SpS1-Silicate dust formation around AGB stars

Extract

VISIR, the VLT Imager and Spectrograph for the Mid-Infrared is a multi-mode instrument, featuring also a high resolution Echelle spectrograph with a spectral resolving power of ≈30 000 or 10 km s⁻¹ at ν ≈ 30 000 GHz (λ ≈ 10 μm). A limited long-slit mode as well as a general cross-dispersed mode are available. The Echelle grating is illuminated with a 200 mm diameter collimated beam. Cross-dispersion is achieved by a pair of grisms in the pre-slit optics. The entire frequency interval corresponding to the “10 μ m-window” from 22 400 to 39 500 GHz is fully accessible, albeit sequentially. This interval contains a multitude of fundamental molecular rotational-vibrational bands such as SiO, OH⁻, H₂O, NH₃, CH₄ and many other hydro-carbonates. Since its commissioning in April 2004, VISIR has been plagued by artifacts introduced from its detector. The cross-dispersed mode is especially handicapped, as it is the most demanding mode for dynamic range of illumination. Now an ambitious upgrade with a pair of newly developed 1k² As:Si detectors is underway, which will fully resurrect the spectroscopic mode. This will also increase the frequency interval accessible in one exposure by 240% while changing from critical to 3-pixel sampling. Even in the absence of extra spectral features this increase is quite valuable for absorption line spectroscopy, as the limiting factor in analysis often is the definition of the photospheric continuum.