Disk and outflow signatures in Orion-KL: the power of high-resolution thermal infrared spectroscopy^*

Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: beuther@mpia-hd.mpg.de

Received: 12 August 2009
Accepted: 4 January 2010

Abstract

Context. The Orion-KL region contains the closest examples of high-mass accretion disk candidates. Studying their properties is an essential step in studying high-mass star formation.

Aims. Resolving the molecular line emission at high spatial and spectral resolution in the immediate environment of the exciting sources to infer the physical properties of the associated gas.

Methods. We used the CRIRES high-resolution spectrograph mounted on the VLT to study the ro-vibrational ¹²CO, ¹³CO, the Pfund β, and H₂ emission between 4.59 and 4.72 μm wavelengths toward the BN object, the disk candidate source n, and a proposed dust density enhancement IRC3.

Results. We detected CO absorption and emission features toward all three targets. Toward the BN object, the data partly confirm the results obtained more than 25 years ago; however, we also identify several new features. While the blue-shifted absorption is likely caused by outflowing gas, toward the BN object we detect CO in emission extending in diameter to ~3300 AU with a velocity structure close to the v_lsr. Although at the observational spectral resolution limit, the ¹³CO line width of that feature increases with energy levels, consistent with a disk origin. If one also attributes the extended CO emission to a disk origin, its extent is consistent with other massive disk candidates in the literature. For source n, we also find the blueshifted CO absorption likely from an outflow. However, it also exhibits a narrower range of redshifted CO absorption and adjacent weak CO emission, consistent with infalling motions. We do not spatially resolve the emission for source n. For both sources we conduct a Boltzmann analysis of the ¹³CO absorption features and find temperatures between 100 and 160 K, and H₂ column densities of a few times 10²³ cm^-2. The observational signatures from IRC3 are very different with only weak absorption against a much weaker continuum source. However, the CO emission is extended and shows wedge-like position velocity signatures consistent with jet-entrainment of molecular gas, potentially associated with the Orion-KL outflow system. We also present and discuss the Pfund β and H₂ emission in the region.

Conclusions. This analysis toward the closest high-mass disk candidates outlines the power of high spectral and spatial resolution mid-infrared spectroscopy for studying the gas properties close to young massive stars. We will extend qualitatively similar studies to larger samples of high-mass young stellar objects to constrain the physical properties of the dense innermost gas structures in more detail and in a statistical sense.