  The emergence of optical interferometers with three and more telescopes
allows image reconstruction of astronomical objects at the milliarcsecond
scale. However, some objects contain components with very different spectral
energy distributions (SED; i.e. different temperatures), which produces strong
chromatic effects on the interferograms that have to be managed with care by
image reconstruction algorithms. For example, the gray approximation for the
image reconstruction process results in a degraded image if the total (u,
v)-coverage given by the spectral supersynthesis is used. The relative flux
contribution of the central object and an extended structure changes with
wavelength for different temperatures. For young stellar objects, the known
characteristics of the central object (i.e., stellar SED), or even the fit of
the spectral index and the relative flux ratio, can be used to model the
central star while reconstructing the image of the extended structure
separately. Methods. We present a new method, called SPARCO (semi-parametric
algorithm for the image reconstruction of chromatic objects), which describes
the spectral characteristics of both the central object and the extended
structure to consider them properly when reconstructing the image of the
surrounding environment. We adapted two image-reconstruction codes (Macim,
Squeeze, and MiRA) to implement this new prescription. SPARCO is applied using
Macim, Squeeze and MiRA on a young stellar object model and also on literature
data on HR5999 in the near-infrared with the VLTI. This method paves the way to
improved aperture-synthesis imaging of several young stellar objects with
existing datasets. More generally, the approach can be used on astrophysical
sources with similar features such as active galactic nuclei, planetary
nebulae, and asymptotic giant branch stars.
