821 research outputs found
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
Thermohaline instability and rotation-induced mixing II- Yields of 3He for low- and intermediate-mass stars
Context. The 3He content of Galactic HII regions is very close to that of the
Sun and the solar system, and only slightly higher than the primordial 3He
abundance as predicted by the standard Big Bang nucleosynthesis. However, the
classical theory of stellar evolution predicts a high production of 3He by
low-mass stars, implying a strong increase of 3He with time in the Galaxy. This
is the well-known "3He problem". Aims. We study the effects of thermohaline and
rotation-induced mixings on the production and destruction of 3He over the
lifetime of low- and intermediate-mass stars at various metallicities. Methods.
We compute stellar evolutionary models in the mass range 1 to 6M\odot for four
metallicities, taking into account thermohaline instability and
rotation-induced mixing. For the thermohaline diffusivity we use the
prescription based on the linear stability analysis, which reproduces Red Giant
Branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing
is treated taking into account meridional circulation and shear turbulence. We
discuss the effects of these processes on internal and surface abundances of
3He and on the net yields. Results. Over the whole mass and metallicity range
investigated, rotation-induced mixing lowers the 3He production, as well as the
upper mass limit at which stars destroy 3He. For low-mass stars, thermohaline
mixing occuring beyond the RGB bump is the dominant process in strongly
reducing the net 3He yield compared to standard computations. Yet these stars
remain net 3He producers. Conclusions. Overall, the net 3He yields are strongly
reduced compared to the standard framework predictions
Inhibition of thermohaline mixing by a magnetic field in Ap star descendants: Implications for the Galactic evolution of 3He
To reconcile the measurements of 3He/H in Galactic HII regions with high
values of 3He in a couple of planetary nebulae, we propose that thermohaline
mixing is inhibited by a fossil magnetic field in red giant stars that are
descendants of Ap stars. We examine the effect of a magnetic field on the
salt-finger instability, using a local analysis. We obtain a threshold for the
magnetic field of 10^4 - 10^5 Gauss, above which it inhibits thermohaline
mixing in red giant stars located at or above the bump. Fields of that order
are expected in the descendants of the Ap stars, taking into account the
contraction of their core. We conclude that in a large fraction of the
descendants of Ap stars thermohaline mixing does not occur. As a consequence
these objects must produce 3He as predicted by the standard theory of stellar
evolution and as observed in the planetary nebulae NGC3242 and J320. The
relative number of such stars with respect to non-magnetic objects that undergo
thermohaline mixing is consistent with the statistical constraint coming from
observations of the carbon isotopic ratio in red giant stars. It also satisfies
the Galactic requirements for the evolution of the 3He abundance.Comment: Accepted for publication in A&A Letters (Vol.476
New Evolutionary Synthesis code. An application to the irregular galaxy NGC 1560
We have developed a new evolutionary synthesis code, which incorporates the
output from chemical evolution models. We compare results of this new code with
other published codes, and we apply it to the irregular galaxy NGC 1560 using
sophisticated chemical evolution models. The code makes important contributions
in two areas: a) the building of synthetic populations with time-dependent star
formation rates and stellar populations of different metallicities; b) the
extension of the set of stellar tracks from the Geneva group by adding the AGB
phases for as well as the very low mass stars. Our code
predicts spectra, broad band colors, and Lick indices by using a spectra
library, which cover a more complete grid of stellar parameters. The
application of the code with the chemical models to the galaxy NGC 1560
constrain the star formation age for its stellar population around 10.0 Gy.Comment: 10 pages, 15 figures, submited to A&
Implications of a Sub-Threshold Resonance for Stellar Beryllium Depletion
Abundance measurements of the light elements lithium, beryllium, and boron
are playing an increasingly important role in the study of stellar physics.
Because these elements are easily destroyed in stars at temperatures 2--4
million K, the abundances in the surface convective zone are diagnostics of the
star's internal workings. Standard stellar models cannot explain depletion
patterns observed in low mass stars, and so are not accounting for all the
relevant physical processes. These processes have important implications for
stellar evolution and primordial lithium production in big bang
nucleosynthesis. Because beryllium is destroyed at slightly higher temperatures
than lithium, observations of both light elements can differentiate between the
various proposed depletion mechanisms. Unfortunately, the reaction rate for the
main destruction channel, 9Be(p,alpha)6Li, is uncertain. A level in the
compound nucleus 10B is only 25.7 keV below the reaction's energetic threshold.
The angular momentum and parity of this level are not well known; current
estimates indicate that the resonance entrance channel is either s- or d-wave.
We show that an s-wave resonance can easily increase the reaction rate by an
order of magnitude at temperatures of approximately 4 million K. Observations
of sub-solar mass stars can constrain the strength of the resonance, as can
experimental measurements at lab energies lower than 30 keV.Comment: 9 pages, 1 ps figure, uses AASTeX macros and epsfig.sty. Reference
added, typos corrected. To appear in ApJ, 10 March 199
Gemini spectroscopy of the outer disk star cluster BH176
BH176 is an old metal-rich star cluster. It is spatially and kinematically
consistent with belonging to the Monoceros Ring. It is larger in size and more
distant from the Galactic plane than typical open clusters, and it does not
belong to the Galactic bulge. Our aim is to determine the origin of this unique
object by accurately determining its distance, metallicity, and age. The best
way to reach this goal is to combine spectroscopic and photometric methods. We
present medium-resolution observations of red clump and red giant branch stars
in BH176 obtained with the Gemini South Multi-Object Spectrograph.We derive
radial velocities, metallicities, effective temperatures, and surface gravities
of the observed stars and use these parameters to distinguish member stars from
field objects. We determine the following parameters for BH176:
km/s, , age Gyr, , distance
kpc, -element abundance dex (the
mean of [Mg/Fe], and [Ca/Fe]). BH176 is a member of old Galactic open clusters
that presumably belong to the thick disk. It may have originated as a massive
star cluster after the encounter of the forming thin disk with a high-velocity
gas cloud or as a satellite dwarf galaxy.Comment: 15 pages, 7 fufures, Accepted for publication in Astronomy &
Astrophysic
A consistent explanation for C/C, Li, and He anomalies in red giant stars
The observations of carbon isotopic ratios in evolved stars suggest that non
standard mixing is acting in low mass stars as they are ascending the red giant
branch. We propose a simple consistent mechanism, based on the most recent
developments in the description of rotation-induced mixing by Zahn (1992),
which simultaneously accounts for the low C/C ratios in globular
cluster and field Pop II giants and for the lithium abundances in metal-poor
giant stars. It also leads to the destruction of He produced on the main
sequence in low mass stars. This should both naturally account for the recent
measurements of He/H in galactic HII regions and allow for high values of
He observed in some planetary nebulae.Comment: 3 pages plus 2 figures, uses aaspp.sty; offprint requests to :
[email protected]
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