Robert D. Gehrz

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.

On extending the mass-metallicity relation of galaxies by 2.5 decades in stellar mass

We report 4.5 � m luminosities for 27 nearby (DP5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 � m luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 � m L-Z relation is 12þ log (O/H) ¼ (5:78 � 0:21) þ (� 0:122 � 0:012)M½4:5� , where M[4.5] is the absolute magnitude at 4.5 � m. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M� -Z) relation is 12 þ log (O/H) ¼ (5:65 � 0:23) þ (0:298 � 0:030)log M� , and extends the SDSSM� - Zrelation to lower mass by about 2.5 dex. We find that the dispersion in the M� -Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L‐Z and M� -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed. Subject headingg galaxies: dwarf — galaxies: evolution — galaxies: irregular — infrared: galaxies Online material: color figures

0.7- to 23-μm photometric observations of P/Halley 1986 III and six recent bright comets

Abstract We report 0.7- to 23-μm observations of P/Halley 1986 III and six other recent bright comets. P/Halley was measured on 47 occasions between 1985 December 12 UT and 1986 May 6 UT, several times within hours of the perihelion passage on 1986 February 9 UT. Our data show that the strength of the 10-μm silicate emission feature and the temperature excess (superheat; S = T obs / T BB ) of the infrared continuum emission are strongly correlated. IR Type I comets have low continuum superheat and muted or undetectable silicate emission features, suggesting that the coma emission from these comets is produced by large grains with radii larger than 1 μm. IR Type II comets have superheated thermal infrared continua and high-contrast silicate emission features, indicating that the coma emission is from small grains with radii between 0.5 and 1 μm. Both types of behavior were exhibited by Comet P/Halley at various times. The relationship between superheat and 10-μm silicate emission may be complex, for although though the strength of these quantities was generally strongly correlated, several comets exhibited occasional episodes when superheat and silicate emission were not correlated. P/Halleys dust coma had an average albedo of 0.20 at a scattering angle of 130°. Our data show that the scattering phase function for typical comet dust is characterized by a moderately strong forward scattering peak, no appreciable backscattering peak, a mean bolometric albedo of ≈0.32, and an albedo of ≈0.15 for scattering angles between 120° and 180°. These characteristics are consistent with laboratory and theoretical results for nonspherical and “fluffy” core-mantle aggregate grains. P/Halleys 10-μm silicate signature showed significant variations in strength and was occasionally weak or absent at heliocentric distances both smaller and larger than 1 AU. Simultaneous measurements of P/Halley and Bradfield 1980 XV with different diaphragms are generally consistent with the steady-state model for nuclear ablation. P/Halleys coma luminosity fluctuated by a factor of nearly 10 on time scales of 1 to 2 days. These variations are consistent with jet-like activity probably associated with nuclear rotation. Dust mass loss rates for the comets studied here are estimated, and we conclude that P/Halley was losing ≥10 6 g sec −1 of dust at a heliocentric distance of 1 AU.

Early science with SOFIA, the stratospheric observatory for infrared astronomy

The Stratospheric Observatory For Infrared Astronomy (SOFIA) is an airborne observatory consisting of a specially modified Boeing 747SP with a 2.7 m telescope, flying at altitudes as high as 13.7 km (45,000 ft). Designed to observe at wavelengths from 0.3 μm to 1.6 mm, SOFIA operates above 99.8% of the water vapor that obscures much of the infrared and submillimeter. SOFIA has seven science instruments under development, including an occultation photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and heterodyne receivers. SOFIA, a joint project between NASA and the German Aerospace Center Deutsches Zentrum fur Luft und-Raumfahrt, began initial science flights in 2010 December, and has conducted 30 science flights in the subsequent year. During this early science period three instruments have flown: the mid-infrared camera FORCAST, the heterodyne spectrometer GREAT, and the occultation photometer HIPO. This Letter provides an overview of the observatory and its early performance.

The Asymmetric Nebula Surrounding the Extreme Red Supergiant Vy Canis Majoris

We present HST/WFPC2 images plus ground-based infrared images and photometry of the very luminous OH/IR star VY Canis Majoris. Our WFPC2 data show a complex distribution of knots and filamentary arcs in the asymmetric reflection nebula around the obscured central star. The reflection arcs may result from multiple, asymmetric ejection episodes due to localized events on VY CMas surface. Such events probably involve magnetic fields and convection, by analogy with solar activity. Surface photometry indicates that the star may have experienced enhanced mass loss over the past 1000 yr. We also demonstrate that the apparent asymmetry of the nebula results from a combination of high extinction and backscattering by dust grains. Thermal-infrared images reveal a more symmetric distribution, elongated along a nearly east-west direction. VY CMa probably has a flattened disklike distribution of dust with a northeast-southwest polar axis and may be experiencing activity analogous to solar prominences. The presence of an axis of symmetry raises interesting questions for a star the size of Saturns orbit. Magnetic fields and surface activity may play an important role in VY CMas mass-loss history.

MASS LOSS FROM M STARS.

M stars mass loss, determining dust shells radii and densities with IR observations of circumstellar emission

Sources of stardust in the Galaxy

Observed mass loss rates and Galactic stellar population distributions are used to estimate the rate of injection of stardust into the ISM. M stars and RLOH/IR Stars produce most of the silicates; most of the carbon and SiC comes from carbon stars. WR stars, novae, and supernovae may eject dust with chemical anomalies. There is little observational evidence for a major stellar source of hydrocarbon grains. The Galactic dust ecology considered by comparing stellar dust injection with depletion by star formation and supernova shocks suggests that dust grains are produced by accretion in molecular clouds at 1 to 5 times the stellar rate.

IRC +10420 - A cool hypergiant near the top of the H-R diagram

New data are reported for the OH/IR star IRC+10420, including optical/infrared imaging, spectroscopy, polarimetry, and photometry. We conclude the following: 1. The optical spectrum is that of a very luminous F supergiant (F Ia+) with a very strong O I blend at 7774 A. Hα is strongly in emission and shows a double-peaked profile similar to the emission seen in stars with rotating equatorial disks. 2. The optical image taken through polarizing filters is elongated, and shows that the star must be intrinsically polarized at a position angle near 90°. The 8.7 μm image is also elongated, but at a position angle near 150°

Giants in the globular cluster ω Centauri: dust production, mass-loss and distance

We present spectral energy distribution modelling of 6875 stars in ω Centauri, obtaining stellar luminosities and temperatures by fitting literature photometry to state-of-the-art MARCS stellar models. By comparison to four different sets of isochrones, we provide a new distance estimate to the cluster of 4850 ± 200 (random error) ± 120 (systematic error) pc, a reddening of E(B − V) = 0.08 ± 0.02 (random) ± 0.02 (systematic) mag and a differential reddening ofE(B − V) 1.2 ± 0.6 0.5 × 10 −6 Myr −1 . Half of the clusters dust production and 30 per cent of its gas production comes from the two most extreme stars - V6 and V42 - for which we present new Gemini/T-ReCS mid-infrared spectroscopy, possibly showing that V42 has carbon-rich dust. The clusters dust tempera- tures are found to be typically 550 K. Mass-loss apparently does not vary significantly with metallicity within the cluster, but shows some correlation with barium enhancement, which appears to occur in cooler stars, and especially on the anomalous RGB. Limits to outflow velocities, dust-to-gas ratios for the dusty objects and the possibility of short-time-scale mass- loss variability are also discussed in the context of mass-loss from low-metallicity stars. The ubiquity of dust around stars near the RGB tip suggests significant dusty mass-loss on the RGB; we estimate that typically 0.20-0.25 Mof mass-loss occurs on the RGB. From obser- vational limits on intracluster material, we suggest the dust is being cleared on a time-scale of 10 5 yr.

The NASA Spitzer Space Telescope

The National Aeronautics and Space Administrations Spitzer Space Telescope (formerly the Space Infrared Telescope Facility) is the fourth and final facility in the Great Observatories Program, joining Hubble Space Telescope (1990), the Compton Gamma-Ray Observatory (1991-2000), and the Chandra X-Ray Observatory (1999). Spitzer, with a sensitivity that is almost three orders of magnitude greater than that of any previous ground-based and space-based infrared observatory, is expected to revolutionize our understanding of the creation of the universe, the formation and evolution of primitive galaxies, the origin of stars and planets, and the chemical evolution of the universe. This review presents a brief overview of the scientific objectives and history of infrared astronomy. We discuss Spitzers expected role in infrared astronomy for the new millennium. We describe pertinent details of the design, construction, launch, in-orbit checkout, and operations of the observatory and summarize some science highlights from the first two and a half years of Spitzer operations. More information about Spitzer can be found at http://spitzer.caltech.edu/.