The X-ray Ridge Surrounding Sgr A* at the Galactic Center

We present the first detailed simulation of the interaction between the supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc region at the Galactic center. The passage of the supernova ejecta through this medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive black hole Sagittarius A* (Sgr A*). We show that the morphology and X-ray intensity of this feature match very well with recently obtained Chandra images, and we infer a supernova remnant age of less than 2,000 years. This young age--a factor 3--4 lower than previous estimates--arises from our inclusion of stellar wind effects in the initial (pre-explosion) conditions in the medium. The supernova does not clear out the central ~ 0.2-pc region around Sgr~A* and does not significantly alter the accretion rate onto the central black hole upon passage through the Galactic center.Comment: 10 pages, 3 figures, submitted to ApJ

Metal Cooling in Simulations of Cosmic Structure Formation

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software, Cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range (10 K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe, and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40% of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the CMB inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure

The Environments around Long-Duration Gamma-Ray Burst Progenitors

Gamma-ray burst (GRB) afterglow observations have allowed us to significantly constrain the engines producing these energetic explosions. Te redshift and position information provided by these afterglows have already allowed us to limit the progenitors of GRBs to only a few models. The afterglows may also provide another observation that can place further constraints on the GRB progenitor: measurements telling us about the environments surrounding GRBs. Current analyses of GRB afterglows suggest that roughly half of long-duration gamma-ray bursts occur in surroundings with density profiles that are uniform. We study the constraints placed by this observation on both the classic ``collapsar'' massive star progenitor and its relative, the ``helium-merger'' progenitor. We study several aspects of wind mass-loss and find that our modifications to the standard Wolf-Rayet mass-loss paradigm are not sufficient to produce constant density profiles. Although this does not rule out the standard ``collapsar'' progenitor, it does suggest a deficiency with this model. We then focus on the He-merger models and find that such progenitors can fit this particular constraint well. We show how detailed observations can not only determine the correct progenitor for GRBs, but also allow us to study binary evolution physics.Comment: 44 pages including 11 figure

Management of Banana Xanthomonas Wilt: Evidence from impact of adoption of cultural control practices in Uganda

Banana Xanthomonas Wilt (BXW) is an important emerging and non-curable infectious plant pathogen in sub-Saharan Africa that can cause up to 100% yield loss, negatively impacting sustainable access to food and income to more than 100 million banana farmers. This study disentangles adopters into partial and full adopters to investigate the factors that are relevant to sustain the adoption process of BXW control practices and quantifies the impact of adopting the practices. Data from a randomly selected sample of 1200 banana farmers in Uganda where the disease is endemic was used. A multinomial logit model was used to determine the factors affecting adoption of control practices and augmented inverse probability weighting was employed to estimate the impacts of adoption on banana productivity and sales. Results show that training a woman farmer and having diverse sources of information about BXW control practices increased adoption of the control practices and reduced the disease incidences. Farmers who adopted all the recommended control practices achieved significantly the highest values of banana production and sales. We conclude that improving information access through farmers’ preferred communication channels, having women-inclusive trainings, and a combination of cultural practices are effective ways for sustaining adoption of the control practices

Spin-Induced Disk Precession in Sagittarius A*

In Sgr A* at the Galactic center, by far the closest and easiest supermassive black hole we can study, the observational evidence is increasingly pointing to the presence of a compact, hot, magnetized disk feeding the accretor. In such low-Mach-number plasmas, forces arising, e.g., from pressure gradients in the plasma, can altogether negate the warping of disks around Kerr black holes caused by the Bardeen-Petterson effect and can lead to coherent precession of the entire disk. In this Letter, we present for the first time highly detailed 3D SPH simulations of the accretion disk evolution in Sgr A*, guided by observational constraints on its physical characteristics, and conclude that indeed the Bardeen-Petterson effect is probably absent in this source. Given what we now understand regarding the emission geometry in this object, we suggest that a ~ 50-500-day modulation in Sgr A*'s spectrum, arising from the disk precession, could be an important observational signature; perhaps the ~ 106-day period seen earlier in its radio flux, if confirmed, could be due to this process. On the other hand, if future observations do not confirm this long modulation in Sgr A*'s spectrum, this would be an indication that either the disk size or orientation is very different from current estimates, or that the black hole is not spinning at all (unlikely), or that our current understanding of how it produces its radiative output is incorrect.Comment: 14 pages, 3 figures; small changes to sections 2.1 and 3; accepted for publication in Ap

Super stellar clusters with a bimodal hydrodynamic solution: an Approximate Analytic Approach

We look for a simple analytic model to distinguish between stellar clusters undergoing a bimodal hydrodynamic solution from those able to drive only a stationary wind. Clusters in the bimodal regime undergo strong radiative cooling within their densest inner regions, which results in the accumulation of the matter injected by supernovae and stellar winds and eventually in the formation of further stellar generations, while their outer regions sustain a stationary wind. The analytic formulae are derived from the basic hydrodynamic equations. Our main assumption, that the density at the star cluster surface scales almost linearly with that at the stagnation radius, is based on results from semi-analytic and full numerical calculations. The analytic formulation allows for the determination of the threshold mechanical luminosity that separates clusters evolving in either of the two solutions. It is possible to fix the stagnation radius by simple analytic expressions and thus to determine the fractions of the deposited matter that clusters evolving in the bimodal regime blow out as a wind or recycle into further stellar generations.Comment: 5 pages, 4 figures, accepted by A&

Gamma-Rays from Single Lobe Supernova Explosions

Multi-dimensional simulations of the neutrino-driven mechanism behind core-collapse supernovae have long shown that the explosions from this mechanism would be asymmetric. Recently, detailed core-collapse simulations have shown that the explosion may be strongest in a single direction. We present a suite of simulations modeling these ``single-lobe'' supernova explosions of a 15 solar mass red supergiant star, focusing on the effect these asymmetries have on the gamma-ray emission and the mixing in the explosion. We discuss how asymmetries in the explosion mechanism might explain many of the observed ``asymmetries'' of supernovae, focusing on features of both supernova 1987A and the Cas A supernova remnant. In particular, we show that single-lobe explosions provide a promising solution to the redshifted iron lines of supernova 1987A. We also show that the extent of mixing for explosive burning products depends sensitively on the angular profile of the velocity asymmetry and can be much more extensive than previously assumed.Comment: 39 pages, 18 figures, submitted to ApJ Version with high resolution figures can be found at http://qso.lanl.gov/~cl

Winds from clu\sters with non-uniform stellar distributions

We present analytic and numerical models of the `cluster wind' resulting from the multiple interactions of the winds ejected by the stars of a dense cluster of massive stars. We consider the case in which the distribution of stars (i.e., the number of stars per unit volume) within the cluster is spherically symmetric, has a power-law radial dependence, and drops discontinuously to zero at the outer radius of the cluster. We carry out comparisons between an analytic model (in which the stars are considered in terms of a spatially continuous injection of mass and energy) and 3D gasdynamic simulations (in which we include 100 stars with identical winds, located in 3D space by statistically sampling the stellar distribution function). From the analytic model, we find that for stellar distributions with steep enough radial dependencies the cluster wind flow develops a very high central density and a non-zero central velocity, and for steeper dependencies it becomes fully supersonic throughout the volume of the cluster (these properties are partially reproduced by the 3D numerical simulations). Therefore, the wind solutions obtained for stratified clusters can differ dramatically from the case of a homogeneous stellar distribution (which produces a cluster wind with zero central velocity, and a fully subsonic flow within the cluster radius). Finally, from our numerical simulations we compute predictions of X-ray emission maps and luminosities, which can be directly compared with observations of cluster wind flows.Comment: 10 pages, 11 figures. MNRAS - Accepted 2007 June 29. Received 2007 June 28; in original form 2007 May 2

Characterizing the universal rigidity of generic frameworks

A framework is a graph and a map from its vertices to E^d (for some d). A framework is universally rigid if any framework in any dimension with the same graph and edge lengths is a Euclidean image of it. We show that a generic universally rigid framework has a positive semi-definite stress matrix of maximal rank. Connelly showed that the existence of such a positive semi-definite stress matrix is sufficient for universal rigidity, so this provides a characterization of universal rigidity for generic frameworks. We also extend our argument to give a new result on the genericity of strict complementarity in semidefinite programming.Comment: 18 pages, v2: updates throughout; v3: published versio

Constraints on Type Ib/c and GRB Progenitors

Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these, we find that single-star models cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.Comment: 36 pages, 6 figure