The enigma of GCIRS 3 - Constraining the properties of the mid-infrared reference star of the central parsec of the Milky Way with optical long baseline interferometry

Context. GCIRS 3 is the most prominent MIR-source in the central parsec of the Galaxy. NIR spectroscopy has failed to solve the enigma of its nature. The properties and peculiarities of extreme individual objects in the central stellar cluster contribute to our knowledge of star and dust formation close to a supermassive black hole. Aims. We initiated an unprecedented interferometric experiment to understand the nature of GCIRS 3, where we investigate its properties as a spectroscopic and interferometric reference star at 10 mu m. Methods. VLT/VISIR imaging separates a compact source from diffuse, surrounding emission. The VLTI/MIDI instrument was used to measure spectroscopically resolved visibility moduli at an angular resolution of similar to 10 mas of that compact 10 mu m source, still unresolved by a single VLT. Recent NIR/MIR photometry data were added to enable simple SED-and full radiative transfer-modeling of the data. Results. The luminosity and size estimates show that IRS 3 is probably a cool carbon star enshrouded by a complex dust distribution. Blackbody temperatures were derived. The coinciding interpretation of single telescope and interferometric data confirm dust emission from several different spatial scales. The interferometric data resolve the inner rim of dust formation. Despite observed deep silicate absorption towards GCIRS 3, we favor a carbon-rich circumstellar dust shell. The silicate absorption most probably takes place in the outer diffuse dust, which is mostly ignored by MIDI measurements, but very observable in complementary VLT/VISIR data. This indicates physically and chemically distinct conditions of the local dust, changing with the distance to GCIRS 3. Conclusions. We have demonstrated that optical long baseline interferometry at infrared wavelengths is an indispensable tool for investigating sources at the Galactic center. Our findings suggest further studies of the composition of interstellar dust and the shape of the 10 mu m silicate feature in this extraordinary region.