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Using varying models for the star formation rate (SFR) of Population (Pop)III and II stars at z>6 we derive the expected redshift history of the global21 cm signal from the inter-galactic medium (IGM). To recover the observedThomson scattering optical depth of the cosmic microwave background (CMB)requires SFRs at the level of ~ 10^-3 Msun/yr/Mpc^3 at z ~ 15 from Pop IIIstars, or ~ 10^-1Msun/yr/Mpc^3 at z ~ 7 from Pop II stars. In the case the SFRis dominated by Pop III stars, the IGM quickly heats above the CMB at z > 12due to heating from supernovae. In addition, Lyman-alpha photons from haloeshosting Pop III stars couple the spin temperature to that of the gas, resultingin a deep absorption signal. If the SFR is dominated by Pop II stars, the IGMslowly heats and exceeds the CMB temperature at z ~ 10. However, the larger andvarying fraction of Pop III stars is able to break this degeneracy. We findthat the impact of the initial mass function (IMF) of Pop III stars on the 21cm signal results in an earlier change to a positive signal if the IMF slope is~ -1.2. Measuring the 21 cm signal at z > 10 with next generation radiotelescopes such as the Square Kilometre Array will be able to investigate thecontribution from Pop III and Pop II stars to the global star formation rate.

Can the 21 cm signal probe Population III and II star formation?