172 research outputs found
Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
The carbon-enhanced metal-poor (CEMP) stars constitute approximately one
fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well
understood. The most widely accepted formation scenario, invokes mass-transfer
of carbon-rich material from a thermally-pulsing asymptotic giant branch
(TPAGB) primary star to a less massive main-sequence companion which is seen
today. Recent studies explore the possibility that an initial mass function
biased toward intermediate-mass stars is required to reproduce the observed
CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also
implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars
which is not seen. We investigate whether the observed CEMP and NEMP to
extremely metal-poor (EMP) ratios can be explained without invoking a change in
the initial mass function.
We confirm earlier findings that with current detailed TPAGB models the large
observed CEMP fraction cannot be accounted for. We find that efficient third
dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at
least a partial explanation to the large observed CEMP fraction while remaining
consistent with the small observed NEMP fraction.Comment: 20 pages, 23 figures, accepted for publication in A&
The Puzzling Frequencies of CEMP and NEMP Stars
We present the results of binary population simulations of carbon- and
nitrogen-enhanced metal-poor (CEMP and NEMP) stars. We show that the observed
paucity of very nitrogen-rich stars puts strong constraints on possible
modifications of the initial mass function at low metallicity.Comment: 3 pages, contribution to "The Origin of the Elements Heavier than
Iron" in honor of the 70th birthday of Roberto Gallino, Torino, Italy,
September 200
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
Critically rotating stars in binaries - an unsolved problem -
In close binaries mass and angular momentum can be transferred from one star
to the other during Roche-lobe overflow. The efficiency of this process is not
well understood and constitutes one of the largest uncertainties in binary
evolution.
One of the problems lies in the transfer of angular momentum, which will spin
up the accreting star. In very tight systems tidal friction can prevent
reaching critical rotation, by locking the spin period to the orbital period.
Accreting stars in systems with orbital periods larger than a few days reach
critical rotation after accreting only a fraction of their mass, unless there
is an effective mechanism to get rid of angular momentum. In low mass stars
magnetic field might help. In more massive stars angular momentum loss will be
accompanied by strong mass loss. This would imply that most interacting
binaries with initial orbital periods larger than a few days evolve very
non-conservatively.
In this contribution we wish to draw attention to the unsolved problems
related to mass and angular momentum transfer in binary systems. We do this by
presenting the first results of an implementation of spin up by accretion into
the TWIN version of the Eggleton stellar evolution code.Comment: 5 pages, 1 figure, to appear in the proceedings of the conference
"Unsolved Problems in Stellar Physics", Cambridge, 2-6 July 200
Policy implications, eligibility and demographic characteristics of people with intellectual disability who have access to self-directed funding in the United States
This study identifies factors (state of residence, personal characteristics, and living situation) associated with access to self-directed funding (SDF) for adults with intellectual disability in the United States. Data from 10,033 participants from 26 states in the 2012–2013 National Core Indicators Adult Consumer Survey were analyzed. We examined state, age group, residence type, disability diagnoses, mental health status, and type of disability support funding used. Availability of SDF for people with ID varied by state and aligned mostly with state-by-state policy data on SDF eligibility and availability. The results of a logistic regression analysis demonstrated that access to SDF was lower in older adults and higher for people who lived in their parents' or relatives' home, an independent home, and with certain personal characteristics. Potential influences from policy and practice, and approaches to increase access to SDF are discussed
Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars
We present the results of binary population simulations of carbon-enhanced
metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers
of the origin of CEMP stars, and conclude that the observed paucity of very
nitrogen-rich stars puts strong constraints on possible modifications of the
initial mass function at low metallicity. The large number fraction of CEMP
stars may instead require much more efficient dredge-up from low-metallicity
asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252
"The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya,
Chin
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
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