Numerical Simulations of Thermohaline Convection: Implications for Extra-Mixing in Low-Mass RGB Stars (original) (raw)
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3 He‐driven Mixing in Low‐Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
The Astrophysical Journal, 2008
We examine the stability and observational consequences of mixing induced by 3 He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have finger-like structures with a length to diameter ratio l/d 10. We identify some potentially serious obstacles for thermohaline convection as the predominant mixing mechanism for giants. We show that rotation-induced horizontal turbulent diffusion may suppress the 3 He-driven thermohaline convection. Another potentially serious problem for it is to explain observational evidence of enhanced extra mixing. The 3 He exhaustion in stars approaching the red giant branch (RGB) tip should make the 3 He mixing inefficient on the asymptotic giant branch (AGB). In spite of this, there are observational data indicating the presence of extra mixing in low-mass AGB stars similar to that operating on the RGB. Overmixing may also occur in carbon-enhanced metal-poor stars.
The Astrophysical Journal, 2011
First results of our 3D numerical simulations of thermohaline convection driven by 3 He burning in a low-mass RGB star at the bump luminosity are presented. They confirm our previous conclusion that this convection has a mixing rate which is a factor of 50 lower than the observationally constrained rate of RGB extra-mixing. It is also shown that the large-scale instabilities of saltfingering mean field (those of the Boussinesq and advection-diffusion equations averaged over length and time scales of many salt fingers), which have been observed to increase the rate of oceanic thermohaline mixing up to one order of magnitude, do not enhance the RGB thermohaline mixing. We speculate on possible alternative solutions of the problem of RGB extra-mixing, among which the most promising one that is related to thermohaline mixing is going to take advantage of the shifting of salt-finger spectrum towards larger diameters by toroidal magnetic field.
Deep mixing at variable speed in stars: is thermohaline diffusion sufficient?
2010
We study the circulation of matter in red giants above the H-burning shell, which is known to yield the appearance at the stellar surface of p-capture isotopes like 7Li, 13C, 17O and the unstable 26Al. These isotopes were observed (either in presolar grains of circumstellar origin or in the photospheres of evolved stars) to display abundance ratios to other nuclei that cannot be accounted for by canonical stellar models. Slow mixing below the convective envelope is the usual explanation invoked for their abundance. Diffusion generated by an inversion in the molecular weight mu is today the most commonly assumed driving mechanism for it. We argue that slow transport reaching moderate temperatures (T < 4×107 K), like the one achievable by diffusive processes induced by a mu inversion can account for some, but not all the observational constraints. In particular the production of Li after the first dredge up and of 26Al in the final evolutionary stages both call for substantially di...
Deep mixing at variable speed in stars: is thermohaline di usion su cient?
2010
We study the circulation of matter in red giants above the H-burning shell, which is known to yield the appearance at the stellar surface of p-capture isotopes like 7Li, 13C, 17O and the unstable 26Al. These isotopes were observed (either in presolar grains of circumstellar origin or in the photospheres of evolved stars) to display abundance ratios to other nuclei that cannot be accounted for by canonical stellar models. Slow mixing below the convective envelope is the usual explanation invoked for their abundance. Diffusion generated by an inversion in the molecular weight μ is today the most commonly assumed driving mechanism for it. We argue that slow transport reaching moderate temperatures (T < 4×107 K), like the one achievable by diffusive processes induced by a μ inversion can account for some, but not all the observational constraints. In particular the production of Li after the first dredge up and of 26Al in the final evolutionary stages both call for substantially diff...
Subsurface convection zones in hot massive stars and their observable consequences
Astronomy & Astrophysics, 2009
Context. We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. Aims. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. Methods. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. Results. We find the iron convection zone (FeCZ) to be more prominent for lower surface gravity, higher luminosity and higher initial metallicity. It is absent for luminosities below about 10 3.2 L ⊙ , 10 3.9 L ⊙ , and 10 4.2 L ⊙ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variability.
Astronomy & Astrophysics, 2010
Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection. Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band in order to characterize and measure the convection pattern on this star. Methods. We use 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and we thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations. Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in the optical and in the H band based on excellent fits to the observed visibility points and closure phases. Moreover, we determine that the Betelgeuse surface in the H band is covered by small to medium scale (5-15 mas) convection-related surface structures and a large (30 mas) convective cell. In this spectral region, H2O molecules are the main absorbers and contribute to the small structures and to the position of the first null of the visibility curve (i.e. the apparent stellar radius).
On the interactions of turbulent convection and rotation in RGB stars
Proceedings of the International Astronomical Union, 2006
We have performed the first three-dimensional non-linear simulation of the turbulent convective envelope of a rotating 0.8 M⊙ RGB star using the ASH code. Adopting a global typical rotation rate of a tenth of the solar rate, we have analyzed the dynamical properties of the convection and the transport of angular momentum within the inner 50% in radius of the convective envelope. The convective patterns consist of a small number of large cell, associated with fast flows (∼3000m/s) and large temperature fluctuations (∼300K) in order to carry outward the large luminosity (L ∼ 400L⊙) of the star. The interactions between convection and rotation give rise to a large radial differential rotation and a meridional circulation possessing one cell per hemisphere, the flow being poleward in both hemisphere. By analysing the redistribution of angular momentum, we find that the meridional circulation transports the angular momentum outward in the radial direction, and poleward in the latitudinal...
3D MHD simulations of subsurface convection in OB stars
Proceedings of The International Astronomical Union, 2011
During their main sequence evolution, massive stars can develop convective regions very close to their surface. These regions are caused by an opacity peak associated with iron ionization. Cantiello et al. (2009) found a possible connection between the presence of sub-photospheric convective motions and small scale stochastic velocities in the photosphere of early-type stars. This supports a physical mechanism where microturbulence is caused by waves that are triggered by subsurface convection zones. They further suggest that clumping in the inner parts of the winds of OB stars could be related to subsurface convection, and that the convective layers may also be responsible for stochastic excitation of non-radial pulsations. Furthermore, magnetic fields produced in the iron convection zone could appear at the surface of such massive stars. Therefore subsurface convection could be responsible for the occurrence of observable phenomena such as line profile variability and discrete absorption components. These phenomena have been observed for decades, but still evade a clear theoretical explanation. Here we present preliminary results from 3D MHD simulations of such subsurface convection.
3D Anelastic Simulations of Convection in Massive Stars
arXiv (Cornell University), 2002
After briefly describing the anelastic approximation and Glatzmaier's code, we present results from our preliminary studies of core convection during the hydrogen burning phase of a 15M ⊙ star, as well as our most recent results concerning convection in the oxygen shell of a 25M ⊙ star.
Astronomy & Astrophysics, 2009
Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection. Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band in order to characterize and measure the convection pattern on this star. Methods. We use 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and we thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations. Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in the optical and in the H band based on excellent fits to the observed visibility points and closure phases. Moreover, we determine that the Betelgeuse surface in the H band is covered by small to medium scale (5-15 mas) convection-related surface structures and a large (≈30 mas) convective cell. In this spectral region, H 2 O molecules are the main absorbers and contribute to the small structures and to the position of the first null of the visibility curve (i.e. the apparent stellar radius).