EX Lupi in quiescence

Aims. EX Lupi is the prototype of EXors, a subclass of low-mass pre-main sequence stars whose episodic eruptions are attributed to temporarily increased accretion. In quiescence the optical and near-infrared properties of EX Lup cannot be distinguished from those of normal T Tau stars. Here we investigate whether it is the circumstellar disk structure that makes EX Lup an atypical Class II object. During outburst the disk might undergo structural changes. Our characterization of the quiescent disk is intended to serve as a reference for studying the physical changes related to one of EX Lupi's strongest known eruptions in 2008 Jan-Sep. Methods. We searched the literature for photometric and spectroscopic observations including ground-based, IRAS, ISO, and Spitzer data. After constructing the optical-infrared spectral energy distribution (SED), we compared it with the typical SEDs of other young stellar objects and modeled it using the Monte Carlo radiative transfer code RADMC. We determined the mineralogical composition of the 10 mu m silicate emission feature and also gave a description of the optical and near-infrared spectra. Results. The SED is similar to that of a typical T Tauri star in most aspects, though EX Lup emits higher flux above 7 mu m. The quiescent phase data suggest low-level variability in the optical-mid-infrared domain. By integrating the optical and infrared fluxes, we derived a bolometric luminosity of 0.7 L-circle dot. The 10 mu m silicate profile could be fitted by a mixture consisting of amorphous silicates, but no crystalline silicates were found. A modestly flaring disk model with a total mass of 0.025 M-circle dot and an outer radius of 150AU was able to reproduce the observed SED. The derived inner radius of 0.2AU is larger than the sublimation radius, and this inner gap sets EX Lup apart from typical T Tauri stars.