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Our mid-infrared and near-infrared surveys over the last five years have helped to strengthen and clarify the relationships between water, methanol, and OH masers and the star formation process. Our surveys show that maser emission seems to be more closely associated with mid-infrared emission than cm radio continuum emission from UC HII regions. We find that masers of all molecular species surveyed trace a wide variety of phenomena and show a proclivity for linear distributions. The vast majority of these linear distributions can be explained by outflows or shocks, and in general do not appear to trace circumstellar disks as was previously thought. Some water and methanol masers that are not associated with radio continuum emission appear to trace infrared-bright hot cores, the earliest observable stage of massive stellar life before the onset of a UC HII region., Comment: 6 pages, 3 figures, to appear in the proceedings of IAU Symposium 227: "Massive Star Birth: A Crossroads of Astrophysics", version with full-resolution images available at http://www.ctio.noao.edu/~debuizer

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 subarcsecond resolution mid infrared images of NGC 4151 at 10.8 micron and 18.2 micron. These images were taken with the University of Florida mid-IR camera/spectrometer OSCIR at the Gemini North 8-m telescope. We resolve emission at both 10.8 micron and 18.2 micron extending ~ 3.5" across at a P.A. of ~ 60 degrees. This coincides with the the narrow line region of NGC 4151 as observed in [OIII] by the Hubble Space Telescope. The most likely explanation for this extended mid-IR emission is dust in the narrow line region heated by a central engine. We find no extended emission associated with the proposed torus and place an upper limit on its mid-IR size of less than or equal to ~ 35 pc.

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.

We present subarcsecond resolution mid-infrared images at 10.8 and 18.2 microns of Cygnus A. These images were obtained with the University of Florida mid-IR camera/spectrometer OSCIR at the Keck II 10-m telescope. Our data show extended mid-IR emission primarily to the east of the nucleus with a possible western extension detected after image deconvolution. This extended emission is closely aligned with the bi-conical structure observed at optical and near-IR wavelengths by the HST. This emission is consistent with dust heated from the central engine of Cygnus A. We also marginally detect large-scale low level emission extending > 1.5 kpc from the nucleus which may be caused by in-situ star formation, line emission, and/or PAH contamination within the bandpass of our wide N-band filter., 20 pages, 8 figures, accepted for publication in the Astrophysical Journal

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 analyze a 10.8 ?m image of M87, obtained with the Gemini 8 m telescope + OSCIR. The image has better than 05 resolution and represents 7 hr of observing time, making it the deepest high-resolution mid-IR image ever. The core is marginally resolved, and we also detect five optically bright knots in the jet. The spectral energy distributions of these features are entirely consistent with synchrotron radiation; we find little evidence of thermal emission from a dusty torus. Four faint jet regions are below the noise level of these observations. We also find evidence for diffuse galactic emission.

We present a mid-infrared imaging survey of 21 sites of massive star formation associated with methanol masers. Images were obtained from the Cerro Tololo Inter-American Observatory 4 m Blanco telescope using the University of Florida imager/spectrometer OSCIR. Of the 10 sites where the methanol masers are distributed in a linear fashion, we find three sources that are elongated at the same position angle as their linear methanol maser distributions. It is believed that these elongated mid-infrared objects are indeed circumstellar disks. It was found that the masers may arise inside the mid-infrared emitting regions of these young stellar objects, indicating that the methanol masers may be pumped by mid-infrared photons. Many mid-infrared sources in our survey have no detectable radio continuum emission, and we advance the hypothesis that these sources are lower mass, nonionizing stars.

We report the detection of extended IR emission at 10.8 and 18.2 µm around the Vega-like source HD 141569. Mid-IR imaging with OSCIR on Keck II shows emission from dust extending out to 100 AU from the B9.5 Ve star. Our modeling of the dust places an upper limit of approximately 2 µm on the diameter of the mid-IR-emitting particles if they are Mie spheres of astronomical silicates. Comparison of our mid-IR images to the near-IR (1.1 µm) NICMOS images of HD 141569 (Weinberger et al. 1999) shows that the mid-IR emission originates at smaller distances from the star than the scattered near-IR light, as also previously observed for the archetype Vega-like source beta Pictoris.

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.

A group of young, active stars in the vicinity of TW Hydrae has recently been identified as a possible physical association with a common origin. Given its proximity ($\sim$50 pc), age ($\sim$10 Myr) and abundance of binary systems, the TW Hya Association is ideally suited to studies of diversity and evolution of circumstellar disks. Here we present mid-infrared observations of 15 candidate members of the group, 11 of which have no previous flux measurements at wavelengths longer than 2$\mu$m. We report the discovery of a possible 10$\mu$m excess in CD -33$^{\circ}$7795, which may be due to a circumstellar disk or a faint, as yet undetected binary companion. Of the other stars, only TW Hya, HD 98800, Hen 3-600A, and HR 4796A -- all of which were detected by IRAS -- show excess thermal emission. Our 10$\mu$m flux measurements for the remaining members of the Association are consistent with photospheric emission, allowing us to rule out dusty inner disks. In light of these findings, we discuss the origin and age of the TW Hya Association as well as implications for disk evolution timescales., Comment: 10 pages and 1 PostScript figure, accepted for publication in The Astrophysical Journal Letters

We report both ROSAT observations and ground-based 10.8 ?m imaging of the wide binary HR 4796, which consists of a main-sequence A-type star with a large amount of circumstellar dust, HR 4796A, and, at a separation of 77, a pre-main-sequence M-type companion, HR 4796B. From the ROSAT data, we find that the X-ray emission is centered on HR 4796B, with LX/Lbol ? 3 ? 10-4. The 10.8 ?m flux, which arises from HR 4796A, displays an excess over the photospheric emission of 0.08 ? 0.02 Jy, a result consistent with the previous characterization of the emission from the circumstellar dust in the wavelength range 12 ?m ? ? ? 100 ?m as a 110 K blackbody. The Hipparcos data can be used to argue that the three main-sequence A-type stars in the Bright Star Catalogue with LIR/Lbol > 10-3 (HR 4796A, ? Pic, and 49 Cet) all have low luminosities for their colors. We argue that approximately 20% of all A-type stars pass through an early phase where they possess an amount of circumstellar dust comparable to that found around HR 4796A or ? Pic. In order to explain the result that the grain emission can be approximated by a 110 K blackbody, we propose that the circumstellar grains are largely composed of ice particles with a typical radius near 100 ?m, and that they sublimate rapidly when they are closer than ~35 AU to HR 4796A. This swarm of ice particles might be a protocometary cloud.

We present 10.8 and 18.2 μm images of HD 233517 (K2 III) made with the Observatory Spectrometer Camera for the Infrared on Keck II. Previous observations with a 3.8 m telescope indicated that a highly inclined disk with FWHM of 15 surrounds this star, but we find that HD 233517 is unresolved in the mid-IR at the highest resolution currently possible at these wavelengths. We measure the source to have FWHM less than 031 at 10.8 μm and less than 041 at 18.2 μm. Combined with more recent analyses and conclusions by others, HD 233517 appears to be a first-ascent giant star with spatially unresolved mid-IR dust emission.

We present here a mid-infrared imaging survey of 26 sites of water maser emission. Observations were obtained at the InfraRed Telescope Facility 3-m telescope with the University of Florida mid-infrared imager/spectrometer OSCIR, and the JPL mid-infrared camera MIRLIN. The main purpose of the survey was to explore the relationship between water masers and the massive star formation process. It is generally believed that water masers predominantly trace outflows and embedded massive stellar objects, but may also exist in circumstellar disks around young stars. We investigate each of these possibilities in light of our mid-infrared imaging. We find that mid-infrared emission seems to be more closely associated with water and OH maser emission than cm radio continuum emission from UC HII regions. We also find from the sample of sources in our survey that, like groups of methanol masers, both water and OH masers have a proclivity for grouping into linear or elongated distributions. We conclude that the vast majority of linearly distributed masers are not tracing circumstellar disks, but outflows and shocks instead., 49 pages; 23 figures; To appear in February 2005 ApJS; To download a version with better quality figures, go to http://www.ctio.noao.edu/~debuizer/

The University of Florida is developing a mid-infrared camera for the 10.4-meter Gran Telescopio CANARIAS. CanariCam has four science modes and two engineering modes, which use the same 320 x 240-pixel, arsenic-doped silicon, blocked-impurity-band detector from Raytheon. Each mode can be remotely selected quickly during an observing sequence. The pixel scale is 0.08 arcsec, resulting in Nyquist sampling of the diffraction-limited point-spread-function at 8 μm, the shortest wavelength for which CanariCam is optimized. The total available field of view for imaging is 26 arcsec x 19 arcsec. The primary science mode will be diffraction-limited imaging using one of several available spectral filters in the 10 μm (8-14 μm) and 20 μm (16-25 μm) atmospheric windows. Any one of four plane gratings can be inserted for low and moderate-resolution (R = 100 - 1300) slit spectroscopy in the 10 and 20-μm regions. Insertion of appropriate field and pupil stops converts the camera into a coronagraph, while insertion of an internal rotating half-wave plate, a field mask, and a Wollaston prism converts the camera into a dual-beam polarimeter.

Mid-infrared (10-25 \micron) imaging of the protostellar binary system L1448N-IRS3(A,B) is presented. Only one source, IRS3(A), was detected at mid-infrared wavelengths -- all of the mid-infrared emission from IRS3(A,B) emanates from IRS3(A). The mid-infrared luminosity of IRS3(A) is $L_{midir} = 1.3(\frac{d}{300\rm{pc}})^2 L_\sun$, which yields a central source mass, depending on the mass infall rate, of $M_* = 0.2 M_\sun \frac{10^{-6}M_\sun yr^{-1}}{\dot M}$. The envelope mass surrounding IRS3(A) is $\sim 0.15 M_\sun$, suggesting that the central source and the envelope are of comparable mass. The locations of IRS3(A) and IRS3(B) on an $M_{env} - L_{bol}$ diagram indicate that IRS3(A) and IRS3(B) appear to be class I and class 0 protostars, respectively., Accepted for publication in The Astrophysical Journal

We present high-resolution (, Accepted for publication by the Astrophysical Journal, 27 pages, 9 figures

We present high angular resolution (~0.5") 10 and 18 micron images of the region around G29.96-0.02 taken from the Gemini North 8-m telescope using the mid-infrared imager and spectrometer OSCIR. These observations were centered on the location of a group of water masers, which delineate the site of a hot molecular core believed to contain an extremely young, massive star. We report here the direct detection of a hot molecular core at mid-infrared wavelengths at this location. The size and extent of the core at 18 microns appears to be very similar to the morphology as seen in integrated NH3 maps. However, our observations indicate that the mid-infrared emission may not be exactly coincident with the NH3 emission., 12 pages, 3 figures, accepted for publication in ApJ Letters

G339.88-1.26 is considered to be a good candidate for a massive star with a circumstellar disk. This has been supported by the observations of linearly distributed methanol maser spots believed to delineate this disk, and mid-infrared observations that have discovered a source at this location that is elongated at the same position angle as the methanol maser distribution. We used the mid-infrared imager/spectrometer OSCIR at Keck to make high-resolution images of G339.88-1.26. We resolve the mid-infrared emission into 3 sources within 1.5 arcsec of the location of the masers. We determine that the methanol masers are most likely not located in a circumstellar disk. Furthermore we find that the observed radio continuum emission most likely comes from two sources in close proximity to each other. One source is an unobscured massive star with an extended HII region that is responsible for the peak in the radio continuum emission. A second source is embedded and centered on the elongation in the radio continuum emission that is believed to be tracing an outflow in this region., Comment: 12 pages, 2 figures, Accepted for publication in ApJ

We have conducted deep mid-infrared imaging of a relatively nearby sample of candidate Vega-like stars using the OSCIR instrument on the CTIO 4-meter and Keck II 10-meter telescopes. Our discovery of a spatially-resolved disk around HR 4796A has already been reported (Jayawardhana et al. 1998). Here we present imaging observations of the other members of the sample, including the discovery that only the primary in the HD 35187 binary system appears to harbor a substantial circumstellar disk and the possible detection of extended disk emission around 49 Ceti. We derive global properties of the dust disks, place constraints on their sizes, and discuss several interesting cases in detail. Although our targets are believed to be main sequence stars, we note that several have large infrared excesses compared to prototype Vega-like systems, and may therefore be somewhat younger. The disk size constraints we derive, in many cases, imply emission from relatively large ($\gtrsim$ 10$\mu$m) particles at mid-infrared wavelengths., Comment: 15 pages and 2 PostScript figures, accepted for publication in The Astronomical Journal

We present the results of the first broadband 10.8 um survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry in order to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster, and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band magnitudes K < 10.5, we detected 35 (~59%) at 10 um. Combining these detections, and upper limits for the non-detections, with existing JHKL data, we identify 1 Class I, 6 flat spectrum, 28 Class II and 5 Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% +/- 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in NGC 2024 formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared excess region of the JHKL color-color diagram for the cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with disks. Of the 14 sources with K - L colors suggestive of protostellar objects, \~29% are protostellar in nature, while ~7% are true Class I YSOs. This may be due to extinction producing very red K - L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests caution must be applied when estimating the sizes and lifetimes of protostellar populations within star forming regions based on K - L colors alone. Finally, we calculate the luminosities of the Class II YSOs in NGC 2024, rho Oph and Taurus and discuss the results., 30 pages, 8 figures, paper to appear in March AJ

This paper presents diffraction-limited 1-18 micron images of the young quadruple star system HD 98800 obtained with the W. M. Keck 10-m telescopes using speckle and adaptive optics imaging at near-IR wavelengths and direct imaging at mid-IR wavelengths. The two components of the visual binary, A and B, both themselves spectroscopic binaries, were separable at all wavelengths, allowing us to determine their stellar and circumstellar properties. Combining these observations with spectroscopic data from the literature, we derive an age of 10 Myr, masses of 0.93 and 0.64 M_sun and an inclination angle of 58 deg for the spectroscopic components of HD 98800 B, and an age of 10 Myr and a mass of 1.1 M_sun for HD 98800 Aa. Our data confirm that the large mid-IR excess is entirely associated with HD 98800 B. This excess exhibits a black body temperature of 150 K and a strong 10 micron silicate emission feature. The theoretical equilibrium radius of large, perfectly absorbing, 150 K grains around HD 98800 B is 2.4 AU, suggesting a circum-spectroscopic binary distribution. Our observations set important upper limits on the size of the inner dust radius of ~2 AU (mid-IR data) and on the quantity of scattered light of 2 microns. The total mass of the dust, located in a circumbinary disk around the HD 98800 B, is >0.002 M_earth. The orbital dynamics of the A-B pair are likely responsible for the disk geometry., 27 pages, 4 tables, 9 figures; small revisions to section 3.2.4

We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that was recently discovered to have a spectacular, nearly edge-on, circumstellar disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998). These new images, obtained with OSCIR at Keck II, show that the disk's size at 10 microns is comparable to its size at 18 microns. Therefore, the 18 micron-emitting dust may also emit some, or all, of the 10 micron radiation. Using these multi-wavelength images, we determine a "characteristic" diameter of 2-3 microns for the mid-infrared-emitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Dynamical modeling of the disk (Wyatt et al. 2000) indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider et al. (1999) from their NICMOS images. We present evidence (~1.8 sigma significance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This "pericenter glow," which must still be confirmed, results from a very small (a few AU) shift of the disk's center of symmetry relative to the central star HR 4796A; one side of the inner boundary of the annulus is shifted towards HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical effects of planets on disks is within reach., Comment: 18 pages. 9 GIF images. Total size ~800 kB. High resolution images available upon request. Accepted for publication in the Astrophysical Journal (scheduled for January 10, 2000)

Recent images of the disks of dust around the young stars HR 4796A and Fomalhaut show, in each case, a double-lobed feature that may be asymmetric (one lobe may be brighter than the other). A symmetric double-lobed structure is that expected from a disk of dust with a central hole that is observed nearly edge-on (i.e., close to the plane of the disk). This paper shows how the gravitational influence of a second body in the system with an eccentric orbit would cause a brightness asymmetry in such a disk by imposing a "forced eccentricity" on the orbits of the constituent dust particles, thus shifting the center of symmetry of the disk away from the star and causing the dust near the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR 4796 disk shows that its 5% brightness asymmetry could be the result of a forced eccentricity as small as 0.02 imposed on the disk by either the binary companion HR 4796B, or by an unseen planet close to the inner edge of the disk. Since it is likely that a forced eccentricity of 0.01 or higher would be imposed on a disk in a system in which there are planets, but no binary companion, the corresponding asymmetry in the disk's structure could serve as a sensitive indicator of these planets that might otherwise remain undetected., Comment: 61 pages, 10 figures, accepted for publication in the Astrophysical Journal (scheduled for January 10, 2000)

We present high-resolution mid-infrared observations of the nearby late-type young binary system Hen 3-600. The binary, at a distance of $\sim$ 50 pc, could be a member of the TW Hydrae Association, the nearest known group of young stars, with an age of a few million years. Our images make it possible for the first time to determine which star in the pair, separated by 1.4'', harbors the mid-infrared excess detected by IRAS. In the near-infrared, where the radiation is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux ratio of 1.6. At 4.8$\mu$m, 10.8$\mu$m, and 18.2$\mu$m, the primary becomes increasingly dominant over the secondary, suggesting that most of the circumstellar dust in the system resides around Hen 3-600A. Comparison of the spectral energy distribution (SED) of Hen 3-600A to the median SED of classical T Tauri stars suggests that its disk may be truncated by the secondary and provides tentative evidence for a central disk hole. The distribution of dust in the Hen 3-600 system may provide important clues to the formation and evolution of protoplanetary disks in close binaries., Comment: 9 pages, 2 PostScript figures, accepted for publication in The Astrophysical Journal Letters

We report the codiscovery of the spatially-resolved dust disk of the Vega-like star HR 4796A. Images of the thermal dust emission at $\lambda = 18 \mu$m show an elongated structure approximately 200 AU in diameter surrounding the central A0V star. The position angle of the disk, $30^{\circ} \pm 10^{\circ}$, is consistent to the position angle of the M companion star, $225^{\circ}$, suggesting that the disk-binary system is being seen nearly along its orbital plane. The surface brightness distribution of the disk is consistent with the presence of an inner disk hole of approximately 50 AU radius, as was originally suggested by Jura et al. on the basis of the infrared spectrum. HR 4796 is a unique system among the Vega-like or $\beta$ Pictoris stars in that the M star companion (a weak-emission T Tauri star) shows that the system is relatively young, $\sim 8 \pm 3$ Myr. The inner disk hole may provide evidence for coagulation of dust into larger bodies on a timescale similar to that suggested for planet formation in the solar system., Comment: 12 pages, 3 PostScript figures, accepted for publication in Astrophysical Journal Letters