574 research outputs found
A New Twist In the Evolution of Low-Mass Stars
We show that the evolutionary track of a low-mass red giant should make an
extended zigzag on the Hertzsprung-Russel diagram just after the bump
luminosity, if fast internal rotation and enhanced extra mixing in the
radiative zone bring the temperature gradient close to the adiabatic one. This
can explain both the location and peculiar surface chemical composition of
Li-rich K giants studied by Kumar, Reddy, & Lambert (2011). We also discuss a
striking resemblance between the photometric and composition peculiarities of
these stars and giant components of RS CVn binaries. We demonstrate that the
observationally constrained values of the temperature gradient in the Li-rich K
giants agree with the required rate of extra mixing only if the turbulence
which is believed to be responsible for this extra mixing is highly
anisotropic, with its associated transport coefficients in the horizontal
direction strongly dominating over those in the vertical direction.Comment: 13 pages, 4 figures, submitted to ApJ Letter
A Model of Magnetic Braking of Solar Rotation That Satisfies Observational Constraints
The model of magnetic braking of solar rotation considered by Charbonneau &
MacGregor (1993) has been modified so that it is able to reproduce for the
first time the rotational evolution of both the fastest and slowest rotators
among solar-type stars in open clusters of different ages, without coming into
conflict with other observational constraints, such as the time evolution of
the atmospheric Li abundance in solar twins and the thinness of the solar
tachocline. This new model assumes that rotation-driven turbulent diffusion,
which is thought to amplify the viscosity and magnetic diffusivity in stellar
radiative zones, is strongly anisotropic with the horizontal components of the
transport coefficients strongly dominating over those in the vertical
direction. Also taken into account is the poloidal field decay that helps to
confine the width of the tachocline at the solar age. The model's properties
are investigated by numerically solving the azimuthal components of the coupled
momentum and magnetic induction equations in two dimensions using a finite
element methodComment: 39 pages, 11 figures, submitted to Ap
Angular Momentum Transport In Solar-Type Stars: Testing the Timescale For Core-Envelope Coupling
We critically examine the constraints on internal angular momentum transport
which can be inferred from the spin down of open cluster stars. The rotation
distribution inferred from rotation velocities and periods are consistent for
larger and more recent samples, but smaller samples of rotation periods appear
biased relative to vsini studies. We therefore focus on whether the rotation
period distributions observed in star forming regions can be evolved into the
observed ones in the Pleiades, NGC2516, M34, M35, M37, and M50 with plausible
assumptions about star-disk coupling and angular momentum loss from magnetized
solar-like winds. Solid body models are consistent with the data for low mass
fully convective stars but highly inconsistent for higher mass stars where the
surface convection zone can decouple for angular momentum purposes from the
radiative interior. The Tayler-Spruit magnetic angular momentum transport
mechanism, commonly employed in models of high mass stars, predicts solid-body
rotation on extremely short timescales and is therefore unlikely to operate in
solar-type pre-MS and MS stars at the predicted rate. Models with core-envelope
decoupling can explain the spin down of 1.0 and 0.8 solar mass slow rotators
with characteristic coupling timescales of 55+-25 Myr and 175+-25 Myr
respectively. The upper envelope of the rotation distribution is more strongly
coupled than the lower envelope of the rotation distribution, in accord with
theoretical predictions that the angular momentum transport timescale should be
shorter for more rapidly rotating stars. Constraints imposed by the solar
rotation curve are also discussed (Abridged)Comment: 42 pages, 16 figures, submitted to Ap
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
MESA and NuGrid Simulations of Classical Nova Outbursts and Nucleosynthesis
Classical novae are the results of surface thermonuclear explosions of
hydrogen accreted by white dwarfs (WDs) from their low-mass main-sequence or
red-giant binary companions. Chemical composition analysis of their ejecta
shows that nova outbursts occur on both carbon-oxygen (CO) and more massive
oxygen-neon (ONe) WDs, and that there is cross-boundary mixing between the
accreted envelope and underlying WD. We demonstrate that the state-of-the-art
stellar evolution code MESA and post-processing nucleosynthesis tools of NuGrid
can successfully be used for modeling of CO and ONe nova outbursts and
nucleosynthesis. The convective boundary mixing (CBM) in our 1D numerical
simulations is implemented using a diffusion coefficient that is exponentially
decreasing with a distance below the bottom of the convective envelope. We show
that this prescription produces maximum temperature evolution profiles and
nucleosynthesis yields in good agreement with those obtained using the commonly
adopted 1D nova model in which the CBM is mimicked by assuming that the
accreted envelope has been pre-mixed with WD's material. In a previous paper,
we have found that 3He can be produced in situ in solar-composition envelopes
accreted with slow rates (dM/dt < 1e-10 M_sun/yr) by cold (T_WD < 1d7 K) CO
WDs, and that convection is triggered by 3He burning before the nova outburst
in this case. Here, we confirm this result for ONe novae. Additionally, we find
that the interplay between the 3He production and destruction in the
solar-composition envelope accreted with an intermediate rate, e.g. dM/dt =
1e-10 M_sun/yr, by the 1.15 M_sun ONe WD with a relatively high initial central
temperature, e.g. T_WD = 15e6 K, leads to the formation of a thick radiative
buffer zone that separates the bottom of the convective envelope from the WD
surface.Comment: 6 pages, 4 figures, STELLA NOVAE: FUTURE AND PAST DECADES Conference
Proceedings, Submitted to ASP Conference Serie
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