65 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
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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