ABSTRACT
The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars that produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be ...formed primarily through white dwarf mergers. In this paper, we use mesa to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 to 5.4 × 108 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 × 108 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10 per cent solar metallicity models, with the exception of carbon and lithium that are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. Additionally, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.
ABSTRACT The importance of νmax (the frequency of maximum oscillation power) for asteroseismology has been demonstrated widely in the previous decade, especially for red giants. With the large amount ...of photometric data from CoRoT (Convection, Rotation, and planetary Transits), Kepler, and Transiting Exoplanet Survey Satellite, several automated algorithms to retrieve νmax values have been introduced. Most of these algorithms correct the granulation background in the power spectrum by fitting a model and subtracting it before measuring νmax. We have developed a method that does not require fitting to the granulation background. Instead, we simply divide the power spectrum by a function of the form $\rm \nu ^{-2}$, to remove the slope due to granulation background, and then smooth to measure νmax. This method is fast and simple and avoids degeneracies associated with fitting. The method is able to measure oscillations in 99.9 per cent of previously studied Kepler red giants, with a systematic offset of 1.5 per cent in νmax values that we are able to calibrate. On comparing the seismic radii from this work with Gaia, we see similar trends to those observed in previous studies. Additionally, our values of width of the power envelope can clearly identify the dipole mode suppressed stars as a distinct population, hence as a way to detect them. We also applied our method to stars with low νmax (0.19–18.35 µHz) and found that it works well to correctly identify the oscillations.
ABSTRACT
When low- and intermediate-mass stars evolve off the main sequence, they expand and cool into the red giant stages of evolution, which include those associated with shell H burning (the red ...giant branch), core He burning (the red clump), and shell He burning (the asymptotic giant branch). The majority of red giants have masses <2 M⊙, and red giants more massive than this are often excluded from major studies. Here, we present a study of the highest mass stars (M > 3.0 M⊙) in the Kepler sample of 16 000 red giants. We begin by re-estimating their global seismic properties with new light curves, highlighting the differences between using the simple aperture photometry and presearch data conditioning of simple aperture photometry light curves provided by Kepler. We use the re-estimated properties to derive new mass estimates for the stars, ending with a final sample of 48 confirmed high-mass stars. We explore their oscillation envelopes, confirming the trends found in recent works such as low mean mode amplitude and wide envelopes. We find, through probabilistic means, that our sample is likely all core He burning stars. We measure their dipole and quadrupole mode visibilities and confirm that the dipole mode visibility tends to decrease with mass.
ABSTRACT
Stellar spectral classification has been highly useful in the study of stars. While there is a currently accepted spectral classification system for carbon stars, the subset of ...hydrogen-deficient carbon (HdC) stars has not been well described by such a system, due predominantly to their rarity and their variability. Here we present the first system for the classification of HdCs based on their spectra, which is made wholly on their observable appearance. We use a combination of dimensionality reduction and clustering algorithms with human classification to create such a system with eight total classes corresponding to temperature, and an additional second axis corresponding to the carbon molecular band strength. We classify over half of the known sample of HdC stars using this, and roughly calibrate the temperatures of each class using their colours. Additionally, we express trends in the occurrence of certain spectral peculiarities such as the presence of hydrogen and lithium lines. We also present three previously unpublished spectra, report the discovery of two new Galactic dustless HdC stars, and additionally discuss one especially unique star that appears to border between the hottest HdCs and the coolest extreme helium stars.
Abstract
R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich variable supergiants thought to be the result of dynamically unstable white dwarf mergers. We attempt to model RCB ...stars through all the relevant timescales by simulating a merger event in
Octo-tiger
, a 3D adaptive mesh refinement (AMR) hydrodynamics code, and mapping the post-merger object into MESA, a 1D stellar evolution code. We then post-process the nucleosynthesis on a much larger nuclear reaction network to study the enhancement of
s
-process elements. We present models that match observations or previous studies in most surface abundances, isotopic ratios, early evolution, and lifetimes. We also observe similar mixing behavior to previous modeling attempts that result in the partial He-burning products visible on the surface in observations. However, we do note that our subsolar models lack any enhancement in
s
-process elements, which we attribute to a lack of hydrogen in the envelope. We also find that the
16
O/
18
O isotopic ratio is very sensitive to initial hydrogen abundance and increases outside of the acceptable range with a hydrogen mass fraction greater than 10
−4
.
The Epilepsy Bioinformatics Study for Anti-epileptogenic Therapy (EpiBioS4Rx) is a longitudinal prospective observational study funded by the National Institute of Health (NIH) to discover and ...validate observational biomarkers of epileptogenesis after traumatic brain injury (TBI). A multidisciplinary approach has been incorporated to investigate acute electrical, neuroanatomical, and blood biomarkers after TBI that may predict the development of post-traumatic epilepsy (PTE). We plan to enroll 300 moderate-severe TBI patients with a frontal and/or temporal lobe hemorrhagic contusion. Acute evaluation with blood, imaging and electroencephalographic monitoring will be performed and then patients will be tracked for 2 years to determine the incidence of PTE. Validation of selected biomarkers that are discovered in planned animal models will be a principal feature of this work. Specific hypotheses regarding the discovery of biomarkers have been set forth in this study. An international cohort of 13 centers spanning 2 continents will be developed to facilitate this study, and for future interventional studies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Interpreting a failure to replicate is complicated by the fact that the failure could be due to the original finding being a false positive, unrecognized moderating influences between the original ...and replication procedures, or faulty implementation of the procedures in the replication. One strategy to maximize replication quality is involving the original authors in study design. We (N = 17 Labs and N = 1,550 participants, after exclusions) experimentally tested whether original author involvement improved replicability of a classic finding from Terror Management Theory (Greenberg et al., 1994). Our results were non-diagnostic of whether original author involvement improves replicability because we were unable to replicate the finding under any conditions. This suggests that the original finding was either a false positive or the conditions necessary to obtain it are not fully understood or no longer exist. Data, materials, analysis code, preregistration, and supplementary documents can be found on the OSF page: https://osf.io/8ccnw/
Context.
R Coronae Borealis (RCB) variables and their non-variable counterparts, the dustless Hydrogen-deficient Carbon (dLHdC) stars have been known to exhibit enhanced
s
-processed material on ...their surfaces, especially Sr, Y, and Ba. No comprehensive work has been done to explore the
s
-process in these types of stars, however one particular RCB star, U Aqr, has been under scrutiny for its extraordinary Sr enhancement.
Aims.
We aim to identify RCB and dLHdC stars that have significantly enhanced Sr abundances, such as U Aqr, and use stellar evolution models to begin to estimate the type of neutron exposure that occurs in a typical HdC star.
Methods.
We compared the strength of the Sr II 4077 Å spectral line to Ca II H to identify the new subclass of Sr-rich HdCs. We additionally used the structural and abundance information from existing RCB MESA models to calculate the neutron exposure parameter, τ.
Results.
We identified six stars in the Sr-rich class. Two are RCBs, and four are dLHdCs. We additionally found that the preferred RCB MESA model has a neutron exposure τ ≃ 0.1 mb
−1
, which is lower than the estimated τ between 0.15 and 0.6 mb
−1
for the Sr-rich star U Aqr found in the literature. We found trends in the neutron exposure corresponding to He-burning shell temperature, metallicity, and assumed
s
-processing site.
Conclusions.
We have found a sub-class of six HdCs known as the Sr-rich class, which tend to lie in the halo, outside the typical distribution of RCBs and dLHdCs. We found that dLHdC stars are more likely to be Sr-rich than RCBs, with an occurrence rate of ~13% for dLHdCs and ~2% for RCBs. This is one of the first potential spectroscopic differences between RCBs and dLHdCs, along with dLHdCs having stronger surface abundances of
18
O. We additionally found neutron exposure trends in our RCB models that will aide in understanding the interplay between model parameters and surface
s
-process elements.
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