2,916 research outputs found
The Asperian Design
Reality is two-fold, composed of the lighted world as revealed in Genesis, and the darker primordiality which preceded it. The illuminated represents that which the human mind can comprehend, manipulate and re-order to its will: a “designed” and mechanical universe of parts. But behind it, in the backspace of reality, remains the darkness. A formless state of pre-creation, the darkness exists as an endless series of intertwining “signatures” – single possibilities waiting to be created in the illuminated forefront of reality. Permitting each and every part of the lighted world to be connected to the rest, it possesses a “design” all of its own. The question is, if we are blind in the dark, how could we ever come to know it
The Field
Life is a battlefield onto which we are thrown at birth, with only fate and fortune settling upon where we land. Wherever we land, whether it's on the front lines or surrounded by a network of defenses, we are all asking the same question: why are we here?
The problem with this question, however, is that we tend to answer it from our own relative positions, and so we all arrive at different conclusions. The many answers we've created have filled the field. They have become banners that are raised high so that we can see and follow them. These banners are always at risk of being toppled though; always at risk of losing their capacity for belief. But then, what if belief is more than a mere by-product of our universal search? What if belief can change the very fabric of this field? That is, what if truth is actually irrelevant, and the only real thing that makes up this battlefield is the belief, and consequently the collective will, which we invest in our answers
Metallicity in the Galactic Center: The Arches cluster
We present a quantitative spectral analysis of five very massive stars in the
Arches cluster, located near the Galactic center, to determine stellar
parameters, stellar wind properties and, most importantly, metallicity content.
The analysis uses a new technique, presented here for the first time, and uses
line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared
spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the
cluster. It relies on the fact that massive stars reach a maximum nitrogen
abundance that is related to initial metallicity when they are in the WNL
phase. We determine the present-day nitrogen abundance of the WNL stars in the
Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity
in the cluster to be solar. This result is invariant to assumptions about the
mass-luminosity relationship, the mass-loss rates, and rotation speeds. In
addition, from this analysis, we find the age of the Arches cluster to be
2-2.5Myr, assuming coeval formation
A large Wolf-Rayet population in NGC300 uncovered by VLT-FORS2
We have detected 58 Wolf-Rayet candidates in the central region of the nearby
spiral galaxy NGC 300, based on deep VLT-FORS2 narrow-band imaging. Our survey
is close to complete except for heavily reddened WR stars. Of the objects in
our list, 16 stars were already spectroscopically confirmed as WR stars by
Schild & Testor and Breysacher et al., to which 4 stars are added using low
resolution FORS2 datasets. The WR population of NGC300 now totals 60,a
threefold increase over previous surveys, with WC/WN>1/3, in reasonable
agreement with Local Group galaxies for a moderately sub-solar metallicity. We
also discuss the WR surface density in the central region of NGC 300. Finally,
analyses are presented for two apparently single WC stars - #29 (alias WR3,
WC5) and #48 (alias WR13, WC4) located close to the nucleus, and at a
deprojected radius of 2.5 kpc, respectively. These are among the first models
of WR stars in galaxies beyond the Local Group, and are compared with early WC
stars in our Galaxy and LMC.Comment: 12 pages, 12 figures, submitted to A&A (includes aa.cls) - version
with higher resolution finding charts available from
ftp://ftp.star.ucl.ac.uk/pub/pac/ngc300.ps.g
Recommended from our members
The Wolf-Rayet population of Westerlund 1
New NTT/SOFI near-IR narrow-band imaging and spectroscopy reveals an additional four Wolf-Rayet (WR) stars in the massive cluster Westerlund 1, bringing the total WR population to 24. Sixteen of the WR stars in Wd1 have been classified WN5–11, while eight are WC8–9. An observed WR to RSG/YHG ratio of ∼3 suggests an age of 4.5–5.0 Myr, with WR stars descended from 40–55MSolar progenitors. On the basis of dust and hard X-ray emission, we estimate that 40–65% are probable members of massive star binary systems
A Spectroscopic Search for the non-nuclear Wolf-Rayet Population of the metal-rich spiral galaxy M83
We present a catalogue of non-nuclear regions containing Wolf-Rayet stars in
the metal-rich spiral galaxy M83 (NGC5236). From a total of 283 candidate
regions identified using HeII 4686 imaging with VLT-FORS2, Multi Object
Spectroscopy of 198 regions was carried out, confirming 132 WR sources. From
this sub-sample, an exceptional content of 1035 +/- 300 WR stars is inferred,
with N(WC)/N(WN) approx 1.2, continuing the trend to larger values at higher
metallicity amongst Local Group galaxies, and greatly exceeding current
evolutionary predictions at high metallicity. Late-type stars dominate the WC
population of M83, with N(WC8-9)/N(WC4-7)=9 and WO subtypes absent, consistent
with metallicity dependent WC winds. Equal numbers of late to early WN stars
are observed, again in contrast to current evolutionary predictions. Several
sources contain large numbers of WR stars. In particular, #74 (alias region 35
from De Vaucouleurs et al. contains 230 WR stars, and is identified as a Super
Star Cluster from inspection of archival HST/ACS images. Omitting this
starburst cluster would result in revised statistics of N(WC)/N(WN) approx 1
and N(WC8-9)/N(WC4-7) approx 6 for the `quiescent' disk population. Including
recent results for the nucleus and accounting for incompleteness in our
spectroscopic sample, we suspect the total WR population of M83 may exceed 3000
stars.Comment: 39 pages, 13 figures, 17 finding charts, accepted for Astronomy &
Astrophysics. Version will full resolution images available at
ftp://astro1.shef.ac.uk/pub/pac/m83.ps.g
EBM and Epistemological Imperialism: Narrowing the divide between evidence and illness
Evidence Based Medicine (EBM) is an approach to clinical practice that relies on the use of systematically reviewed published clinical research of high quality. Whilst there is some speculation as to whether a true consensus definition of EBM exists (Loughlin (2008)(1)), a commonly cited explanation “the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients’ (Sackett et al (1996)(2)). Most approaches to “EBM” incorporate the use of an evidence hierarchy that presupposes that some forms of evidence are better than others (Guyatt and Rennie (2002)(3)), that meta-analyses and randomised controlled trials (RCTs) will guide a better level of care than expert or local knowledge.
Although EBM is pervasive throughout all health literature a number of ethical (Gupta (2009)(4)), epistemological (Loughlin (2008)(1)), and clinical practice critiques (Tobin (2008)(5)) have emerged. Criticisms of EBM on ethical grounds have previously been summarised by Kerridge (2010)(6) and include ; “that the implicit and explicit requirement for RCTs may lead to unnecessary research being done where sufficient evidence already exists;... that methods privileged by EBM, most notably the RCT, are methodologically unable to answer questions related to individual patients;.... that evidence hierarchies are inadequate and misleading;.... that the dataset that EBM draws from is systematically bias[ed],.... that the translation of evidence into practice through clinical practice guidelines and decision aids is both ethically and epistemologically problematic...[and] that evidence is not value-neutral and cannot be easily translated into practice.
Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67
We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical
non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux
distributions derived from a stellar analysis of WR124, are compared with
nebular properties via photo-ionization modelling. Our study demonstrates the
significant role that line blanketing plays in affecting the Lyman ionizing
energy distribution of WR stars, of particular relevance to the study of HII
regions containing young stellar populations.
We confirm previous results that non-line blanketed WR energy distributions
fail to explain the observed nebular properties of M1-67, such that the
predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is
carried out using the method of Hillier & Miller (1998), with stellar
properties in accord with previous results, except that the inclusion of
clumping in the stellar wind reduces its wind performance factor to only
approx2. The ionizing spectrum of the line blanketed model is much softer than
for a comparable temperature unblanketed case, such that negligible flux is
emitted with energy above the HeI 504 edge. Photo-ionization modelling,
incorporating the observed radial density distribution for M1-67 reveals
excellent agreement with the observed nebular electron temperature, ionization
balance and line strengths. An alternative stellar model of WR124 is
calculated, following the technique of de Koter et al. (1997), augmented to
include line blanketing following Schmutz et al. (1991). Good consistency is
reached regarding the stellar properties of WR124, but agreement with the
nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller
code.Comment: 12 pages, 5 figures, latex2e style file, Astronomy & Astrophysics
(accepted
Properties of Hot Stars in the Wolf-Rayet galaxy NGC5253 from ISO Spectroscopy
ISO-SWS spectroscopy of the WR galaxy NGC5253 is presented, and analysed to
provide estimates of its hot young star population. Our approach differs from
previous investigations in that we are able to distinguish between the regions
in which different infrared fine-structure lines form, using complementary
ground-based observations. The high excitation nebular [SIV] emission is formed
in a very compact region, which we attribute to the central super-star-nucleus,
and lower excitation [NeII] nebular emission originates in the galactic core.
We use photo-ionization modelling coupled with the latest theoretical O-star
flux distributions to derive effective stellar temperatures and ionization
parameters of Teff>38kK, logQ=8.25 for the compact nucleus, with Teff=35kK,
logQ<8 for the larger core. Results are supported by more sophisticated
calculations using evolutionary synthesis models. We assess the contribution
that Wolf-Rayet stars may make to highly ionized nebular lines (e.g. [OIV]).
From our Br(alpha) flux, the 2" nucleus contains the equivalent of
approximately 1000 O7V star equivalents and the starburst there is 2-3Myr old;
the 20" core contains about 2500 O7V star equivalents, with a representative
age of 5Myr. The Lyman ionizing flux of the nucleus is equivalent to the 30
Doradus region. These quantities are in good agreement with the observed mid-IR
dust luminosity of 7.8x10^8 L(sun) Since this structure of hot clusters
embedded in cooler emission may be common in dwarf starbursts, observing a
galaxy solely with a large aperture may result in confusion. Neglecting the
spatial distribution of nebular emission in NGC5253, implies `global' stellar
temperatures (or ages) of 36kK (4.8Myr) and 39kK (2.9 or 4.4Myr) from the
observed [NeIII/II] and [SIV/III] line ratios, assuming logQ=8.Comment: 16 pages, 7 figures, uses mn.sty, to appear in MNRA
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