\text { 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 \sim 38 R _ { Star } and implies a turbulence of \xi \sim 0.8 km/s .