S. E. Strom

The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry

We report on the properties of pre-main-sequence objects in the Taurus molecular clouds as observed in seven mid- and far-infrared bands with the Spitzer Space Telescope. There are 215 previously identified members of the Taurus star-forming region in our ~44 deg^2 map; these members exhibit a range of Spitzer colors that we take to define young stars still surrounded by circumstellar dust (noting that ~20% of the bona fide Taurus members exhibit no detectable dust excesses). We looked for new objects in the survey field with similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet imaging, and found 148 new candidate members of Taurus. We have obtained follow-up spectroscopy for about half the candidate sample, thus far confirming 34 new members, three probable new members, and 10 possible new members, an increase of 15%–20% in Taurus members. Of the objects for which we have spectroscopy, seven are now confirmed extragalactic objects, and one is a background Be star. The remaining 93 candidate objects await additional analysis and/or data to be confirmed or rejected as Taurus members. Most of the new members are Class II M stars and are located along the same cloud filaments as the previously identified Taurus members. Among non-members with Spitzer colors similar to young, dusty stars are evolved Be stars, planetary nebulae, carbon stars, galaxies, and active galactic nuclei.

NGC 6611: a cluster caught in the act

We have combined optical CCD photometry and spectroscopy with infrared imaging photometry to study the young cluster NGC 6611. We use these data to derive improved values for the reddening law (R=3.75) and the distance modulus (m−M=11.5), and to construct a physical Hertzprung-Russell (HR) diagram from which we can probe the ages, masses, and evolutionary states of this stellar ensemble. The HR diagram shows a strong population of high-mass stars, the most massive of which has a mass of roughly 80 M ○. , similar to what we find in other Galactic and Magellanic Cloud clusters and associations. The age of the massive stellar population in NGC 6611 is approximately 2 million yr, with an age spread of perhaps, 1 million yr, although the data are also consistent with there being no discernible age spread among the most massive stars

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 Systems, that was proposed in 2000, begun in 2001, and executed aboard the Spitzer Space Telescope between 2003 and 2006. This program exploits the sensitivity of Spitzer to carry out mid-infrared spectrophotometric observations of solar-type stars. With a sample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace the evolution of circumstellar gas and dust from primordial planet-building stages in young circumstellar disks through to older collisionally generated debris disks. When completed, our program will help define the timescales over which terrestrial and gas giant planets are built, constrain the frequency of planetesimal collisions as a function of time, and establish the diversity of mature planetary architectures. In addition to the observational program, we have coordinated a concomitant theoretical effort aimed at understanding the dynamics of circumstellar dust with and without the effects of embedded planets, dust spectral energy distributions, and atomic and molecular gas line emission. Together with the observations, these efforts will provide an astronomical context for understanding whether our solar system—and its habitable planet—is a common or a rare circumstance. Additional information about the FEPS project can be found on the team Web site.

The Formation and Evolution of Planetary Systems: First Results from a Spitzer Legacy Science Program

We present 3–160 � m photometry obtained with the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) instruments for the first five targets from the Spitzer Space Telescope Legacy Science Program ‘‘Formation and Evolution of Planetary Systems’’ and 4–35 � m spectrophotometry obtained with the Infrared Spectrograph (IRS) for two sources. We discuss in detail our observations of the debris disks surrounding HD 105 (G0 V, 30 � 10 Myr) and HD 150706 (G3 V, � 700 � 300 Myr). For HD 105, possible interpretations include large bodies clearing the dust inside of 45 AU or a reservoir of gas capable of sculpting the dust distribution. The disk surrounding HD 150706 also exhibits evidence of a large inner hole in its dust distribution. Of the four survey targets without previously detected IR excess, spanning ages 30 Myr to 3 Gyr, the new detection of excess in just one system of intermediate age suggests a variety of initial conditions or divergent evolutionary paths for debris disk systems orbiting solar-type stars. Subject heading gs: circumstellar matter — infrared: stars — planetary systems: protoplanetary disks

The North American and Pelican Nebulae. II. MIPS Observations and Analysis

We present observations of ~7 deg^2 of the North American and Pelican Nebulae region at 24, 70, and 160 μm with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). We incorporate the MIPS observations with earlier Spitzer Infrared Array Camera (IRAC) observations, as well as archival near-infrared (IR) and optical data. We use the MIPS data to identify 1286 young stellar object (YSO) candidates. IRAC data alone can identify 806 more YSO candidates, for a total of 2076 YSO candidates. Prior to the Spitzer observations, there were only ~200 YSOs known in this region. Three subregions within the complex are highlighted as clusters: the Gulf of Mexico, the Pelican, and the Pelicans Hat. The Gulf of Mexico cluster is subject to the highest extinction (AV at least ~30) and has the widest range of infrared colors of the three clusters, including the largest excesses and by far the most point-source detections at 70 μm. Just 3% of the cluster members were previously identified; we have redefined this cluster as about 10-100 times larger (in projected area) than was previously realized.

The star-forming history of the young cluster NGC 2264

UBVRI H-alpha photographic photometry was obtained for a sample of low-mass stars in the young open cluster NGC 2264 in order to investigate the star-forming history of this region. A theoretical H-R diagram was constructed for the sample of probable cluster members. Isochrones and evolutionary tracks were adopted from Cohen and Kuhi (1979). Evidence for a significant age spread in the cluster was found amounting to over ten million yr. In addition, the derived star formation rate as a function of stellar mass suggests that the principal star-forming mass range in NGC 2264 has proceeded sequentially in time from the lowest to the highest masses. The low-mass cluster stars were the first cluster members to form in significant numbers, although their present birth rate is much lower now than it was about ten million yr ago. The star-formation rate has risen to a peak at successively higher masses and then declined.

Star Formation: Answering Fundamental Questions During the Spitzer Warm Mission Phase

Through existing studies of star‐forming regions, Spitzer has created rich databases which have already profoundly influenced our ability to understand the star and planet formation process on micro and macro scales. However, it is essential to note that Spitzer observations to date have focused largely on deep observations of regions of recent star formation associated directly with well‐known molecular clouds located within 500 pc. What has not been done is to explore to sufficient depth or breadth a representative sample of the much larger regions surrounding the more massive of these molecular clouds. Also, while there have been targeted studies of specific distant star forming regions, in general, there has been little attention devoted to mapping and characterizing the stellar populations and star‐forming histories of the surrounding giant molecular clouds (GMCs). As a result, we have yet to develop an understanding of the major physical processes that control star formation on the scale or spiral a...

On the relationship between stellar rotation and radius in young clusters

We examine the early angular momentum history of stars in young clusters via 197 photometric periods in the Orion Flanking Fields, 83 photometric periods in NGC 2264, and 256 measurements of v sin i in the ONC. We show that PMS stars, even those without observable disks, apparently do not conserve stellar angular momentum as they evolve down their convective tracks, but instead evolve at nearly constant angular velocity. This result is inconsistent with expectations that convective stars lacking disks should spin up as they contract, but paradoxically consistent with disk-locking models. We briefly explore possible resolutions, including disk locking, birthline effects, stellar winds, and planetary companions. We have found no plausible explanations for this paradox.

The Formation and Evolution of Planetary Systems: SIRTF Legacy Science in the VLT Era

We will utilize the sensitivity of SIRTF through the Legacy Science Program to carry out spectrophotometric observations of solar-type stars aimed at (1) defining the timescales over which terrestrial and gas giant planets are built, from measurements diagnostic of dust/gas masses and radial distributions; and (2) establishing the diversity of planetary architectures and the frequency of planetesimal collisions as a function of time through observations of circumstellar debris disks Together, these observations will provide an astronomical context for understanding whether our solar system — and its habitable planet — is a common or a rare circumstance.

Imaging of Embedded Clusters: Mon R2

Isolated clusters of young stars embedded within molecular clouds provide unique opportunities to study star formation. They are ideal for establishing the timescales for star formation as well as determining the emergent mass spectrum of stellar groups. Such embedded clusters, bound by parent molecular gas, preserve a record of the mass distribution in a localized region of space. We present results from our near-IR survey of the Mon R2 molecular core studied by Xie (1992). Through analysis of color-color and color-magnitude diagrams based on JHK imaging photometry, we determine the fraction of young stars surrounded by accretion disks, and investigate the age and mass distribution of the cluster.