1,172 research outputs found
A low energy core-collapse supernova without a hydrogen envelope
The final fate of massive stars depends on many factors, including mass,
rotation rate, magnetic fields and metallicity. Theory suggests that some
massive stars (initially greater than 25-30 solar masses) end up as Wolf-Rayet
stars which are deficient in hydrogen because of mass loss through strong
stellar winds. The most massive of these stars have cores which may form a
black hole and theory predicts that the resulting explosion produces ejecta of
low kinetic energy, a faint optical display and a small mass fraction of
radioactive nickel(1,2,3). An alternative origin for low energy supernovae is
the collapse of the oxygen-neon core of a relatively lowmass star (7-9 solar
masses) through electron capture(4,5). However no weak, hydrogen deficient,
core-collapse supernovae are known. Here we report that such faint, low energy
core-collapse supernovae do exist, and show that SN2008ha is the faintest
hydrogen poor supernova ever observed. We propose that other similar events
have been observed but they have been misclassified as peculiar thermonuclear
supernovae (sometimes labelled SN2002cx-like events(6)). This discovery could
link these faint supernovae to some long duration gamma-ray bursts. Extremely
faint, hydrogen-stripped core-collapse supernovae have been proposed to produce
those long gamma-ray bursts whose afterglows do not show evidence of
association with supernovae (7,8,9).Comment: Submitted 12 January 2009 - Accepted 24 March 200
Vitamin D receptor ChIP-seq in primary CD4+ cells: relationship to serum 25-hydroxyvitamin D levels and autoimmune disease
PMCID: PMC3710212This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited
An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae
Type Ia Supernovae (SNe Ia) form an observationally uniform class of stellar
explosions, in that more luminous objects have smaller decline-rates. This
one-parameter behavior allows SNe Ia to be calibrated as cosmological `standard
candles', and led to the discovery of an accelerating Universe. Recent
investigations, however, have revealed that the true nature of SNe Ia is more
complicated. Theoretically, it has been suggested that the initial
thermonuclear sparks are ignited at an offset from the centre of the
white-dwarf (WD) progenitor, possibly as a result of convection before the
explosion. Observationally, the diversity seen in the spectral evolution of SNe
Ia beyond the luminosity decline-rate relation is an unresolved issue. Here we
report that the spectral diversity is a consequence of random directions from
which an asymmetric explosion is viewed. Our findings suggest that the spectral
evolution diversity is no longer a concern in using SNe Ia as cosmological
standard candles. Furthermore, this indicates that ignition at an offset from
the centre of is a generic feature of SNe Ia.Comment: To appear in Nature, 1st July 2010 issue. 36 pages including
supplementary materials. 4 figures, 3 supplementary figures, 1 supplementary
tabl
Signal Transduction Pathways in the Pentameric Ligand-Gated Ion Channels
The mechanisms of allosteric action within pentameric ligand-gated ion channels (pLGICs) remain to be determined. Using crystallography, site-directed mutagenesis, and two-electrode voltage clamp measurements, we identified two functionally relevant sites in the extracellular (EC) domain of the bacterial pLGIC from Gloeobacter violaceus (GLIC). One site is at the C-loop region, where the NQN mutation (D91N, E177Q, and D178N) eliminated inter-subunit salt bridges in the open-channel GLIC structure and thereby shifted the channel activation to a higher agonist concentration. The other site is below the C-loop, where binding of the anesthetic ketamine inhibited GLIC currents in a concentration dependent manner. To understand how a perturbation signal in the EC domain, either resulting from the NQN mutation or ketamine binding, is transduced to the channel gate, we have used the Perturbation-based Markovian Transmission (PMT) model to determine dynamic responses of the GLIC channel and signaling pathways upon initial perturbations in the EC domain of GLIC. Despite the existence of many possible routes for the initial perturbation signal to reach the channel gate, the PMT model in combination with Yen's algorithm revealed that perturbation signals with the highest probability flow travel either via the β1-β2 loop or through pre-TM1. The β1-β2 loop occurs in either intra- or inter-subunit pathways, while pre-TM1 occurs exclusively in inter-subunit pathways. Residues involved in both types of pathways are well supported by previous experimental data on nAChR. The direct coupling between pre-TM1 and TM2 of the adjacent subunit adds new insight into the allosteric signaling mechanism in pLGICs. © 2013 Mowrey et al
A highly magnified candidate for a young galaxy seen when the Universe was 500 Myrs old
The early Universe at redshift z\sim6-11 marks the reionization of the
intergalactic medium, following the formation of the first generation of stars.
However, those young galaxies at a cosmic age of \lesssim 500 million years
(Myr, at z \gtrsim 10) remain largely unexplored as they are at or beyond the
sensitivity limits of current large telescopes. Gravitational lensing by galaxy
clusters enables the detection of high-redshift galaxies that are fainter than
what otherwise could be found in the deepest images of the sky. We report the
discovery of an object found in the multi-band observations of the cluster
MACS1149+22 that has a high probability of being a gravitationally magnified
object from the early universe. The object is firmly detected (12 sigma) in the
two reddest bands of HST/WFC3, and not detected below 1.2 {\mu}m, matching the
characteristics of z\sim9 objects. We derive a robust photometric redshift of z
= 9.6 \pm 0.2, corresponding to a cosmic age of 490 \pm 15Myr (i.e., 3.6% of
the age of the Universe). The large number of bands used to derive the redshift
estimate make it one of the most accurate estimates ever obtained for such a
distant object. The significant magnification by cluster lensing (a factor of
\sim15) allows us to analyze the object's ultra-violet and optical luminosity
in its rest-frame, thus enabling us to constrain on its stellar mass,
star-formation rate and age. If the galaxy is indeed at such a large redshift,
then its age is less than 200 Myr (at the 95% confidence level), implying a
formation redshift of zf \lesssim 14. The object is the first z>9 candidate
that is bright enough for detailed spectroscopic studies with JWST,
demonstrating the unique potential of galaxy cluster fields for finding highly
magnified, intrinsically faint galaxies at the highest redshifts.Comment: Submitted to the Nature Journal. 39 Pages, 13 figure
Constraints on the Progenitor System of the Type Ia Supernova SN 2011fe/PTF11kly
Type Ia supernovae (SNe) serve as a fundamental pillar of modern cosmology,
owing to their large luminosity and a well-defined relationship between
light-curve shape and peak brightness. The precision distance measurements
enabled by SNe Ia first revealed the accelerating expansion of the universe,
now widely believed (though hardly understood) to require the presence of a
mysterious "dark" energy. General consensus holds that Type Ia SNe result from
thermonuclear explosions of a white dwarf (WD) in a binary system; however,
little is known of the precise nature of the companion star and the physical
properties of the progenitor system. Here we make use of extensive historical
imaging obtained at the location of SN 2011fe/PTF11kly, the closest SN Ia
discovered in the digital imaging era, to constrain the visible-light
luminosity of the progenitor to be 10-100 times fainter than previous limits on
other SN Ia progenitors. This directly rules out luminous red giants and the
vast majority of helium stars as the mass-donating companion to the exploding
white dwarf. Any evolved red companion must have been born with mass less than
3.5 times the mass of the Sun. These observations favour a scenario where the
exploding WD of SN 2011fe/PTF11kly, accreted matter either from another WD, or
by Roche-lobe overflow from a subgiant or main-sequence companion star.Comment: 22 pages, 6 figures, submitte
Type Ia Supernovae as Stellar Endpoints and Cosmological Tools
Empirically, Type Ia supernovae are the most useful, precise, and mature
tools for determining astronomical distances. Acting as calibrated candles they
revealed the presence of dark energy and are being used to measure its
properties. However, the nature of the SN Ia explosion, and the progenitors
involved, have remained elusive, even after seven decades of research. But now
new large surveys are bringing about a paradigm shift --- we can finally
compare samples of hundreds of supernovae to isolate critical variables. As a
result of this, and advances in modeling, breakthroughs in understanding all
aspects of SNe Ia are finally starting to happen.Comment: Invited review for Nature Communications. Final published version.
Shortened, update
Outcome of anal symptoms and anorectal function following two obstetric anal sphincter injuries (OASIS)-a nested case-controlled study.
INTRODUCTION AND HYPOTHESIS: Obstetric anal sphincter injury (OASI) is a significant risk factor for developing anal incontinence. It can therefore be hypothesised that recurrent OASI in a subsequent delivery may predispose women to further anal sphincter dysfunction. METHODS: A nested case-controlled study based on data collected prospectively between 2006 and 2019. Women matched for age and ethnicity, with a history of one OASI and no sphincter damage in a subsequent delivery (control) were compared to women sustaining a second OASI. Assessment was carried out using the St Mark's score (SMIS), anorectal manometry and endoanal ultrasound scan (findings quantified using the modified Starck score). RESULTS: Eighty-four women were included and equally distributed between the two groups, who were followed up 12 weeks postnatally. No difference in SMIS scores was found. Maximum resting pressure (MRP, mmHg) and maximum squeeze pressure (MSP, mmHg) were significantly reduced in the study group. Median (IQR) MRP in the study group was 40.0 (31.3-54.0) versus 46.0 (39.3-61.5) in the control group (p = 0.030). Median (IQR) MSP was 73.0 (58.3-93.5) in the study group versus 92.5 (70.5-110.8) (p = 0.006) in the control group. A significant difference (p = 0.002) was found in the modified Starck score between the study group (median 0.0 [IQR 0.0-6.0]) and control group (median 0.0 [IQR 0.0-0.0]). CONCLUSIONS: We have demonstrated that women with recurrent OASI do not have significant anorectal symptoms compared to those with one OASI 12 weeks after delivery, but worse anal sphincter function and integrity. Therefore, on long-term follow-up, symptoms may possibly develop. This information will be useful when counselling women in a subsequent pregnancy
A hybrid type Ia supernova with an early flash triggered by helium-shell detonation.
Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen. The uniformity of their light curves makes these supernovae powerful cosmological distance indicators, but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved. For example, the recent detection of the early ultraviolet pulse of a peculiar, subluminous type Ia supernova has been claimed as evidence for an interaction between a red-giant or a main-sequence companion and ejecta from a white-dwarf explosion. Here we report observations of a prominent but red optical flash that appears about half a day after the explosion of a type Ia supernova. This supernova shows hybrid features of different supernova subclasses, namely a light curve that is typical of normal-brightness supernovae, but with strong titanium absorption, which is commonly seen in the spectra of subluminous ones. We argue that this early flash does not occur through previously suggested mechanisms such as the companion-ejecta interaction. Instead, our simulations show that it could occur through detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf, or on a sub-Chandrasekhar-mass white dwarf merging with a less-massive white dwarf. Our finding provides evidence that one branch of previously proposed explosion models-the helium-ignition branch-does exist in nature, and that such a model may account for the explosions of white dwarfs in a mass range wider than previously supposed
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