The influence of streaming velocities and Lyman-Werner radiation on the formation of the first stars

Abstract

The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures $T_{\mathrm{vir}}<10^4$~K. Cooling in these minihaloes is dominated by molecular hydrogen (H${}_2$), and so Population III star formation is only possible in those minihaloes that form enough H${}_2$ to cool on a short timescale. As H${}_2$ cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale $M_{\min }$ above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H${}_2$ by a high redshift Lyman-Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on $M_{\min }$ and on $M_{\mathrm{ave}}$ (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on $M_{\min }$ than the LW background, but also that both effects are additive. We also provide fitting functions quantifying the dependence of $M_{\mathrm{ave}}$ and $M_{\min }$ on the streaming velocity and the strength of the LW background.

15 pages, 12+1 figures, 2 fit-formulas, accepted by MNRAS

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