Stellar mass dependence of the 21-cm signal around the first star and
  its impact on the global signal
  The 21-cm signal in the vicinity of the first stars is expected to reflect
properties of the first stars. In this study we pay special attention to
tracing the time evolution of the ionizing photons' escape fraction, which
affects the distribution of neutral hydrogen, by performing radiation
hydrodynamics (RHD) simulations resolving dense gas in a halo. We find that the
radial profile of 21-cm differential brightness temperature is quite sensitive
to the stellar and halo masses, which reflects the time evolution of the escape
fraction. In the case of a less massive star, ionizing photons hardly escape
from its host halo due to the absorption by dense halo gas, thus an deep 21-cm
absorption feature at just outside the halo lasts a long time. Whereas photons
from a massive star well working to heat the ambient intergalactic medium turn
out to cause a spatially extended 21-cm emission signature. Although individual
signals are found to be undetectable with the Square Kilometre Array, our
analysis using the results from the RHD simulations indicates that the
properties of the first stars are imprinted on the 21-cm global signal: its
amplitude depends not only on the cosmic star formation rate density, but also
on the typical mass of the first stars due to the stellar-mass-dependent
heating rate. Thus, we suggest that the initial mass function of the first
stars is an essential factor in understanding the global signal.
