John R. Stauffer

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.

The initial mass function in the Taurus star-forming region

By combining a deep optical imaging (I; z 0 ) survey of 8 deg 2 in the Taurus star-forming region with data from the Two-Micron All-Sky Survey (2MASS) and follow-up spectroscopy, we have performed a search for low-mass Taurus members that is complete to 0.02 Mfor reddenings of AV d4. We report the discovery of nine new members with spectral types of M5.75-M9.5, corresponding to masses of 0.1-0.015 Mby recent evolutionary models. The new M9.5 member is the least massive brown dwarf found to date in the Taurus star-forming region. We derive an initial mass function (IMF) for the fields surveyed in this work and in our previous studies, which encompass 54% of the known Taurus membership. We compare the Taurus IMF with a similarly derived one for the Trapezium Cluster and to mass functions for the M35 and Pleiades open clusters. While the IMFs in all of these regions flatten near � 0.8 M� , the mass function in Taurus is more nar- row and sharply peaked at this mass. Our survey indicates that Taurus has � 2 � fewer brown dwarfs at 0.02- 0.08 Mthan the Trapezium. We discuss the implications of these results for theories of the IMF, and suggest that the lower frequency of brown dwarfs in Taurus relative to the Trapezium may result from the low-den- sity star-forming environment, leading to larger minimum Jeans masses. Subject headings: infrared: stars — stars: evolution — stars: formation — stars: low-mass, brown dwarfs — stars: luminosity function, mass function — stars: pre-main-sequence

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelengthrangefrom3.6to24 � m,allowingustodetectdiskemissionoveratypicalrangeofradii � 0.1to � 20AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be ‘‘transition’’ objects, with essentially photospheric fluxes at wavelengths � 4.5 � m but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the youngerTaurusregion;thedecreaseininfraredexcessislargerat6–8 � mthanat24 � m,suggestingthatgraingrowth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 � m excesses around a few intermediate-mass stars, which may represent so-called ‘‘debris disk’’ systems. Our observations provide new constraints on disk evolution through an important age range. Subject headingg accretion, accretion disks — planetary systems: protoplanetary disks — stars: pre–main-sequence

Strong water absorption in the dayside emission spectrum of the planet HD 189733b

Recent observations of the extrasolar planet HD 189733b did not reveal the presence of water in the emission spectrum of the planet. Yet models of such ‘hot-Jupiter’ planets predict an abundance of atmospheric water vapour. Validating and constraining these models is crucial to understanding the physics and chemistry of planetary atmospheres in extreme environments. Indications of the presence of water in the atmosphere of HD 189733b have recently been found in transmission spectra, where the planet’s atmosphere selectively absorbs the light of the parent star, and in broadband photometry. Here we report the detection of strong water absorption in a high-signal-to-noise, mid-infrared emission spectrum of the planet itself. We find both a strong downturn in the flux ratio below 10 µm and discrete spectral features that are characteristic of strong absorption by water vapour. The differences between these and previous observations are significant and admit the possibility that predicted planetary-scale dynamical weather structures may alter the emission spectrum over time. Models that match the observed spectrum and the broadband photometry suggest that heat redistribution from the dayside to the nightside is weak. Reconciling this with the high nightside temperature will require a better understanding of atmospheric circulation or possible additional energy sources.

THE SPITZER SPACE TELESCOPE SURVEY OF THE ORION A AND B MOLECULAR CLOUDS. I. A CENSUS OF DUSTY YOUNG STELLAR OBJECTS AND A STUDY OF THEIR MID-INFRARED VARIABILITY

We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments on board Spitzer. In total, five distinct fields were mapped, covering 9 deg^2 in five mid-IR bands spanning 3-24 μm. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630, and 1622 dark clouds, and the NGC 2023, 2024, 2068, and 2071 nebulae. These data are merged with the Two Micron All Sky Survey point source catalog to generate a catalog of eight-band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main-sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in two to three epochs over six months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 μm bands. We find that 50% of the dusty YSOs show variability. The variations are typically small (~0.2 mag) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8, and 8 μm variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots.

A SAMPLE OF VERY YOUNG FIELD L DWARFS AND IMPLICATIONS FOR THE BROWN DWARF LITHIUM TEST AT EARLY AGES

Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6% ± 1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (decl: < 0°) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, β Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few x 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well-determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

Spitzer IRAC Photometry of M, L, and T Dwarfs

We present the results of a program to acquire photometry for 86 late M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_(5.8) versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one-parameter family in T_(eff). Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of 5 (~70M_J to ~15M_J), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 μm fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 μm. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of K_s, IRAC 3.6 μm, and 4.5 μm bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_k versus K_s-[4.5] relation, which shows a smooth progression over spectral type, and splits the M, L, and T types cleanly.

New low-mass members of the Taurus star-forming region

Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 deg2 of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have AV ≤ 8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M☉). Its spectrum contains extremely strong emission in Hα (Wλ ~ 950 A), as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong Hα are suggestive of intense accretion.

Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC 2391: Lithium Depletion and Hα Emission

We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC 2391. Of these, 26 spectra are totally new, 14 had been analyzed in a previous paper, and another four are in common with that paper. These spectra, taken at the Cerro Tololo 4 m and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, the presence of Li, and Hα emission. Among these new cluster members is CTIO-160 (M7), the first IC 2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708 A doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. On the basis of comparison of several theoretical models, we derive an age of 50 ± 5 Myr for IC 2391. We also estimate new ages for the α Per and Pleiades clusters; our ages are 85 ± 10 and 130 ± 20 Myr, respectively. We derive an estimate of the initial mass function of IC 2391 that extends to below the substellar limit and compare it to those of other well-studied young open clusters. The index of the power-law mass function for IC 2391 is α = 0.96 ± 0.12, valid in the range 0.5-0.072 M☉.We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC 2391. Of these, 26 spectra are totally new, 14 were already analyzed in a previous paper and another four are in common. These spectra, taken at the Cerro Tololo 4-meter and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, presence of Li, and Hα emission. Among these new cluster members is CTIO-160 (M7), the first IC 2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708A doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. Based – 2 – on comparison to several theoretical models, we derive an age of 50±5 Myr for IC 2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our ages are 85±10 Myr and 130±20 Myr, respectively. We derive an estimate of the initial mass function of IC 2391 that extends to below the substellar limit, and compare it to those of other well-studied young open clusters. The index of the power law mass function for IC 2391 is α=0.96±0.12, valid in the range 0.5 to 0.072 M ⊙. Subject headings: stars: low mass, brown dwarfs – stars: pre-main-sequence – stars: luminosity, mass functions – open clusters and associations: individual: IC 2391

The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones

We continue our series of papers on open cluster distances with a critical assessment of the accuracy of main-sequence fitting using isochrones that employ empirical corrections to the color-temperature relations. We use four nearby open clusters with multicolor photometry and accurate metallicities and present a new metallicity for Praesepe ([Fe/H]=+0.11±0.03) from high-resolution spectra. The internal precision of distance estimates is about a factor of 5 better than the case without the color calibrations. After taking into account all major systematic errors, we obtain distances accurate to about 2%–3% when there exists a good metallicity estimate. Metallicities accurate to better than 0.1 dex may be obtained from BVICKs photometry alone. We also derive a helium abundance for the Pleiades of Y=0.279±0.015, which is equal within the errors to the Suns initial helium abundance and that of the Hyades. Our best estimates of distances are (m-M)0=6.33±0.04,8.03±0.04, and 9.61±0.03 to Praesepe, NGC 2516, and M67, respectively. Our Pleiades distance at the spectroscopic metallicity,(m-M)0=5.66±0.01(internal)±0.05(systematic), is in excellent agreement with several geometric distance measurements. We have made calibrated isochrones for -0.3≤[Fe/H]≤+0.2 available online.