W49A North - Global or Local or No Collapse?

We attempt to fit observations with 5" resolution of the J=2-1 transition of CS in the directions of H II regions A, B, and G of W49A North as well as observations with 20" resolution of the J=2-1, 3-2, 5-4, and 7-6 transitions in the directions of H II regions A and G by using radiative transfer calculations. These calculations predict the intensity profiles resulting from several spherical clouds along the line of sight. We consider three models: global collapse of a very large (5 pc radius) cloud, localized collapse from smaller (1 pc) clouds around individual H II regions, and multiple, static clouds. For all three models we can find combinations of parameters that reproduce the CS profiles reasonably well provided that the component clouds have a core-envelope structure with a temperature gradient. Cores with high temperature and high molecular hydrogen density are needed to match the higher transitions (e.g. J=7-6) observed towards A and G. The lower temperature, low density gas needed to create the inverse P-Cygni profile seen in the CS J=2-1 line (with 5" beam) towards H II region G arises from different components in the 3 models. The infalling envelope of cloud G plus cloud B creates the absorption in global collapse, cloud B is responsible in local collapse, and a separate cloud, G', is needed in the case of many static clouds. The exact nature of the velocity field in the envelopes for the case of local collapse is not important as long as it is in the range of 1 to 5 km/s for a turbulent velocity of about 6 km/s. High resolution observations of the J=1-0 and 5-4 transitions of CS and C34S may distinguish between these three models. Modeling existing observations of HCO+ and C18O does not allow one to distinguish between the three models but does indicate the existence of a bipolar outflow.Comment: 42 pages, 27 figures, accepted for publication in the ApJS August 2004, v153 issu

Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival HST optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact wind nebula which is strongly elongated and a feature of large-scale diffuse emission trailing from the pulsar. This latter comet tail-shaped feature coexists with enhanced radio emission and is oriented nearly perpendicular to the major axis of the pulsar wind nebula. We propose the following scenario to explain these features. The bright, compact nebula is likely powered by a toroidal pulsar wind of relativistic particles which is partially confined by the ram-pressure from the supersonic motion of the pulsar. The particles, after being forced out from the compact nebula (the head of the ``comet''), are eventually dumped into a bubble (the tail), which is primarily responsible for the extended diffuse X-ray and radio emission. The ram-pressure confinement also allows a natural explanation for the observed X-ray luminosity of the compact nebula and for the unusually small X-ray to spin-down luminosity ratio, compared to similarly energetic pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times 10^6 (with an uncertainty of a factor about 2, consistent with that inferred from the modeling of the Crab Nebula.Comment: 15 pages plus 4 figures. The postscript file of the whole paper is available at http://xray.astro.umass.edu/wqd/papers/n157b/n157b.ps. accepted for publication in Ap

Re-identification of G35.6-0.4 as a supernova remnant

G35.6-0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an HII region. Observations from the VLA Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6-0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with alpha -0.47 +/- 0.07. G35.6-0.4 does not have obvious associated infra-red emission, so it is identified as a Galactic supernova remnant, not an HII region. It is approximately 15 x 11 arcmin**2 in extent, showing partial limb brightening.Comment: 4 pages, accepted by MNRA

Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC

We present the first X-ray detection of SNR 0450-70.9 the largest known supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical images and high-dispersion spectra, and radio continuum maps. Optical images of SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments along the eastern and western rims and within the shell interior. The interior filaments have higher [S II]/Halpha ratios and form an apparent inner shell morphology. The X-ray emission region is smaller than the full extent of the optical shell, with the brightest X-ray emission found within the small interior shell and on the western rim of the large shell. The expansion velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s. The radio image shows central brightening and a fairly flat radio spectral index over the SNR. However, no point X-ray or radio source corresponding to a pulsar is detected and the X-ray emission is predominantly thermal. Therefore, these phenomena can be most reasonably explained in terms of the advanced age of the large SNR. Using hydrodynamic models combined with a nonequilibrium ionization model for thermal X-ray emission, we derived a lower limit on the SNR age of about 45,000 yr, well into the later stages of SNR evolution. Despite this, the temperature and density derived from spectral fits to the X-ray emission indicate that the remnant is still overpressured, and thus that the development is largely driven by hot gas in the SNR interior.Comment: Accepted for publication in The Astrophysical Journa

A Continuum Formulation of Stress Correlations of Dislocations in Two Dimensions

The Continuum Dislocation Dynamics theory (CDD) of crystal plasticity, utilizing a second-order dislocation density tensor, is a powerful tool in understanding and modeling the dynamic behavior of dislocations on microscopic scales. Using this model, a number of benchmark systems have been tested. All results show excellent agreement with both analytic solutions, where available, as well as discrete simulations. While accurate solutions have been found for effectively one dimensional systems, fully two- and three-dimensional systems increase the complexity of the problem. In order to predict the behavior of the continuum density accurately, it must be properly understood as an ensemble average over discrete distributions. In this work, an overview of a simplified, integrated form of the CDD method is presented, along with an overview of one-dimensional results compared with both analytic solutions and discrete simulation. Then, the results from CDD for a distribution of one-dimensional glide planes in a two-dimensional elastic medium is presented. Using comparisons with Discrete Dislocation Dynamics (DDD) in a few simple systems, the multi-component stress field which must be considered for dislocation density motion is derived and demonstrated

Radio Emission from the Composite Supernova Remnant G326.3-1.8 (MSH15-56)

High resolution radio observations of the composite supernova remnant (SNR) G326.3-1.8 or MSH 15-56 with the Australia Telescope Compact Array show details of both the shell and the bright plerion which is offset about 1/3 of the distance from the center of the SNR to the shell. The shell appears to be composed of thin filaments, typical of older shell SNRs. The central part of the elongated plerion is composed of a bundle of parallel ridges which bulge out at the ends and form a distinct ring structure on the northwestern end. The magnetic field with a strength of order 45 microGauss, is directed along the axis of the ridges but circles around the northwestern ring. This plerion is large and bright in the radio but is not detected in x-ray or optical wavelengths. There is, however, a faint hard x-ray feature closer to the shell outside the plerion. Perhaps if the supernova explosion left a rapidly moving magnetar with large energy input but initially rapid decay of both relativistic particles and magnetic field, the observed differences with wavelength could be explained.Comment: 15 pages, 10 figures, accepted by Ap

The population of X-ray supernova remnants in the Large Magellanic Cloud

We present a comprehensive X-ray study of the population of supernova remnants (SNRs) in the LMC. Using primarily XMM-Newton, we conduct a systematic spectral analysis of LMC SNRs to gain new insights on their evolution and the interplay with their host galaxy. We combined all the archival XMM observations of the LMC with those of our Very Large Programme survey. We produced X-ray images and spectra of 51 SNRs, out of a list of 59. Using a careful modelling of the background, we consistently analysed all the X-ray spectra and measure temperatures, luminosities, and chemical compositions. We investigated the spatial distribution of SNRs in the LMC and the connection with their environment, characterised by various SFHs. We tentatively typed all LMC SNRs to constrain the ratio of core-collapse to type Ia SN rates in the LMC. We compared the X-ray-derived column densities to HI maps to probe the three-dimensional structure of the LMC. This work provides the first homogeneous catalogue of X-ray spectral properties of LMC SNRs. It offers a complete census of LMC SNRs exhibiting Fe K lines (13% of the sample), or revealing contribution from hot SN ejecta (39%). Abundances in the LMC ISM are found to be 0.2-0.5 solar, with a lower [$\alpha$/Fe] than in the Milky Way. The ratio of CC/type Ia SN in the LMC is $N_{\mathrm{CC}}/N_{\mathrm{Ia}} = 1.35(_{-0.24}^{+0.11})$, lower than in local SN surveys and galaxy clusters. Comparison of X-ray luminosity functions of SNRs in Local Group galaxies reveals an intriguing excess of bright objects in the LMC. We confirm that 30 Doradus and the LMC Bar are offset from the main disc of the LMC, to the far and near sides, respectively. (abridged)Comment: Accepted for publication in Astronomy and Astrophysics. 54 pages, 18 figures, 12 tables. The resolution of the figures has been reduced compared to the journal version; v2: New title, minor text edits; v3: Correct version

Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 SNR

The N206 supernova remnant (SNR) in the Large Magellanic Cloud (LMC) has long been considered a prototypical "mixed morphology" SNR. Recent observations, however, have added a new twist to this familiar plot: an elongated, radially-oriented radio feature seen in projection against the SNR face. Utilizing the high resolution and sensitivity available with the Hubble Space Telescope, Chandra, and XMM-Newton, we have obtained optical emission-line images and spatially resolved X-ray spectral maps for this intriguing SNR. Our findings present the SNR itself as a remnant in the mid to late stages of its evolution. X-ray emission associated with the radio "linear feature" strongly suggests it to be a pulsar-wind nebula (PWN). A small X-ray knot is discovered at the outer tip of this feature. The feature's elongated morphology and the surrounding wedge-shaped X-ray enhancement strongly suggest a bow-shock PWN structure.Comment: 41 pages including 7 figures, accepted for publication by the Astrophysical Journa