275 research outputs found
A Very Large Telescope imaging and spectroscopic survey of the Wolf-Rayet population in NGC 7793
We present a VLT/FORS1 imaging and spectroscopic survey of the Wolf-Rayet
(WR) population in the Sculptor group spiral galaxy NGC 7793. We identify 74
emission line candidates from archival narrow-band imaging, from which 39 were
observed with the Multi Object Spectroscopy (MOS) mode of FORS1. 85% of these
sources displayed WR features. Additional slits were used to observe HII
regions, enabling an estimate of the metallicity gradient of NGC 7793 using
strong line calibrations, from which a central oxygen content of log (O/H) + 12
= 8.6 was obtained, falling to 8.25 at R_25. We have estimated WR populations
using a calibration of line luminosities of Large Magellanic Cloud stars,
revealing ~27 WN and ~25 WC stars from 29 sources spectroscopically observed.
Photometric properties of the remaining candidates suggest an additional ~27 WN
and ~8 WC stars. A comparison with the WR census of the LMC suggests that our
imaging survey has identified 80% of WN stars and 90% for the WC subclass.
Allowing for incompleteness, NGC 7793 hosts ~105 WR stars for which
N(WC)/N(WN)~0.5. From our spectroscopy of HII regions in NGC 7793, we revise
the global Halpha star formation rate of Kennicutt et al. upward by 50% to 0.45
M_sun/yr. This allows us to obtain N(WR)/N(O)~0.018, which is somewhat lower
than that resulting from the WR census by Schild et al. of another Sculptor
group spiral NGC 300, whose global physical properties are similar to NGC 7793.
Finally, we also report the fortuitous detection of a bright (m_V = 20.8 mag)
background quasar Q2358-32 at z~2.02 resulting from CIV 1548-51 redshifted to
the 4684 passband.Comment: 17 pages, 13 figures, accepted for MNRAS (detailed finding charts
omitted)
Reduced Wolf-Rayet Line Luminosities at Low Metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Magellanic Clouds
are presented, from which HeII 4686 line luminosities have been derived, and
compared with observations of other Magellanic Cloud WR stars. SMC WN3-4 stars
possess line luminosities which are a factor of 4 times lower than LMC
counterparts, incorporating several binary SMC WN3-4 stars. Similar results are
found for WN5-6 stars, despite reduced statistics, incorporating observations
of single LMC WN5-9 stars. CIV 5808 line luminosities of carbon sequence WR
stars in the SMC and IC1613 (both WO subtypes) are a factor of 3 lower than LMC
WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are
also obtained for the sole LMC WO star. We demonstrate how reduced line
luminosities at low metallicity follow naturally if WR winds are Z-dependent,
as recent results suggest. We apply mass loss-Z scalings to atmospheric non-LTE
models of Milky Way and LMC WR stars to predict the wind signatures of WR stars
in the metal-poor star forming WR galaxy IZw18. WN HeII 4686 line luminosities
are 7-20 times lower than in Z-rich counterparts of identical bolometric
luminosity, whilst WC CIV 5808 line luminosities are 3-6 times lower.
Significant He^+ Lyman continuum fluxes are predicted for Z-poor early-type WR
stars. Consequently, our results suggest the need for larger population of WR
stars in IZw18 than is presently assumed, particularly for WN stars,
potentially posing a severe challenge to evolutionary models at very low Z.
Finally, reduced wind strengths from WR stars at low Z impacts upon the
immediate circumstellar environment of long duration GRB afterglows,
particularly since the host galaxies of high-redshift GRBs tend to be Z-poor.Comment: 14 pages, 12 figures, accepted for A&A, revision fixes error with Eqn
Galactic constraints on supernova progenitor models
Aims. To estimate the mean masses of oxygen and iron ejected per each type of supernovae (SNe) event from observations of the elemental abundance patterns in the Galactic disk and constrain the relevant SNe progenitor models.
Methods. We undertake a statistical analysis of the radial abundance distributions in the Galactic disk within a theoretical framework for Galactic chemical evolution which incorporates the influence of spiral arms. This framework has been shown to recover the non-linear behaviour in radial gradients, the mean masses of oxygen and iron ejected during SNe explosions to be estimated, and constraints to be placed on SNe progenitor models.
Results. (i) The mean mass of oxygen ejected per core-collapse SNe (CC SNe) event (which are concentrated within spiral arms) is similar to 0.27 M-circle dot; (ii) the mean mass of iron ejected by tardy Type Ia SNe (SNeIa, whose progenitors are older/longer-lived stars with ages greater than or similar to 100 Myr and up to several Gyr, which do not concentrate within spiral arms) is similar to 0.58 M-circle dot; (iii) the upper mass of iron ejected by prompt SNeIa (SNe whose progenitors are younger/shorter-lived stars with ages less than or similar to 100 Myr, which are concentrated within spiral arms) is <= 0.23 M-circle dot per event; (iv) the corresponding mean mass of iron produced by CC SNe is <= 0.04 M-circle dot per event; (v) short-lived SNe (core-collapse or prompt SNeIa) supply similar to 85% of the Galactic disk's iron.
Conclusions. The inferred low mean mass of oxygen ejected per CC SNe event implies a low upper mass limit for the corresponding progenitors of similar to 23 M-circle dot, otherwise the Galactic disk would be overabundant in oxygen. This inference is the consequence of the non-linear dependence between the upper limit of the progenitor initial mass and the mean mass of oxygen ejected per CC SNe explosion. The low mean mass of iron ejected by prompt SNeIa, relative to the mass produced by tardy SNeIa (similar to 2.5 times lower), prejudices the idea that both sub-populations of SNeIa have the same physical nature. We suggest that, perhaps, prompt SNeIa are more akin to CC SNe, and discuss the implications of such a suggestion
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters
NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high
spatial resolution near-IR photometry. We derive high T* for the WN stars in
NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~
2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars
make a disproportionate contribution to the global budget of their host
clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region.
Comparisons with stellar models calculated for the main-sequence evolution of
85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3
systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar
masses are supported by dynamical mass determinations for the components of NGC
3603 A1. We consider the predicted L_X 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 in such a dense environment. From simulated star clusters, whose
constituents are randomly sampled from the Kroupa IMF, both clusters are
consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster
is either too old, exhibits a deficiency of very massive stars, or more likely
stellar masses have been underestimated - M_init for the most luminous stars in
the Arches cluster approach 200 Msun according to contemporary stellar &
photometric results. The potential for stars greatly exceeding 150 Msun within
metal-poor galaxies suggests that such pair-instability SNe could occur within
the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher
resolution figures is available from
http://pacrowther.staff.shef.ac.uk/R136.pdf See also
http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST
Extreme adaptive optics astrometry of R136
We compared high-contrast near-infrared images of the core of R136 taken by VLT/SPHERE, in two epochs separated by 3.06 years. For the first time we monitored the dynamics of the detected sources in the core of R136 from a ground-based telescope with adaptive optics. The aim of these observations was to search for High prOper Motion cAndidates (HOMAs) in the central region of R136 (râ<â6âł) where it has been challenging for other instruments. Two bright sources (Kâ<â15 mag and Vâ<â16 mag) are located near R136a1 and R136c (massive WR stars) and have been identified as potential HOMAs. These sources have significantly shifted in the images with respect to the mean shift of all reliable detected sources and their neighbours, and six times their own astrometric errors. We calculate their proper motions to be 1.36â
屉
0.22 mas yrâ2 (321â
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52 km sâ1) and 1.15â
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0.11 mas yrâ2 (273â
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26 km sâ1). We discuss different possible scenarios to explain the magnitude of such extreme proper motions, and argue for the necessity to conduct future observations to conclude on the nature of HOMAs in the core of R136
The VLT-FLAMES Tarantula Survey I: Introduction and observational overview
The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has
obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30
Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our
scientific motivations and give an overview of the survey targets, including
optical and near-infrared photometry and comprehensive details of the data
reduction. One of the principal objectives was to detect massive binary systems
via variations in their radial velocities, thus shaping the multi-epoch
observing strategy. Spectral classifications are given for the massive
emission-line stars observed by the survey, including the discovery of a new
Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To
illustrate the diversity of objects encompassed by the survey, we investigate
the spectral properties of sixteen targets identified by Gruendl & Chu from
Spitzer photometry as candidate young stellar objects or stars with notable
mid-infrared excesses. Detailed spectral classification and quantitative
analysis of the O- and B-type stars in the VFTS sample, paying particular
attention to the effects of rotational mixing and binarity, will be presented
in a series of future articles to address fundamental questions in both stellar
and cluster evolution.Comment: Accepted by A&A, 52 pages (main body: 19 pages, supplementary tables:
33 pages), v3: two classifications updated to match a parallel pape
Topical emollient therapy with sunflower seed oil alters the skin microbiota of young children with severe acute malnutrition in Bangladesh: A randomised, controlled study.
BACKGROUND: Topical emollient therapy with sunflower seed oil (SSO) reduces risk of sepsis and mortality in very preterm infants in low- or middle-income countries (LMICs). Proposed mechanisms include modulation of skin and possibly gut barrier function. The skin and gut microbiota play important roles in regulating barrier function, but the effects of emollient therapy on these microbiotas are poorly understood. METHODS: We characterised microbiota structure and diversity with 16S rRNA gene amplicon sequence data and ecological statistics in 20 children with severe acute malnutrition (SAM) aged 2-24 months, at four skin sites and in stool, during a randomised, controlled trial of emollient therapy with SSO in Bangladesh. Microbes associated with therapy were identified with tree-based sparse discriminant analysis. RESULTS: The skin microbiota of Bangladeshi children with SAM was highly diverse and displayed significant variation in structure as a function of physical distance between sites. Microbiota structure differed between the study groups (Pâ=â0.005), was more diverse in emollient-treated subjects-including on the forehead which did not receive direct treatment-and changed with each day (Pâ=â0.005) at all skin sites. Overall, Prevotellaceae were the most differentially affected by emollient treatment; several genera within this family became more abundant in the emollient group than in the controls across several skin sites. Gut microbiota structure was associated with sample day (Pâ=â0.045) and subject age (Pâ=â0.045), but was not significantly affected by emollient treatment (Pâ=â0.060). CONCLUSIONS: Emollient therapy altered the skin microbiota in a consistent and temporally coherent manner. We speculate that therapy with SSO enhances skin barrier function in part through alterations in the microbiota, and through systemic mechanisms. Strategies to strengthen skin and gut barrier function in populations at risk, such as children in LMICs like Bangladesh, might include deliberate manipulation of their skin microbiota. TRIAL REGISTRATION: ClinicalTrials.gov: NCT02616289
Microfluidic wound bandage: Localized oxygen modulation of collagen maturation
Restoring tissue oxygenation has the potential to improve poorly healing wounds with impaired microvasculature. Compared with more established wound therapy using hyperbaric oxygen chambers, topical oxygen therapy has lower cost and better patient comfort, although topical devices have provided inconsistent results. To provide controlled topical oxygen while minimizing moisture loss, a major issue for topical oxygen, we have devised a novel wound bandage based on microfluidic diffusion delivery of oxygen. In addition to modulating oxygen from 0 to 100% in 60 seconds rise time, the microfluidic oxygen bandage provides a conformal seal around the wound. When 100% oxygen is delivered, it penetrates wound tissues as measured in agar phantom and in vivo wounds. Using this microfluidic bandage, we applied the oxygen modulation to 8âmm excisional wounds prepared on diabetic mice. Treatment with the microfluidic bandage demonstrated improved collagen maturity in the wound bed, although only marginal differences were observed in total collagen, microvasculature, and external closure rates. Our results show that proper topical oxygen can improve wound parameters underneath the surface. Because of the ease of fabrication, the oxygen bandage represents an economical yet practical method for oxygen wound research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97268/1/wrr12021.pd
Can pneumococcal meningitis surveillance be used to assess the impact of pneumococcal conjugate vaccine on total invasive pneumococcal disease? A case-study from South Africa, 2005â2016
INTRODUCTION : South Africa introduced seven-valent pneumococcal conjugate vaccine (PCV7) in 2009 and PCV13 in 2011. We aimed to compare the estimated impact of PCV on pneumococcal meningitis (PM) to impact of PCV on total invasive pneumococcal disease (tIPD) based on risk reduction after PCV introduction.
METHODS : We conducted national, laboratory-based surveillance for tIPD during 2005â2016. We estimated and compared rates of PCV13 and non-PCV13 serotype disease among tIPD and PM in individuals aged <5âŻyears and â„5âŻyears, and compared these rates between the 2005â2008 pre-PCV introduction period and two time points after PCV introduction, 2012 and 2016.
RESULTS : We enrolled 45,853 tIPD cases; 17,251 (38%) were PM. By 2016, IPD caused by all serotypes decreased 55% (95%CI â57% to â53%) for tIPD, and 54% for PM (95%CI â58% to â51%), 0.7% difference between estimates (pâŻ=âŻ0.7). No significant differences were observed between PCV7-serotype disease reduction in tIPD and PM in both age groups or the additional 6 serotypes included in PCV13 in <5âŻyear olds in 2012 and 2016. In 2012 there was a significant difference between increases in non-PCV13 serotype disease in those â„5âŻyears for tIPD and PM (32% greater increase in PM, pâŻ<âŻ0.001), but this difference was absent by 2016. There was a significant difference between the estimated decrease in additional PCV13 type disease in 2016 between tIPD and PM for those aged â„5âŻyears (28% greater reduction in PM, pâŻ=âŻ0.008).
CONCLUSION : PM showed similar reductions to tIPD seven years after PCV introduction in vaccine serotype disease in those <5âŻyears, and increases in non-vaccine serotype disease in all ages.The National Institute for Communicable Diseases a division of the National Health Laboratory Service, South Africa; the United States Agency for International Developmentâs Antimicrobial Resistance Initiative, United States of America, transferred via a cooperative agreement [U60/CCU022088] from the United States Centers for Disease Control and Prevention, United States if Ameriva; and the United States Centers for Disease Control and Prevention [U62/CCU022901], United States of America.http://www.elsevier.com/locate/vaccine2020-09-10hj2019School of Health Systems and Public Health (SHSPH
Empathy, engagement, entrainment: the interaction dynamics of aesthetic experience
A recent version of the view that aesthetic experience is based in empathy as inner
imitation explains aesthetic experience as the automatic simulation of actions,
emotions, and bodily sensations depicted in an artwork by motor neurons in the brain. Criticizing the simulation theory for committing to an erroneous concept of empathy and failing to distinguish regular from aesthetic experiences of art, I advance an alternative, dynamic approach and claim that aesthetic experience is enacted and skillful, based in the recognition of othersâ experiences as distinct from oneâs own. In combining insights from mainly psychology, phenomenology, and cognitive science, the dynamic approach aims to explain the emergence of aesthetic experience in terms of the reciprocal interaction between viewer and artwork. I argue that aesthetic experience emerges by participatory sense-making and revolves around movement as a means for creating meaning. While entrainment merely plays a preparatory part in this, aesthetic engagement constitutes the phenomenological side of coupling to an artwork and provides the context for exploration, and eventually for moving, seeing, and feeling with art. I submit that aesthetic experience emerges from bodily and emotional engagement with works of art via the complementary processes of the perceptionâaction and motionâemotion loops. The former involves the embodied
visual exploration of an artwork in physical space, and progressively structures and organizes visual experience by way of perceptual feedback from body movements made in response to the artwork. The latter concerns the movement qualities and shapes of implicit and explicit bodily responses to an artwork that cue emotion and thereby modulate over-all affect and attitude. The two processes cause the viewer to bodily and emotionally move with and be moved by individual works of art, and consequently to recognize another psychological orientation than her own, which explains how art can cause feelings of insight or awe and disclose aspects of life that are unfamiliar or novel to the viewer
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