  The young planetary system {\beta} Pictoris is surrounded by a circumstellar
disk of dust and gas. Because both dust and gas have a lifetime shorter than
the system age, they need to be replenished continuously. The gas composition
is partly known, but its location and its origin are still a puzzle. The gas
source could be the exocomets (or so-called falling and evaporating bodies,
FEBs), which are observed as transient features in absorption lines of
refractory elements (Mg, Ca, and Fe) when they transit in front of the star at
several tens of stellar radii.
  Nearly 1700 high-resolution spectra of {\beta} Pictoris have been obtained
from 2003 to 2015 using the HARPS spectrograph. In these spectra, the
circumstellar disk is always detected as a stable component among the numerous
variable absorption signatures of transiting exocomets.
  Summing all the 1700 spectra allowed us to reach a signal-to-noise ratio
higher than 1000, which is an unprecedentedly high number for a {\beta}
Pictoris spectrum. It revealed many weak Fe I absorption lines of the
circumstellar gas in more than ten excited states. These weak lines bring new
information on the physical properties of the neutral iron gas in the
circumstellar disk. The population of the first excited levels follows a
Boltzmann distribution with a slope consistent with a gas temperature of about
1300 K; this temperature corresponds to a distance to the star of ~ 38 RStar
and implies a turbulence of {\xi} ~ 0.8 km/s.
