EX Lupi in quiescence

Aims. EX Lup 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 which 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 to study 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. A mineralogical decomposition of the 10 µm silicate emission feature and also the description of the optical and near-infrared spectra were performed. Results. The SED is in general similar to that of a typical T Tauri star, though above 7 µm EX Lup emits higher flux. The quiescent phase data suggest low level variability in the optical–mid-infrared domain. Integrating the optical and infrared fluxes we derived a bolometric luminosity of 0.7 L ⊙. The 10 µm silicate profile could be fitted by a mixture consisting of amorphous silicates, no crystalline silicates were found. A modestly flaring disk model with a total mass of 0.025 M ⊙ and an outer radius of 150 AU was able to reproduce the observed SED. The derived inner radius of 0.2 AU is larger than the sublimation radius, and this inner gap sets EX Lup aside from typical T Tauri stars.