778 research outputs found
VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136
Using the Very Large Telescope's Spectrograph for INtegral Field Observation
in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR
spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of
the very young (~1 Myr), massive and dense cluster R136, in the Large
Magellanic Cloud (LMC). We also de-archived available images that were obtained
with the Hubble Space Telescope's Space Telescope Imaging Spectrograph
(HST/STIS), and extracted high-quality, differential photometry of our target
stars to check for any variability related to binary motion.
Previous studies, relying on spatially unresolved, integrated, optical
spectroscopy, had reported that one of these stars was likely to be a 4.377-day
binary. Our study set out to identify the culprit and any other short-period
system among our targets. However, none displays significant photometric
variability, and only one star, BAT99-112 (R136c), located on the outer fringe
of R136, displays a marginal variability in its radial velocities; we
tentatively report an 8.2-day period. The binary status of BAT99-112 is
supported by the fact that it is one of the brightest X-ray sources among all
known WR stars in the LMC, consistent with it being a colliding-wind system.
Follow-up observations have been proposed to confirm the orbital period of this
potentially very massive system.Comment: 9 pages, 6 figures; accepted for publication in MNRA
Exploring leadership communication in the United Arab Emirates: Issues of culture and gender
This study aims to identify what makes a successful leader in the UAE within the paradigms of leadership styles and leadership communication. In order to do this, we explore, and potentially challenge, a number of the existing leadership stereotypes that are germane to the region, along with the stereotypical discourse strategies that have been associated with the ways in which men and women enact leadership. We wanted to find out if Emirati nationals would prefer the type of discursive leadership that has been associated with women leaders, that is transformational leadership and collaborative communication, or if they would prefer the discursive leadership that has been stereotypically associated with men in equivalent positions, that is laissez-faire, transactional or paternalisticleadership styles, together with the use of a direct and competitive style of communication. Our findings show that a Western understanding of leadership may be too strictly delineated to account for leadership styles in the Gulf, and also that leaders in the region may effectively draw on a combined set of communication strategies that have been stereotypically attributed elsewhere to male and female leaders
Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
Multiplicity is one of the most fundamental observable properties of massive
O-type stars and offers a promising way to discriminate between massive star
formation theories. Nevertheless, companions at separations between 1 and 100
mas remain mostly unknown due to intrinsic observational limitations. [...] The
Southern MAssive Stars at High angular resolution survey (SMASH+) was designed
to fill this gap by providing the first systematic interferometric survey of
Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162
O-type stars with NACO/SAM, respectively probing the separation ranges 1-45 and
30-250mas and brightness contrasts of Delta H < 4 and Delta H < 5. Taking
advantage of NACO's field-of-view, we further uniformly searched for visual
companions in an 8''-radius down to Delta H = 8. This paper describes the
observations and data analysis, reports the discovery of almost 200 new
companions in the separation range from 1mas to 8'' and presents the catalog of
detections, including the first resolved measurements of over a dozen known
long-period spectroscopic binaries.
Excluding known runaway stars for which no companions are detected, 96
objects in our main sample (DEC < 0 deg; H<7.5) were observed both with PIONIER
and NACO/SAM. The fraction of these stars with at least one resolved companion
within 200mas is 0.53. Accounting for known but unresolved spectroscopic or
eclipsing companions, the multiplicity fraction at separation < 8'' increases
to f_m = 0.91 +/- 0.03. The fraction of luminosity class V stars that have a
bound companion reaches 100% at 30mas while their average number of physically
connected companions within 8'' is f_c = 2.2 +/- 0.3. This demonstrates that
massive stars form nearly exclusively in multiple systems. Additionally, the
nine non-thermal (NT) radio emitters observed by SMASH+ are all resolved [...]Comment: 57 pages, 20 figures, 7 tables; accepted for publication in ApJ
Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST
From stellar spectra, a variety of physical properties of stars can be
derived. In particular, the chemical composition of stellar atmospheres can be
inferred from absorption line analyses. These provide key information on large
scales, such as the formation of our Galaxy, down to the small-scale
nucleosynthesis processes that take place in stars and supernovae. By extending
the observed wavelength range toward bluer wavelengths, we optimize such
studies to also include critical absorption lines in metal-poor stars, and
allow for studies of heavy elements (Z>38) whose formation processes remain
poorly constrained. In this context, spectrographs optimized for observing blue
wavelength ranges are essential, since many absorption lines at redder
wavelengths are too weak to be detected in metal-poor stars. This means that
some elements cannot be studied in the visual-redder regions, and important
scientific tracers and science cases are lost. The present era of large public
surveys will target millions of stars. Here we describe the requirements
driving the design of the forthcoming survey instrument 4MOST, a multi-object
spectrograph commissioned for the ESO VISTA 4m-telescope. We focus here on
high-density, wide-area survey of stars and the science that can be achieved
with high-resolution stellar spectroscopy. Scientific and technical
requirements that governed the design are described along with a thorough line
blending analysis. For the high-resolution spectrograph, we find that a
sampling of >2.5 (pixels per resolving element), spectral resolution of 18000
or higher, and a wavelength range covering 393-436 nm, is the most
well-balanced solution for the instrument. A spectrograph with these
characteristics will enable accurate abundance analysis (+/-0.1 dex) in the
blue and allow us to confront the outlined scientific questions. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A
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
Linear and nonlinear rheology of wormlike micelles
Several surfactant molecules self-assemble in solution to form long,
cylindrical, flexible wormlike micelles. These micelles can be entangled with
each other leading to viscoelastic phases. The rheological properties of such
phases are very interesting and have been the subject of a large number of
experimental and theoretical studies in recent years. We shall report on our
recent work on the macrorheology, microrheology and nonlinear flow behaviour of
dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium
Tosilate). This system forms elongated micelles and exhibits strong
viscoelasticity at low concentrations ( 0.9 wt%) without the addition of
electrolytes. Microrheology measurements of have been done using
diffusing wave spectroscopy which will be compared with the conventional
frequency sweep measurements done using a cone and plate rheometer. The second
part of the paper deals with the nonlinear rheology where the measured shear
stress is a nonmonotonic function of the shear rate . In
stress-controlled experiments, the shear stress shows a plateau for
larger than some critical strain rate, similar to the earlier
reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a
signature of mechanical instability in the form of shear bands. We have carried
out extensive experiments under controlled strain rate conditions, to study the
time-dependence of shear stress. The measured time series of shear stress has
been analysed in terms of correlation integrals and Lyapunov exponents to show
unambiguously that the behaviour is typical of low dimensional dynamical
systems.Comment: 15 pages, 10 eps figure
Yeast-Derived Formulations Are Differentially Fermented by the Canine and Feline Microbiome As Assessed in a Novel in Vitro Colonic Fermentation Model
The current study evaluated the effect of five yeast-derived formulations (T1-T5) on microbial metabolism and composition of the canine and feline gut microbiota using a novel in vitro colonic incubation approach. This novel in vitro model allowed for growth of the entire spectrum of dog- and cat-derived bacteria from the inoculum, thus offering an excellent platform to evaluate effects of nutritional interventions on the gut microbiota. Further, yeast-derived ingredients differentially increased production of acetate, propionate, butyrate, ammonium, and branched short-chain fatty acids, with T5 and T1 consistently stimulating propionate and butyrate, respectively. 16S-targeted Illumina sequencing coupled with flow cytometry provided unprecedented high-resolution quantitative insights in canine and feline microbiota modulation by yeast-derived ingredients, revealing that effects on propionate production were related to Prevotellaceae, Tannerellaceae, Bacteroidaceae, and Veillonellaceae members, while effects on butyrate production were related to Erysipelotrichaceae, Lachnospiraceae, Ruminococcaceae, and Fusobacteriaceae. Overall, these findings strengthen the health-promoting potential of yeast-derived ingredients
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