2,132 research outputs found
The First Galaxies: Clues from Element Abundances
It has recently become possible to measure directly the abundances of several
chemical elements in a variety of environments at redshifts up to z = 5. In
this review I summarise the latest observations of Lyman break galaxies, damped
Lyman alpha systems and the Lyman alpha forest with a view to uncovering any
clues which these data may offer to the first episodes of star formation. The
picture which is emerging is one where the universe at z = 3 already included
many of the components of today's galaxies--even at these early times we see
evidence for Populations I and II stars, while the `smoking gun' for Population
III objects may be hidden in the chemical composition of the lowest density
regions of the IGM, yet to be deciphered.Comment: 15 pages, LaTex, 8 Postscript Figures. To appear in the Philosophical
Transactions of The Royal Society, Series
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
Experimental behaviour of a steel structure under natural fire
Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.http://www.sciencedirect.com/science/article/B6V37-4KN5C4D-1/1/8f781d0c96159d54029bef7c9ec451b
Morphology and Composition of the Helix Nebula
We present new narrow-band filter imagery in H-alpha and [N II] 6584 along
with UV and optical spectrophotometry measurements from 1200 to 9600 Angstroms
of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large
diameter and low surface brightness. Detailed models of the observable ionized
nebula support the recent claim that the Helix is actually a flattened disk
whose thickness is roughly one-third its diameter with an inner region
containing hot, highly ionized gas which is generally invisible in narrow-band
images. The outer visible ring structure is of lower ionization and temperature
and is brighter because of a thickening in the disk. We also confirm a central
star effective temperature and luminosity of 120,000K and 100L_sun, and we
estimate a lower limit to the nebular mass to be 0.30M_sun. Abundance
measurements indicate the following values: He/H=0.12 (+/-0.017),
O/H=4.60x10^-4 (+/-0.18), C/O=0.87 (+/-0.12), N/O=0.54 (+/-0.14), Ne/O=0.33
(+/-0.04), S/O=3.22x10^-3 (+/-0.26), and Ar/O=6.74x10^-3 (+/-0.76). Our carbon
abundance measurements represent the first of their kind for the Helix Nebula.
The S/O ratio which we derive is anomalously low; such values are found in only
a few other planetary nebulae. The central star properties, the super-solar
values of He/H and N/O, and a solar level of C/O are consistent with a 6.5M_sun
progenitor which underwent three phases of dredge-up and hot bottom burning
before forming the planetary nebula.Comment: 50-page manuscript plus 11 postscript figures. This revised version
corrects a typo in earlier submission. Nothing else has changed. Accepted for
publication in the Astrophysical Journa
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
Sulfur, Chlorine, & Argon Abundances in Planetary Nebulae. I: Observations and Abundances in a Northern Sample
This paper is the first of a series specifically studying the abundances of
sulfur, chlorine, and argon in Type II planetary nebulae (PNe) in the Galactic
disk. Ratios of S/O, Cl/O, and Ar/O constitute important tests of differential
nucleosynthesis of these elements and serve as strict constraints on massive
star yield predictions. We present new ground-based optical spectra extending
from 3600-9600 Angstroms for a sample of 19 Type II northern PNe. This range
includes the strong near infrared lines of [S III] 9069,9532, which allows us
to test extensively their effectiveness as sulfur abundance indicators. We also
introduce a new, model-tested ionization correction factor for sulfur. For the
present sample, we find average values of S/O=1.2E-2(+/- 0.71E-2),
Cl/O=3.3E-4(+/- 1.6E-4), and Ar/O=5.0E-3(+/- 1.9E-3).Comment: 44 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Review: ‘Gimme five’: future challenges in multiple sclerosis. ECTRIMS Lecture 2009
This article is based on the ECTRIMS lecture given at the 25th ECTRIMS meeting which was held in Düsseldorf, Germany, from 9 to 12 September 2009. Five challenges have been identified: (1) safeguarding the principles of medical ethics; (2) optimizing the risk/benefit ratio; (3) bridging the gap between multiple sclerosis and experimental autoimmune encephalitis; (4) promoting neuroprotection and repair; and (5) tailoring multiple sclerosis therapy to the individual patient. Each of these challenges will be discussed and placed in the context of current research into the pathogenesis and treatment of multiple sclerosis
Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
We present a catalog of 5324 massive stars in the Small Magellanic Cloud
(SMC), with accurate spectral types compiled from the literature, and a
photometric catalog for a subset of 3654 of these stars, with the goal of
exploring their infrared properties. The photometric catalog consists of stars
with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which
we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands.
We compare the color magnitude diagrams and color-color diagrams to those of
the Large Magellanic Cloud (LMC), finding that the brightest infrared sources
in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars,
luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less
infrared excess, the red supergiants being less dusty and the sgB[e] stars
being on average less luminous. Among the objects detected at 24 um are a few
very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy
distributions, and all 3 known luminous blue variables. We detect a distinct Be
star sequence, displaced to the red, and suggest a novel method of confirming
Be star candidates photometrically. We find a higher fraction of Oe and Be
stars among O and early-B stars in the SMC, respectively, when compared to the
LMC, and that the SMC Be stars occur at higher luminosities. We estimate
mass-loss rates for the red supergiants, confirming the correlation with
luminosity even at the metallicity of the SMC. Finally, we confirm the new
class of stars displaying composite A & F type spectra, the sgB[e] nature of
2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue
variable with cold dust.Comment: 23 pages, 17 figures, 5 tables, accepted for publication in the
Astronomical Journa
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