Che-Yu Chen

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 deg^2) including the body, wing, and tail in seven bands from 3.6 to 160 μm using IRAC and MIPS on the Spitzer Space Telescope. The data were reduced and mosaicked, and the point sources were measured using customized routines specific for large surveys. We have made the resulting mosaics and point-source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 μm/24 μm ratio is somewhat lower than the average and the 70 μm/160 μm ratio is somewhat higher than the average. The global infrared spectral energy distribution (SED) shows that the SMC has approximately 1/3 the aromatic emission/polycyclic aromatic hydrocarbon abundance of most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average SED of H II/star formation regions is compared to the equivalent Large Magellanic Cloud average H II/star formation region SED. These preliminary results will be expanded in detail in subsequent papers.

The SAGE‐Spec Spitzer Legacy programme: the life‐cycle of dust and gas in the Large Magellanic Cloud – Point source classification I

We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H_(II) regions and 12 other objects, seven of which remain unclassified.

The Dust and Gas Around β Pictoris

We have obtained Spitzer IRS 5.5-35 ?m spectroscopy of the debris disk around ? Pictoris. In addition to the 10 ?m silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 ?m. This is the first time that the silicate bands at wavelengths longer than 10 ?m have ever been seen in the ? Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 ? upper limit of <17 M? on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around ? Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

The youngest massive protostars in the Large Magellanic Cloud

We demonstrate the unique capabilities of Herschel to study very young luminous extragalactic young stellar objects (YSOs) by analyzing a central strip of the Large Magellanic Cloud obtained through the HERITAGE science demonstration program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500 μm photometry with 2MASS (1.25-2.17 μm) and Spitzer IRAC and MIPS (3.6-70 μm) to construct complete spectral energy distributions (SEDs) of compact sources. From these, we identify 207 candidate embedded YSOs in the observed region, ∼40% never-before identified. We discuss their position in far-infrared color-magnitude space, comparing with previously studied, spectroscopically confirmed YSOs and maser emission. All have red colors indicating massive cool envelopes and great youth. We analyze four example YSOs, determining their physical properties by fitting their SEDs with radiative transfer models. Fitting full SEDs including the Herschel data requires us to increase the ◦�� �

THE EXCEPTIONALLY LARGE DEBRIS DISK AROUND γ OPHIUCHI

Spitzer images resolve the debris disk around γ Ophiuchi at both 24 and 70 μm. The resolved images suggest a disk radius of ~520 AU at 70 μm and 260 AU at 24 μm. The images, along with a consistent fit to the spectral energy distribution of the disk from 20 to 350 μm, show that the primary disk structure is inclined by ~50° from the plane of the sky at a position angle of 55° ± 2°. Among a group of 12 debris disks that have similar host star spectral types, ages, and infrared fractional luminosities, the observed sizes in the infrared and color temperatures indicate that evolution of the debris disks is influenced by multiple parameters in addition to the protoplanetary disk initial mass.

The Herschel revolution: Unveiling the morphology of the high-mass star-formation sites N44 and N63 in the LMC

Aims. We study the structure of the medium surrounding sites of high-mass star formation to determine the interrelation between the H ii regions and the environment from which they were formed. The density distribution of the surroundings is key in determining how the radiation of the newly formed stars interacts with the surroundings in a way that allows it to be used as a star-formation tracer. Methods. We present new Herschel/SPIRE 250 μm, 350 μm and 500 μm data of LHA 120-N44 and LHA 120-N63 in the LMC. We construct average spectral energy distributions (SEDs) for annuli centered on the IR bright part of the star-formation sites. The annuli cover ∼10–∼100 pc. We use a phenomenological dust model to fit these SEDs to derive the dust column-densities, characterize the incident radiation field and the abundance of polycyclic aromatic hydrocarbon molecules. We see a decrease of a factor 5 in the radiation field energy-density as a function of radial distance around N63. The source N44 does not show a systematic trend. We construct a simple geometrical model to derive the 3D density profile of the surroundings of these two regions. Results. Herschel/SPIRE data have proven very efficient in deriving the dust-mass distribution. We find that the radiation field in the two sources behaves very differently. N63 is more or less spherically symmetric and the average radiation field drops with distance. N44 shows no systematic decrease of the radiation intensity, which is probably due to the inhomogeneity of the surrounding molecular material and to the complex distribution of several star-forming clusters in the region.

Modelling dust polarization observations of molecular clouds through MHD simulations

The BLASTPol observations of Vela C have provided the most detailed characterization of the polarization fraction

Geometry, Kinematics, and Magnetization of Simulated Prestellar Cores

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Fantastic Striations and Where to Find Them: The Origin of Magnetically Aligned Striations in Interstellar Clouds

and dispersion in polarization angles

The SAGE-Spec Spitzer Legacy Program: The Life Cycle of Dust and Gas in the Large Magellanic Cloud

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