Publication Abstracts

Tsuruta 1964

Tsuruta, S., 1964: Neutron Star Models. Ph.D. thesis. Columbia University.

When this research was started the main objective was to construct hot neutron star models and to investigate the detectability of these stars. Since then there has arisen the possibility that some of the galactic X-ray sources recently discovered might be neutron stars. Hence both cold and hot models were constructed and their properties were investigated. For this purpose, equilibrium nuclear configurations of dense matter with densities in the range 106 ≲ ρ ≲ 1012 g/cm3 for (a), zero temperature and (b), temperatures in the range 2×109 ≲ T ≲ 1010°K were studied. Equilibrium concentrations of sub-atomic particles with densities in the range 1012 ≲ ρ ≲ 1017 g/cm3 at zero temperature were also studied, and then a number of different equations of state to be applied to a dense star were constructed.

In the temperature and density range considered, stability shifts to more neutron-rich nuclei with increase in density. Nuclei of smaller Z become more abundant and the abundances of nuclei near a peak tend to become comparable to that of the peak nucleus, with an increase of temperature. When T ≲ 5×109°K. the transition from heavy nuclei to neutrons takes place at ρ ~ 3×1011 g/cm3. At higher temperatures this transition would occur at lower density. The molecular weight per electron, μe, is rather intensitive to temperature changes. On the other hand its variation with density is somewhat larger (μe ~ 2 for ρ ~ 106 g/cm3, while for ρ ~ 3×1011 g/cm3 μe ~ 3). At higher densities (ρ > 1013 g/cm3) neutrons become contaminated with other baryons, mesons and leptons.

In "real" gas models the pressure is less than that for ideal gas models in the range 1012 ≲ ρ ≲ 1015 g/cm3, but the situation is reversed for ρ > 1015 g/cm3. The relativistic limitation on the equation of state prevents the pressure from increasing too rapidly. The properties of neutron stars depend primarily on the mass and the interaction between the constituent particles, and the effects of hyperons and of the relativistic limit are minor. The envelope of electrons and heavy ions is important in some of the least massive neutron stars. The mass and radius of stable neutron stars range from about 0.2-2 M and 25-5 km, respectively. All the models constructed develop a central singularity at a finite mass and radius, but all the stable models investigated in this research do not show the Schwarzschild singularity. A small local maximum above the Oppenheimer-Volkoff crushing point is observed. Red shifts are calculated to be from about 1% (least massive models) to 30% (most massive models) which indicates that general relativistic effects are not negligible even near the surface.

Surface properties, temperature effects and cooling are studied for six models of possible stabe neutron stars. For this purpose the opacity is calculated by means of Cox's opacity code. Model atmospheres are constructed both for a pure ion and a pure magnesium compsition. It is found that the non-degenerate layers are only a few meters thick and in no case exceed 1% of the stellar radius. When the surface temperature is about 106~7°K the internal temperature is about 107.5~9°K. For a surface temperature of ~5×107°K the internal energy and optical luminosity are ~1050 ergs and ~106 L, while for the lower surface temperature of ~2×104°K these two values are ~104 ergs and 10-8 L. The cooling process is mainly neutrino emission when T > 109°K but at lower temperatures than this it is primarily optical radiation. The age of a neutron star of a given temperature depends on its mass, interaction potential and surface composition, among which the dependence on mass is the greatest. The cooling behavior is quite complicated. As a consequence, the possible age of a detectable neutron star can be anywhere from about 1 day to about 106 years. Low mass neutron stars would be almost impossible to detect, but intermediate and large mass neutron stars located even far away (~103 parsecs) can keep sufficiently luminous long enough to allow their detection by instruments above the earth's atmosphere, and it should not be too difficult to observe them, were the sensitivity of present detectors increased by a factor of, say, 100.

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BibTeX Citation

  author={Tsuruta, S.},
  title={Neutron Star Models},
  school={Columbia University},
  address={New York, N.Y.},

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RIS Citation

ID  - ts05200y
AU  - Tsuruta, S.
PY  - 1964
BT  - Neutron Star Models
PB  - Columbia University
CY  - New York, N.Y.
ER  -

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