Constraints on the Symmetry Energy Using the Mass-Radius Relation of Neutron Stars

The nuclear symmetry energy is intimately connected with nuclear astrophysics. This contribution focuses on the estimation of the symmetry energy from experiment and how it is related to the structure of neutron stars. The most important connection is between the radii of neutron stars and the pressure of neutron star matter in the vicinity of the nuclear saturation density $n_s$. This pressure is essentially controlled by the nuclear symmetry energy parameters $S_v$ and $L$, the first two coefficients of a Taylor expansion of the symmetry energy around $n_s$. We discuss constraints on these parameters that can be found from nuclear experiments. We demonstrate that these constraints are largely model-independent by deriving them qualitatively from a simple nuclear model. We also summarize how recent theoretical studies of pure neutron matter can reinforce these constraints. To date, several different astrophysical measurements of neutron star radii have been attempted. Attention is focused on photospheric radius expansion bursts and on thermal emissions from quiescent low-mass X-ray binaries. While none of these observations can, at the present time, determine individual neutron star radii to better than 20% accuracy, the body of observations can be used with Bayesian techniques to effectively constrain them to higher precision. These techniques invert the structure equations and obtain estimates of the pressure-density relation of neutron star matter, not only near $n_s$, but up to the highest densities found in neutron star interiors. The estimates we derive for neutron star radii are in concordance with predictions from nuclear experiment and theory.Comment: 24 pages, 13 figure

Taxation and Household Portfolio Composition: U.S. Evidence from the 1980s and 1990s

This paper explores the relationship between household marginal income tax rates, the set of assets that households own, and the portfolio shares accounted for by each of these assets. It analyzes data from the 1983, 1989, 1992, and 1995 Surveys of Consumer Finances and develops a new algorithm for imputing federal marginal tax rates to households in these surveys. The empirical findings suggest that a household's marginal tax rate has an important effect its asset allocation decisions. The probability that a household owns tax-advantaged assets is strongly related to its tax rate on ordinary income. In addition, the amount of investment through tax-deferred accounts such as 401(k) plans and IRAs is an increasing function of the household's marginal tax rate. Holdings of corporate stock, which is taxed less heavily than interest bearing assets, and of tax-exempt bonds are also increasing in the household's marginal tax rate. Holdings of heavily taxed assets, such as corporate bonds and interest-bearing accounts, decline as a share of wealth as a household's marginal tax rate increases.

Household Portfolio Allocation Over the Life Cycle

In this paper, we analyze the relationship between age and portfolio structure for households in the US. We focus on both the probability that households of different ages own particular portfolio assets and the fraction of their net worth allocated to each asset category. We distinguish between age and cohort effects using data from the repeated cross-sections of the Federal Reserve Board's Surveys of Consumer Finances. We present two broad conclusions. First, there are important differences across asset classes in both the age-specific probabilities of asset ownership and in the portfolio shares of different assets at different ages. The notnion that all assets can be treated as identical from the standpoint of analyzing household wealth accumulation is not supported by the data. Institutional factors, asset liquidity, and evolving investor tastes must be recognized in modeling asset demand. These factors could affect analyses of overall household saving as well as the composition of this saving. Second, there are evident differences in the asset ownership probabilities of different birth cohorts. Currently, older households were more likely to hold corporate stock, and less likely to hold tax-exempt bonds, than younger households at any given age. These differences across cohorts are important to recognize when analyzing asset accumulation profiles.