Pavel A. Denissenkov

Supermassive stars as a source of abundance anomalies of proton-capture elements in globular clusters

We propose that the abundance anomalies of proton-capture elements in globular clusters, such as the C-N, Na-O, Mg-Al and Na-F anti-correlations, were produced by super-massive stars with M ~ 10,000 Msun. Such stars could form in the runaway collisions of massive stars that sank to the cluster center as a result of dynamical friction, or via the direct monolithic collapse of the low-metallicity gas cloud from which the cluster formed. To explain the observed abundance anomalies, we assume that the super-massive stars had lost significant parts of their initial masses when only a small mass fraction of hydrogen, Delta X ~ 0.15, was transformed into helium. We speculate that the required mass loss might be caused by the super-Eddington radiation continuum-driven stellar wind or by the diffusive mode of the Jeans instability.

Canonical Extra Mixing in Low-Mass Red Giants

We have used the latest observational data on the evolutionary variations of the surface chemical composition in low-mass metal-poor stars, both in the field and in globular clusters, to constrain the basic properties of extra mixing in upper red giant branch (RGB) stars. Two different models of extra mixing have been incorporated into a stellar evolution code: a parametrical diffusion model and a model with rotation-induced turbulent diffusion. Application of the first model to the interpretation of the observed variations of the surface abundances of Li, C, and N and of the isotopic ratio 12C/13C in field stars has revealed that, for the majority of upper RGB stars, the depth and rate of extra mixing do not appear to vary appreciably from star to star. Furthermore, comparisons of our calculations with the results obtained by other authors show that at least the extra mixing depth does not seem to depend strongly on metallicity. Therefore, we propose to call this universal nonconvective mixing process canonical extra mixing. We also put forward the hypothesis that some of the upper RGB stars may be experiencing enhanced extra mixing, which is much faster (by a factor of ~100) and somewhat deeper than canonical extra mixing. This could explain the phenomenon of Li-rich giants. Enhanced extra mixing could also contribute to the O-Na and Mg-Al anticorrelations that are seen in some globular cluster red giants. A possible mechanism of extra mixing in upper RGB stars may be turbulent diffusion or/and meridional circulation induced by rotation. In this case, enhanced extra mixing requires rotational velocities that are ≈10 times as fast as those that are sufficient for the occurrence of canonical extra mixing. Observations do not exclude this possibility because (1) the dispersion in the surface rotational velocities of field Li-rich giants span a range of a factor of ~10 and (2) the extremely fast rotation of blue horizontal branch stars in globular clusters may require that their RGB precursors had been spun up appreciably by an external source. Star-to-star abundance variations in globular clusters may well have been produced as the result of both evolutionary and primordial processes. In the primordial scenario, the nuclearly processed material that is accreted by low-mass main-sequence stars may have originated primarily in earlier generations of massive asymptotic giant branch stars that had undergone hot bottom burning of their envelopes and partly in mass-losing upper RGB stars that had been just a bit more massive than the present-day main-sequence turnoff stars and had experienced extra mixing in the past.

NUMERICAL SIMULATIONS OF THERMOHALINE CONVECTION: IMPLICATIONS FOR EXTRA-MIXING IN LOW-MASS RGB STARS

Low-mass stars are known to experience extra-mixing in their radiative zones on the red-giant branch (RGB) above the bump luminosity. To determine if the salt-fingering transport of chemical composition driven by 3He burning is efficient enough to produce RGB extra-mixing, 2D numerical simulations of thermohaline convection for physical conditions corresponding to the RGB case have been carried out. We have found that the effective ratio of a salt-fingers length to its diameter a_eff 7). On the other hand, using the thermohaline diffusion coefficient from linear stability analysis together with a=a_obs is able to describe the RGB extra-mixing at all metallicities so well that it is tempting to believe that it may represent the true mechanism. In view of these results, follow-up 3D numerical simulations of thermohaline convection for the RGB case are clearly needed.

The Abundance Evolution of Oxygen, Sodium, and Magnesium in Extremely Metal Poor Intermediate-Mass Stars: Implications for the Self-Pollution Scenario in Globular Clusters

We present full stellar evolution and parametric models of the surface abundance evolution of 16O, 22Ne, 23Na, and the magnesium isotopes in an extremely metal poor intermediate-mass star (ZAMS = 5 ☉, where ZAMS stands for the zero-age main sequence, and Z = 0.0001). 16O and 22Ne are injected into the envelope by the third dredge-up following thermal pulses on the asymptotic giant branch. These species and the initially present 24Mg are depleted by hot bottom burning (HBB) during the interpulse phase. As a result, 23Na, 25Mg, and 26Mg are enhanced. If the HBB temperatures are sufficiently high for this process to deplete oxygen efficiently, 23Na is first produced and then depleted during the interpulse phase. Although the simultaneous depletion of 16O and enhancement of 23Na 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 anticorrelation observed in globular cluster stars. However, a very robust prediction of our models are low 24Mg/25Mg and 24Mg/26Mg ratios whenever significant 16O depletion can be achieved. This seems to be in stark contrast to recent observations of the magnesium isotopic ratios in the globular cluster NGC 6752.

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 is consistent for larger and more recent samples, but smaller samples of rotation periods appear biased toward shorter periods relative to v sin i 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, NGC 2516, M 34, M 35, M 37, and M 50 with plausible assumptions about star–disk coupling and angular momentum loss from magnetized solar-like winds. Solid-body (SB) 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 SB rotation on extremely short timescales of less than 1 Myr and is therefore unlikely to operate in solar-type pre-main-sequence (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. We argue that neither hydrodynamic mechanisms nor our revised and less efficient prescription for the Tayler–Spruit dynamo can reproduce both spin-down and the internal solar rotation profile by themselves. It is likely that a successful model of angular momentum evolution will involve more than one mechanism. Further observational studies, especially of clusters younger than 100 Myr, will provide important additional constraints on the internal rotation of stars and could firmly rule out or confirm the operation of major classes of theoretical mechanisms.

Enhanced extra mixing in low-mass red giants: Lithium production and thermal stability

We show that canonical extra mixing with a diffusion coefficient Dmix ≈ 109 cm2 s-1, which is thought to start working in the majority of low-mass stars when they reach the bump luminosities on the red giant branch (RGB), cannot lead to an Li flash or a thermal instability, as has been proposed. The abundance levels of 7Li measured in the most extreme Li-rich giants can be reproduced with models including enhanced extra mixing with a diffusion coefficient Dmix ≈ 1011 cm2 s-1. We propose that if extra mixing in RGB stars is driven by rotation, then enhanced extra mixing and Li enrichment in some of these stars can be triggered by their spinning up by an external source of angular momentum. As plausible mechanisms of the spinning up, we consider tidal synchronization of a red giants spin and orbital rotation in a close binary system and engulfment of a massive planet. The most convincing theoretical argument in favor of our hypothesis is a finding that a 10-fold increase of the spin angular velocity of a solar metallicity upper RGB star results in appropriate changes of both extra-mixing depth and rate, exactly as required for efficient Li production. We regard the existence of binary and single RGB stars with rotational velocities approaching ~10% of their equatorial Keplerian velocities, as well as the much larger proportion of Li-rich giants (~50%) among rapidly rotating objects, as the observational support for our hypothesis.

THERMOHALINE MIXING: DOES IT REALLY GOVERN THE ATMOSPHERIC CHEMICAL COMPOSITION OF LOW-MASS RED GIANTS?

First results of our three-dimensional numerical simulations of thermohaline convection driven by 3He burning in a low-mass red giant branch (RGB) star at the bump luminosity are presented. They confirm our previous conclusion that this convection has a mixing rate that 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 the salt-fingering mean field (those of the Boussinesq and advection-diffusion equations averaged over length and timescales 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 takes advantage of the shifting of the salt-finger spectrum toward larger diameters by toroidal magnetic field.

MESA Models of Classical Nova Outbursts: The Multicycle Evolution and Effects of Convective Boundary Mixing

Novae are cataclysmic variables driven by accretion of H-rich material onto a white dwarf (WD) star from its low-mass main-sequence binary companion. New time-domain observational capabilities, such as the Palomar Transient Factory and Pan-STARRS, have revealed a diversity of their behavior that should be theoretically addressed. Nova outbursts depend sensitively on nuclear physics data, and more readily available nova simulations are needed in order to effectively prioritize experimental effort in nuclear astrophysics. In this paper, we use the MESA stellar evolution code to construct multicycle nova evolution sequences with CO WD cores. We explore a range of WD masses and accretion rates as well as the effect of different cooling times before the onset of accretion. In addition, we study the dependence on the elemental abundance distribution of accreted material and convective boundary mixing at the core-envelope interface. Models with such convective boundary mixing display an enrichment of the accreted envelope with C and O from the underlying WD that is commensurate with observations. We compare our results with the previous work and investigate a new scenario for novae with the 3He-triggered convection.

A MODEL OF MAGNETIC BRAKING OF SOLAR ROTATION THAT SATISFIES OBSERVATIONAL CONSTRAINTS

The model of magnetic braking of solar rotation considered by Charbonneau & MacGregor 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 models properties are investigated by numerically solving the azimuthal components of the coupled momentum and magnetic induction equations in two dimensions using a finite element method.

The primordial and evolutionary abundance variations in globular-cluster stars: a problem with two unknowns

Abundances of the proton-capture elements and their isotopes in globular-cluster stars correlate with each other in such a manner as if their variations were produced in high-temperature hydrogen burning at the same time in the past. In addition to these primordial abundance variations, the RGB stars in globular clusters, like their eld counterparts, show the evolutionary variations of the C and N abundances and 12 C/ 13 C isotopic ratio. The latter are caused by extra mixing operating in the RGB star’s radiative zone that separates the H-burning shell from the bottom of its convective envelope. We demonstrate that among the potential sources of the primordial abundance variations in globular-cluster stars proposed so far, such as the hot-bottom burning in massive AGB stars and H burning in the convective cores of supermassive and fastrotating massive MS stars, only the supermassive MS stars with M > 10 4 M can explain all the abundance correlations without any ne-tuning of free parameters. We use our assumed chemical composition for the pristine gas in M13 (NGC 6205) and its mixtures with 50% and 90% of the material partially processed in H burning in the 6 10 4 M MS model star as the initial compositions for the normal, intermediate and extreme populations of low-mass stars in this globular cluster, as suggested by its O-Na anti-correlation. We evolve these stars from the zero-age MS to the RGB tip with the thermohaline and parametric prescriptions for the RGB extra mixing. We nd that the 3 He-driven thermohaline convection cannot explain the evolutionary decline of [C/Fe] in M 13 RGB stars, which, on the other hand, is well reproduced with the universal values for the mixing depth and rate calibrated using the observed decrease of [C/Fe] with MV in the globular cluster NGC5466 that does not have the primordial abundance variations.