The infrared emission of circumstellar envelopes, dark silhouettes, and photoionized disks in HII regions

We have modeled the infrared (IR) spectral energy distribution ( SED) of circumstellar disks embedded in a H II region and photoevaporated by the external ultraviolet radiation. The model applies to the photoevaporated disks (proplyds) in the Orion Nebula, most of them illuminated by the O6.5 star theta(1) Ori C. First we calculate the IR emission of a pre main-sequence star surrounded by a dusty globule that is immersed within an H II region. The globule is assumed to be spherical, homogeneous, optically thin at IR wavelengths, and photoevaporated according to the Dyson model. Second, we consider the IR emission of a disk directly exposed to the nebular environment. The reprocessing disk is passive and treated according to the Chiang & Goldreich model. We improve over the Chiang & Goldreich treatment by tracing the propagation of the various radiative fluxes (from the star exciting the H II region, nebular, and grazing from the disk central star) through the disk superheated atmosphere. Since the opposite disk sides receive different amounts of radiation, the flaring angle and the surface temperature distributions are different, resulting in well-distinguished SEDs for the two disk faces. Finally, we combine the globule and disk models to estimate the IR emission of proplyds. The energy input from the central star and the nebular environment increase the disk flaring angle, and therefore also the amount of stellar radiation intercepted by the disk. The relative intensity of the disk versus envelope emission varies with the tilt angle relative to the directions of theta(1) Ori C and the Earth. We explore the dependence of the SEDs upon the tilt angle with respect to the Earth, the distance from theta(1) Ori C, the size on the envelope, the inner disk radius, and the temperature of the central star. The resulting SEDs are characterized by a broad peak of emission at 30-60 mum and are in general significantly different from those of isolated disks in low-mass star-forming regions like Taurus-Auriga. Our model indicates that in the presence of an external radiation field, relatively evolved Class 2 objects may display a SED peaking at mid-IR and far-IR wavelengths. Also, the model can account for the strong mid-IR excess we have recently detected at 10 mum from embedded disks in the Orion Nebula.