Modeling Spitzer Observations of Vv Ser. Ii. an Extended Quantum Heated Nebula and a Disk Shadow

We present mid-infrared Spitzer IRAC and MIPS images of the UX Orionis star VV Ser and the surrounding cloud. The 5.6–70μm images show bright, localized and nebulous emission extended over 4 centered on VV Ser. We interpret the nebulosity as being due to transiently heated grains excited by UV photons emitted by VV Ser. A companion paper describes how the physical structure of the VV Ser disk has been constrained using a wide range of observational data modeled by an axisymmetric Monte Carlo radiative transfer code. The model of the system in particular constrains the strength of UV radiation field throughout the envelope, and the nebulosity thus presents an opportunity for directly constraining the excitation mechanism and opacity law of PAHs in molecular clouds. In this paper we employ the model to study the nebulosity surrounding VV Ser using quantumheated PAH molecules and Very Small Grains (VSGs) consisting of amorphous carbon in the thermal cooling approximation. Imprinted on the nebulosity is a wedge-shaped dark band, centered on the star. We interpret this dark wedge as the shadow cast by the inner regions of a near-edge-on disk in UV light, allowing the PAHs to be excited only outside of this shadow. The presence of a disk shadow strongly constrains the inclination as well as the position angle of the disk. The nebulosity at 5.6–8.0μm and the 2175 Å absorption feature seen in an archival spectrum from the International Ultraviolet Explorer can be fit using only PAHs, consistent with the main carrier of the 2175 Å feature being due to the graphite-like structure of the PAHs. The PAH component is found to be relatively smoothly distributed in the cloud, while the population of VSGs emitting at 20–70μm is strongly concentrated ∼ 50 to the south east of VV Ser. Although depending on the adopted PAH opacity, the abundance of PAHs in the surrounding cloud is constrained to 5 ± 2% of the total dust mass, given the opacity. The extent of the nebulosity constrains the density of the gas surrounding the VV Ser disk to 500± 200 cm for a gas-to-dust ratio of 100. This low density suggests that the quantum heated material is not part of the original envelope of VV Ser and that it is rather a quiescent part of the Serpens molecular cloud that the star has passed into after being formed. Although relatively rare, quantum heated nebulosities surrounding single, well defined stars are therefore well-suited for gaining unique insights into the physics of very small particles in molecular clouds. Subject headings: accretion, accretion disks – circumstellar matter – stars: formation, pre-mainsequence – infrared: stars