AAS 199th meeting, Washington, DC, January 2002
Session 105. HEAD I: Measuring Neutron Star Radii vi Thermal Emission: Constraining the Nuclear Equation of State
Special Session Oral, Wednesday, January 9, 2002, 10:00-11:30am, International Ballroom Center

## [105.01] How Neutron Star Radius Measurements Can Constrain the Nuclear Equation of State

J. M. Lattimer (SUNY - Stony Brook)

Precision measurements of neutron star structure may soon become possible from isolated neutron stars like RX J185635-3754 or from transient X-ray sources in globular clusters. Analyses of thermal emission, coupled with atmosphere models, yield estimates of the `radiation radius' R\inf, which is related to the mass M and R by R\inf=R/\sqrt{1-2GM/Rc2}. Further information, such as from redshifts or observations of binary companions, may make possible simultaneous measurements of mass and radius.

It is demonstrated that knowledge of the neutron star radius to an accuracy of 1 km or better permits a useful determination of the pressure of neutron star matter in the range 1-2ns, where ns=0.16 fm-3 (2.7 1014 g cm-3) is the nuclear saturation density. This pressure primarily depends upon dSv(n)/dn, where Sv(n) is the density-dependent nuclear symmetry energy. In principle, Sv(n) can be inferred from nuclear systematics (masses, neutron skins, fission barrier heights). A new precision measurement of the neutron skin thickness of 208Pb being carried out at Jefferson Lab will significantly reduce experimental uncertainties. An interesting collaboration to fix Sv(n), developed from objects differing in size by 1020, is thus at hand. Better knowledge of Sv(n) is needed to predict properties of neutron-rich nuclei found in supernovae, r-process simulations, and neutron star crusts.

It is shown that a neutron star's moment of inertia, the fraction of the moment of inertia residing in the star's crust, and the stellar binding energy are nearly universal functions of the star's mass and radius, irrespective of the underlying equation of state. These features can be understood by considering analytic solutions of Einstein's equations, due to Buchdahl and Tolman. Further constraints from neutrino observations of proto-neutron stars and from observations of pulsar glitches are thus possible. The neutron star maximum mass, however, depends upon the pressure at several times ns and cannot be gleaned from radius measurements alone.

The author(s) of this abstract have provided an email address for comments about the abstract: lattimer@astro.sunysb.edu