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1992Apj. . .387. .372G the Astrophysical Journal, 387:372-393 .372G The Astrophysical Journal, 387:372-393,1992 March 1 © 1992. The American Astronomical Society. All rights reserved. Printed in U.S.A. .387. 1992ApJ. STANDARD SOLAR MODEL D. B. Guenther, P. Demarque, Y.-C. Kim, and M. H. Pinsonneault Center for Solar and Space Research, Department of Astronomy, Yale University, P.O. Box 6666, New Haven, CT 06511 Received 1991 June 3 ; accepted 1991 August 29 ABSTRACT A set of solar models have been constructed, each based on a single modification to the physics of a refer- ence solar model. In addition, a model combining several of the improvements has been calculated to provide a “best” solar model. Improvements were made to the nuclear reaction rates, the equation of state, the opa- cities, and the treatment of the atmosphere. The impact on both the structure and the frequencies of the low-/ p-modes of the model to these improvements are discussed. We find that the combined solar model, which is based on the best physics available to us (and does not contain any ad hoc assumptions), reproduces the observed oscillation spectrum (for low-/) within the errors associated with the uncertainties in the model physics (primarily opacities). Subject headings: equation of state — Sun: interior — Sun: oscillations 1. INTRODUCTION abundance and the radius of late-type stellar models strongly The enterprise of constructing solar models has a long and depends on the efficiency of convective energy transport. rich history; the introduction of new and improved physics has Helium can be seen only in stars with surface temperatures hot paralleled the development of successively more accurate enough to ionize helium, where, unfortunately, non-LTE models of the Sun. Solar models are crucial in our understand- effects and gravitational settling of helium make abundance ing of stars and stellar evolution, and continue to provide the determinations difficult. Because helium is not visible in the best means of calibrating stellar models. In this paper we optical spectrum of the Sun and because its abundance sensiti- compare the properties of solar models constructed with the vely affects the luminosity of a solar model, it is used as an best available physics with the most sensitive test of the outer adjustable parameter of the solar model. The mixing-length layers: namely, the frequencies of nonradial solar oscillations. theory characterizes the efficiency of convection in carrying the We demonstrate that our “ best ” solar model reproduces the energy flux with a single parameter of order unity (a), which is low-/ p-mode oscillation frequencies within the errors associ- the characteristic distance (in pressure scale heights) over ated with the physics of the model. We also discuss the impact which a convective element travels before merging with its of the individual improvements on the structure and the fre- surroundings. Because it sets the temperature gradients in the quencies. superadiabatic layers, it is responsible for fixing the location of Lane (1869) first wrote down and solved the equations of a a star’s surface, i.e., its radius. Thus the mixing-length theory gas sphere in hydrostatic equilibrium to determine (without the also provides, in the standard solar model, a convenient means aid of the Stefan-Boltzmann law, which had not yet been of adjusting the solar radius to compensate for the other errors published) the Sun’s surface temperature and density (he in the physics of the outer layers (Demarque & Percy 1964). 3 obtained Tsurface = 30,000 K and psurface = 0.0004 g cm " ). The mixing-length parameter and the helium abundance are Advances in atomic theory and the subsequent calculation of adjusted to produce a model which has the Sun’s observed absorption coefficients for various atoms enabled Eddington radius and luminosity; these values are then used in model (1926) to include a description of the transport of energy by calculations of other stars (Demarque & Larson 1964). It is radiation in his static solar and stellar models. The Sun’s within this context that the standard solar model first appeared energy source was not incorporated in solar models until the in the literature. discovery of thermonuclear reactions (Bethe 1939). The follow- Today, though, a standard solar model has come to mean ing decade was highlighted by the realization that the proton- many things to many authors (see, e.g., Sackmann, Boothroyd, proton chain, not the CN cycle, provides the principal source & Fowler 1990). For example, it can represent a conservative of nuclear energy in the Sun. Schwarzschild, Howard, & Härm model of the Sun based on well-established physics; it can (1957) calculated the first evolutionary models of the Sun, start- represent an ever-improving model of the Sun, upgraded with ing from the zero-age main sequence. The mixing-length each improvement made to current physics; or it can represent theory of convection was first applied to the solar convection a solar model which best fits the observables of the Sun but zone by Böhm-Vitense (1958). Needless to say, the intro- relies on ad hoc adjustments to the physics. In this work we duction of computers resulted in a dramatic advance in the define our reference standard solar model to be a conservative sophistication of the models, albeit, with relatively minor model of the Sun based on well-established physics and calcu- modifications to the actual structure predictions of the model. late several improved solar models which upgrade the refer- Computers have enabled more physics and more accurately ence model by incorporating cutting-edge physics. modeled physics to be included in models of the Sun. It is well known that the solar neutrino flux, as measured Models of other stars are constructed using the convective with the Davis chlorine detector (Davis 1978), is a factor of 2-3 mixing-length parameter and the helium abundance (for disk times smaller than the predictions of solar models. As a conse- population stars) determined from the solar model. The lumi- quence, over the past 20 years, hundreds of articles have been nosity of stellar models strongly depends on the initial helium written describing various modifications to the standard solar 372 © American Astronomical Society • Provided by the NASA Astrophysics Data System .372G STANDARD SOLAR MODEL 373 .387. model which attempt to reconcile these discrepancies. In 1988, the frequencies of their p-mode spectra (of low-/) to the Sun’s . two independent studies of the solar model (Bahcall & Ulrich p-mode spectrum. 1988; Turck-Chièze, Cahen, & Cassé 1988) focused on neu- In § 2 we describe the physics and modeling procedure of our trino flux predictions and concluded that, once again, the stan- reference standard solar model and the augmented models. dard physical assumptions of the standard solar model cannot The augmented models include improvements to the atmo- 1992ApJ. be adjusted within the errors to account for the observed neu- sphere model, the nuclear reaction rates, the equation of state trino flux. This has led many to conclude that indeed the stan- (MHD and Debye-Hückel), and opacities (Cox, Kurucz, and dard solar model is correct and it is the neutrino physics which OPAL). In § 3 we describe the structural differences among the is wrong (Bahcall & Pinsonneault 1991). In fact, the solar models and compare the low-/ p-mode oscillation spectra of model and neutrino flux observations may ultimately establish the models to the Sun’s. In § 4 we summarize our results and the existence of resonant neutrino oscillations and the validity comment on future directions. of nonstandard theories of electroweak interactions (Bahcall 1989; Mikheyev & Smirnov 1986; Wolfenstein 1978). 2. THE SOLAR MODELS In recent years the very accurate determination of the fre- quencies of the Sun’s p-modes (for example, Libbrecht, 2.1. The Reference Standard Solar Model Woodard, & Kaufman 1990) and the persistent 1% discrep- Our reference standard solar model (STD model) is con- ancy between these observations and model predictions has structed using the default physics and parameters of the Yale goaded solar modelers to adopt a more conservative approach, Rotating Stellar Evolution Code (Prather 1976; Pinsonneault that of establishing the sensitivity of the model and its observa- 1988), in its nonrotating configuration, i.e., physics and param- bles to variations or adjustments in the model parameters: age eters normally used for stellar evolutionary calculations. (Guenther 1989, 1991b); composition, opacities, and equation YREC (Yale Rotating Stellar Evolution Code) solves the con- of state (Christensen-Dalsgaard, Däppen, & Lebreton 1988; servation and transport equations of stellar structure using the Cox, Guzik, & Raby 1990; Guenther, Jaífe, & Demarque 1989; Henyey relaxation method. The nuclear reaction network is Guenther & Sarajedini 1988; Kim, Demarque, & Guenther solved separately for each shell in the model. 1991); envelope structure (Guenther 1991a; Ulrich & Rhodes In the outer regions (log T < 5.5), the equation-of-state rou- 1977); and survey (Bahcall & Ulrich 1988; Christensen- tines determine particle densities by solving the Saha equation Dalsgaard 1988; Ulrich & Rhodes 1983). The immediate goal for the single ionization state of H and metals, and the single is not to produce the ultimate fix to the standard solar model and double ionization states of He. In the inner regions the but to try to understand the importance of individual param- elements are assumed to be fully ionized. A small transition eters to determining the accuracy with which one can expect region is defined (5.5 < log T < 6.0) where the interior formu- the standard solar model to predict the frequencies of p-modes. lation and exterior formulation are weighted with a ramp func- Ulrich & Rhodes (1983) were able so show, in a definitive tion and then averaged. Radiation pressure and electron manner, that the standard solar model, and several non- degeneracy corrections are included but perturbations due to standard solar models, could not be easily tweaked within the Coulomb interactions in the plasma (i.e., Debye-Hückel known errors of the physics at that time, to reproduce the corrections) are ignored.
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