) theory, this effect is empirically captured by a linear relation between the slope of the low-mass stellar IMF, ${\alpha }_1$, and the metal mass fraction, $Z$. This linear ${\alpha }_1$-$Z$ relation has been calibrated up to $\approx 2Z_{\odot }$, though higher metallicity environments are known to exist. We show that if the linear ${\alpha }_1$-$Z$ relation extends to higher metallicities ($[Z]\gtrsim 0.5$), massive star formation is suppressed entirely. Alternatively, fragmentation efficiency may saturate beyond some metallicity threshold if gravitational collapse cascades rapidly enough. To model this behavior, we propose a logistic function describing the transition from metallicity-sensitive to metallicity-insensitive fragmentation regimes. We provide a user-friendly public code, pyIGIMF, which enables the instantaneous computation of the IGIMF theory with the logistic ${\alpha }_1$-$Z$ relation.

Accepted for publication by Research in Astronomy and Astrophysics. 27 pages, 18 figures, 1 table