372 research outputs found
The treatment of mixing in core helium burning models -- III. Suppressing core breathing pulses with a new constraint on overshoot
Theoretical predictions for the core helium burning phase of stellar
evolution are highly sensitive to the uncertain treatment of mixing at
convective boundaries. In the last few years, interest in constraining the
uncertain structure of their deep interiors has been renewed by insights from
asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of
growth of helium-burning convective cores based on the higher buoyancy of
material ingested from outside the convective core. In this paper we test the
implications of such a limit for stellar models with a range of initial mass
and metallicity. We find that the constraint on mixing beyond the Schwarzschild
boundary has a significant effect on the evolution late in core helium burning,
when core breathing pulses occur and the ingestion rate of helium is fastest.
Ordinarily, core breathing pulses prolong the core helium burning lifetime to
such an extent that models are at odds with observations of globular cluster
populations. Across a wide range of initial stellar masses (), applying the Spruit constraint reduces the core
helium burning lifetime because core breathing pulses are either avoided or
their number and severity reduced. The constraint suggested by Spruit therefore
helps to resolve significant discrepancies between observations and theoretical
predictions. Specifically, we find improved agreement for , the observed
ratio of asymptotic giant branch to horizontal branch stars in globular
clusters; the luminosity difference between these two groups; and in
asteroseismology, the mixed-mode period spacing detected in red clump stars in
the \textit{Kepler} field.Comment: Accepted for publication in MNRAS; 11 pages, 6 figure
Diagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters
We conduct a series of comparisons between spectroscopic and photometric
observations of globular clusters and stellar models to examine their
predictive power. Data from medium-to-high resolution spectroscopic surveys of
lithium allow us to investigate first dredge-up and extra mixing in two
clusters well separated in metallicity. Abundances at first dredge-up are
satisfactorily reproduced but there is preliminary evidence to suggest that the
models overestimate the luminosity at which the surface composition first
changes in the lowest-metallicity system. Our models also begin extra mixing at
luminosities that are too high, demonstrating a significant discrepancy with
observations at low metallicity. We model the abundance changes during extra
mixing as a thermohaline process and determine that the usual diffusive form of
this mechanism cannot simultaneously reproduce both the carbon and lithium
observations. Hubble Space Telescope photometry provides turnoff and bump
magnitudes in a large number of globular clusters and offers the opportunity to
better test stellar modelling as function of metallicity. We directly compare
the predicted main-sequence turn-off and bump magnitudes as well as the
distance-independent parameter . We
require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some
clusters and cannot match the bump in low-metallicity systems. Changes to the
distance modulus, metallicity scale and bolometric corrections may impact on
the direct comparisons but , which is
also underestimated from the models, can only be improved through changes to
the input physics. Overshooting at the base of the convective envelope with an
efficiency that is metallicity dependent is required to reproduce the
empirically determined value of .Comment: 20 pages, 11 Figures, 4 Tables, Accepted for publication in MNRA
The treatment of mixing in core helium burning models - I. Implications for asteroseismology
The detection of mixed oscillation modes offers a unique insight into the
internal structure of core helium burning (CHeB) stars. The stellar structure
during CHeB is very uncertain because the growth of the convective core, and/or
the development of a semiconvection zone, is critically dependent on the
treatment of convective boundaries. In this study we calculate a suite of
stellar structure models and their non-radial pulsations to investigate why the
predicted asymptotic g-mode period spacing is
systematically lower than is inferred from Kepler field stars. We find that
only models with large convective cores, such as those calculated with our
newly proposed "maximal-overshoot" scheme, can match the average
reported. However, we also find another possible solution that is related to
the method used to determine : mode trapping can raise the
observationally inferred well above its true value. Even after
accounting for these two proposed resolutions to the discrepancy in average
, models still predict more CHeB stars with low ( s) than are observed. We establish two possible remedies for this: i)
there may be a difficulty in determining for early CHeB stars
(when is lowest) because of the effect that the sharp composition
profile at the hydrogen burning shell has on the pulsations, or ii) the mass of
the helium core at the flash is higher than predicted. Our conclusions
highlight the need for the reporting of selection effects in asteroseismic
population studies in order to safely use this information to constrain stellar
evolution theory.Comment: 24 pages. 24 figures. Published in MNRA
Redefining the endophenotype concept to accommodate transdiagnostic vulnerabilities and etiological complexity
In psychopathology research, endophenotypes are a subset of biomarkers that indicate genetic vulnerability independent of clinical state. To date, an explicit expectation is that endophenotypes be specific to single disorders. We evaluate this expectation considering recent advances in psychiatric genetics, recognition that transdiagnostic vulnerability traits are often more useful than clinical diagnoses in psychiatric genetics, and appreciation for etiological complexity across genetic, neural, hormonal and environmental levels of analysis. We suggest that the disorder-specificity requirement of endophenotypes be relaxed, that neural functions are preferable to behaviors as starting points in searches for endophenotypes, and that future research should focus on interactive effects of multiple endophenotypes on complex psychiatric disorders, some of which are \u27phenocopies\u27 with distinct etiologies
22q11.2 duplication: A review of neuropsychiatric correlates and a newly observed case of prototypic sociopathy
Callous-unemotional (CU) traits are highly disabling behavioral characteristics, common predictors of delinquency and criminality, and pathognomonic for antisocial personality disorder. They are highly heritable, but their specific molecular genetic causes are unknown. Here, we briefly review the literature on neuropsychiatric correlates of 22q11.2 duplication and describe a newly identified case of a 737-kb microduplication within the low copy repeat (LCR) B-D region, involving a 13-yr-old early adoptee with mild developmental delay and severe, chronic antisocial behavior of early childhood onset. When psychiatric symptoms have been reported in relation to duplications in this specific region, 19% of the reports feature aggression-but never previously CU traits-as a component of the phenotype. We discuss the potential implications of gain of function in this chromosomal region for heritable origins of sociopathy and their possible relation to genetic influences on aggression
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