Dawn E. Peterson

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 Survey of Interstellar Clouds in the Gould Belt. II. The Cepheus Flare Observed with IRAC and MIPS

We present Spitzer Infrared Array Camera (IRAC; ~2 deg^2) and Multiband Imaging Photometer for Spitzer (MIPS; ~8 deg^2) observations of the Cepheus Flare, which is associated with the Gould Belt, at an approximate distance of ~300 pc. Around 6500 sources are detected in all four IRAC bands, of which ~900 have MIPS 24 μm detections. We identify 133 young stellar object (YSO) candidates using color-magnitude diagram techniques, and a large number of the YSO candidates are associated with the NGC 7023 reflection nebula. Cross-identifications were made with the Guide Star Catalog II and the IRAS Faint Source Catalog, and spectral energy distributions (SEDs) were constructed. SED modeling was conducted to estimate the degree of infrared excess. It was found that a large majority of disks were optically thick accreting disks, suggesting that there has been little disk evolution in these sources. Nearest-neighbor clustering analysis identified four small protostellar groups (L1228, L1228N, L1251A, and L1251B) with 5-15 members each and the larger NGC 7023 association with 32 YSO members. The star-formation efficiency for cores with clusters of protostars and for those without clusters was found to be ~8% and ~1%, respectively. The cores L1155, L1241, and L1247 are confirmed to be starless down to our luminosity limit of L _(bol) = 0.06 L ⊙.

A FIRST LOOK AT THE AURIGA-CALIFORNIA GIANT MOLECULAR CLOUD WITH HERSCHEL * AND THE CSO: CENSUS OF THE YOUNG STELLAR OBJECTS AND THE DENSE GAS

We have mapped the Auriga/California molecular cloud with the Herschel PACS and SPIRE cameras and the Bolocam 1.1 mm camera on the Caltech Submillimeter Observatory with the eventual goal of quantifying the star formation and cloud structure in this giant molecular cloud (GMC) that is comparable in size and mass to the Orion GMC, but which appears to be forming far fewer stars. We have tabulated 60 compact 70/160 μm sources that are likely pre-main-sequence objects and correlated those with Spitzer and WISE mid-IR sources. At 1.1 mm, we find 18 cold, compact sources and discuss their properties. The most important result from this part of our study is that we find a modest number of additional compact young objects beyond those identified at shorter wavelengths with Spitzer. We also describe the dust column density and temperature structure derived from our photometric maps. The column density peaks at a few × 10 22 cm −2 (NH2) and is distributed in a clear filamentary structure along which nearly all of the pre-main-sequence objects are found. We compare the young stellar object surface density to the gas column density and find a strong nonlinear correlation between them. The dust temperature in the densest parts of the filaments drops to ∼10 K from values ∼14–15 K in the low-density parts of the cloud. We also derive the cumulative mass fraction and probability density function of material in the cloud, which we compare with similar data on other star-forming clouds.

Physical Conditions of Accreting Gas in T Tauri Star Systems

We present results from a low-resolution ( -->R 300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTSs) in the Taurus-Auriga star-forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pa -->nup/Paβ, Br -->nup/Brγ, and Brγ/Pa -->nup H I line ratios for all observations exhibit a scatter of 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or global value was determined for each ratio in both the Paschen and Brackett series, as well as the Brγ/Pa -->nup line ratios. A comparison of observed line ratio values was made to those predicted by the temperature- and electron density-dependent models of case B hydrogen recombination line theory. The measured line ratios are statistically well fit by a tightly constrained range of temperatures ( -->T 2000 K) and electron densities ( -->109 cm −3 < ne 1010 cm−3). A comparison of the observed line ratio values to the values predicted by the optically thick and thin local thermodynamic equilibrium cases rules out these conditions for the emitting H I gas. Therefore, the emission is consistent with having an origin in a non-LTE recombining gas. While the range of electron densities is consistent with the gas densities predicted by existing magnetospheric accretion models, the temperature range constrained by the case B comparison is considerably lower than that expected for accreting gas. The cooler gas temperatures will require a nonthermal excitation process (e.g., coronal/accretion-related X-rays and UV photons) to power the observed line emission.

The Spitzer Survey of Interstellar Clouds in the Gould Belt. IV. Lupus V and VI Observed with IRAC and MIPS

We present Goulds Belt (GB) Spitzer IRAC and MIPS observations of the Lupus V and VI clouds and discuss them in combination with near-infrared (2MASS) data. Our observations complement those obtained for other Lupus clouds within the frame of the Spitzer Core to Disk (c2d) Legacy Survey. We found 43 young stellar object (YSO) candidates in Lupus V and 45 in Lupus VI, including two transition disks, using the standard c2d/GB selection method. None of these sources was classified as a pre-main-sequence star from previous optical, near-IR, and X-ray surveys. A large majority of these YSO candidates appear to be surrounded by thin disks (Class III; ~79% in Lupus V and ~87% in Lupus VI). These Class III abundances differ significantly from those observed for the other Lupus clouds and c2d/GB surveyed star-forming regions, where objects with optically thick disks (Class II) dominate the young population. We investigate various scenarios that can explain this discrepancy. In particular, we show that disk photoevaporation due to nearby OB stars is not responsible for the high fraction of Class III objects. The gas surface densities measured for Lupus V and VI lie below the star formation threshold (AV 8.6 mag), while this is not the case for other Lupus clouds. Thus, few Myr older age for the YSOs in Lupus V and VI with respect to other Lupus clouds is the most likely explanation of the high fraction of Class III objects in these clouds, while a higher characteristic stellar mass might be a contributing factor. Better constraints on the age and binary fraction of the Lupus clouds might solve the puzzle but require further observations.

The Spitzer Survey of Interstellar Clouds in the Gould Belt. VI. The Auriga-California Molecular Cloud Observed with IRAC and MIPS

We present observations of the Auriga-California Molecular Cloud (AMC) at 3.6, 4.5, 5.8, 8.0, 24, 70, and 160 μm observed with the IRAC and MIPS detectors as part of the Spitzer Gould Belt Legacy Survey. The total mapped areas are 2.5 deg2 with IRAC and 10.47 deg2 with MIPS. This giant molecular cloud is one of two in the nearby Gould Belt of star-forming regions, the other being the Orion A Molecular Cloud (OMC). We compare source counts, colors, and magnitudes in our observed region to a subset of the SWIRE data that was processed through our pipeline. Using color-magnitude and color-color diagrams, we find evidence for a substantial population of 166 young stellar objects (YSOs) in the cloud, many of which were previously unknown. Most of this population is concentrated around the LkHα 101 cluster and the filament extending from it. We present a quantitative description of the degree of clustering and discuss the relative fraction of YSOs in earlier (Class I and F) and later (Class II) classes compared to other clouds. We perform simple SED modeling of the YSOs with disks to compare the mid-IR properties to disks in other clouds and identify 14 classical transition disk candidates. Although the AMC is similar in mass, size, and distance to the OMC, it is forming about 15-20 times fewer stars.

A Universal Spin–Mass Relation for Brown Dwarfs and Planets

While brown dwarfs show similarities with stars in their early life, their spin evolution is much more akin to that of planets. We have used lightcurves from the K2 mission to measure new rotation periods for 18 young brown dwarfs in the Taurus star-forming region. Our sample spans masses from 0.02 to 0.08 Msol and has been characterised extensively in the past. To search for periods, we utilize three different methods (autocorrelation, periodogram, Gaussian Processes). The median period for brown dwarfs with disks is twice as long as for those without (3.1 vs. 1.6 d), a signature of rotational braking by the disk, albeit with small numbers. With an overall median period of 1.9 d, brown dwarfs in Taurus rotate slower than their counterparts in somewhat older (3-10 Myr) star-forming regions, consistent with spin-up of the latter due to contraction and angular momentum conservation, a clear sign that disk braking overall is inefficient and/or temporary in this mass domain. We confirm the presence of a linear increase of the typical rotation period as a function of mass in the sub-stellar regime. The rotational velocities, when calculated forward to the age of the solar system assuming angular momentum conservation, fit the known spin-mass relation for solar system planets and extra-solar planetary-mass objects. This spin-mass trend holds over six orders of magnitude in mass, including objects from several different formation paths. Our result implies that brown dwarfs by and large retain their primordial angular momentum through the first few Myr of their evolution.

ERRATUM: “THE SPITZER SURVEY OF INTERSTELLAR CLOUDS IN THE GOULD BELT. VI. THE AURIGA-CALIFORNIA MOLECULAR CLOUD OBSERVED WITH IRAC AND MIPS” (2014, ApJ, 786, 37)

Hannah Broekhoven-Fiene1, Brenda C. Matthews1,2, Paul M. Harvey3, Robert A. Gutermuth4, Tracy L. Huard5,6, Nicholas F. H. Tothill7, David Nutter8, Tyler L. Bourke9, James Di Francesco2, Jes K. Jørgensen10,11, Lori E. Allen12, Nicholas L. Chapman13, Lucas A. Cieza14, Michael M. Dunham15, Bruno Merı́n16, Jennifer F. Miller5,9, Susan Terebey17, Dawn E. Peterson18, and Karl R. Stapelfeldt19 1 Department of Physics and Astronomy, University of Victoria, Victoria, BC V8W 3P6, Canada 2 National Research Council Herzberg Astronomy and Astrophysics, Victoria, BC V9E 2E7, Canada 3 Astronomy Department, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA 4 Department of Astronomy, University of Massachusetts, Amherst, MA 01003-9305, USA 5 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 6 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA 7 School of Computing, Engineering and Mathematics, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia 8 School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK 9 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 10 Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø., Denmark 11 Centre for Star and Planet Formation, Natural History Museum of Denmark, Øster Voldgade 5-7, DK-1350 Copenhagen K., Denmark 12 National Optical Astronomy Observatories, Tucson, AZ 85719, USA 13 Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University,