William F. Hoffmann

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

The Spitzer Space Telescope Mission

The Spitzer Space Telescope, NASAs Great Observatory for infrared astronomy, was launched 2003 August 25 and is returning excellent scientific data from its Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity achievable with a cryogenic telescope in space with the great imaging and spectroscopic power of modern detector arrays to provide the user community with huge gains in capability for exploration of the cosmos in the infrared. The observatory systems are largely performing as expected, and the projected cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit scientific, technical, and operational performance of Spitzer. Subsequent papers in this special issue describe the Spitzer instruments in detail and highlight many of the exciting scientific results obtained during the first 6 months of the Spitzer mission.

CONSTRAINING THE LIFETIME OF CIRCUMSTELLAR DISKS IN THE TERRESTRIAL PLANET ZONE: A MID-INFRARED SURVEY OF THE 30 Myr OLD TUCANA-HOROLOGIUM ASSOCIATION

We have conducted an N-band survey of 14 young stars in the ~30 Myr old Tucana-Horologium association to search for evidence of warm, circumstellar dust disks. Using the MIRAC-BLINC camera on the Magellan I (Baade) 6.5 m telescope, we find that none of the stars have a statistically significant N-band excess compared to the predicted stellar photospheric flux. Using three different sets of assumptions, this null result rules out the existence of the following around these post-T Tauri stars: (1) optically thick disks with inner hole radii of 0.1 AU, (2) optically thin disks with masses of less than 10-6 M⊕ (in ~1 μm sized grains) within 10 AU of these stars, and (3) scaled-up analogs of the solar system zodiacal dust cloud with more than 4000 times the emitting area. Our survey was sensitive to dust disks in the terrestrial planet zone with fractional luminosity of log(Ldust/L*) ~ 10-2.9, yet none were found. Combined with results from previous surveys, these data suggest that circumstellar dust disks become so optically thin as to be undetectable at N band before age ~20 Myr. We also present N-band photometry for several members of other young associations and a subsample of targets that will be observed with the Spitzer Space Telescope by the Formation and Evolution of Planetary Systems Legacy Science Program. Finally, we present an absolute calibration of MIRAC-BLINC for four filters (L, N, 11.6, and Qs) on the Cohen-Walker-Witteborn system.

Semi-annual oscillations in Saturn's low-latitude stratospheric temperatures.

Observations of oscillations of temperature and wind in planetary atmospheres provide a means of generalizing models for atmospheric dynamics in a diverse set of planets in the Solar System and elsewhere. An equatorial oscillation similar to one in the Earth’s atmosphere has been discovered in Jupiter. Here we report the existence of similar oscillations in Saturn’s atmosphere, from an analysis of over two decades of spatially resolved observations of its 7.8-μm methane and 12.2-μm ethane stratospheric emissions, where we compare zonal-mean stratospheric brightness temperatures at planetographic latitudes of 3.6° and 15.5° in both the northern and the southern hemispheres. These results support the interpretation of vertical and meridional variability of temperatures in Saturn’s stratosphere as a manifestation of a wave phenomenon similar to that on the Earth and in Jupiter. The period of this oscillation is 14.8 ± 1.2 terrestrial years, roughly half of Saturn’s year, suggesting the influence of seasonal forcing, as is the case with the Earth’s semi-annual oscillation.

Imaging circumstellar environments with a nulling interferometer

Extrasolar planets must be imaged directly if their nature is to be better understood. But this will be difficult, as the bright light from the parent star (or rather its diffracted halo in the imaging apparatus) can easily overwhelm nearby faint sources. Bracewell has proposed a way of selectively removing starlight before detection, by superposing the light from two telescopes so that the stellar wavefronts interfere destructively. Such a ‘nulling’ interferometer could be used in space to search for extrasolar Earth-like planets through their thermal emission and to determine through spectroscopic analysis if they possess the atmospheric signatures of life. Here we report mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique. Images of unresolved stars are seen to disappear almost completely, while light from a nearby source as close as 0.2 arcsec remains, as shown by images of Betelgeuse. With this star cancelled, there remains the thermal image of its surrounding, small dust nebula. In the future, larger ground-based interferometers that correct for atmospheric distortions (using adaptive optics) should achieve better cancellation, allowing direct detection of warm, Jupiter-size planets and faint zodiacal dust around other nearby stars.

Earth-Based Observations of the Galileo Probe Entry Site

Earth-based observations of Jupiter indicate that the Galileo probe probably entered Jupiters atmosphere just inside a region that has less cloud cover and drier conditions than more than 99 percent of the rest of the planet. The visual appearance of the clouds at the site was generally dark at longer wavelengths. The tropospheric and stratospheric temperature fields have a strong longitudinal wave structure that is expected to manifest itself in the vertical temperature profile.

Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility

The National Aeronautics and Space Administration (NASA) Infrared Telescope Facility was used to investigate the collision of comet Shoemaker-Levy 9 with Jupiter from 12 July to 7 August 1994. Strong thermal infrared emission lasting several minutes was observed after the impacts of fragments C, G, and R. All impacts warmed the stratosphere and some the troposphere up to several degrees. The abundance of stratospheric ammonia increased by more than 50 times. Impact-related particles extended up to a level where the atmospheric pressure measured several millibars. The north polar near-infrared aurora brightened by nearly a factor of 5 a week after the impacts.

Mid-Infrared (8-21 micron) Imaging of Proto-Planetary Nebulae

We present mid-infrared (8-21 μm) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 μm emission feature. Both of the sources are well resolved and show evidence for axial symmetry. From our images, we calculate temperature and optical depth maps and estimate the abundance of the 11 μm and 21 μm feature carriers. In both sources, the dust temperatures range from ~160-200 K. The optical depths in IRAS 07134 are about a factor of 3 lower than those in IRAS 22272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylindrical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 M☉ for IRAS 07134 and 0.42 M☉ for IRAS 22272. Although the mid-IR emission primarily comes from warm (T ≈ 190 K) dust, our models require a significant cooler dust (T ≈ 80 K) component to fit the observed mid- and far-IR spectral energy distributions.

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC.

Subarcsecond Mid-Infrared Structure of the Dust Shell around IRAS 22272+5435*

We report sub-arcsecond imaging of extended mid-infrared emission from a proto-planetary nebula (PPN), \iras 22272+5435, performed at the MMT observatory with its newly upgraded 6.5 m aperture telescope and at the Keck observatory. The mid-infrared emission structure is resolved into two emission peaks separated by