The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud: spectroscopy, orbital analysis, formation, and evolution

Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy budget of galaxies and probe a critical phase in the evolution of massive stars prior to core-collapse. It is not known whether core He-burning WR stars (classical WR, cWR) form predominantly through wind-stripping (w-WR) or binary stripping (b-WR). With spectroscopy of WR binaries so-far largely avoided due to its complexity, our study focuses on the 44 WR binaries / binary candidates of the Large Magellanic Cloud (LMC, metallicity Z~0.5 Zsun), identified on the basis of radial velocity variations, composite spectra, or high X-ray luminosities. Relying on a diverse spectroscopic database, we aim to derive the physical and orbital parameters of our targets, confronting evolution models of evolved massive stars at sub-solar metallicity, and constraining the impact of binary interaction in forming them. Spectroscopy is performed using the Potsdam Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement is performed using the code Spectangular or the shift-and-add algorithm. Evolutionary status is interpreted using the Binary Population and Spectral Synthesis (BPASS) code, exploring binary interaction and chemically-homogeneous evolution. No obvious dichotomy in the locations of apparently-single and binary WN stars on the Hertzsprung-Russell diagram is apparent. According to commonly used stellar evolution models (BPASS, Geneva), most apparently-single WN stars could not have formed as single stars, implying that they were stripped by an undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing (e.g., during the red supergiant phase) are strongly underestimated in standard stellar evolution models.Comment: accepted to A&A on 10.05.2019; 69 pages (25 main paper + 44 appendix); Corrigendum: Shenar et al. 2020, A&A, 641, 2: An unfortunate typo in the implementation of the "transformed radius" caused errors of up to ~0.5dex in the derived mass-loss rates. This has now been correcte

Does the marine biosphere mix the ocean?

Author Posting. © Sears Foundation for Marine Research, 2006. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 64 (2006): 541-561, doi:10.1357/002224006778715720.Ocean mixing is thought to control the climatically important oceanic overturning circulation. Here we argue the marine biosphere, by a mechanism like the bioturbation occurring in marine sediments, mixes the oceans as effectively as the winds and tides. This statement is derived ultimately from an estimated 62.7 TeraWatts of chemical power provided to the marine environment in net primary production. Various approaches argue something like 1% (.63 TeraWatts) of this power is invested in aphotic ocean mechanical energy, a rate comparable to wind and tidal inputs

Aqueous phase methylation as a potential source of methylmercury in wet deposition

Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V for personal use, not for redistribution. The definitive version was published in Atmospheric Environment 41 (2007): 1663-1668, doi:10.1016/j.atmosenv.2006.10.032.The source of monomethylmercury (MMHg) in wet deposition is unknown. Volatilization of gaseous MMHg, evasion and demethylation of dimethylmercury, and methylation of Hg0 have been either proposed or tested unsuccessfully as potential sources. Here, we show that MMHg in precipitation, sampled across a wide geographical range in North America, is related positively to an operationally defined and measured reactive Hg species (HgR), but connected weakly to total Hg. The mean molar ratio of MMHg:HgR measured in continental precipitation (0.025 ± 0.006) is comparable to the MMHg:Hg(II) ratio estimated from first-order rate constants for acetate-mediated Hg methylation and MMHg photolysis (0.025 ± 0.002). This suggests MMHg may be formed in the atmosphere through a reaction between labile Hg(II) complexes and an unknown methylating agent(s), potentially acetate or similar molecules. Availability of Hg(II) appears to limit the reaction, and accordingly, increased atmospheric loadings of Hg could lead to enhanced MMHg in precipitation.This study was supported by a grant from the National Science Foundation-Office of Polar Programs (0425562) and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding from the Doherty Foundation

The racist bodily imaginary: the image of the body-in-pieces in (post)apartheid culture

This paper outlines a reoccurring motif within the racist imaginary of (post)apartheid culture: the black body-in-pieces. This disturbing visual idiom is approached from three conceptual perspectives. By linking ideas prevalent in Frantz Fanon’s description of colonial racism with psychoanalytic concepts such as Lacan’s notion of the corps morcelé, the paper offers, firstly, an account of the black body-in-pieces as fantasmatic preoccupation of the (post)apartheid imaginary. The role of such images is approached, secondly, through the lens of affect theory which eschews a representational ‘reading’ of such images in favour of attention to their asignifying intensities and the role they play in effectively constituting such bodies. Lastly, Judith Butler’s discussion of war photography and the conditions of grievability introduces an ethical dimension to the discussion and helps draw attention to the unsavory relations of enjoyment occasioned by such images

Universality of Level Spacing Distributions in Classical Chaos

We suggest that random matrix theory applied to a classical action matrix can be used in classical physics to distinguish chaotic from non-chaotic behavior. We consider the 2-D stadium billiard system as well as the 2-D anharmonic and harmonic oscillator. By unfolding of the spectrum of such matrix we compute the level spacing distribution, the spectral auto-correlation and spectral rigidity. We observe Poissonian behavior in the integrable case and Wignerian behavior in the chaotic case. We present numerical evidence that the action matrix of the stadium billiard displays GOE behavior and give an explanation for it. The findings present evidence for universality of level fluctuations - known from quantum chaos - also to hold in classical physics

Cygnus X-3 with ISO: investigating the wind

We observed the energetic binary Cygnus X-3 in both quiescent and flaring states between 4 and 16 microns using the ISO satellite. We find that the quiescent source shows the thermal free-free spectrum typical of a hot, fast stellar wind, such as from a massive helium star. The quiescent mass-loss rate due to a spherically symmetric, non-accelerating wind is found to be in the range 0.4-2.9 x 10E-4 solar masses per year, consistent with other infrared and radio observations, but considerably larger than the 10E-5 solar masses per year deduced from both the orbital change and the X-ray column density. There is rapid, large amplitude flaring at 4.5 and 11.5 microns at the same time as enhanced radio and X-ray activity, with the infrared spectrum apparently becoming flatter in the flaring state. We believe non-thermal processes are operating, perhaps along with enhanced thermal emission.Comment: Accepted for publication in MNRAS, 11 pages, 6 figure

The Onfp Class in the Magellanic Clouds

The Onfp class of rotationally broadened, hot spectra was defined some time ago in the Galaxy, where its membership to date numbers only eight. The principal defining characteristic is a broad, centrally reversed He II $\lambda$4686 emission profile; other emission and absorption lines are also rotationally broadened. Recent surveys in the Magellanic Clouds (MCs) have brought the class membership there, including some related spectra, to 28. We present a survey of the spectral morphology and rotational velocities, as a first step toward elucidating the nature of this class. Evolved, rapidly rotating hot stars are not expected theoretically, because the stellar winds should brake the rotation. Luminosity classification of these spectra is not possible, because the principal criterion (He II $\lambda$4686) is peculiar; however, the MCs provide reliable absolute magnitudes, which show that they span the entire range from dwarfs to supergiants. The Onfp line-broadening distribution is distinct and shifted toward larger values from those of normal O dwarfs and supergiants with >99.99% confidence. All cases with multiple observations show line-profile variations, which even remove some objects from the class temporarily. Some of them are spectroscopic binaries; it is possible that the peculiar profiles may have multiple causes among different objects. The origin and future of these stars are intriguing; for instance, they could be stellar mergers and/or gamma-ray-burst progenitors.Comment: 27 pages, 8 figures, 2 tables; AJ accepte

A 2.3-Day Periodic Variability in the Apparently Single Wolf-Rayet Star WR 134: Collapsed Companion or Rotational Modulation?

We present the results of an intensive campaign of spectroscopic and photometric monitoring of the peculiar Wolf-Rayet star WR 134 from 1989 to 1997. This unprecedentedly large data set allows us to confirm unambiguously the existence of a coherent 2.25 +/- 0.05 day periodicity in the line-profile changes of He II 4686, although the global pattern of variability is different from one epoch to another. This period is only marginally detected in the photometric data set. Assuming the 2.25 day periodic variability to be induced by orbital motion of a collapsed companion, we develop a simple model aiming at investigating (i) the effect of this strongly ionizing, accreting companion on the Wolf-Rayet wind structure, and (ii) the expected emergent X-ray luminosity. We argue that the predicted and observed X-ray fluxes can only be matched if the accretion on the collapsed star is significantly inhibited. Additionally, we performed simulations of line-profile variations caused by the orbital revolution of a localized, strongly ionized wind cavity surrounding the X-ray source. A reasonable fit is achieved between the observed and modeled phase-dependent line profiles of He II 4686. However, the derived size of the photoionized zone substantially exceeds our expectations, given the observed low-level X-ray flux. Alternatively, we explore rotational modulation of a persistent, largely anisotropic outflow as the origin of the observed cyclical variability. Although qualitative, this hypothesis leads to greater consistency with the observations.Comment: 34 pages, 16 figures. Accepted by the Astrophysical Journa

Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary gamma Velorum

We present XMM-Newton observations of gamma^2 Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Ray binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate 8x10^{-6} M_sun/yr for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12 Ang) show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5 MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of CV (25.3 Ang) and CVI (31.6 Ang) radiative recombination continua at both phases indicates the presence of a cool (~40,000 K) recombination region located far out in the binary system.Comment: 16 page