Can the 21 cm signal probe Population III and II star formation?
  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 global
21 cm signal from the inter-galactic medium (IGM). To recover the observed
Thomson 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 III
stars, or ~ 10^-1Msun/yr/Mpc^3 at z ~ 7 from Pop II stars. In the case the SFR
is dominated by Pop III stars, the IGM quickly heats above the CMB at z > 12
due to heating from supernovae. In addition, Lyman-alpha photons from haloes
hosting Pop III stars couple the spin temperature to that of the gas, resulting
in a deep absorption signal. If the SFR is dominated by Pop II stars, the IGM
slowly heats and exceeds the CMB temperature at z ~ 10. However, the larger and
varying fraction of Pop III stars is able to break this degeneracy. We find
that the impact of the initial mass function (IMF) of Pop III stars on the 21
cm 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 radio
telescopes such as the Square Kilometre Array will be able to investigate the
contribution from Pop III and Pop II stars to the global star formation rate.
