Timothy Young Brooke

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.

Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun-like Stars

We present data obtained with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope (Spitzer) for a sample of 74 young (t < 30 Myr old) Sun-like (0.7 < M*/M☉ < 1.5) stars. These are a subset of the observations that comprise the Spitzer Legacy science program entitled the Formation and Evolution of Planetary Systems (FEPS). Using IRAC, we study the fraction of young stars that exhibit 3.6-8.0 μm infrared emission in excess of that expected from the stellar photosphere, as a function of age from 3 to 30 Myr. The most straightforward interpretation of such excess emission is the presence of hot (300-1000 K) dust in the inner regions (<3 AU) of a circumstellar disk. Five out of the 74 young stars show a strong infrared excess, four of which have estimated ages of 3-10 Myr. While we detect excesses from five optically thick disks and photospheric emission from the remainder of our sample, we do not detect any excess emission from optically thin disks at these wavelengths. We compare our results with accretion disk fractions detected in previous studies and use the ensemble results to place additional constraints on the dissipation timescales for optically thick, primordial disks.

The Spitzer c2d Survey of Nearby Dense Cores. I. First Direct Detection of the Embedded Source in IRAM 04191+1522

We report the first detections of the Class 0 protostellar source IRAM 04191+1522 at wavelengths shortward of 60 μm with the Spitzer Space Telescope. We see extended emission in the Spitzer images that suggests the presence of an outflow cavity in the circumstellar envelope. We combine the Spitzer observations with existing data to form a complete data set ranging from 3.6 to 1300 μm and use these data to construct radiative transfer models of the source. We conclude that the internal luminosity of IRAM 04191+1522, defined to be the sum of the luminosity from the internal sources (a star and a disk), is L_(int) = 0.08 ± 0.04 L_⊙, placing it among the lowest luminosity protostars known. Although it was discovered before the launch of the Spitzer Space Telescope, IRAM 04191+1522 falls within a new class of very low luminosity objects being discovered by Spitzer. Unlike the two other well-studied objects in this class, which are associated either with weak, compact outflows or no outflows at all, IRAM 04191+1522 has a well-defined molecular outflow with properties consistent with those expected based on relations derived from higher luminosity (L_(int) ≥ 1 L_⊙) protostars. We discuss the difficulties in understanding IRAM 04191+1522 in the context of the standard model of star formation and suggest a possible explanation for the very low luminosity of this source.

SPITZER SPACE TELESCOPE SPECTROSCOPY OF ICES TOWARD LOW-MASS EMBEDDED PROTOSTARS

Sensitive 5-38 μm Spitzer Space Telescope and ground-based 3-5 μm spectra of the embedded low-mass protostars B5 IRS1 and HH 46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO_2 bending mode at 15 μm is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN^- bands, however. The observed CO2 profile indicates an intimate mixture with H_(2)O, but not necessarily with CH_(3)OH, in contrast to some high-mass protostars. This is consistent with the low CH_(3)OH abundance derived from the ground-based L-band spectra. The CO_2 : H_(2)O column density ratios are high in both B5 IRS1 and HH 46 IRS (~35%). Clearly, the Spitzer spectra are essential for studying ice evolution in low-mass protostellar environments and for eventually determining the relation between interstellar and solar system ices.

PRIMORDIAL CIRCUMSTELLAR DISKS IN BINARY SYSTEMS: EVIDENCE FOR REDUCED LIFETIMES

We combine the results from several multiplicity surveys of pre-main-sequence stars located in four nearby star-forming regions with Spitzer data from three different Legacy Projects. This allows us to construct a sample of 349 targets, including 125 binaries, which we use to to investigate the effect of companions on the evolution of circumstellar disks. We find that the distribution of projected separations of systems with Spitzer excesses is significantly different (P ~ 2.4e-5, according to the K-S test for binaries with separations less than 400 AU) from that of systems lacking evidence for a disk. As expected, systems with projected separations less than 40 AU are half as likely to retain at least one disk than are systems with projected separations in the 40-400 AU range. These results represent the first statistically significant evidence for a correlation between binary separation and the presence of an inner disk (r~ 1 AU). Several factors (e.g., the incompleteness of the census of close binaries, the use of unresolved disk indicators, and projection effects) have previously masked this correlation in smaller samples. We discuss the implications of our findings for circumstellar disk lifetimes and the formation of planets in multiple systems.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VII. Ophiuchus Observed with MIPS

We present maps of 14.4 deg^2 of the Ophiuchus dark clouds observed by the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS). These high-quality maps depict both numerous point sources and extended dust emission within the star-forming and non–star-forming portions of these clouds. Using PSF-fitting photometry, we detect 5779 sources at 24 μm and 81 sources at 70 μm at the 10 σ level of significance. Three hundred twenty-three candidate young stellar objects (YSOs) were identified according to their positions on the MIPS/2MASS K versus color-magnitude diagrams, as compared to 24 μm detections in the SWIRE extragalactic survey. We find that more than half of the YSO candidates, and almost all those with protostellar Class I spectral energy distributions, are confined to the known cluster and aggregates.

The Spitzer Gould Belt Survey of Large Nearby Interstellar Clouds: Discovery of a Dense Embedded Cluster in the Serpens-Aquila Rift

We report the discovery of a nearby, embedded cluster of young stellar objects, associated filamentary infrared dark cloud, and 4.5 μm shock emission knots from outflows detected in Spitzer IRAC mid-infrared imaging of the Serpens-Aquila Rift obtained as part of the Spitzer Gould Belt Legacy Survey. We also present radial velocity measurements of the region from molecular line observations obtained with the Submillimeter Array (SMA) that suggest the cluster is comoving with the Serpens Main embedded cluster 3° to the north. We therefore assign it the same distance, 260 pc. The core of the new cluster, which we call Serpens South, is composed of an unusually large fraction of protostars (77%) at high mean surface density (>430 pc−2) and short median nearest neighbor spacing (3700 AU). We perform basic cluster structure characterization using nearest neighbor surface density mapping of the YSOs and compare our findings to other known clusters with equivalent analyses available in the literature.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VI. Perseus Observed with MIPS

We present observations of 10.6 deg^2 of the Perseus molecular cloud at 24, 70, and 160 μm with Spitzer MIPS. The images show prominent, complex extended emission dominated by illuminating B stars on the east side of the cloud and by cold filaments of 160 μm emission on the west side. Of 3950 point sources identified at 24 μm, 1141 have 2MASS counterparts. A quarter of these populate regions of the K_s versus K_s - [24] diagram that are distinct from stellar photospheres and background galaxies and thus are likely to be cloud members with infrared excess. Nearly half (46%) of these 24 μm excess sources are distributed outside the IC 348 and NGC 1333 clusters. A significant number of IRAS PSC objects are not recovered by Spitzer MIPS, most often because the IRAS objects were confused by bright nebulosity. The intercluster region contains several tightly clumped (r ~ 0.1 pc) young stellar aggregates whose members exhibit a wide variety of infrared SEDs characteristic of different circumstellar environments. This could be explained by a significant age spread among the aggregate members, or if the members formed at the same time, a remarkably rapid circumstellar evolution would be required to account for the association of Class I and Class III sources at ages ≲1 Myr. We highlight important results for the HH 211 flow, where the bow shocks are detected at both 24 and 70 μm, and for the debris disk candidate BD +31 643, where the MIPS data show the linear nebulosity to be an unrelated interstellar feature. Our data, mosaics, and catalogs are available at the Spitzer Science Archive for use by interested members of the community.

The Formation and Evolution of Planetary Systems (FEPS): Discovery of an Unusual Debris System Associated with HD 12039

We report the discovery of a debris system associated with the ~30 Myr old G3/5V star HD 12039 using Spitzer Space Telescope observations from 3.6-160 μm. An observed infrared excess (L_(IR)/L^* = 1 × 10^(-4)) above the expected photosphere for λ ≳ 14 μm is fit by thermally emitting material with a color temperature of T ~ 110 K, warmer than the majority of debris disks identified to date around Sun-like stars. The object is not detected at 70 μm with a 3 σ upper limit 6 times the expected photospheric flux. The spectrum of the infrared excess can be explained by warm, optically thin material comprised of blackbody-like grains of size ≳ 7 μm that reside in a belt orbiting the star at 4-6 AU. An alternate model dominated by smaller grains, near the blowout size ɑ ~ 0.5 μm, located at 30-40 AU is also possible but requires the dust to have been produced recently, since such small grains will be expelled from the system by radiation pressure in approximately a few times 10^2 yr.

Formation and evolution of planetary systems: Cold outer disks associated with Sun-like stars

We present the discovery of debris systems around three Sun-like stars based on observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, The Formation and Evolution of Planetary Systems (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 ?m band but are consistent with photospheric emission at ?33 ?m. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7-3 Gyr. On the basis of models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the possible presence of planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d = 11 pc) that are indistinguishable from a bare photosphere at both 24 and 70 ?m. The observations place strong upper limits on the presence of any cold dust in this nearby system (LIR/L < 10-5.2).