123 research outputs found
The interaction of core-collapse supernova ejecta with a stellar companion
The progenitors of many core-collapse supernovae (CCSNe) are expected to be
in binary systems. By performing a series of three-dimensional hydrodynamical
simulations, we investigate how CCSN explosions affect their binary companion.
We find that the amount of removed stellar mass, the resulting impact velocity,
and the chemical contamination of the companion that results from the impact of
the SN ejecta, strongly increases with decreasing binary separation and
increasing explosion energy. Also, it is foud that the impact effects of CCSN
ejecta on the structure of main-sequence (MS) companions, and thus their long
term post-explosion evolution, is in general not be dramatic.Comment: 5 pages, 3 figures, poster contribution: IAU Symposium 346 "High Mass
X-ray Binaries: illuminating the passage from massive binaries to merging
compact objects", Vienna, Austria, 27-31 August 2018. arXiv admin note:
substantial text overlap with arXiv:1509.0363
Confinement of the Sun's interior magnetic field: some exact boundary-layer solutions
High-latitude laminar confinement of the Sun's interior magnetic field is
shown to be possible, as originally proposed by Gough and McIntyre (1998) but
contrary to a recent claim by Brun and Zahn (A&A 2006). Mean downwelling as
weak as 2x10^-6cm/s -- gyroscopically pumped by turbulent stresses in the
overlying convection zone and/or tachocline -- can hold the field in
advective-diffusive balance within a confinement layer of thickness scale ~
1.5Mm ~ 0.002 x (solar radius) while transmitting a retrograde torque to the
Ferraro-constrained interior. The confinement layer sits at the base of the
high-latitude tachocline, near the top of the radiative envelope and just above
the `tachopause' marking the top of the helium settling layer. A family of
exact, laminar, frictionless, axisymmetric confinement-layer solutions is
obtained for uniform downwelling in the limit of strong rotation and
stratification. A scale analysis shows that the flow is dynamically stable and
the assumption of laminar flow realistic. The solution remains valid for
downwelling values of the order of 10^-5cm/s but not much larger. This suggests
that the confinement layer may be unable to accept a much larger mass
throughput. Such a restriction would imply an upper limit on possible internal
field strengths, perhaps of the order of hundreds of gauss, and would have
implications also for ventilation and lithium burning.
The solutions have interesting chirality properties not mentioned in the
paper owing to space restrictions, but described at
http://www.atmos-dynamics.damtp.cam.ac.uk/people/mem/papers/SQBO/solarfigure.htmlComment: 6 pages, 3 figures, to appear in conference proceedings: Unsolved
Problems in Stellar Physic
Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis
The stellar population in the Galactic halo is characterised by a large
fraction of CEMP stars. Most CEMP stars are enriched in -elements (CEMP-
stars), and some of these are also enriched in -elements (CEMP- stars).
One formation scenario proposed for CEMP stars invokes wind mass transfer in
the past from a TP-AGB primary star to a less massive companion star which is
presently observed. We generate low-metallicity populations of binary stars to
reproduce the observed CEMP-star fraction. In addition, we aim to constrain our
wind mass-transfer model and investigate under which conditions our synthetic
populations reproduce observed abundance distributions. We compare the CEMP
fractions and the abundance distributions determined from our synthetic
populations with observations. Several physical parameters of the binary
stellar population of the halo are uncertain, e.g. the initial mass function,
the mass-ratio and orbital-period distributions, and the binary fraction. We
vary the assumptions in our model about these parameters, as well as the wind
mass-transfer process, and study the consequent variations of our synthetic
CEMP population. The CEMP fractions calculated in our synthetic populations
vary between 7% and 17%, a range consistent with the CEMP fractions among very
metal-poor stars recently derived from the SDSS/SEGUE data sample. The results
of our comparison between the modelled and observed abundance distributions are
different for CEMP- stars and for CEMP- stars. For the latter, our
simulations qualitatively reproduce the observed distributions of C, Na, Sr,
Ba, Eu, and Pb. Contrarily, for CEMP- stars our model cannot reproduce the
large abundances of neutron-rich elements such as Ba, Eu, and Pb. This result
is consistent with previous studies, and suggests that CEMP- stars
experienced a different nucleosynthesis history to CEMP- stars.Comment: 17 pages, 11 figures, accepted for publication on Astronomy and
Astrophysic
Thermohaline mixing in low-mass giants: RGB and beyond
Thermohaline mixing has recently been proposed to occur in low mass red
giants, with large consequence for the chemical yields of low mass stars. We
investigate the role of thermohaline mixing during the evolution of stars
between 1 Msun and 3 Msun. We use a stellar evolution code which includes
rotational mixing and internal magnetic fields. We confirm that thermohaline
mixing has the potential to destroy most of the helium 3 which is produced
earlier on the main sequence during the red giant stage, in stars below
1.5Msun. We find this process to continue during core helium burning and
beyond. We find rotational and magnetic mixing to be negligible compared to the
thermohaline mixing in the relevant layers, even if the interaction of
thermohaline motions with the differential rotation may be essential to
establish the time scale of thermohaline mixing in red giants.Comment: Proceedings of the Conference "Unsolved problems in stellar physics"
- Cambridge, July 200
Does GD 356 have a Terrestrial Planetary Companion?
GD 356 is unique among magnetic white dwarfs because it shows Zeeman-split
Balmer lines in pure emission. The lines originate from a region of nearly
uniform field strength (delta B/B is approximately 0.1) that covers 10 per cent
of the stellar surface in which there is a temperature inversion. The energy
source that heats the photosphere remains a mystery but it is likely to be
associated with the presence of a companion. Based on current models we use
archival Spitzer IRAC observations to place a new and stringent upper limit of
12 Jupiter masses for the mass of such a companion. In the light of this result
and the recent discovery of a 115 min photometric period for GD 356, we exclude
previous models that invoke accretion and revisit the unipolar inductor model
that has been proposed for this system. In this model a highly conducting
planet with a metallic core orbits the magnetic white dwarf and, as it cuts
through field lines, a current is set flowing between the two bodies. This
current dissipates in the photosphere of the white dwarf and causes a
temperature inversion. Such a planet is unlikely to have survived the RGB/AGB
phases of evolution so we argue that it may have formed from the circumstellar
disc of a disrupted He or CO core during a rare merger of two white dwarfs. GD
356 would then be a white dwarf counterpart of the millisecond binary pulsar
PSR 1257+12 which is known to host a planetary system.Comment: 9 pages, 4 figures, accepted by MNRA
The Asymptotic Giant Branches of GCs: Selective Entry Only
The handful of available observations of AGB stars in Galactic Globular
Clusters suggest that the GC AGB populations are dominated by cyanogen-weak
stars. This contrasts strongly with the distributions in the RGB (and other)
populations, which generally show a 50:50 bimodality in CN band strength. If it
is true that the AGB populations show very different distributions then it
presents a serious problem for low mass stellar evolution theory, since such a
surface abundance change going from the RGB to AGB is not predicted by stellar
models. However this is only a tentative conclusion, since it is based on very
small AGB sample sizes. To test whether this problem really exists we have
carried out an observational campaign specifically targeting AGB stars in GCs.
We have obtained medium resolution spectra for about 250 AGB stars across 9
Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We
present some of the preliminary findings of the study for the second parameter
trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there
is a deficiency of stars with strong CN bands on the AGB. To confirm that this
phenomenon is robust and not just confined to CN band strengths and their
vagaries, we have made observations using FLAMES/VLT to measure elemental
abundances for NGC 6752.We present some initial results from this study also.
Our sodium abundance results show conclusively that only a subset of stars in
GCs experience the AGB phase of evolution. This is the first direct, concrete
confirmation of the phenomenon.Comment: 4 pages, to appear in conference proceedings of "Reading the book of
globular clusters with the lens of stellar evolution", Rome, 26-28 November
201
Health and healthcare for people with disabilities in the UK during the COVID-19 pandemic
Background:
While emerging evidence shows increased mortality from COVID-19 among people with disability, evidence regarding whether there are disability-related inequalities in health during the pandemic is lacking.
Objective:
This study compares access to COVID-19 and non-COVID-19 related health care and mental health of people with and without disability.
Methods:
Longitudinal analysis of 12,703 adults (16–64 years) who participated in W9 (2017–2019) and the April and/or May COVID-19 special surveys of the UK Understanding Society study. Descriptive analyses and Poisson regression (adjusted for age, gender, ethnicity and financial stress) were conducted to estimate associations between disability (measured at Wave 9) and a number of different COVID-19-related health and health care outcomes (COVID-19 symptoms, testing and hospitalisation), mental health and loneliness, and non-COVID-19 related health care (e.g. outpatient and inpatient hospital care, prescription medications).
Results:
Results from the fully-adjusted regression models found that people with disability were more likely: to be hospitalised if symptomatic (adjusted PRR 3.0 95% 1.07–8.43); to experience current symptoms of psychological distress (PRR 1.15, 95% CI 1.05–1.26) and to report being lonely (PRR 1.75, 95% CI 1.46–2.09) compared to non-disabled people. People with disability reported much higher levels of comorbidities than people without disability. However, inability to access health care and treatment were similar.
Conclusions:
As the UK opens up, it is important that health care services and social policy address the poor mental health and social isolation of people with disability so that the inequalities occurring early in the pandemic do not become further entrenched
Towards a unified model of stellar rotation II: Model-dependent characteristics of stellar populations
Rotation has a number of important effects on the evolution of stars. Apart
from structural changes because of the centrifugal force, turbulent mixing and
meridional circulation caused by rotation can dramatically affect a star's
chemical evolution. This leads to changes in the surface temperature and
luminosity as well as modifying its lifetime. Observationally rotation
decreases the surface gravity, causes enhanced mass loss and leads to surface
abundance anomalies of various chemical isotopes. The replication of these
physical effects with simple stellar evolution models is very difficult and has
resulted in the use of numerous different formulations to describe the physics.
Using stellar evolution calculations based on several physical models we
discuss the features of the resulting simulated stellar populations which can
help to distinguish between the models.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
Spindown of massive rotating stars
Models of rapidly rotating massive stars at low metallicities show
significantly different evolution and higher metal yields compared to
non-rotating stars. We estimate the spin-down time-scale of rapid rotating
non-convective stars supporting an alpha-Omega dynamo. The magnetic dynamo
gives rise to mass loss in a magnetically controlled stellar wind and hence
stellar spin down owing to loss of angular momentum. The dynamo is maintained
by strong horizontal rotation-driven turbulence which dominates over the Parker
instability. We calculate the spin-down time-scale and find that it could be
relatively short, a small fraction of the main-sequence lifetime. The spin-down
time-scale decreases dramatically for higher surface rotations suggesting that
rapid rotators may only exhibit such high surface velocities for a short time,
only a small fraction of their main-sequence lifetime.Comment: Accepted by MNRA
The changing of the guard: groupwork with people who have intellectual disabilities
This paper considers the impact of service systems on group activities. It describes an inter-professional groupwork project facilitated by a social worker and a community nurse. The project provided an emancipatory experience for a group of adults who had intellectual disabilities. The group was charged with the task of reviewing and updating the recruitment and interview processes used by a 'Learning Disability Partnership Board', when employing new support workers.
The paper begins with a brief history of intellectual disability and provides a context to the underpinning philosophical belief that people should be encouraged and supported to inhabit valued social roles no matter what disability they may have. It then identifies the ways in which the sponsoring health, education and social care services impacted on the creation and development of a groupwork project. It might have been expected that the nature of the intellectual disability would have been the major influence on group process. However the paper reveals that organisational constraints had a significant impact on group functioning. Issues including, staffing budgets and transport contracts impacted on group process and function.
The results of the project show how, with adequate support, people with intellectual disability can make important decisions that have long-reaching impacts on the services
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