You-Hua Chu

10 MK Gas in M17 and the Rosette Nebula: X-Ray Flows in Galactic H II Regions

We present the first high spatial resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M17) and the Rosette Nebula (NGC 2237-2246), obtained with the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into more than 900 (M17) and 300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. The diffuse emission has luminosity LX 3.4 × 1033 ergs s-1 in M17 with plasma energy components at kT 0.13 and 0.6 keV (1.5 and 7 MK), while in Rosette it has LX 6 × 1032 ergs s-1 with plasma energy components at kT 0.06 and 0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

A Neutral Hydrogen Survey of the Large Magellanic Cloud: Aperture Synthesis and Multibeam Data Combined

Recent H I surveys of the Large Magellanic Cloud (LMC) with the Australia Telescope Compact Array and the Parkes multibeam receiver have focused, respectively, on the small-scale ( 1°) structure of the galaxy. Using a Fourier-plane technique, we have merged both data sets, providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc upward. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km s-1), brightness temperature sensitivity (2.4 K), and column density sensitivity (7.2 × 1018 cm-2 per 1.649 km s-1 channel) allow for studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbors to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. This paper presents the merged data and size spectrum of H I clouds, which is similar to the typical size spectrum of the holes and shells in the H I distribution. The H I clouds in the LMC have been identified by defining a cloud to be an object composed of all pixels in right ascension, declination, and velocity that are simply connected and that lie above the threshold brightness temperature.

X-rays from superbubbles in the Large Magellanic Cloud

Diffuse X-ray emission not associated with known supernova remnants (SNRs) are found in seven Large Magellanic Cloud H II complexes encompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30 Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34th ergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30 Dor, have simple ring morphologies, indicating shell structures. Modeling these as superbubbles, it is found that the X-ray luminosities expected from their hot interiors fall an order of magnitude below the observed values. SNRs close to the center of a superbubble add very little emission, but it is calculated that off-center SNRs hitting the ionized shell could explain the observed emission. 60 refs.

Kinematic structure of the 30 Doradus giant H II region

We have used the echelle CCD spectrograph on the Cerro Tololo Inter-American Observatory (CTIO) 4 m telescope to map the nebular velocity field in the 30 Doradus giant H II region. The kinematics of 30 Dor are very complex. The outer regions are charaterized by a smooth velocity field, but its turbulent velocity, 30-40 km/s Full Width Half Maximum (FWHM), is considerably higher than those in most smaller H II regions. In the central 9 min core, multiple velocity components are observed at most positions. The velocity field is dominated by a large number of expanding structures, ranging in size from 1 to 100 pc and expansion velocities of 20-200 km/s, and often organized into large hierarchical networks. The integral of these complex expanding structures in 30 Dor produces a surprisingly simple profile with a broad Gaussian core and faint extended wings. Several fast-expanding shells, with diameters of 2-20 pc, expansion velocities of 100-300 km/s, and kinetic energies of 0.5-10 x 10(exp 50) ergs have been identified. The large fast-expanding shells and networks are coincident with extended X-ray sources and are probably associated with supernova remnants embedded in supershells produced by the combined effects of stellar winds and supernovae from OB associations. We have used the intensity-calibrated echelle spectra to determine the basic physical and dynamical properties of the kinematic features in 30 Dor. The expanding shells contain roughly half of the kinetic energy in the 30 Dor complex, and this energy is several times higher than the gravitational binding energy of the region. The energetic requirements of the gas are consistent with the observed stellar content of 30 Dor, if the gas is accelerated by a combination of stellar winds and supernovae. Extrapolating the current energy injection rate in the nebula over the lifetime of the OB complex suggest that 30 Dor and its vicinity will evolve into a supergiant shell as seen in the LMC and other nearby galaxies.

The Magellanic Mopra Assessment (MAGMA). I. the molecular cloud population of the large magellanic cloud

We present the properties of an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) mapped at 11?pc resolution in the CO(1-0) line. Targets were chosen based on a limiting CO flux and peak brightness as measured by the NANTEN survey. The observations were conducted with the ATNF Mopra Telescope as part of the Magellanic Mopra Assessment. We identify clouds as regions of connected CO emission and find that the distributions of cloud sizes, fluxes, and masses are sensitive to the choice of decomposition parameters. In all cases, however, the luminosity function of CO clouds is steeper than dN/dLL ?2, suggesting that a substantial fraction of mass is in low-mass clouds. A correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. We argue that the correlation between virial mass and CO luminosity is the result of comparing two covariant quantities, with the correlation appearing tighter on larger scales where a size-linewidth relation holds. The virial parameter (the ratio of a clouds kinetic to self-gravitational energy) shows a wide range of values and exhibits no clear trends with the CO luminosity or the likelihood of hosting young stellar object (YSO) candidates, casting further doubt on the assumption of virialization for molecular clouds in the LMC. Higher CO luminosity increases the likelihood of a cloud harboring a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming the close link between giant molecular clouds and massive star formation.

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

He II emission in extragalactic H II regions

Spectroscopic observations confirming the presence of nebular He II 4686-A emission in the SMC H II region N76 are presented, and it is demonstrated that the He II emission associated with the WO star in IC 1613 is also extended. The properties of four H II regions are discussed. The close correlation of the emission with specific stars indicates that photoionization by the stars themselves is the excitation mechanism, and it is proposed that this may be true for those nebulae ionized by WO stars and some rare high-excitation WN stars. The existence of these nebulae with He II emission increases the likelihood that the 4686-A emission frequently observed in dwarf emission-line galaxies is nebular in origin. This prompts the conclusion that the radiation field associated with star-forming regions can be harder than previously suspected, and reopens the issue of whether photoionization by stars in young galaxies can account for the ionization observed in QSO absorption-line systems. 42 refs.

HYDRA II: A FAINT AND COMPACT MILKY WAY DWARF GALAXY FOUND IN THE SURVEY OF THE MAGELLANIC STELLAR HISTORY

© 2015. The American Astronomical Society. All rights reserved.We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the Magellanic Stellar History conducted with the Dark Energy Camera on the Blanco 4 m Telescope. The new satellite is compact (r h = 68 ± 11 pc) and faint (M V = -4.8 ± 0.3), but well within the realm of dwarf galaxies. The stellar distribution of Hydra II in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 ± 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.

A debris disk around the central star of the Helix Nebula

Excess emission from a point-like source coincident with the central star of the Helix Nebula is detected with Spitzer at 8, 24, and 70 µm. At 24 µm, the central source is superposed on an extended diffuse emission region. While the [O IV] 25.89 µm line contributes to the diffuse emission, a 1035 µm spectrum of the central source shows a strong thermal continuum. The excess emission from the star most likely originates from a dust disk with blackbody temperatures of 90–130 K. Assuming a simple optically thin debris disk model, the dust is distributed in a ring between �35 and �150 AU from the central star, possibly arising from collisions of Kuiper-Belt-like Objects or the break-up of comets from an Oort-like cloud that have survived from the post-main-sequence evolution of the central star. Subject headings: infrared: stars – stars: individual (WD 2226 210) – white dwarfs

Chandra Reveals the X-Ray Glint in the Cat’s Eye

We have obtained Chandra ACIS-S observations of NGC 6543, the Cats Eye Nebula. The X-ray emission from NGC 6543 is clearly resolved into a point source at the central star and diffuse emission confined within the central elliptical shell and two extensions along the major axis. Spectral analysis of the diffuse component shows that the abundances of the X-ray-emitting gas are similar to those of the fast (1750 km s-1) stellar wind but not those of the nebula. Furthermore, the temperature of this gas is ~1.7 × 106 K, which is 100 times lower than the expected postshock temperature of the fast stellar wind. The combination of low temperature and wind abundances is puzzling. The thermal pressure of this hot gas is about twice the pressure in the cool nebular shell; thus, the hot gas plays an essential role in the ongoing evolution of the nebula.