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

Par-Lup3-4 is a very low-mass star (spectral type M5) in the Lupus III star-forming region. The object is underluminous by ~4 mag when compared to objects of similar mass in the same association. To better understand the origin of its underluminosity, we have analyzed high angular resolution near-IR imaging data and mid-IR spectroscopy. We have also compared the SED of the target (from the optical to the sub-millimeter regime) to a grid of radiative transfer models of circumstellar disks. The diffraction-limited infrared observations do not show obvious extended emission, allowing us to put an upper limit to the disk outer radius of ~20AU. The lack of extended emission, together with the non detection of a strong 9.8 microns silicate in absorption indicates that Par-Lup3-4 is probably in a Class II (rather than Class I) evolutionary stage. 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 microns. We can fit the whole SED with a single disk model with an inclination of 81+/-6 degrees which provides a natural explanation for the under-luminosity of the target. Our analysis allows to put constraints on the disk inner radius, Rin < 0.05 AU, which is very close to the dust sublimation radius, and the maximum size of the dust grains, a_max > 10 microns, which indicates that dust processing has already taken place in Par-Lup3-4.