Hyperons and massive neutron stars: the role of hyperon potentials (1111.6049v3)

Abstract: The constituents of cold dense matter are still far from being understood. However, neutron star observations such as the recently observed pulsar PSR J1614-2230 with a mass of 1.97+/-0.04 M_solar help to considerably constrain the hadronic equation of state (EoS). We systematically investigate the influence of the hyperon potentials on the stiffness of the EoS. We find that they have but little influence on the maximum mass compared to the inclusion of an additional vector meson mediating repulsive interaction amongst hyperons. The new mass limit can only be reached with this additional meson regardless of the hyperon potentials. Further, we investigate the impact of the nuclear compression modulus and the effective mass of the nucleon at saturation density on the high density regime of the EoS. We show that the maximum mass of purely nucleonic stars is very sensitive to the effective nucleon mass but only very little to the compression modulus.

• The paper demonstrates that including extra vector meson interactions significantly stiffens the equation of state in hyperon-rich neutron stars.
• It employs the Relativistic Mean Field model to quantify how hyperon potentials affect maximum neutron star mass by approximately 0.2 solar masses.
• Findings reveal that the effective nucleon mass, rather than compression modulus, is crucial in governing neutron star mass predictions under hyperonic influences.

Overview of Hyperons and Massive Neutron Stars: The Role of Hyperon Potentials

The research paper by Weissenborn, Chatterjee, and Schaffner-Bielich explores a crucial aspect in the field of nuclear astrophysics, focusing on the equation of state (EoS) of dense hadronic matter and its implications for neutron stars, particularly those harboring hyperons. This detailed investigation is motivated by recent observational advances, such as the precise mass determination of the millisecond pulsar PSR J1614-2230, which signifies a neutron star mass of $1.97 \pm 0.04$ M$_\odot$. This mass measurement challenges conventional models and theoretical predictions, necessitating a reevaluation of the components and forces at play within neutron star interiors.

Theoretical Underpinnings

The authors employ the Relativistic Mean Field (RMF) model, a robust framework for modeling the interactions among nucleons and hyperons mediated by mesonic exchanges. Their inquiry primarily scrutinizes how hyperon potentials—specifically, the hyperon-nucleon and hyperon-hyperon interactions—inform the stiffness of the EoS. The inclusion of hyperons in the stellar core generally softens the EoS, ostensibly limiting the maximum mass a neutron star can attain. Notably, to account for the observed $2.0$ M$_\odot$ mass, new interactions, particularly those involving additional vector mesons, are considered to generate requisite repulsive forces among hyperons.

Numerical Insights

Strong numerical results from the paper underscore that hyperon potentials minimally impact maximum mass predictions as compared to the incorporation of an additional vector meson ($\phi$ meson). For instance, variations in potential depths of hyperons yield maximum mass discrepancies around 0.2 M$_\odot$ within traditional models, failing to reconcile with observed masses. Conversely, mesonic inclusion can amplify the maximum neutron star mass significantly, by approximately 0.2 M$_\odot$, thus aligning theoretical outcomes with empirical data.

Dependence on Nuclear Parameters

Furthermore, they paper sensitivities of the EoS to nuclear saturation properties, particularly nucleon effective mass and compression modulus. The paper finds that maximum neutron star masses are strongly contingent on effective nucleon mass rather than the compression modulus, counter to previous assertions. This insight is paramount for models striving to achieve compatibility with empirical neutron star mass constraints while accounting for the presence of hyperons.

Theoretical and Practical Implications

By evaluating the conditions permitting hyperons within massive neutron stars, this research provides essential parameters for RMF models to attain accurate EoS predictions. The emphasis on incorporating additional vector meson interaction underscores a theoretical pivot necessary for harmonizing nuclear physics with compact astrophysical observations. Practically, these findings inform future nuclear model constructions, potentially influencing the design of experiments targeting hypernuclear matter and neutron star composition.

Speculative Outlook

Speculatively, this paper might suggest pathways for extending beyond hadronic descriptions, gesturing towards the inclusion of quark matter phases or more exotic states in neutron stars. Future observable discoveries, such as a confirmed $2.4$ M$_\odot$ neutron star, would further necessitate theoretical advancements potentially invoking beyond-standard model physics, thereby enriching our understanding of quantum chromodynamics in extreme environments.

In conclusion, Weissenborn and colleagues provide a detailed examination of hyperon contributions to the EoS, offering substantive insights into reconciling heavy pulsar observations with theoretical frameworks—a crucial step for advancing the field of dense matter astrophysics.