Heavy neutron stars from light scalars

• DOI: 10.48550/arxiv.2307.14418
• Publication Date: 2025-02-20
• Authors: Reuven Balkin; Javi Serra; Konstantin Spring...; Stefan Stelzl; Andreas Weiler
• Keywords: New Light Particles; High Energy Astrophysical Phenomena (astro-ph.HE); Nuclear Theory; FOS: Physical sciences; QC770-798; General Relativity and Quantum Cosmology (gr-qc); General Relativity and Quantum Cosmology; Nuclear Theory (nucl-th); High Energy Physics - Phenomenology; High Energy Physics - Phenomenology (hep-ph); Nuclear and particle physics. Atomic energy. Radioactivity; Axions and ALPs; Astrophysics - High Energy Astrophysical Phenomena; Finite Temperature or Finite Density

Abstract We study how light scalar fields can change the stellar landscape by triggering a new phase of nuclear matter. Scalars coupled to nucleons can develop a non-trivial expectation value at finite baryon density. This sourcing of a scalar reduces the nucleon mass and provides an additional energy density and pressure source. Under generic conditions, a new ground state of nuclear matter emerges, with striking implications for the configuration of stellar remnants. Notably, neutron stars in the new ground state can be significantly heavier than QCD equations of state currently predict. We also find hybrid stellar compositions and stable self-bound objects with sizes as small as the Compton wavelength of the scalar. We discuss several specific realizations of this scenario: the QCD axion and lighter generalizations thereof and linearly or quadratically coupled scalar fields effectively equivalent to a class of scalar-tensor modification of gravity. Lastly, we explore phenomenological signatures relevant to electromagnetic and gravitational wave observations of neutron stars, such as atypical compactness and instability gaps in radii.