274 research outputs found
The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
It is now widely accepted that globular cluster red giant branch stars owe
their strange abundance patterns to a combination of pollution from progenitor
stars and in situ extra mixing. In this hybrid theory a first generation of
stars imprint abundance patterns into the gas from which a second generation
forms. The hybrid theory suggests that extra mixing is operating in both
populations and we use the variation of [C/Fe] with luminosity to examine how
efficient this mixing is. We investigate the observed red giant branches of M3,
M13, M92, M15 and NGC 5466 as a means to test a theory of thermohaline mixing.
The second parameter pair M3 and M13 are of intermediate metallicity and our
models are able to account for the evolution of carbon along the RGB in both
clusters. Although, in order to fit the most carbon-depleted main-sequence
stars in M13 we require a model whose initial [C/Fe] abundance leads to a
carbon abundance lower than is observed. Furthermore our results suggest that
stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3).
In the metal-poor regime only NGC 5466 can be tentatively explained by
thermohaline mixing operating in multiple populations. We find thermohaline
mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15.
It appears as if extra mixing is occurring before the luminosity function bump
in these clusters. To reconcile the data with the models would require first
dredge-up to be deeper than found in extant models.Comment: 13 Pages, 3 figures. Accepted for publication in the Astrophysical
Journa
From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries
Stellar models which incorporate simple diffusion or shear induced mixing are
used to describe canonical extra mixing in low mass red giants of low and solar
metallicity. These models are able to simultaneously explain the observed Li
and CN abundance changes along upper red giant branch (RGB) in field
low-metallicity stars and match photometry, rotation and carbon isotopic ratios
for stars in the old open cluster M67. The shear mixing model requires that
main sequence (MS) progenitors of upper RGB stars possessed rapidly rotating
radiative cores and that specific angular momentum was conserved in each of
their mass shells during their evolution. We surmise that solar-type stars will
not experience canonical extra mixing on the RGB because their more efficient
MS spin-down resulted in solid-body rotation, as revealed by helioseismological
data for the Sun. Thus, RGB stars in the old, high metallicity cluster NGC 6791
should show no evidence for mixing in their carbon isotopic ratios.
We develop the idea that canonical extra mixing in a giant component of a
binary system may be switched to its enhanced mode with much faster and
somewhat deeper mixing as a result of the giant's tidal spin-up. This scenario
can explain photometric and composition peculiarities of RS CVn binaries. The
tidally enforced enhanced extra mixing might contribute to the star-to-star
abundance variations of O, Na and Al in globular clusters. This idea may be
tested with observations of carbon isotopic ratios and CN abundances in RS CVn
binaries.Comment: 47 pages, 19 figures, accepted for publication in Ap
Effects of rotation and magnetic fields on the lithium abundance and asteroseismic properties of exoplanet-host stars
Aims: The effects of rotation and magnetic fields on the surface abundances
of solar-type stars are studied in order to investigate whether the reported
difference in lithium content of exoplanet-host stars can be related to their
rotational history. Moreover, the asteroseismic properties predicted for stars
with and without exoplanets are compared to determine how such a scenario,
which relates the lithium abundances and the rotational history of the star,
can be further challenged by observations of solar-like oscillations. Methods:
Based on observations of rotational periods of solar-type stars, slow rotators
on the zero age main sequence (ZAMS) are modelled with a comprehensive
treatment of only the shellular rotation, while fast rotators are modelled
including both shellular rotation and magnetic fields. Assuming a possible link
between low rotation rates on the ZAMS and the presence of planets as a result
of a longer disc-locking phase during the pre-main sequence (PMS), we compare
the surface abundances and asteroseismic properties of slow and fast rotating
models, which correspond to exoplanet-host stars and stars without detected
planets, respectively. Results: We confirm previous suggestions that the
difference in the lithium content of stars with and without detected planets
can be related to their different rotational history. The larger efficiency of
rotational mixing predicted in exoplanet-host stars explains their lithium
depletion and also leads to changes in the structure and chemical composition
of the central stellar layers. Asteroseismic observations can reveal these
changes and can help us distinguish between different possible explanations for
the lower lithium content of exoplanet-host stars.Comment: 4 pages, 4 figures, A&A lette
Hydra Observations of Aluminum Abundances in the Red Giants of the Globular Clusters M80 and NGC 6752
Aluminum and other metal abundances were determined in 21 red giants in the
globular clusters NGC 6752 and M80 as part of a larger study to determine
whether the aluminum distribution on the red giant branch is related to the
second parameter effect that causes clusters of similar metallicity to display
different horizontal branch morphologies. The observations were obtained of the
Al I lines near 6700 Angstroms with the CTIO Blanco 4-m telescope and Hydra
multi-object spectrograph. The spectra have a resolving power of 18000 or 9400,
with typical S/N ratios of 100-200. Mean [Fe/H] values obtained from the
spectra are -1.58 for NGC 6752 and -1.73 for M80; this represents the first
spectroscopic iron abundance determination for M80. Both NGC 6752 and M80
display a spread in aluminum abundance, with mean [Al/Fe] ratios of +0.51 and
+0.37, respectively. No trend in the variation of the mean Al abundance with
position on the giant branch is discernible in either cluster with our small
sample.Comment: 29 pages, 4 figures, Accepted to AJ, uses aastex V5.
The Abundance Evolution of Oxygen, Sodium and Magnesium in Extremely Metal-Poor Intermediate Mass Stars: Implications for the Self-Polution Scenario in Globular Clusters
We present full stellar evolution and parametric models of the surface
abundance evolution of O16, Ne22, Na23 and the magnesium isotopes in an
extremely metal-poor intermediate mass star M_ZAMS=5M_sun, Z=0.0001. O16 and
Ne22 are injected into the envelope by the third dredge-up following thermal
pulses on the asymptotic giant branch. These species and the initially present
Mg24 are depleted by hot bottom burning (HBB) during the interpulse phase. As a
result, Na23, Mg25 and Mg26 are enhanced. If the HBB temperatures are
sufficiently high for this process to deplete oxygen efficiently, Na23 is first
produced and then depleted during the interpulse phase. Although the
simultaneous depletion of O16 and enhancement of Na23 is possible, the required
fine tuning of the dredge-up and HBB casts some doubt on the robustness of this
process as the origin of the O-Na anti-correlation observed in globular cluster
stars. However, a very robust prediction of our models are low Mg24/Mg25 and
Mg24/Mg26 ratios whenever significant O16 depletion can be achieved. This seems
to be in stark contrast with recent observations of the magnesium isotopic
ratios in the globular cluster NGC6752.Comment: ApJ Letters, in pres
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