The formation scenario for massive stars is still under discussion. To further constrain current theories, it is vital to spatially resolve the structures from which material accretes onto massive young stellar objects (MYSOs). Due to the small angular extent of MYSOs, one needs to overcome the limitations of conventional thermal infrared imaging, regarding spatial resolution, in order to get observational access to the inner structure of these objects. We employed mid-infrared interferometry, using the MIDI instrument on the ESO/VLTI, to investigate the Kleinmann-Wright Object, a massive young stellar object previously identified as a Herbig Be star precursor. Dispersed visibility curves in the N-band (8-13 μm) have been obtained at 5 interferometric baselines. We show that the mid-infrared emission region is resolved. A qualitative analysis of the data indicates a non-rotationally symmetric structure, e.g. the projection of an inclined disk. We employed extensive radiative transfer simulations based on spectral energy distribution fitting. Since SED-only fitting usually yields degenerate results, we first employed a statistical analysis of the parameters provided by the radiative transfer models. In addition, we compared the ten best-fitting self-consistent models to the interferometric observations. Our analysis of the Kleinmann-Wright Object suggests the existence of a circumstellar disk of 0.1 M at an intermediate inclination of 76 • , while an additional dusty envelope is not necessary for fitting the data. Furthermore, we demonstrate that the combination of IR interferometry with radiative transfer simulations has the potential to resolve ambiguities arising from the analysis of spectral energy distributions alone.

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CRIRES at the VLT is one of the few adaptive optics enabled instruments that offer a resolving power of 10 5 from 1 − 5 µm. An instrument upgrade (CRIRES+) is proposed to implement cross-dispersion capabilities, spectro-polarimetry modes, a new detector mosaic, and a new gas absorption cell. CRIRES+ will boost the simultaneous wavelength coverage of the current instrument (∼ λ/70 in a single-order) by a factor of 10 in the cross-dispersed configuration, while still retaining a 10 arcsec slit suitable for long-slit spectroscopy. CRIRES+ dramatically enhances the instrument's observing efficiency, and opens new scientific opportunities. These include high-precision radial-velocity studies on the 3 m/s level to characterize extra-solar planets and their athmospheres, which demand for specialized, highly accurate wavelength calibration techniques. In this paper, we present a newly developed absorption gas-cell to enable high-precision wavelength calibration for CRIRES+. We also discuss the strategies and developments to cover the full operational spectral range (1−5 µm), employing hollow-cathode emission lamps, Fabry-Pérotétalons, and absorption gas-cells.