The near-infrared reflected spectrum of source i in Orion-KL

Abstract

Context. Source I in the Orion-KL nebula is believed to be the nearest example of a massive star still in the main accretion phase. It is thus one of the best cases for studying the properties of massive protostars to constrain high-mass star formation theories. Near-infrared radiation from source I escapes through the cavity opened by the OMC1 outflow and is scattered by dust towards our line of sight. Aims. The reflected spectrum offers a unique possibility of observing the emission from the innermost regions of the system and probing the nature of source I and its immediate surroundings. Methods. We obtained moderately high spectral-resolution (λ/Δλ ∼ 9000) observations of the near infrared diffuse emission in several locations around source I/Orion-KL. We observed a widespread rich absorption line spectrum that we compare with cool stellar photospheres and protostellar accretion disk models. Results. The spectrum is broadly similar to strongly veiled, cool, low-gravity stellar photospheres in the range Teff ∼ 3500-4500 K, luminosity class I-III. An exact match explaining all features has not been found, and a plausible explanation is that a range of different temperatures contribute to the observed absorption spectrum. The 1D velocity dispersions implied by the absorption spectra, σ ∼ 30 km s-1, can be explained by the emission from a disk around a massive, m-* ∼ 10 M, protostar that is accreting at a high rate, m ∼ 3 × 10-3 M yr-1. Conclusions. Our observations suggest that the near-infrared reflection spectrum observed in the Orion-KL region is produced close to source I and scattered to our line of sight in the OMC1 outflow cavity. The spectrum allows us to exclude source I being a very large, massive protostar rotating at breakup speed. We suggest that the absorption spectrum is produced in a disk surrounding a ∼10 M protostar, accreting from its disk at a high rate of a few × 10-3 M/yr). © 2010 ESO.