An Infrared View of the EXor Variables: The Case of V1118 Ori

Abstract

We investigate the relationship between the IR observed properties of the EXor variables and the mechanisms active during their evolutionary stage typical of the pre-main-sequence phase. To this aim, we have constructed a catalog containing all the IR (1-100 μm) photometric and spectroscopic observations appearing during the last 30 years in the literature. Moreover, new results of our monitoring program based on near- and mid-IR photometry and near-IR spectroscopy and polarimetry of one object (V1118 Ori) typical of the EXor class are presented, complementing those given in a previous paper and related to a different activity period. Our catalog indicates how the database accumulated so far, stemming from a fortuitous monitoring of the stellar activity, is inadequate for any statistical study of the EXor events. Nevertheless, all the observational evidence can be interpreted into a coherent scheme. The sources that present the largest brightness variations tend to become bluer while brightening. The scenario of the disk accretion hypothesis based on the viscous friction between particles agrees with the observational evidence. The new results on V1118 Ori confirm such a general view. The striking novelty is represented by a near-IR spectrum of V1118 Ori taken 1 yr after the last monitored outburst: any emission line previously detected has now totally disappeared at our sensitivity. For the same source, mid-IR photometry is provided here for the first time and allows us to construct a meaningful SED. The first polarimetric data indicate that the source is intrinsically polarized and its spotted and magnetized surface becomes recognizable during the less active phases.