Context. Because of inherent difficulties involved in observations and numerical simulations of the formation of massive stars,
an understanding of the early evolutionary phases of these objects remains elusive. In particular, observationally probing
circumstellar material at distances ≲100 AU from the central star is exceedingly difficult, as such objects are rare (and
thus, on average, far away) and typically deeply embedded. Long-baseline mid-infrared interferometry provides one way of obtaining
the necessary spatial resolution at appropriate wavelengths for studying this class of objects; however, interpreting such
observations is often difficult due to sparse spatial-frequency coverage.
Aims. We aim to characterize the distribution
and composition of circumstellar material around young massive stars and to investigate exactly which physical structures
in these objects are probed by long-baseline mid-infrared interferometric observations.
Methods. We used the two-telescope
interferometric instrument MIDI of the Very Large Telescope Interferometer of the European Southern Observatory to observe
a sample of 24 intermediate- and high-mass young stellar objects in the N band (8-13 μm). We had successful fringe detections
for 20 objects and present spectrally-resolved correlated fluxes and visibility levels for projected baselines of up to 128
m. We fit the visibilities with geometric models to derive the sizes of the emitting regions, as well as the orientation and
elongation of the circumstellar material. Fourteen objects in the sample show the 10 μm silicate feature in absorption in
the total and correlated flux spectra. For 13 of these objects, we were able to fit the correlated flux spectra with a simple
absorption model, allowing us to constrain the composition and absorptive properties of the circumstellar material.
Nearly all of the massive young stellar objects observed show significant deviations from spherical symmetry at mid-infrared
wavelengths. In general, the mid-infrared emission can trace both disks and outflows, and in many cases it may be difficult
to disentangle these components on the basis of interferometric data alone, because of the sparse spatial frequency coverage
normally provided by current long-baseline interferometers. For the majority of the objects in this sample, the absorption
occurs on spatial scales larger than those probed by MIDI. Finally, the physical extent of the mid-infrared emission around
these sources is correlated with the total luminosity, albeit with significant scatter.
material is ubiquitous at distances ≲100 AU around young massive stars. Long-baseline mid-infrared interferometry provides
the resolving power necessary for observing this material directly. However, in particular for deeply-embedded sources, caution
must be used when attempting to attribute mid-infrared emission to specific physical structures, such as a circumstellar disk
or an outflow.