Molecular dynamics simulation of mechanical behavior of nanoporous copper foams

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.Includes bibliographical references (leaves 43-44).Metallic foams have a variety of unique mechanical properties that make them prime candidates for many different applications. Recent developments in manufacturing have allowed for the creation of nanoporous foams but still relatively little is known about foam behavior on the nano-scale. To determine if macro-scale metallic foam theory scaled appropriately to nanoporous foams, the mechanical behavior of nanoporous copper was simulated. Molecular dynamics (MD) simulations were performed at room temperature using the Mishin potential on tetrakaidecahedron foam structures. In closed-celled foams close agreement is observed between simulated Young's moduli and constitutive theory. Nanoporous closed-celled foams also show mechanical failure mechanisms similar to those seen on the macro-scale.by Charles Cantrell.S.B

Evidence for a black-hole in the X-ray transient XTE J1859+226

We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 (V406 Vul). Photometric observations taken during 2000 and 2008 reveals the presence of the secondary star's ellipsoidal modulation. Further photometry obtained in 2010 shows the system ~1 mag brighter than its quiescence level and the ellipsoidal modulation diluted by strong flaring activity. Spectroscopic data obtained with the 10.4-m GTC in 2010 reveals radial velocity variations of ~500 km/s over 3 h. A simultaneous fit to the photometry and spectroscopy using sinusoids to represent the secondary star's ellipsoidal and radial velocity variations, yields an orbital period of 6.58+-0.05 h and a secondary star's radial velocity semi-amplitude of K_2= 541+-70 km/s. The implied mass function is f(M)=4.5+-0.6 Msun, significantly lower than previously reported but consistent with the presence of a black hole in XTE J1859+226. The lack of eclipses sets an upper limit to the inclination of 70 degrees which yields a lower limit to the black hole mass of 5.42 Msun.Comment: Accepted for publication in MNRAS. Contains 5 pages and 4 figure

THE INCLINATION OF THE SOFT X-RAY TRANSIENT A0620–00 AND THE MASS OF ITS BLACK HOLE

We analyze photometry of the soft X-ray transient A0620 – 00 spanning nearly 30 years, including previously published and previously unpublished data. Previous attempts to determine the inclination of A0620 using subsets of these data have yielded a wide range of measured values of i. Differences in the measured value of i have been due to changes in the shape of the light curve and uncertainty regarding the contamination from the disk. We give a new technique for estimating the disk fraction and find that disk light is significant in all light curves, even in the infrared. We also find that all changes in the shape and normalization of the light curve originate in a variable disk component. After accounting for this disk component, we find that all the data, including light curves of significantly different shapes, point to a consistent value of i. Combining results from many separate data sets, we find i = 51fdg0 ± 0fdg9, implying M = 6.6 ± 0.25 M ☉. Using our dynamical model and zero-disk stellar VIH magnitudes, we find d = 1.06 ± 0.12 kpc. Understanding the disk origin of nonellipsoidal variability may assist with making reliable determinations of i in other systems, and the fluctuations in disk light may provide a new observational tool for understanding the three-dimensional structure of the accretion disk.National Science Foundation (U.S.) (NSF Graduate Research Fellowship DGE-0202738)National Science Foundation (U.S.) (NSF/AST grant 0407063)National Science Foundation (U.S.) (NSF/AST grant 0707627

The Spin of the Black Hole in the Soft X-Ray Transient A0620-00

During its year-long outburst in 1975–76, the transient source A0620–00 reached an intensity of 50 Crab, an all-time record for any X-ray binary. The source has been quiescent since then. We have recently determined accurate values for the black hole (BH) mass, orbital inclination angle, and distance. Building on these results, we have measured the radius of the inner edge of the accretion disk around the BH primary by fitting its thermal continuum spectrum to our version of the relativistic Novikov–Thorne thin-disk model. We have thereby estimated the spin of the BH. Although our spin estimate depends on a single high-quality spectrum, which was obtained in 1975 by OSO-8, we are confident of our result because of the consistent values of the inner-disk radius that we have obtained for hundreds of observations of other sources: H1743-322, XTE J1550-564, and notably LMC X-3. We have determined the dimensionless spin parameter of the BH to be a∗ = 0.12 ± 0.19, with a∗ −0.59 at the 3σ level of confidence. This result takes into account all sources of observational and model-parameter uncertainties. Despite the low spin, the intensity and properties of the radio counterpart, both in outburst and quiescence, attest to the presence of a strong jet. If jets are driven by BH spin, then current models indicate that jet power should be a steeply increasing function of a∗. Consequently, the low spin of A0620–00 suggests that its jet may be disk driven.Astronom

Multiwavelength Observations of A0620-00 in Quiescence

[Abridged.] We present multiwavelength observations of the black hole binary system, A0620-00. Using the Cosmic Origins Spectrograph on the Hubble Space Telescope, we have obtained the first FUV spectrum of A0620-00. The observed spectrum is flat in the FUV and very faint (with continuum fluxes \simeq 1e - 17 ergs/cm^2/s/A). We compiled the dereddened, broadband spectral energy distribution of A0620-00 and compared it to previous SEDs as well as theoretical models. The SEDs show that the source varies at all wavelengths for which we have multiple samples. Contrary to previous observations, the optical-UV spectrum does not continue to drop to shorter wavelengths, but instead shows a recovery and an increasingly blue spectrum in the FUV. We created an optical-UV spectrum of A0620-00 with the donor star contribution removed. The non-stellar spectrum peaks at \simeq3000 {\deg}A. The peak can be fit with a T=10,000 K blackbody with a small emitting area, probably originating in the hot spot where the accretion stream impacts the outer disk. However, one or more components in addition to the blackbody are needed to fit the FUV upturn and the red optical fluxes in the optical-UV spectrum. By comparing the mass accretion rate determined from the hot spot luminosity to the mean accretion rate inferred from the outburst history, we find that the latter is an order of magnitude smaller than the former, indicating that \sim90% of the accreted mass must be lost from the system if the predictions of the disk instability model and the estimated interoutburst interval are correct. The mass accretion rate at the hot spot is 10^5 the accretion rate at the black hole inferred from the X-ray luminosity. To reconcile these requires that outflows carry away virtually all of the accreted mass, a very low rate of mass transfer from the outer cold disk into the inner hot region, and/or radiatively inefficient accretion.Comment: ApJ, accepte

Ethanol and High-Value Terpene Co-Production from Lignocellulosic Biomass of Cymbopogon flexuosus and Cymbopogon martinii

Cymbopogon flexuosus, lemongrass, and C. martinii, palmarosa, are perennial grasses grown to produce essential oils for the fragrance industry. The objectives of this study were (1) to evaluate biomass and oil yields as a function of nitrogen and sulfur fertilization, and (2) to characterize their utility for lignocellulosic ethanol compared to Panicum virgatum (switchgrass). Mean biomass yields were 12.83 Mg lemongrass ha⁻¹ and 15.11 Mg palmarosa ha⁻¹ during the second harvest year resulting in theoretical biofuel yields of 2541 and 2569 L ethanol ha⁻¹ respectively compared to reported 1749–3691 L ethanol ha⁻¹ for switchgrass. Pretreated lemongrass yielded 198 mL ethanol (g biomass)⁻¹ and pretreated palmarosa yielded 170 mL ethanol (g biomass)⁻¹. Additionally, lemongrass yielded 85.7 kg essential oil ha⁻¹ and palmarosa yielded 67.0 kg ha⁻¹ with an estimated value of USD $857 and$1005 ha⁻¹. These data suggest that dual-use crops such as lemongrass and palmarosa may increase the economic viability of lignocellulosic biofuels

Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets

The spins of ten stellar black holes have been measured using the continuum-fitting method. These black holes are located in two distinct classes of X-ray binary systems, one that is persistently X-ray bright and another that is transient. Both the persistent and transient black holes remain for long periods in a state where their spectra are dominated by a thermal accretion disk component. The spin of a black hole of known mass and distance can be measured by fitting this thermal continuum spectrum to the thin-disk model of Novikov and Thorne; the key fit parameter is the radius of the inner edge of the black hole's accretion disk. Strong observational and theoretical evidence links the inner-disk radius to the radius of the innermost stable circular orbit, which is trivially related to the dimensionless spin parameter a_* of the black hole (|a_*| < 1). The ten spins that have so far been measured by this continuum-fitting method range widely from a_* \approx 0 to a_* > 0.95. The robustness of the method is demonstrated by the dozens or hundreds of independent and consistent measurements of spin that have been obtained for several black holes, and through careful consideration of many sources of systematic error. Among the results discussed is a dichotomy between the transient and persistent black holes; the latter have higher spins and larger masses. Also discussed is recently discovered evidence in the transient sources for a correlation between the power of ballistic jets and black hole spin.Comment: 30 pages. Accepted for publication in Space Science Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI "The Physics of Accretion onto Black Holes" (Springer Publisher). Changes to Sections 5.2, 6.1 and 7.4. Section 7.4 responds to Russell et al. 2013 (MNRAS, 431, 405) who find no evidence for a correlation between the power of ballistic jets and black hole spi

BlackCAT: A catalogue of stellar-mass black holes in X-ray transients

During the last ~50 years, the population of black hole candidates in X-ray binaries has increased considerably with 59 Galactic objects detected in transient low-mass X-ray binaries, plus a few in persistent systems (including ~5 extragalactic binaries). We collect near-infrared, optical and X-ray information spread over hundreds of references in order to study the population of black holes in X-ray transients as a whole. We present the most updated catalogue of black hole transients, which contains X-ray, optical and near-infrared observations together with their astrometric and dynamical properties. It provides new useful information in both statistical and observational parameters providing a thorough and complete overview of the black hole population in the Milky Way. Analysing the distances and spatial distribution of the observed systems, we estimate a total population of ~1300 Galactic black hole transients. This means that we have already discovered less than ~5% of the total Galactic distribution. The complete version of this catalogue will be continuously updated online and in the Virtual Observatory, including finding charts and data in other wavelengths.Comment: http://www.astro.puc.cl/BlackCAT - Accepted for publication in Astronomy & Astrophysics. 20 pages, 8 figures, 5 Table

Agricultural Research Service Weed Science Research: Past, Present, and Future

The U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) has been a leader in weed science research covering topics ranging from the development and use of integrated weed management (IWM) tactics to basic mechanistic studies, including biotic resistance of desirable plant communities and herbicide resistance. ARS weed scientists have worked in agricultural and natural ecosystems, including agronomic and horticultural crops, pastures, forests, wild lands, aquatic habitats, wetlands, and riparian areas. Through strong partnerships with academia, state agencies, private industry, and numerous federal programs, ARS weed scientists have made contributions to discoveries in the newest fields of robotics and genetics, as well as the traditional and fundamental subjects of weed-crop competition and physiology and integration of weed control tactics and practices. Weed science at ARS is often overshadowed by other research topics; thus, few are aware of the long history of ARS weed science and its important contributions. This review is the result of a symposium held at the Weed Science Society of America\u27s 62nd Annual Meeting in 2022 that included 10 separate presentations in a virtual Weed Science Webinar Series. The overarching themes of management tactics (IWM, biological control, and automation), basic mechanisms (competition, invasive plant genetics, and herbicide resistance), and ecosystem impacts (invasive plant spread, climate change, conservation, and restoration) represent core ARS weed science research that is dynamic and efficacious and has been a significant component of the agency\u27s national and international efforts. This review highlights current studies and future directions that exemplify the science and collaborative relationships both within and outside ARS. Given the constraints of weeds and invasive plants on all aspects of food, feed, and fiber systems, there is an acknowledged need to face new challenges, including agriculture and natural resources sustainability, economic resilience and reliability, and societal health and well-being

A retrospective analysis of normal saline and lactated ringers as resuscitation fluid in sepsis

BackgroundThe Surviving Sepsis Campaign suggested preferential resuscitation with balanced crystalloids, such as Lactated Ringer’s (LR), although the level of recommendation was weak, and the quality of evidence was low. Past studies reported an association of unbalanced solutions, such as normal saline (NS), with increased AKI risks, metabolic acidosis, and prolonged ICU stay, although some of the findings are conflicting. We have compared the outcomes with the preferential use of normal saline vs. ringer’s lactate in a cohort of sepsis patients.MethodWe performed a retrospective cohort analysis of patients visiting the ED of 19 different Mayo Clinic sites between August 2018 to November 2020 with sepsis and receiving at least 30 mL/kg fluid in the first 6 h. Patients were divided into two cohorts based on the type of resuscitation fluid (LR vs. NS) and propensity-matching was done based on clinical characteristics as well as fluid amount (with 5 ml/kg). Single variable logistic regression (categorical outcomes) and Cox proportional hazards regression models were used to compare the primary and secondary outcomes between the 2 groups.ResultsOut of 2022 patients meeting our inclusion criteria; 1,428 (70.6%) received NS, and 594 (29.4%) received LR as the predominant fluid (&gt;30 mL/kg). Patients receiving predominantly NS were more likely to be male and older in age. The LR cohort had a higher BMI, lactate level and incidence of septic shock. Propensity-matched analysis did not show a difference in 30-day and in-hospital mortality rate, mechanical ventilation, oxygen therapy, or CRRT requirement. We did observe longer hospital LOS in the LR group (median 5 vs. 4 days, p = 0.047 and higher requirement for ICU post-admission (OR: 0.70; 95% CI: 0.51–0.96; p = 0.026) in the NS group. However, these did not remain statistically significant after adjustment for multiple testing.ConclusionIn our matched cohort, we did not show any statistically significant difference in mortality rates, hospital LOS, ICU admission after diagnosis, mechanical ventilation, oxygen therapy and RRT between sepsis patients receiving lactated ringers and normal saline as predominant resuscitation fluid. Further large-scale prospective studies are needed to solidify the current guidelines on the use of balanced crystalloids