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FU Orionis stars are a small group of pre–main-sequence stars known for large-amplitude optical variability. These objects also exhibit multiwavelength phenomena suggestive of active accretion from a circumstellar disk. We present high spatial resolution mid-IR imaging and spectroscopy, submillimeter photometry, and 3–4 μm photometry of four FU Ori–class objects, RNO 1B and C, Z CMa, and Par 21, and one object classified as a pre–FU Ori star, V380 Ori. We resolve multiple IR sources and extended emission in the RNO 1B/C system, and we discuss in detail their association with disk activity and the source of the Infrared Astronomical Satellite far-IR and radio maser emission in this field. We derive dust temperatures and masses for all sources and discuss how dust composition and morphology is related to the evolutionary stage of these objects.

We present arcsecond resolution, 10.8, 11.7, and 18.2 μm images of the central few parsecs of the Galaxy. These images show for the first time the clumpiness of the dust in the western arc. The 11.7 and 18.2 μm images of part of the western arc were used to evaluate the dust temperature and optical depth distribution of this region. We see several mid-infrared emission peaks that coincide with dust temperature peaks and regions of low optical depth, and we infer the presence of embedded sources in the western arc. Minimum luminosity estimates for two of these sources (5 × 104 L⊙ and 2 × 104 L⊙) suggest that the circumnuclear ring is being locally heated by relatively young stars.

We have obtained high spatial resolution mid-IR observations of four intermediate-mass pre–main-sequence stars: HD 259431, AS 310, LkHα 234, and MWC 1080. Our observations reveal the morphology of the circumstellar (500–10,000 AU) environments of these young stars. We resolve companion stars and complex diffuse emission from dust that can account for on average 50% of the IR emission detected in previous low-resolution observations. We calculate new spectral indices for these objects and show that they differ from those expected from simple geometrically flat disk models. We also show mid-IR spectral energy distributions that reveal the presence of silicate emission and absorption features for several objects. Finally, temperature maps constructed from our observations suggest the presence of transiently heated grains, and optical depth maps point to the locations of the most embedded objects.

Coronagraphic imaging of the nearest Herbig Be star with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope, Ks (2.15 ?m) imaging with ADONIS at the 3.6 m telescope at La Silla, and mid-infrared imaging with OSCIR using the 4 m Blanco Telescope at Cerro Tololo Inter-American Observatory confirm the detection of the disk reported by Pantin et al. and map the disk out to 5'' (~515 AU) in the optical and at Ks. While the source is unresolved at 10 and 18 ?m, it can be traced to 15 at 11.7 ?m. We confirm the change in the radial dependence of the disk surface brightness near 27 seen at 1.6 ?m by Augereau et al. at Ks. No such break in the power law is seen in the optical. The STIS data reveal spiral dark lane structure, making HD 100546 the third near?zero-age main-sequence Herbig Ae/Be star with structure more than 100 AU from the star. We also optically detect a low surface brightness envelope extending 10'' (1000 AU) from the star, in addition to nebulosity, which is probably associated with DC 292.6-7.9. The survival of the envelope through essentially the entire pre?main-sequence lifetime of the star, coupled with the absence of physical companions within 1500 AU of the star, suggests that envelope lifetimes owe more to the star-forming environment than to mass-loss activity from the Herbig Ae/Be star.

We report high angular resolution observations of complex mid-infrared (4.8?18 ?m) structure in the ? Carinae Nebula. This massive (~120 M?), luminous star is surrounded by a large bipolar nebula extending as much as ~22'' in the mid-infrared. Our observations reveal the presence of several condensations within the complex core known as the Homunculus, as well as what may be a toroidal structure visible at 18 ?m. This structure may be related to a similar torus seen at near-infrared wavelengths. Our high angular resolution photometry reveals for the first time the spatial variation of the silicate feature in the nucleus of the ? Car Nebula. Finally, we present the first high-resolution mid-infrared color-temperature and optical depth maps for the nebula and discuss their significance in relation to the origin of the emission and evolutionary status of the star.