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

摘要

Context. Extreme-ultraviolet (EUV) and X-ray photons fromclassical T Tauri stars are powerful ionization and heating agents thatdrive disk chemistry, disk instabilities, and photoevaporative flows.The mid-infrared fine-structure line of [Ne II] at 12.81m 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, ,with stellar and circumstellar disk parameters. Methods. We have conducted a study of [Ne II]lineemission based on a sample of 92pre-main sequence stars mostlybelonging to the infrared Class II, but including 13accretingtransition disk objects, and also 14objects that drive known jetsand outflows. Results. We find several significant correlations between and stellar parameters, in particularand the wind mass loss rate, .Most correlations are, however, strongly dominated by systematicscatter of unknown origin. While there is a positive correlationbetween and ,the stellar mass accretion rate, ,induces a correlation only if we combine the largely different subsetsof jet sources and stars without jets. Our results indeed suggest that is bi-modally distributed, with separate distributions for the two subsamples. The jet sources show systematically higher ,by 1-2orders of magnitude with respect to objects without jets.Jet-driving stars also tend to show higher mass accretion rates. Wetherefore hypothesize that the trend with only reflects a trend with that is more physically relevant for [Ne II]emission. Conclusions. The [Ne II] luminositiesmeasured for objects without known outflows and jets are found to agreewith simplified calculations of [Ne II] emissionfrom disk surface layers if the measured stellar X-rays are responsiblefor heating and ionizing the gas. The large scatter in may be introduced by variations of disk properties and the irradiationspectrum, as previously suggested. If these additional factors can besufficiently well constrained, then the [Ne II] 12.81mlineshould be an important diagnostic for disk surface ionization andheating, at least in the inner disk region. This applies in particularto transition disks also included in our sample. The systematicallyenhanced [Ne II]flux from jet sourcesclearly suggests a role for the jets themselves, as previouslydemonstrated by a spatially resolved observation of the outflow systemin the TTau triple. Key words: stars: formation - stars: pre-main sequence - protoplanetary disks