Tim Brooke

The spitzer C2D survey of weak-line T tauri stars. I. Initial results

Using the Spitzer Space Telescope, we have observed 90 weak-line and classical T Tauri stars in the vicinity of the Ophiuchus, Lupus, Chamaeleon, and Taurus star-forming regions as part of the Cores to Disks (c2d) Spitzer Legacy project. In addition to the Spitzer data, we have obtained contemporaneous optical photometry to assist in constructing spectral energy distributions. These objects were specifically chosen as solar-type young stars with low levels of Hα emission, strong X-ray emission, and lithium absorption, i.e., weak-line T Tauri stars, most of which were undetected in the mid- to far-IR by the IRAS survey. Weak-line T Tauri stars are potentially extremely important objects in determining the timescale over which disk evolution may take place. Our objective is to determine whether these young stars are diskless or have remnant disks that are below the detection threshold of previous infrared missions. We find that only 5/83 weak-line T Tauri stars have detectable excess emission between 3.6 and 70 μm, which would indicate the presence of dust from the inner few tenths of an AU out to the planet-forming regions a few tens of AU from the star. Of these sources, two have small excesses at 24 μm consistent with optically thin disks; the others have optically thick disks already detected by previous IR surveys. All of the seven classical T Tauri stars show excess emission at 24 and 70 μm although their properties vary at shorter wavelengths. Our initial results show that disks are rare among young stars selected for their weak Hα emission.

The formation and evolution of planetary systems: Placing our solar system in context with Spitzer

We provide an overview of the Spitzer Legacy Program, Formation and Evolution of Planetary nSystems, that was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between n2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric nobservations of solar-type stars. With a sample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace nthe evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks nthrough to older collisionally generated debris disks. When completed, our program will help define the timescales nover which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a nfunction of time, and establish the diversity of mature planetary architectures. In addition to the observational nprogram, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of ncircumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic nand molecular gas line emission. Together with the observations, these efforts will provide an astronomical context nfor understanding whether our solar system—and its habitable planet—is a common or a rare circumstance. nAdditional information about the FEPS project can be found on the team Web site.

THE SPITZER c2d SURVEY OF WEAK-LINE T TAURI STARS. III. THE TRANSITION FROM PRIMORDIAL DISKS TO DEBRIS DISKS

We present 3.6 to 70 μm Spitzer photometry of 154 weak-line T Tauri stars (WTTSs) in the Chamaeleon, Lupus, Ophiuchus, and Taurus star formation regions, all of which are within 200 pc of the Sun. For a comparative study, we also include 33 classical T Tauri stars which are located in the same star-forming regions. Spitzer sensitivities allow us to robustly detect the photosphere in the IRAC bands (3.6 to 8 μm) and the 24 μm MIPS band. In the 70 μm MIPS band, we are able to detect dust emission brighter than roughly 40 times the photosphere. These observations represent the most sensitive WTTSs survey in the mid- to far-infrared to date and reveal the frequency of outer disks (r = 3-50 AU) around WTTSs. The 70 μm photometry for half the c2d WTTSs sample (the on-cloud objects), which were not included in the earlier papers in this series, those of Padgett et al. and Cieza et al., are presented here for the first time. We find a disk frequency of 19% for on-cloud WTTSs, but just 5% for off-cloud WTTSs, similar to the value reported in the earlier works. WTTSs exhibit spectral energy distributions that are quite diverse, spanning the range from optically thick to optically thin disks. Most disks become more tenuous than L disk/L * = 2 × 10–3 in 2 Myr and more tenuous than L disk/L * = 5 × 10–4 in 4 Myr.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds VIII. Serpens Observed with MIPS

We present maps of 1.5 deg2 of the Serpens dark cloud at 24, 70, and 160 μm observed with the Spitzer Space Telescope MIPS camera. We describe the observations and briefly discuss the data processing carried out by the c2d team on these data. More than 2400 compact sources have been extracted at 24 μm, nearly 100 at 70 μm, and four at 160 μm. We estimate completeness limits for our 24 μm survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets: a 0.50 deg 2 set on a low-extinction region near the dark cloud, and a 5.3 deg2 subset of the SWIRE ELAIS N1 data that was processed through our pipeline. These results show that there is an easily identifiable population of young stellar object candidates in the Serpens cloud that is not present in either of the reference data sets. We also show a comparison of visual extinction and cool dust emission illustrating a close correlation between the two and find that the most embedded YSO candidates are located in the areas of highest visual extinction.