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Register a new userNuclear Masses and Neutron Stars
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Added by
Matthias Hempel Ph.D.
Publication date
2013
fields
Physics
and research's language is
English
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Precision mass spectrometry of neutron-rich nuclei is of great relevance for astrophysics. Masses of exotic nuclides impose constraints on models for the nuclear interaction and thus affect the description of the equation of state of nuclear matter, which can be extended to describe neutron-star matter. With knowledge of the masses of nuclides near shell closures, one can also derive the neutron-star crustal composition. The Penning-trap mass spectrometer ISOLTRAP at CERN-ISOLDE has recently achieved a breakthrough measuring the mass of 82Zn, which allowed constraining neutron-star crust composition to deeper layers (Wolf et al., PRL 110, 2013). We perform a more detailed study on the sequence of nuclei in the outer crust of neutron stars with input from different nuclear models to illustrate the sensitivity to masses and the robustness of neutron-star models. The dominant role of the N=50 and N=82 closed neutron shells for the crustal composition is confirmed.
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The slope of the nuclear symmetry energy at saturation density $L$ is pointed out as a crucial quantity to determine the mass and width of neutron-star crusts. This letter clarifies the relation between $L$ and the core-crust transition. We confirm that the transition density is soundly correlated with $L$ despite differences between models, and we propose a clear understanding of this correlation based on a generalised liquid drop model. Using a large number of nuclear models, we evaluate the dispersion affecting the correlation between the transition pressure $P_t$ and $L$. From a detailed analysis it is shown that this correlation is weak due to a cancellation between different terms. The correlation between the isovector coefficients $K_{rm sym}$ and $L$ plays a crucial role in this discussion.
We combine equation of state of dense matter up to twice nuclear saturation density ($n_{rm sat}=0.16, text{fm}^{-3}$) obtained using chiral effective field theory ($chi$EFT), and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-by-order calculations up to next-to-next-to-next-to-leading order in the $chi$EFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities, and provide stringent limits on the maximum and minimum radii of $sim1.4,{rm M}_{odot}$ and $sim2.0,{rm M}_{odot}$ stars. Including $chi$EFT predictions from $n_{rm sat}$ to $2,n_{rm sat}$ reduces the permitted ranges of the radius of a $1.4,{rm M}_{odot}$ star, $R_{1.4}$, by $sim3.5, text{km}$. If observations indicate $R_{1.4}<11.2, text{km}$, our study implies that either the squared speed of sound $c^2_{s}>1/2$ for densities above $2,n_{rm sat}$, or that $chi$EFT breaks down below $2,n_{rm sat}$. We also comment on the nature of the secondary compact object in GW190814 with mass $simeq 2.6,{rm M}_{odot}$, and discuss the implications of massive neutron stars $>2.1 ,{rm M}_{odot},(2.6,{rm M}_{odot})$ in future radio and gravitational-wave searches. Some form of strongly interacting matter with $c^2_{s}>0.35, (0.55)$ must be realized in the cores of such massive neutron stars. In the absence of phase transitions below $2,n_{rm sat}$, the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by $chi$EFT for the baryon density $n_{rm B}$ in the range $1-2,n_{rm sat}$. Together they imply that the rapid stiffening required to support a high maximum mass should occur only when $n_{rm B} gtrsim 1.5-1.8,n_{rm sat}$.
We review the current status and recent progress of microscopic many-body approaches and phenomenological models, which are employed to construct the equation of state of neutron stars. The equation of state is relevant for the description of their structure and dynamical properties, and it rules also the dynamics of core-collapse supernovae and binary neutron star mergers. We describe neutron star matter assuming that the main degrees of freedom are nucleons and hyperons, disregarding the appearance of quark matter. We compare the theoretical predictions of the different equation-of-state models with the currently available data coming from both terrestrial laboratory experiments and recent astrophysical observations. We also analyse the importance of the nuclear strong interaction and equation of state for the cooling properties of neutron stars. We discuss the main open challenges in the description of the equation of state, mainly focusing on the limits of the different many-body techniques, the so-called hyperon puzzle, and the dependence of the direct URCA processes on the equation of state.
We present in this article an overview of the problem of neutron star masses. After a brief appraisal of the methods employed to determine the masses of neutron stars in binary systems, the existing sample of measured masses is presented, with a highlight on some very well-determined cases. We discuss the analysis made to uncover the underlying distribution and a few robust results that stand out from them. The issues related to some particular groups of neutron stars originated from different channels of stellar evolution are shown. Our conclusions are that last centurys paradigm that there a single, $1.4 M_{odot}$ scale is too simple. A bimodal or even more complex distribution is actually present. It is confirmed that some neutron stars have masses of $sim 2 M_{odot}$, and, while there is still no firm conclusion on the maximum and minimum values produced in nature, the field has entered a mature stage in which all these and related questions can soon be given an answer.
Nuclear masses are one of the key ingredients of nuclear physics that go into astrophysical simulations of the $r$ process. Nuclear masses effect $r$-process abundances by entering into calculations of Q-values, neutron capture rates, photo-dissociation rates, beta-decay rates, branching ratios and the properties of fission. Most of the thousands of short-lived neutron-rich nuclei which are believed to participate in the $r$ process lack any experimental verification, thus the identification of the most influential nuclei is of paramount importance. We have conducted mass sensitivity studies near the $N=82$ closed shell in the context of a main $r$-process. Our studies take into account how an uncertainty in a single nuclear mass propagates to influence the relevant quantities of neighboring nuclei and finally to $r$-process abundances. We identify influential nuclei in various astrophysical conditions using the FRDM mass model. We show that our conclusions regarding these key nuclei are still retained when a superposition of astrophysical trajectories is considered.
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