James Thomas Radomski

Mid-infrared images of β Pictoris and the possible role of planetesimal collisions in the central disk

When viewed in optical starlight scattered by dust, the nearly edge-on debris disk surrounding the A5V star β Pictoris (distance 19.3 pc; ref. 1) extends farther than 1,450 au from the star. Its large-scale complexity has been well characterized, but the detailed structure of the disks central ∼200-au region has remained elusive. This region is of special interest, because planets may have formed there during the stars 10–20-million-year lifetime, perhaps resulting in both the observed tilt of 4.6 degrees relative to the large-scale main disk and the partial clearing of the innermost dust. A peculiarity of the central disk (also possibly related to the presence of planets) is the asymmetry in the brightness of the ‘wings’, in which the southwestern wing is brighter and more extended at 12 µm than the northeastern wing. Here we present thermal infrared images of the central disk that imply that the brightness asymmetry results from the presence of a bright clump composed of particles that may differ in size from dust elsewhere in the disk. We suggest that this clump results from the collisional grinding of resonantly trapped planetesimals or the cataclysmic break-up of a planetesimal.

Resolved Mid-Infrared Emission in the Narrow-Line Region of NGC 4151

We present subarcsecond-resolution mid-infrared images of NGC 4151 at 10.8 and 18.2 μm. 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 and 18.2 μm, extending ~35 across at a P.A. of ~60°. This coincides with the narrow-line region of NGC 4151, as observed in [O III] 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 35 pc.

Deep 10 micron imaging of M87

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.

Understanding the 8 μm versus Paα Relationship on Subarcsecond Scales in Luminous Infrared Galaxies

14 pages, 9 figures, 4 tables.-- Based on observations obtained with T-ReCS instrument at the Gemini South Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: NSF (United States), PPARC (UK), NRC (Canada), CONICYT (Chile) ARC (Australia), CNPq (Brazil), and CONICET (Argentina).-- Also based on observations with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-266555.

Observations of Massive Star-Forming Regions with Water Masers: Mid-Infrared Imaging

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 H II 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.

Mid-infrared, spatially resolved spectroscopy of the nucleus of the Circinus galaxy

High spatial resolution spectroscopy at 8-13 μm with T-ReCS on Gemini-S has revealed striking variations in the mid-infrared emission and absorption in the nucleus of the Circinus galaxy (hereafter Circinus) on subarcsecond scales. The core of Circinus is compact and obscured by a substantial column of cool silicate dust. Weak extended emission to the east and west coincides with the coronal line region and arises from featureless dust grains which are probably heated by line emission in the coronal emission zone. The extended emission on the east side of the nucleus displays a much deeper silicate absorption than that on the west, indicating significant columns of cool material along the line of sight and corresponding to an additional extinction of A V ∼ 25 mag. Emission bands from aromatic hydrocarbons are not subject to this additional extinction, are relatively weak in the core and in the coronal line region, and are much more spatially extended than the continuum dust emission; they presumably arise in the circumnuclear star-forming regions. These data are interpreted in terms of an inclined disc-like structure around the nucleus extending over tens of parsecs and possibly related to the inner disc found from observations of water masers by Greenhill et al..

High-Resolution Mid-Infrared Morphology of Cygnus A

We present subarcsecond-resolution mid-infrared images at 10.8 and 18.2 μm of Cyg 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 biconical structure observed at optical and near-IR wavelengths by the Hubble Space Telescope. This emission is consistent with dust heated from the central engine of Cyg A. We also marginally detect large-scale low-level emission extending more than 1.5 kpc from the nucleus, which may be caused by in situ star formation, line emission, and/or polycyclic aromatic hydrocarbon contamination within the bandpass of our wide N-band filter.

High spatial resolution T-ReCS mid-infrared imaging of luminous infrared galaxies

We present diffraction-limited (FWHM ~ 03) Gemini/T-ReCS mid-infrared (MIR; N-band or narrowband at 8.7 μm) imaging of four luminous infrared galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kiloparsecs is strikingly similar to that traced by Paα and generally consists of bright nuclear emission and several compact circumnuclear and/or extranuclear H II regions. The central MIR emission is dominated by these powerful H II regions, consistent with the majority of active galactic nuclei in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear H II regions detected in LIRGs follow the extrapolation of the 8 μm versus Paα relation found for M51 H II knots. The integrated central 3-7 kpc of galaxies, however, present elevated 8 μm/Paα ratios with respect to individual H II regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8 μm emission, not directly related to the ionizing stellar population, can be as luminous as that from the resolved H II regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8 μm emission of nearby galaxies, not that of the H II regions alone.

Mid-Infrared Detection of a Hot Molecular Core in G29.96–0.02

We present high angular resolution (~05) 10 and 18 ?m images of the region around G29.96-0.02 taken from the Gemini North Observatory 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 ?m 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 and 18 μm Images of Dust Surrounding HD 32297

We present the first subarcsecond-resolution images at multiple mid-IR wavelengths of the thermally emitting dust around the A0 star HD 32297. Our observations with T-ReCS at Gemini South reveal a nearly edge-on resolved disk at both 11.7 and 18.3 μm that extends ~150 AU in radius. The mid-IR is the third wavelength region in which this disk has been resolved, following coronagraphic observations by others of the source at optical and near-IR wavelengths. The global mid-IR colors and detailed consideration of the radial colortemperature distribution imply that the central part of the disk out to ~80 AU is relatively deficient in dust.