45 research outputs found
Anisotropic mass ejection in binary mergers
We investigate the mass loss from a rotationally distorted envelope following
the early, rapid in-spiral of a companion star inside a common envelope. For
initially wide, massive binaries (M_1+M_2=20M_{\odot}, P\sim 10 yr), the
primary has a convective envelope at the onset of mass transfer and is able to
store much of the available orbital angular momentum in its expanded envelope.
Three-dimensional smoothed particle hydrodynamics calculations show that mass
loss is enhanced at mid-latitudes due to shock reflection from a torus-shaped
outer envelope. Mass ejection in the equatorial plane is completely suppressed
if the shock wave is too weak to penetrate the outer envelope in the equatorial
direction (typically when the energy deposited in the star is less than about
one-third of the binding energy of the envelope). We present a parameter study
to show how the geometry of the ejecta depends on the angular momentum and the
energy deposited in the envelope during a merging event. Applications to the
nearly axisymmetric, but very non-spherical nebulae around SN1987A and Sheridan
25 are discussed, as well as possible links to RY Scuti and the Small
Magellanic Cloud object R4.Comment: 10 pages, 11 figures, accepted for publication in MNRAS. Figs 1, 2
and 10 reduced in siz
Experimental investigation of the corrosion behavior of Eurofer97 steel in contact with Lithium ceramic breeder pebbles under specific Helium Cooled Pebble Bed breeding zone atmosphere
Gamma-Ray Bursts from tidally spun-up Wolf-Rayet stars?
The collapsar model requires rapidly rotating Wolf-Rayet stars as progenitors
of long gamma-ray bursts. However, Galactic Wolf-Rayet stars rapidly lose
angular momentum due to their intense stellar winds. We investigate whether the
tidal interaction of a Wolf-Rayet star with a compact object in a binary system
can spin up the Wolf-Rayet star enough to produce a collapsar. We compute the
evolution of close Wolf-Rayet binaries, including tidal angular momentum
exchange, differential rotation of the Wolf-Rayet star, internal magnetic
fields, stellar wind mass loss, and mass transfer. The Wolf-Rayet companion is
approximated as a point mass. We then employ a population synthesis code to
infer the occurrence rates of the various relevant binary evolution channels.
We find that the simple scenario -- i.e., the Wolf-Rayet star being tidally
spun up and producing a collapsar -- does not occur at solar metallicity and
may only occur with low probability at low metallicity. It is limited by the
widening of the binary orbit induced by the strong Wolf-Rayet wind or by the
radius evolution of the Wolf-Rayet star that most often leads to a binary
merger. The tidal effects enhance the merger rate of Wolf-Rayet stars with
black holes such that it becomes comparable to the occurrence rate of long
gamma-ray bursts.Comment: 9 pages, 11 figures, accepted for publication in A&
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio
Gravitational waves from intermediate-mass black holes in young clusters
Massive young clusters (YCs) are expected to host intermediate-mass black
holes (IMBHs) born via runaway collapse. These IMBHs are likely in binaries and
can undergo mergers with other compact objects, such as stellar mass black
holes (BHs) and neutron stars (NSs). We derive the frequency of such mergers
starting from information available in the Local Universe. Mergers of IMBH-NS
and IMBH-BH binaries are sources of gravitational waves (GWs), which might
allow us to reveal the presence of IMBHs. We thus examine their detectability
by current and future GW observatories, both ground- and space-based. In
particular, as representative of different classes of instruments we consider
Initial and Advanced LIGO, the Einstein gravitational-wave Telescope (ET) and
the Laser Interferometer Space Antenna (LISA). We find that IMBH mergers are
unlikely to be detected with instruments operating at the current sensitivity
(Initial LIGO). LISA detections are disfavored by the mass range of IMBH-NS and
IMBH-BH binaries: less than one event per year is expected to be observed by
such instrument. Advanced LIGO is expected to observe a few merger events
involving IMBH binaries in a 1-year long observation. Advanced LIGO is
particularly suited for mergers of relatively light IMBHs (~100 Msun) with
stellar mass BHs. The number of mergers detectable with ET is much larger: tens
(hundreds) of IMBH-NS (IMBH-BH) mergers might be observed per year, according
to the runaway collapse scenario for the formation of IMBHs. We note that our
results are affected by large uncertainties, produced by poor observational
constraints on many of the physical processes involved in this study, such as
the evolution of the YC density with redshift.[abridged]Comment: 29 pages, 4 figures, accepted for publication in Ap
Measuring the value of placements to employers: A cost-benefit approach
This article explores the concept and measurement of placement value, underexplored in theory and practice to date. The article makes a theoretical contribution to the placement value discourse by examining and articulating the placement value concept. It also offers a practical contribution by exploring a piloted tool to evaluate employer placement value, developed as part of a project funded by the Higher Education Funding Council for England. It examines the immaturity of the placement value concept against contemporary value discourse, including service- and goods-dominant logic frameworks (exploring value-in-use and value-in-exchange) and calls for greater attention to be paid to placement value to support the sustainable provision of placements.N/
Acoustic and optical variations during rapid downward motion episodes in the deep north-western Mediterranean Sea
An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site
of the ANTARES neutrino telescope near Toulon, France, thus providing a unique
opportunity to compare high-resolution acoustic and optical observations
between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward
vertical currents of magnitudes up to 0.03 m s-1 in late winter and early
spring 2006. In the same period, observations were made of enhanced levels of
acoustic reflection, interpreted as suspended particles including zooplankton,
by a factor of about 10 and of horizontal currents reaching 0.35 m s-1. These
observations coincided with high light levels detected by the telescope,
interpreted as increased bioluminescence. During winter 2006 deep dense-water
formation occurred in the Ligurian subbasin, thus providing a possible
explanation for these observations. However, the 10-20 days quasi-periodic
episodes of high levels of acoustic reflection, light and large vertical
currents continuing into the summer are not direct evidence of this process. It
is hypothesized that the main process allowing for suspended material to be
moved vertically later in the year is local advection, linked with topographic
boundary current instabilities along the rim of the 'Northern Current'.Comment: 30 pages, 7 figure
Functional Characterization of the HuR:CD83 mRNA Interaction
Maturation of dendritic cells (DC) is characterized by expression of CD83, a surface protein that appears to be necessary for the effective activation of naïve T-cells and T-helper cells by DC. Lately it was shown that CD83 expression is regulated on the posttranscriptional level by interaction of the shuttle protein HuR with a novel posttranscriptional regulatory RNA element (PRE), which is located in the coding region of the CD83 transcript. Interestingly, this interaction commits the CD83 mRNA to efficient nuclear export via the CRM1 pathway. To date, however, the structural basis of this interaction, which potentially involves three distinct RNA recognition motifs (RRM1–3) in HuR and a complex three-pronged RNA stem-loop element in CD83 mRNA, has not been investigated in detail. In the present work we analyzed this interaction in vitro and in vivo using various HuR- and CD83 mRNA mutants. We are able to demonstrate that both, RRM1 and RRM2 are crucial for binding, whereas RRM3 as well as the HuR hinge region contributed only marginally to this protein∶RNA interaction. Furthermore, mutation of uridine rich patches within the PRE did not disturb HuR:CD83 mRNA complex formation while, in contrast, the deletion of specific PRE subfragments from the CD83 mRNA prevented HuR binding in vitro and in vivo. Interestingly, the observed inhibition of HuR binding to CD83 mRNA does not lead to a nuclear trapping of the transcript but rather redirected this transcript from the CRM1- towards the NXF1/TAP-specific nuclear export pathway. Thus, the presence of a functional PRE permits nucleocytoplasmic trafficking of the CD83 transcript via the CRM1 pathway
Differences in the genetic control of early egg development and reproduction between C. elegans and its parthenogenetic relative D. coronatus
Rotating Stars in Relativity
Rotating relativistic stars have been studied extensively in recent years,
both theoretically and observationally, because of the information one could
obtain about the equation of state of matter at extremely high densities and
because they are considered to be promising sources of gravitational waves. The
latest theoretical understanding of rotating stars in relativity is reviewed in
this updated article. The sections on the equilibrium properties and on the
nonaxisymmetric instabilities in f-modes and r-modes have been updated and
several new sections have been added on analytic solutions for the exterior
spacetime, rotating stars in LMXBs, rotating strange stars, and on rotating
stars in numerical relativity.Comment: 101 pages, 18 figures. The full online-readable version of this
article, including several animations, will be published in Living Reviews in
Relativity at http://www.livingreviews.org