Chemical abundances in the protoplanetary disc LV2 (Orion): Clues to the causes of the abundance anomaly in H II regions

Abstract

Optical integral field spectroscopy of the archetype protoplanetary disc LV2 in the Orion nebula is presented, taken with the Very Large Telescope (VLT) FLAMES/Argus fibre array. The detection of recombination lines (RLs) of Cii and Oii from this class of objects is reported, and the lines are utilized as abundance diagnostics. The study is complemented with the analysis of Hubble Space Telescope (HST) Faint Object Spectrograph ultraviolet and optical spectra of the target contained within the Argus field of view. By subtracting the local nebula background the intrinsic spectrum of the proplyd is obtained and its elemental composition is derived for the first time. The proplyd is found to be overabundant in carbon, oxygen and neon compared to the Orion nebula and the Sun. The simultaneous coverage over LV2 of the Ciii]λ1908 and [Oiii]λ5007 collisionally excited lines (CELs) and Cii and Oii RLs has enabled us to measure the abundances of C2 + and O2 + for LV2 with both sets of lines. The two methods yield consistent results for the intrinsic proplyd spectrum, but not for the proplyd spectrum contaminated by the generic nebula spectrum, thus providing one example where the long-standing abundance anomaly plaguing metallicity studies of Hii regions has been resolved. These results would indicate that the standard forbidden-line methods used in the derivation of light metal abundances in Hii regions in our own and other galaxies underestimate the true gas metallicity. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.