On the origin of [NeII] 12.81 μm emission from pre-main sequence stars: Disks, jets, and accretion

Context. Extreme-ultraviolet (EUV) and X-ray photons from classical T Tauri stars are powerful ionization and heating agents that drive disk chemistry, disk instabilities, and photoevaporative flows. The mid-infrared fine-structure line of [Ne ii] at 12.81 μm has been proposed to trace gas in disk surface layers heated and ionized by stellar X-ray and EUV radiation.
Aims: We aim at locating the origin of [Ne ii] line emission in circumstellar environments by studying distributions of [Ne ii] emission and correlating the inferred [Ne ii] luminosities, L_{[Ne II]}, with stellar and circumstellar disk parameters.
Methods: We have conducted a study of [Ne ii] line emission based on a sample of 92 pre-main sequence stars mostly belonging to the infrared Class II, but including 13 accreting transition disk objects, and also 14 objects that drive known jets and outflows.
Results: We find several significant correlations between L_{[Ne II]} and stellar parameters, in particular L_X and the wind mass loss rate, dot{M}_loss. Most correlations are, however, strongly dominated by systematic scatter of unknown origin. While there is a positive correlation between L_{[Ne II]} and L_X, the stellar mass accretion rate, dot{M}_acc, induces a correlation only if we combine the largely different subsets of jet sources and stars without jets. Our results indeed suggest that L_{[Ne II]} is bi-modally distributed, with separate distributions for the two subsamples. The jet sources show systematically higher L_{[Ne II]}, by 1-2 orders of magnitude with respect to objects without jets. Jet-driving stars also tend to show higher mass accretion rates. We therefore hypothesize that the trend with dot{M}_acc only reflects a trend with dot{M}_loss that is more physically relevant for [Ne ii] emission.
Conclusions: The [Ne ii] luminosities measured for objects without known outflows and jets are found to agree with simplified calculations of [Ne ii] emission from disk surface layers if the measured stellar X-rays are responsible for heating and ionizing the gas. The large scatter in L_{[Ne II]} may be introduced by variations of disk properties and the irradiation spectrum, as previously suggested. If these additional factors can be sufficiently well constrained, then the [Ne ii] 12.81 μm line should be an important diagnostic for disk surface ionization and heating, at least in the inner disk region. This applies in particular to transition disks also included in our sample. The systematically enhanced [Ne ii] flux from jet sources clearly suggests a role for the jets themselves, as previously demonstrated by a spatially resolved observation of the outflow system in the T Tau triple.

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