Radio Recombination Line Observations at $1.0-1.5$ GHz with FAST
  HII regions made of gas ionized by radiations from young massive stars, are
widely distributed in the Milky Way. They are tracers for star formation, and
their distributions are correlated with the Galactic spiral structure. Radio
recombination lines (RRLs) of hydrogen and other atoms allows for the precisest
determination of physical parameters such as temperature and density. However,
RRLs at around 1.4 GHz from HII regions are weak and their detections are
difficult. As a result, only a limited number of detections have been obtained
yet. The 19-beam receiver on board of the Five-hundred-meter Aperture Spherical
radio Telescope (FAST) can simultaneously cover 23 RRLs for H$n{\alpha}$,
He$n{\alpha}$, and C$n{\alpha}$ ($n = 164 - 186$), respectively. This, combined
with its unparalleled collecting area, makes FAST the most powerful telescope
to detect weak RRLs. In this pilot survey, we use FAST to observe nine HII
regions at L band. We allocate 20 minutes pointing time for each source to
achieve a sensitivity of around 9 mK in a velocity resolution of 2.0 km/s. In
total, 21 RRLs for H$n{\alpha}$ and C$n{\alpha}$ at $1.0 - 1.5$ GHz have been
simultaneously detected with strong emission signals. Overall, the detection
rates for the H167${\alpha}$ and C167${\alpha}$ RRLs are 100%, while that for
the He167${\alpha}$ RRL is 33.3%. Using hydrogen and helium RRLs, we measure
the electron density, electron temperature, and pressure for three HII regions.
This pilot survey demonstrates the capability of FAST in RRL measurements, and
a statistically meaningful sample with RRL detection, through which knowledges
about Galactic spiral structure and evolution can be obtained, is expected in
the future.
