EPoS Contribution

On the metallicity-dependence of radiation forces and photoionization feedback in massive star formation Rolf Kuiper UDE, Duisburg, DE
The formation of the most massive stars in the primordial to present-day Universe is hindered by the strong radiative feedback of the forming star itself. In previous work, we modeled the formation of a massive star from a collapsing cloud of dusty gas at solar metallicity, for the first time including the star's impact due to radiation forces and photoionization feedback simultaneously. Here, we will report on our follow-up investigations of the effect of radiation forces and photoionization feedback in environments of different metallicity. The simulations cover the global collapse, disk formation, and the whole stellar accretion phase, i.e. until radiative impact eventually is able to stop further accretion. Hence, we can extract the final mass of the star. While radiation forces are found to be responsible for stopping accretion onto the most massive stars in the present-day Universe, the relative impact of photoionization increases strongly toward environments of lower metallicity and ultimately dominate the stellar impact for metallicities approximately lower than a few times 0.01 the solar value. This result from our direct numerical simulations of the star formation process agrees well with previous estimates using semi-analytical arguments. Interestingly, as a take away message, the overall star formation efficiency is less constrained by the metallicity dependence of the radiative feedback than by the differences in the initial conditions of the different environments.

Caption: Final stellar mass as function of metallicity or dust-to-gas mass ratio, respectively (Kuiper et al., in prep.). Squares connected by long-dashed lines denote simulation results only taking radiation forces into account. Diamonds connected by short-dashed lines denote simulation results only taking photoionization feedback into account. Circles connected by solid lines denote simulation results when taking into account both forms of radiative feedback. Remark: The counterintuitive result that the addition of photoionization feedback yields a higher star formation efficiency in the present-day case than without this feedback component is already known from our previous work as the "scissors handle effect" (Kuiper & Hosokawa 2018).
Collaborators: T. Hosokawa, Kyoto U, JP H. Fukushima, Tokyo U, JP	Relevant topic(s): Feedback High-Mass SF
Relevant Big Question: What shapes the upper end of the initial mass function in environments of different metallicity?