Forbidden line diagnostics of photoevaporative disc winds
  Photoevaporation driven by high energy radiation from the central star plays
an important role in the evolution of protoplanetary discs. Photoevaporative
winds have been unambiguously detected through blue-shifted emission lines, but
their detailed properties remain uncertain. Here we present a new empirical
approach to make observational predictions of these thermal winds, seeking to
fill the gap between theory and observations. We use a self-similar model of an
isothermal wind to compute line profiles of several characteristic emission
lines (the [Ne${\rm{\scriptsize II}}$] line at 12.81 $\mu$m, and optical
forbidden lines such as [O${\rm{\scriptsize I}}$] 6300 $\mathring{A}$ and
[S${\rm{\scriptsize II}}$] 4068/4076 $\mathring{A}$), studying how the lines
are affected by parameters such as the gas temperature, disc inclinations, and
density profile. Our model successfully reproduces blue-shifted lines with
$v_{\rm peak} \lesssim 10$ km/s, which decrease with increasing disc
inclination. The line widths increase with increasing disc inclinations and
range from $\Delta v \sim 15-30$ km/s. The predicted blue-shifts are mostly
sensitive to the gas sound speed. The observed [Ne${\rm{\scriptsize II}}$] line
profiles are consistent with a thermal wind and point towards a relatively high
sound speed, as expected for EUV photoevaporation. However, the observed
[O${\rm{\scriptsize I}}$] line profiles require lower temperatures, as expected
in X-ray photoevaporation, and show a wider scatter that is difficult to
reconcile with a single wind model; it seems likely that these lines trace
different components of a multi-phase wind. We also note that the spectral
resolution of current observations remains an important limiting factor in
these studies, and that higher resolution spectra are required if emission
lines are to further our understanding of protoplanetary disc winds.
