Chemical abundances in Orion protoplanetary discs: integral field
  spectroscopy and photoevaporation models of HST 10
  Photoevaporating protoplanetary discs (proplyds) in the vicinity of hot
massive stars, such as those found in Orion, are important objects of study for
the fields of star formation, early disc evolution, planetary formation, and H
II region astrophysics. Their element abundances are largely unknown, unlike
those of the main-sequence stars or the host Orion nebula. We present a
spectroscopic analysis of the Orion proplyd HST 10, based on integral field
observations with the Very Large Telescope/FLAMES fibre array with 0.31" x
0.31" spatial pixels. The proplyd and its vicinity are imaged in a variety of
emission lines across a 6.8" x 4.3" area. The reddening, electron density and
temperature are mapped out from various line diagnostics. The abundances of
helium, and eight heavy elements are measured relative to hydrogen using the
direct method based on the [O III] electron temperature. The abundance ratios
of O/H and S/H are derived without resort to ionization correction factors. We
construct dynamic photoevaporation models of HST 10 with the Cloudy
microphysics code that validate the oxygen and sulfur abundances. With the
exception of [O I] 6300-A and [S II] 4069-A, the model fit is satisfactory for
all spectral lines arising from the proplyd. The models show that the classic
ionization correction factor for neon significantly underestimates (0.4 dex)
this element's abundance in the low ionization conditions of HST 10. Apart from
iron, whose gas-phase abundance is ~0.3 dex lower than in the local Orion
nebula, most other elements in the proplyd do not show substantially different
gas-phase abundances from the nebula. The abundances of carbon, oxygen and neon
in HST 10 are practically the same as those in B-type stars in Orion.
