1,117 research outputs found
Episodic absorption in the outflow of V603 Aquilae
No full textWe report on the time-dependent behaviour of ultraviolet spectral lines in Hubble Space Telescope Goddard High-Resolution Spectrograph data of the classical nova V603 Aql. In particular, episodic blueshifted absorption (extending to ∼−2500 km s−1) is present, with a variability time-scale down to ∼1 min. The data provide a rare opportunity to study the rapid evolution of absorption structures that may be associated with accretion-disc winds in cataclysmic variables. At least three absorption events are recorded (at blueward velocities only) over ∼5 h, each lasting ∼10–15 min. The derived velocity, acceleration and optical depth properties provide an empirical picture of stochastically variable structures in the outflow, with no evidence for short-term (less than ∼1 h) cyclic or modulated behaviour in the overall absorption properties. In contrast, the emission components of the ultraviolet resonance lines are very stable in velocity and strength in this low-inclination system. On at least two occasions there is an intriguing short-term ‘flare’ in the ultraviolet continuum flux (of up to ∼40 per cent). Though there is no clear one-to-one relation in these data between the continuum fluctuations and the occurrence of the absorption events, the time-scales for the two variable phenomena are essentially the same. The irregular absorption episodes in the ultraviolet data of V603 Aql presently defy a clear physical interpretation. Their overall characteristics are discussed in the context of instabilities in radiation-pressure-driven disc winds
The e-MERLIN L-band Legacy Survey of Cygnus OB2
No full textThe Cygnus OB2 Radio Survey (COBRaS) is an e-MERLIN Legacy survey awarded ~ 300 hours worth of observing time. Split between L-(42 hrs) and C-(252 hrs) band, the project aims to push the limits of e-MERLIN’s enhanced capabilities by providing highly sensitive, targeted, deep-field radio maps of the core of the Cygnus OB2 association. The L-band (21cm) observations, completed throughout the Spring of 2014, serve as the focus of the work presented here. This thesis primarily aims to investigate the radio emission from massive (M 8 M⊙ ) stars in the Cyg OB association, in order to obtain a better understanding of their mass-loss rates and binarity. A description of the entire data reduction process of the COBRaS 21cm observations is given, as is a discussion of the mitigation of radio frequency interference (RFI). In the advent of new radio telescopes, the technical skills, research and tools developed throughout this process aims to serve the wider radio astronomy community. Further technical work is presented in the form of novel source extraction and cross-correlation scripts that deliver the COBRaS L-band all source catalogue (CLASC). Reaching resolutions of ~180 mas, precise positions and 21cm flux densities are presented for 41 sources found within a 0.48 x 0.48 deg region, centred on the Cyg OB2 association. In obtaining a 3σ sensitivity limit of ~ 100 μJy, these observations significantly improve upon previous surveys of the region. Accurate determinations of the mass-loss rates of single massive OB stars are used to investigate their stellar wind properties, helping to resolve current discrepancies in the mass-loss rates via clumped and structured hot star winds. Finally, an analysis of the non-thermal radiation from the colliding winds of massive star binaries seeks to clarify our understanding of such systems and their radio emission processes
Generalized TASE-RK methods for stiff problems
No full textA family of Time-Accurate and Stable Explicit (TASE) methods for the numerical integration of Initial Value Problems in stiff Ordinary Differential Equations (ODEs) y'(t) = f (t, y) was recently introduced in [1]. The main idea was to make local extrapolation of a stabilized Euler method. More recently, in [3] a similar approach by considering the stabilization of arbitrary explicit Runge-Kutta methods (TASE-RK) was analyzed. In this case the explicit Runge-Kutta method integrates a transformed ODE obtained by multiplying the vector field f (t, y) by a certain operator which approximates the identity mapping up to a given order p. The main inconvenience of both approaches is that to reach order p the solution of p2 linear systems plus the evaluation of p derivatives are required per integration step.In order to substantially reduce the computational costs of the former approaches in the linear system solution, but maintaining the good accuracy and stability properties, a new family of TASE-RK methods which allow to introduce a few more free parameters are considered. The formulation of the methods was conceived to be implemented not only in sequential mode but it admits parallelism in a straightforward way. Furthermore, since these methods are linearly implicit, connections to the class of W-methods [19] are properly established. The order conditions for the new class of methods are widely studied by using the rooted tree theory. For p = 3, 4, new methods with p sequential stages and order p are derived and compared on semidiscrete 1D and 2D Partial Differential Equations (PDEs) to those in [1,3] and other standard Rosenbrock and W-methods in the literature.(c) 2023 The Author(s). Published by Elsevier B.V. on behalf of IMACS. This is an open access article under the CC BY-NC-ND license (http:// creativecommons .org /licenses /by-nc -nd /4 .0/)
Terminal Velocities for a Large Sample of O Stars, B Supergiants, and Wolf-Rayet Stars
No full textIt is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles. These estimators are used to determine terminal velocities, v(infinity), for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars, the values are typically 15-20 percent smaller than the extreme violet edge velocities, v(edge), while for WR stars v(infinity) = 0.76 v(edge) on average. New mass-loss rates for WR stars which are thermal radio emitters are given, taking into account the new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. The relationships between v(infinity), the surface escape velocities, and effective temperatures are examined
The morphological effects of sediment diversions the Lower Mississippi River
No full textThe wetlands in the Mississippi delta (USA) are drastically subsiding and eroding. Many projects and researches are ongoing to determine how this “drowning effect” of the delta can be stopped. One of the solutions that could be feasible is implementing sediment diversions in the levees of the Lower Mississippi River in order to divert sediment into the delta. This thesis addresses the morphological effects of river diversions on the Lower Mississippi River. The main objective is to analyze and optimize trade-offs between delta building and river navigability. For this purpose a 2DH numerical model with Delf3D has been created; the model simulates the hydro- and morphodynamic behavior. The river reach which has been studied is the final 110 km of the river from Point a la Hache at River Kilometer 78 (RK 78) down to the mouth of the river (RK -30), below Head of Passes. The hydrodynamic model has been calibrated and verified with flow and stage data from daily observations on the river. With the available sediment data a calibration has been carried out of the morphological behavior in the river. The model has been used to simulate several scenarios to get insight in the problems in the delta. From analysis of the model results the river bed in the study area can be divided into three categories. Upstream of RK 4 the bed is subject to erosion, around RK 4 the bed is practically in equilibrium and downstream of RK 4 the bed is subject to sedimentation. The reach downstream of RK 4 is the dredging reach; after analyzing the long-term simulation of 20 years it is not expected that the dredging quantities will decrease in the future. Closing off West Bay diversion has a positive effect on the dredging quantities. The best diversion site for this study area is found in the inner bend of the river upstream of Empire (RK 47) at RK 53. This site is most efficient and diverts the largest quantities of sand through the diversion.Hydraulic EngineeringCivil Engineering and Geoscience
The discordance of mass-loss estimates for galactic O-type stars
No full textWe have determined accurate values of the product of the mass-loss rate and the ion fraction of P+4, Mq(P+4), for a sample of 40 Galactic O-type stars by fitting stellar wind profiles to observations of the P v resonance doublet obtained with FUSE, ORFEUS BEFS, and Copernicus. When P+4 is the dominant ion in the wind [i.e., 0.5 less than or similar to q(P+4) <= 1],. Mq(P+4) approximates the mass-loss rate to within a factor of less than or similar to 2. Theory predicts that P+4 is the dominant ion in the winds of O7-O9.7 stars, although an empirical estimator suggests that the range O4-O7 may be more appropriate. However, we find that the mass-loss rates obtained from P v wind profiles are systematically smaller than those obtained from fits to H alpha emission profiles or radio free-free emission by median factors of similar to 130 (if P+4 is dominant between O7 and O9.7) or similar to 20 (if P+4 is dominant between O4 and O7). These discordant measurements can be reconciled if the winds of O stars in the relevant temperature range are strongly clumped on small spatial scales. We use a simplified two-component model to investigate the volume filling factors of the denser regions. This clumping implies that mass-loss rates determined from "rho(2)'' diagnostics have been systematically overestimated by factors of 10 or more, at least for a subset of O stars. Reductions in the mass-loss rates of this size have important implications for the evolution of massive stars and quantitative estimates of the feedback that hot-star winds provide to their interstellar environments
Mass-loss rates from mid-infrared excesses in LMC and SMC O stars
No full textWe use a combination of BVJHK and Spitzer [3.6], [5.8] and [8.0] photometry to determine infrared (IR) excesses for a sample of 58 Large Magellanic Cloud and 46 Small Magellanic Cloud O stars. This sample is ideal for determining IR excesses because the very small line-of-sight reddening minimizes uncertainties due to extinction corrections. We use the core–halo model developed by Lamers & Waters to translate the excesses into mass-loss rates and demonstrate that the results of this simple model agree with the more sophisticated CMFGEN models to within a factor of 2. Taken at face value, the derived mass-loss rates are larger than those predicted by Vink et al., and the magnitude of the disagreement increases with decreasing luminosity. However, the IR excesses need not imply large mass-loss rates. Instead, we argue that they probably indicate that the outer atmospheres of O stars contain complex structures and that their winds are launched with much smaller velocity gradients than normally assumed. If this is the case, it could affect the theoretical and observational interpretations of the ‘weak wind’ problem, where classical mass-loss indicators suggest that the mass-loss rates of lower luminosity O stars are far less than expected
The Dunston Manuscript
No full textVariously titled 80 Slides and a Memory, Fit for a King and others this is the definitive performed version as an invited reading in 2001 to the Urban Design Group. It describes in blank verse the early stages of the Gateshead Garden Festival illustrated with slide
Associating Gender with Neighbourhood Deprivation in Lagos State, Nigeria
No full textAJOL has removed this paper from the website after it was found to be published in another academic publication. AJOL has requestd that both journal editorial boards investigate this matter further with the author
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