Multi-wavelength study of the disk around the very low-mass star Par-Lup3-4

Context. Par-Lup3-4 is a very low-mass star (spectral type M5) in the Lupus III star-forming region. It shows spectroscopic evidence of accretion and mass-loss. In the optical and near-infrared, the object is underluminous by ≈4 mag when compared to objects of similar mass in the same association. Aims. The aim of this work is to characterize the circumstellar environment of Par-Lup3-4 to better understand the origin of its underluminosity. Methods.We have analyzed high angular resolution near-IR observations and searched for extended emission from a disk and/or an envelope. We have studied the spectral energy distribution (SED) of the target from the optical to the sub-millimeter regime, and compared it to a grid of radiative transfer models of circumstellar disks. Since the target is strongly variable, we modeled two different near-infrared datasets. Results. The SED of Par-Lup3-4 resembles that of objects with edge-on disks seen in scattered light, that is, a double peaked-SED and a dip at ∼10 μm. The diffraction-limited infrared observations do not show obvious extended emission, allowing us to put an upper limit of ∼20AU to the disk outer radius. Par-Lup3-4 is probably in a Class II (rather than a Class I) evolutionary stage, which is indicated by the lack of extended emission together with the non detection of a strong 9.8 μm silicate in absorption. This last feature is indeed seen in emission. We fitted the whole SED of Par-Lup3-4 with a single disk model. Our modeling predicts a disk inclination of 81° ± 6°, which agrees well with previous estimates, and provides a natural explanation for the under-luminosity of the target. The detection of the silicate feature in emission at such a high inclination might be related to a more complex disk structure (e.g. asymmetries, inhomogeneities) than the one assumed here. Our analysis allows us to put constraints on the disk inner radius, R_in ≤ 0.05 AU, which is very close to the dust sublimation radius, and the maximum size of the dust grains, a_max ≥ 10 μm, which indicates that dust processing has already taken place in Par-Lup3-4. Some of the derived disk parameters vary depending on the modeled near-infrared data-set, which emphasizes the importance of taking variability into account when modeling the SED of young stellar objects.