Spatial Study with the Very Large Telescope of a New Resolved Edge-on Circumstellar Dust Disk Discovered at the Periphery of the ρ Ophiuchi Dark Cloud*

摘要

We report the discovery in near-infrared (NIR) with SofI at the New Technology Telescope (NTT) of a resolved circumstellar dust disk around a 2MASS source at the periphery of the ρ Ophiuchi dark cloud. We present follow-up observations in J, H, and Ks bands, obtained with ISAAC at the Very Large Telescope (VLT) under 04 seeing conditions, which unveil a dark dust lane oriented east-west between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 215 (300 AU at 140 pc) and a width of 12 (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (AV = 2.1 ± 2.6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called "pixel NIR color mapping" (PICMap for short), which allows us to visualize directly the NIR colors of the nebula pixels. Thanks to this method, we identify a ridge, 03 (40 AU at 140 pc) to the north of the dark lane and parallel to it, that displays an NIR color excess. This ridge corresponds to an unusual increase of brightness from J to Ks, which is also visible in the NTT observations obtained 130 days before the VLT ones. We also find that the northern nebula shows ~3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered-light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i = 86° ± 1°, correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar, asymmetrical NIR color properties of this object.