John Faulkner

Lithium dilution through main-sequence mass loss

The consequences of main-sequence mass loss for surface lithium abundances in stars are investigated through the use of evolving, mass-losing stellar models. In particular, we attempt to model lithium depletion observed in three areas: the Sun, the Hyades G dwarfs, and the Hyades F dwarfs. Mass loss as an explanation for lithium depletion has mixed success. It is concluded that mass loss can be a marginal explanation for the solar lithium problem

Main-sequence evolution with efficient central energy transport

Simultaneous solution of the solar neutrino and pulsation problems strongly suggests that in the center of the Sun, energy is transported by a more efficient means than is provided by current, conventional physics. Here, in a first, isothermal approximation, the main-sequence consequences are explored of supposing that the Sun and other 1 solar mass stars contain a relatively efficient means of central energy transport. It is found that, for given initial compositions, the ages of models reaching a given turn-off surface temperature may easily be reduced from the conventionally deduced values by 15-20 percent. The models leave the vicinity of the zero-age main sequence with central hydrogen much less exhausted than usual. This indicates that the possession of a near-isothermal core and the attempted violation of the Schoenberg-Chandrasekhar theorem dominate over mere hydrogen exhaustion in determining the time for departure from the main sequence. Areas of astrophysics that may be affected by these results are indicated. 29 references.

Sub-giant branch evolution and efficient central energy transport

We gather and present observational evidence of a problem with the conventional understanding of the late main-sequence and sub-giant branch evolution of globular cluster stars. The difficulties with these supposedly well-understood phases of stellar evolution manifest themselves in both sub-giant and red-giant branch anomalies (relative to main-sequence normalization) in recent, accurate, CCD-derived luminosity functions. Attempts to reproduce these features using conventional theory and fairly conventional modifications have failed

Is there a classical Hyades lithium problem

The distribution of surface lithium abundances in the Hyades cluster, as a function of effective temperature, has resisted a satisfactory theoretical explanation for more than two decades. Trends in the historically recomputed values of stellar opacity have stimulated a systematic investigation of the effects of larger opacities on lithium depletion during the premain-sequence evolution of low-mass stars. It is shown that the calculated depletion depends quite strongly on the opacities employed, and that both surface and interior opacities play an important role. By performing standard evolutionary calculations with parameterized increments in the opacities, the observed Hyades Li and T(eff) relation for G and K dwarfs have been determined in a manner consistent with the other observational constraints. The solution presented herein requires an increase in interior opacities at about 4 million K over older values by approximately less than 37 percent. Such a change is not incompatible with the historical trends in opacity computations and suggests that the resolution of this problem lies solely in the opacities, without recourse to other physical mechanisms such as turbulent diffusion or extra mixing. 26 refs.

Why Do Stars Become Red Giants

The reasons why main sequence dwarfs evolve into red giants have been known for 30 or 40 years, but there are two excuses for a brief rediscussion of the problem. Firstly, there are quite a number of misconceptions around. Secondly, even the correct physical processes are not obviously correct; is there some reasonably simple model, not based on knowledge of the computer results, which can explain the expansion? We present here a fairly simple model, not entirely free from hindsight, which may be a little more helpful than platitudes about the complexity of the equations.

The Theory of Novae and Nova-Like Systems

Recent observational and theoretical developments in the study of novae, particularly dwarf novae, are discussed. Mechanisms promoting mass transfer include (i) nuclear evolution or (ii) envelope instability of the red star and (iii) gravitational radiation of orbital angular momentum. Growing observational evidence against (ii) is supported by recent theoretical work on the medium and long term response of stellar radii to mass loss. Mechanisms (i) and (iii) may operate alone or in concert, depending on the circumstances.

Resolution of the Classical Hyades Lithium Problem

For the very first time, it has recently proved possible to make a well-motivated, physically plausible, and self-consistent prediction of the Hyades G- and K-dwarf(Li,Teff) relationship that matches the long-unexplained observations. The method employs the latest Iglesias & Rogers (OPAL) interior opacities and Alexander surface opacities (whose respective values are now themselves close to empirical predictions or estimates made earlier in this Hyades project), King’s recently discovered [O/Fe] enhancement (another prediction!) and utterly conventional PMS (pre-mainsequence) evolution unaided by arbitrary and ad hoc adjustable parameters.

Unstable degenerates revisited

Chandrasekhar & Tooper (1964) found that a sequence of increasingly massive helium white dwarfs suffers a general relativistic pulsational instability when the radius R(M) has been reduced to a value such that R/Rs≃246, where R S is the Schwarzschild radius 2GM/c 2 . We show that this curious dimensionless number is essentially (2M p /m e ) 2/3 ≃238, where M p and me are the masses of the proton and electron respectively. The case of different compositions is also covered by our analysis; in general, the corresponding result is (μ e M p /m e ) 2/3 , where μ e is the mean molecular weight per electron

Interior opacities and the standard solar model

We investigate the sensitivity to changes in the interior opacities of a number of solar parameters commonly inferred from standard solar models. In particular we discuss the sensitivity of the central temperature, the radius at the base of the convective zone, and the degree of lithium depletion to changes in the opacity prescription. We examine the impact of the new OPAL opacities on these quantities, and we find that further interior opacity increases concentrated around 2 × 10 6 K are indicated from the solar oscillation data. We provide some additional OPAL opacities needed to produce accurate standard solar models