2009A&A...505..695F

The infrared [NeII] and [NeIII] fine structure lines at 12.81 µm and 15.55 µm have recently been theoretically predicted to trace the circumstellar disk gas subject to X-ray heating and ionization. We observationally investigate the origin of the neon fine structure line emission by comparing observations with models of X-ray irradiated disks and by searching for empirical correlations between the line luminosities and stellar and circumstellar parameters. We select a sample of 28 young stellar objects in the ρ Ophiuchi star formation region for which good quality infrared spectra and X-ray data have been obtained, the former with the Spitzer/IRS and the latter with the deep rho Ophiuchi XMM-Newton observation (DROXO). We measure neon line fluxes and X-ray luminosities; we complement these data with stellar/circumstellar parameters obtained by fitting the spectral energy distributions of our objects (from optical to millimeter wavelengths) with star/disk/envelope models. We detect the [NeII] and the [NeIII] lines in 10 and 1 cases, respectively. Line luminosities show no correlation with X-ray emission. The luminosity of the [NeII] line for one star, and that of both the [NeII] and [NeIII] lines for a second star, match the predictions of published models of X-ray irradiated disks; for the remaining 8 objects the [NeII] emission is 1-3dex higher than predicted on the basis of their L_X. However, the stellar/circumstellar characteristics assumed in published models do not match those of most of the stars in our sample. ClassI objects show significantly stronger [NeII] lines than ClassII and ClassIII ones. A correlation is moreover found between the [NeII] line emission and the disk mass accretion rates estimated from the spectral energy distributions. This might point toward a role of accretion-generated UV emission in the generation of the line or to other mechanisms related to mass inflows from circumstellar disks and envelopes and/or to the associated mass outflows (winds and jets). The X-ray luminosity is clearly not the only parameter that determines the [NeII] emission. For more exacting tests of X-ray irradiated disk models, these must be computed for the stellar and circumstellar characteristics of the observed objects. Explaining the strong [NeII] emission of ClassI objects likely requires the inclusion in the models of additional physical components such as the envelope, inflows, and outflows.