Shih-Ping Lai

The Spitzer c2d Survey of Large, Nearby, Insterstellar Clouds. II. Serpens Observed with IRAC

We present maps of 0.89?deg2 of the Serpens dark cloud at 3.6, 4.5, 5.8, and 8.0??m observed with the Spitzer Space Telescope Infrared Array Camera (IRAC). We discuss in detail the data processing carried out by the c2d team on IRAC data. More than 100,000 compact sources have been extracted, but we confine most of our discussion to the most reliable subset of these sources. This includes those that are detected above 7?? in all four IRAC bands or those detected in the two shorter IRAC bands together with 2MASS. We estimate completeness limits for our 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 0.10 deg2 set of low-extinction regions near the dark cloud and a 1 deg2 subset of the SWIRE Elais N1 data that was processed through our pipeline. We find that it is possible to identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, most of which were previously unknown. In addition to the dense area of new star formation known before in the core region (cluster A), we also find a moderately rich area to the south (cluster B). Our mapped area also includes the Herbig Ae star VV Ser, whose Spitzer images have been carefully modeled in a separate study. The extreme sensitivity of Spitzer IRAC allows us to search to very low luminosity limits for young substellar objects. The comparison of the Serpens region with the reference areas suggests that a population of infrared excess sources exists in Serpens at least down to luminosities of L ~ 10-3 L? and possibly lower.

A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A

Context. Rotationally supported disks are critical in the star forma tion process. The questions of when do they form and what factors influence or hinder their formation have been studied but are largely unanswered. Observations of early stage YSOs are needed to probe disk formation. Aims. VLA1623 is a triple non-coeval protostellar system, with a weak magnetic field perpendicular to the outflow, whose Class 0 component, VLA1623A, shows a disk-like structure in continuum with signatures of rotation in line emission. We aim to determine whether this structure is in part or in whole a rotationally s upported disk, i.e. a Keplerian disk, and what are its charac teristics. Methods. ALMA Cycle 0 Early Science 1.3 mm continuum and C 18 O (2-1) observations in the extended configuration are prese nted here and used to perform an analysis of the disk-like structure using PV diagrams and thin disk modelling with the addition of foreground absorption. Results. The PV diagrams of the C 18 O line emission suggest the presence of a rotationally supported component with a radius of at least 50 AU. Kinematical modelling of the line emission shows that the disk out to 180 AU is actually rotationally support ed, with the rotation being well described by Keplerian rotation out to at leat 150 AU, and the central source mass to be∼0.2 M⊙ for an inclination of 55 ◦ . Pure infall and conserved angular momentum rotation models are excluded. Conclusions. VLA1623A, a very young Class 0 source, presents a disk with an outer radius Rout = 180 AU with a Keplerian velocity )

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 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 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.

Interferometric Mapping of Magnetic Fields in Star-forming Regions. III. Dust and CO Polarization in DR 21(OH)

We present the polarization detections in DR 21(OH) from both the thermal dust emission at 1.3 mm and the CO J = 2 ? 1 line obtained with the Berkeley-Illinois-Maryland Association array. Our results are consistent with the prediction of the Goldreich-Kylafis effect that the CO polarization is either parallel or perpendicular to the magnetic field direction. The detection of the polarized CO emission is over a more extended region than the dust polarization, while the dust polarization provides an aid in resolving the ambiguity of the CO polarization. The combined results suggest that the magnetic field direction in DR 21(OH) is parallel to the CO polarization and therefore parallel to the major axis of DR 21(OH). The strong correlation between the CO and dust polarization suggests that magnetic fields are remarkably uniform throughout the envelope and the cores. The dispersion in polarization position angles implies a magnetic field strength in the plane of the sky of about 1 mG, compared to about 0.5 mG inferred for the line-of-sight field from previous CN Zeeman observations. Our CO data also show that both MM 1 and MM 2 power high-velocity outflows with v 25 km s-1 relative to the systemic velocity.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. 6. 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.

Interferometric Mapping of Magnetic Fields in Star-forming Regions. II. NGC 2024 FIR 5

We present the —rst interferometric polarization maps of the NGC 2024 FIR 5 molecular core, obtained with the BIMA array at approximately 2A resolution. We measure an average position angle of [60i ^ 6i in the main core of FIR 5 and 54i ^ 9i in the eastern wing of FIR 5. The morphology of the polarization angles in the main core of FIR 5 suggests that the —eld lines are parabolic, with a symmetry axis approximately parallel to the major axis of the putative disk in FIR 5, which is consistent with the theoretical scenario that the gravitational collapse pulled the —eld lines into an hourglass shape. The polarization percentage decreases toward regions with high intensity and close to the center of the core, suggesting that the dust alignment efficiency may decrease at high density. The plane-of-sky —eld strength can be estimated with the modi—ed Chandrasekhar-Fermi formula, and the small dispersion of the polarization angles in FIR 5 suggests that the magnetic —eld is strong mG) and perhaps domi(Z2 nates the turbulent motions in the core. . . . . . .

The spitzer c2d survey of large, nearby, interstellar clouds. I. Chamaeleon II observed with MIPS

We present maps of over 1.5 deg2 in Chamaeleon (Cha) II at 24, 70, and 160 μm observed with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) and a 1.2 deg2 millimeter map from SIMBA on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy Teams data reduction pipeline is described in detail. Over 1500 24 μm sources and 41 70 μm sources were detected by MIPS with fluxes greater than 10 σ. More than 40 potential YSOs are identified with a MIPS and 2MASS color-color diagram and by their spectral indices, including two previously unknown sources with 24 μm excesses. Our new SIMBA millimeter map of Cha II shows that only a small fraction of the gas is in compact structures with high column densities. The extended emission seen by MIPS is compared with previous CO observations. Some selected interesting sources, including two detected at 1 mm, associated with Cha II are discussed in detail, and their SEDs are presented. The classification of these sources using MIPS data is found to be consistent with that of previous studies.

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.