•Gray wolf optimizer (GWO) is employed in solving the optimal reactive power dispatch (ORPD) problems.•Three case studies have been utilized to show the effectiveness of GWO.•GWO able to find minimum ...loss and voltage deviation solution than those determined by other techniques.
This paper presents the use of a new meta-heuristic technique namely gray wolf optimizer (GWO) which is inspired from gray wolves’ leadership and hunting behaviors to solve optimal reactive power dispatch (ORPD) problem. ORPD problem is a well-known nonlinear optimization problem in power system. GWO is utilized to find the best combination of control variables such as generator voltages, tap changing transformers’ ratios as well as the amount of reactive compensation devices so that the loss and voltage deviation minimizations can be achieved. In this paper, two case studies of IEEE 30-bus system and IEEE 118-bus system are used to show the effectiveness of GWO technique compared to other techniques available in literature. The results of this research show that GWO is able to achieve less power loss and voltage deviation than those determined by other techniques.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In this study, a multi-strategy driven reinforced hierarchical operator for a grey wolf optimizer (RHGWO) is proposed to solve the feature selection (FS) problem, whereby tedious data are converted ...into information and often modeled as a combinatorial optimization problem. First, a multi-strategy mechanism is proposed to provide the GWO algorithm with exploration capabilities, including memory-based diversity and Lévy flight-based extension search. Next, a hierarchical segmentation technique is proposed to allocate exploration and exploitation, thereby providing exploration capability for superior wolves to search diverse regions and exploitation capability for inferior wolves to converge to the promising area. Subsequently, a chaotic elite learning strategy is designed for leaders to prevent misdirection. Finally, a more rational nonlinear parameter transformation is designed. Multiple experiments validate the adaptability and versatility of the proposed RHGWO algorithm.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Context.
Classical Wolf-Rayet (WR) stars are massive, hydrogen-depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity
Z
, stars exceeding a certain ...initial mass
M
single
WR
(Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low
Z
, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30−40% in the SMC (
Z
= 0.002) and LMC (
Z
= 0.006) are comparable to that of the Galaxy (
Z
= 0.014), and no evidence for the dominance of the binary channel at low
Z
could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass
M
spec
WR
(Z) needed for the stripped product to appear as a WR star.
Aims.
By constraining
M
spec
WR
(Z) and
M
single
WR
(Z), we estimate the importance of binaries in forming WR stars as a function of
Z
.
Methods.
We calibrated
M
spec
WR
using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of
M
single
WR
values were explored using various evolution codes. We estimated the additional contribution of the binary channel by considering the interval
M
spec
WR
(Z),
M
single
WR
(Z), which characterizes the initial-mass range in which the binary channel can form additional WR stars.
Results.
The WR-phenomenon ceases below luminosities of log
L
≈ 4.9, 5.25, and 5.6
L
⊙
in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17
M
⊙
and minimum initial masses of
M
spec
WR
= 18, 23, 37
M
⊙
. Stripped stars with lower initial masses in the respective galaxies would tend not to appear as WR stars. The minimum mass necessary for self-stripping,
M
single
WR
(Z), is strongly model-dependent, but it lies in the range 20−30, 30−60, and ≳40
M
⊙
for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of
Z
that cannot be conclusively claimed to be monotonically increasing with decreasing
Z
.
Conclusions.
The WR spectral appearance arises from the presence of strong winds. Therefore, both
M
spec
WR
and
M
single
WR
increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low
Z
, or that the WR binary fraction grows with decreasing
Z
.
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This paper proposes an innovative tuning approach for fuzzy control systems (CSs) with a reduced parametric sensitivity using the Grey Wolf Optimizer (GWO) algorithm. The CSs consist of servo system ...processes controlled by Takagi-Sugeno-Kang proportional-integral fuzzy controllers (TSK PI-FCs). The process models have second-order dynamics with an integral component, variable parameters, a saturation, and dead-zone static nonlinearity. The sensitivity analysis employs output sensitivity functions of the sensitivity models defined with respect to the parametric variations of the processes. The GWO algorithm is used in solving the optimization problems, where the objective functions include the output sensitivity functions. GWO's motivation is based on its low-computational cost. The tuning approach is validated in an experimental case study of a position control for a laboratory nonlinear servo system, and TSK PI-FCs with a reduced process small time constant sensitivity are offered.
Since antiquity, the she-wolf has served as the potent symbol of Rome. For more than two thousand years, the legendary animal that rescued Romulus and Remus has been the subject of historical and ...political accounts, literary treatments in poetry and prose, and visual representations in every medium. In She-Wolf: The Story of a Roman Icon, Cristina Mazzoni examines the evolution of the she-wolf as a symbol in western history, art, and literature, from antiquity to contemporary times. Used, for example, as an icon of Roman imperial power, papal authority, and the distance between the present and the past, the she-wolf has also served as an allegory for greed, good politics, excessive female sexuality, and, most recently, modern, multi-cultural Rome. Mazzoni engagingly analyzes the various role guises of the she-wolf over time in the first comprehensive study in any language on this subject.
Spectroscopic analyses of hydrogen-rich WN 5–6 stars within the young star clusters NGC 3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and ...high spatial resolution near-IR photometry, including Multi-Conjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC 3603 (T*∼ 42 ± 2 kK) and R136 (T*∼ 53 ± 3 kK) plus clumping-corrected mass-loss rates of 2–5 × 10−5 M⊙ yr−1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ∼7 per cent of the ionizing flux of the entire 30 Doradus region. Comparisons with stellar models calculated for the main-sequence evolution of 85–500 M⊙ accounting for rotation suggest ages of ∼1.5 Myr and initial masses in the range 105–170 M⊙ for three systems in NGC 3603, plus 165–320 M⊙ for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high-density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC 3603 and R136 are consistent with an tentative upper mass limit of ∼300 M⊙. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated – initial masses for the most luminous stars in the Arches cluster approach 200 M⊙ according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150 M⊙ within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe, as has been claimed for SN 2007bi.
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ABSTRACT We present a new empirical prescription for the mass-loss rates of carbon- and oxygen-sequence Wolf-Rayet stars as a function of their luminosity, surface chemical composition, and initial ...metallicity. The new prescription is based on results of detailed spectral analyses of WC and WO stars and improves the often applied Nugis and Lamers relation. We find that the mass-loss rates of WC and WO stars (with X = 0 and Y 0.98) can be expressed as (L/L ) + 0.44 log Y + 0.25 log (ZFe/ZFe, ). This relation is based on mass-loss determinations that assume a volume-filling factor of 0.1, but the prescription can easily be scaled to account for other volume-filling factors. The residual of the fit is = 0.06 dex. We investigated whether the relation can also describe the mass loss of hydrogen-free WN stars and showed that it can when an adjustment of the metallicity dependence ( ) is applied. Compared to that of Nugis and Lamers, is less sensitive to the luminosity and the surface abundance, implying a stronger mass loss of massive stars in their late stages of evolution. The modest metallicity dependence implies that if WC or WO stars are formed in metal-deficient environments, their mass-loss rates are higher than currently anticipated. These effects may result in the formation of a larger number of SNe Ic and fewer black holes and may favor the production of superluminous SNe Ic through interaction with C- and O-rich circumstellar material or dense stellar wind.
Long-standing myths exist about the origins of wolf–coyote hybrids and related Canis species in Ontario. Specifically, there is a perceived controversy whether they are the product of natural ...hybridization that occurred between wolves and coyotes in the wild during the last century or the descendants of animals that escaped or were released from captive colonies or controlled breeding experiments. We review the relevant evidence and conclude that captive colonies and controlled breeding experiments were unlikely to have played any role in the origins of wolf–coyote hybrids and related Canis species in Ontario.
Context. It is now well established that the majority of massive stars reside in multiple systems. However, the effect of multiplicity is not sufficiently understood, resulting in a plethora of ...uncertainties about the end stages of massive-star evolution. In order to investigate these uncertainties, it is useful to study massive stars just before their demise. Classical Wolf-Rayet stars represent the final end stages of stars at the upper-mass end. The multiplicity fraction of these stars was reported to be ∼0.4 in the Galaxy but no correction for observational biases has been attempted. Aims. The aim of this study is to conduct a homogeneous radial-velocity survey of a magnitude-limited (V ≤ 12) sample of Galactic Wolf-Rayet stars to derive their bias-corrected multiplicity properties. The present paper focuses on 12 northern Galactic carbon-rich (WC) Wolf-Rayet stars observable with the 1.2 m Mercator telescope on the island of La Palma. Methods. We homogeneously measured relative radial velocities (RVs) for carbon-rich Wolf-Rayet stars using cross-correlation. Variations in the derived RVs were used to flag binary candidates. We investigated probable orbital configurations and provide a first correction of observational biases through Monte-Carlo simulations. Results. Of the 12 northern Galactic WC stars in our sample, seven show peak-to-peak RV variations larger than 10 km s−1, which we adopt as our detection threshold. This results in an observed spectroscopic multiplicity fraction of 0.58 with a binomial error of 0.14. In our campaign, we find a clear lack of short-period (P < ∼100 d), indicating that a large number of Galactic WC binaries likely reside in long-period systems. Finally, our simulations show that at the 10% significance level, the intrinsic multiplicity fraction of the Galactic WC population is at least 0.72.
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