Modeling lithium rich carbon stars in the Large Magellanic Cloud: an independent distance indicator ?

We present the first quantitative results explaining the presence in the Large Magellanic Cloud of some asymptotic giant branch stars that share the properties of lithium rich carbon stars. A self-consistent description of time-dependent mixing, overshooting, and nuclear burning was required. We identify a narrow range of masses and luminosities for this peculiar stars. Comparison of these models with the luminosities of the few Li-rich C stars in the Large Magellanic Cloud provides an independent distance indicator for the LMCComment: 7 pages, 2 figure

Hot bottom burning in asymptotic giant branch stars and its effect on oxygen isotopic abundances

A self-consistent calculation of asymptotic giant branch (AGB) evolution was carried out, including nucleosynthesis at the base of the convective envelope (hot bottom burning). Hot bottom burning was found to occur for stars between ~4.5 and ~7 M_☉, producing envelopes with ^(18)O/^(16)O ≾ 10^(-6) and 10^(-3) ≾ ^(17)O/^(l6)O ≾ 10^(-1). The ^(17)O abundance depends sensitively on the nuclear ^(17)O-destruction rate; this rate is only loosely constrained by the requirement that first and second dredge-up models match O-isotope observations of red giant branch (RGB) stars (Boothroyd, Sackmann, & Wasserburg 1994). In some cases, high mass-loss rates can terminate hot bottom burning before further ^(17)O enrichment takes place or even before all ^(18)O is destroyed. These predictions are in accord with the very limited stellar observations of J type carbon stars on the AGB and with some of the circumstellar Al_2O_3 grains from meteorites. In contrast, precise data from a number of grains and data from most low-mass Sand C AGB stars (≾ 1.7 M_☉) lie in a region of the ^(18)O/^(l6)O versus ^(17)O/^(16)O diagram that is not accessible by first and second dredge-up or by hot bottom burning. We conclude that for AGB stars, the standard models of stellar evolution are not in accord with these observations. We surmise that an additional mixing mechanism must exist that transports material from the cool bottom of the stellar convective envelope to a depth at which ^(18)O is destroyed. This "cool bottom processing" mechanism on the AGB is similar to extra mixing mechanisms proposed to explain the excess ^(13)C (and depleted ^(12)C) observed in the earlier RGB stage of evolution and the large ^7Li depletion observed in low-mass main-sequence stars

Predictions of oxygen isotope ratios in stars and of oxygen-rich interstellar grains in meteorites

We carried out detailed, self-consistent calculations for stars from 1 to 9 M_☉ over a wide range of metallicities, following the evolution and nucleosynthesis from the pre-main sequence to the asymptotic giant branch (AGB), in order to provide a self-consistent grid for evaluating stellar oxygen isotopic variations. These were calculated for first and second dredge-up, and for some masses also for third dredge-up and "hot bottom" convective envelope burning on the AGB. We demonstrate that ^(16)O/^(17)O in red giant envelopes is primarily a function of the star's mass, while ^(16)O/^(18)O is primarily a function of the initial composition. Uncertainties in the ^(17)O-destruction rate have no effect on the ^(16)O/^(17)O ratio for stars from 1 to 2.5 M_☉, but do affect the ratios for higher masses: the stellar ^(16)O/^(17)O observations are consistent with the Landré et al. (1990) rates using ƒ = 0.2 for ^(17)O(p, y)^(18)F and ^(17)O(p, ɑ)^(14)N, and with the Caughlan & Fowler (1988) rates using ƒ ~ 1. The stellar ^(16)O/^(18)O observations require ƒ ~ 0 in the Caughlan & Fowler ^(18)O(p, ɑ)^(15)N rate. First dredge-up has the largest effect on the oxygen isotope ratios, decreasing ^(16)O/^(17)O significantly from the initial value and increasing ^(16)O/^(18)O slightly. Second and third dredge-up have only minor effects for solar metallicity stars. The absence of very low observed ^(16)O/^(18)O ratios is consistent with a major increase in the ^(18)O(ɑ, y)^(22)Ne rate over the Caughlan & Fowler (1988) value. Hot bottom burning in stars above about 5 M_☉ can cause a huge increase in ^(16)O/^(18)O (to ≳10^6), and possibly a significant decrease in ^(16)O/^(17)O; these are accompanied by a huge increase in ^7Li and a value of ^(12)C/^(13)C ≈ 3. The oxygen isotope ratios in the Al_2O_3 grains (Orgueil grain B, the Murchison 83-5 grain, and the new Bishunpur B39 grain) can be accounted for if they originated in stars that did NOT have the same initial ^(16)O/^(18)O ratio. Thus one cannot assume uniform isotope ratios, even for stars of nearly solar composition. The grains' ^(16)O/^(17)O ratios, together with the ^(26)Mg excesses that indicate grain formation in a ^(26)Al-rich environment, indicate that the Orgueil grain B and Murchison 83-5 grain originated in stars of roughly 1.5 M_☉, during third dredge-up on the AGB. The new Bishunpur B39 grain originated in a star of either 2 or of 4-7 M_☉

Deep Circulation in Red Giant Stars: A Solution to the Carbon and Oxygen Isotope Puzzles?

The long-standing puzzle of low ^(12)C/^(13)C in low-mass red giant branch (RGB) stars, and the more recent puzzle of low ^(18)O/^(16)O ratios in asymptotic giant branch (AGB) stars and in circumstellar Al_2O_3 grains preserved in meteorites, can be resolved by deep circulation currents below the bottom of the standard convective envelope. These currents transport matter from the nonburning bottom of the convective envelope down to regions where some CNO processing can take place ("cool bottom processing"). Modeling circulation with separate downward and upward streams, we found that, to resolve both discrepancies, the base of the extra mixing had to reach a temperature TP close to that of the H-burning shell, namely, Δ log T ≈ 0.17 from the base of the H-shell for both RGB and AGB stars. While the envelope composition depends sensitively on TP, it is insensitive to the speed or geometry of mixing. This indicates that our stream circulation model is generic, so that more sophisticated mixing models with the same TP would yield similar results. On the AGB, our models predict that stars with low ^(18)O/^(16)O can be either S or C stars but must have low ^(12)C/^(13)C (~4) and elevated ^(14)N. Cool bottom processing also destroys ^3He, so that galactic (D + ^3He) decreases with time; this removes the strongest lower limit on the baryon density Ω_b from big bang nucleosynthesis models

Orientational order on curved surfaces - the high temperature region

We study orientational order, subject to thermal fluctuations, on a fixed curved surface. We derive, in particular, the average density of zeros of Gaussian distributed vector fields on a closed Riemannian manifold. Results are compared with the density of disclination charges obtained from a Coulomb gas model. Our model describes the disordered state of two dimensional objects with orientational degrees of freedom, such as vector ordering in Langmuir monolayers and lipid bilayers above the hexatic to fluid transition.Comment: final version, 13 Pages, 2 figures, uses iopart.cl

Luminosities of AGB Variables

The prevailing evidence suggests that most large-amplitude AGB variables follow the period luminosity (PL) relation that has been established for Miras in the LMC and galactic globular clusters. Hipparcos observations indicate that most Miras in the solar neighbourhood are consistent with such a relation. There are two groups of stars with luminosities that are apparently greater than the PL relation would predict: (1) in the LMC and SMC there are large amplitude variables, with long periods, P> 420 days, which are probably undergoing hot bottom burning, but which are very clearly more luminous than the PL relation (these are visually bright and are likely to be among the first stars discovered in more distant intermediate age populations); (2) in the solar neighbourhood there are short period, P<235 days, red stars which are probably more luminous than the PL relation. Similar short-period red stars, with high luminosities, have not been identified in the Magellanic Clouds.Comment: 8 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie

Our Sun. IV. The Standard Model and Helioseismology: Consequences of Uncertainties in Input Physics and in Observed Solar Parameters

Helioseismology provides a powerful tool to explore the deep interior of the Sun: for example, the adiabatic sound speed can be inferred with an accuracy of a few parts in 10,000. This has become a serious challenge to theoretical models of the Sun. Therefore, we have undertaken a self-consistent, systematic study of sources of uncertainties in the standard solar model, which must be understood before the helioseismic observations can be used as constraints on theory. We find that the largest uncertainty in the sound speed in the solar interior, namely, 3 parts in 1000, arises from uncertainties in the observed photospheric abundances of the elements; uncertainties of 1 part in 1000 arise from (1) the 4% uncertainty in the OPAL opacities, (2) the 5% uncertainty in the basic pp nuclear reaction rate, (3) the 15% uncertainty in the diffusion constants for the gravitational settling of helium, and (4) the 50% uncertainties in diffusion constants for the heavier elements. (Other investigators have shown that similar uncertainties arise from uncertainties in the interior equation of state and in rotation-induced turbulent mixing.) The predicted pre-main-sequence solar lithium depletion is a factor of order 20 (an order of magnitude larger than that predicted by earlier models that neglected gravitational settling and used older opacities), and is uncertain by a factor of 2. The predicted neutrino capture rate is uncertain by 30% for the Cl-37 experiment and by 3% for the Ga-71 experiments (not including uncertainties in the capture cross sections), while the B-8 neutrino flux is uncertain by 30%.Comment: LaTeX, 38 pages (including 8 figures); ApJ, in press. Added figures/color figurea available at http://www.cita.utoronto.ca/~boothroy/sun4.htm

Solar Model Uncertainties, MSW Analysis, and Future Solar Neutrino Experiments

Various theoretical uncertainties in the standard solar model and in the Mikheyev-Smirnov-Wolfenstein (MSW) analysis are discussed. It is shown that two methods of estimating the solar neutrino flux uncertainties are equivalent: (a) a simple parametrization of the uncertainties using the core temperature and the nuclear production cross sections; (b) the Monte Carlo method of Bahcall and Ulrich. In the MSW analysis, we emphasize proper treatments of correlation of theoretical uncertainties between flux components and between different detectors, the Earth effect, and multiple solutions in a combined $\chi^2$ procedure. The MSW solutions for various standard and nonstandard solar models are also shown. The MSW predictions of the global solutions for the future solar neutrino experiments are given, emphasizing the measurement of the energy spectrum and the day-night effect in Sudbury Neutrino Observatory and Super-Kamiokande to distinguish the two solutions.Comment: (Revtex 3.0, 43 pages + 26 figures (uuencoded ps files attached), Easy way: ps files of entire text with embedded figures available by anonymous ftp://upenn5.hep.upenn.edu/pub/hata/papers/msw_analysis.u