[Paper Review] Phase Transitions in Dense Baryonic Matter and Cooling of Rotating Neutron Stars

Fridolin Weber, Rodrigo Negreiros|arXiv (Cornell University)|Nov 26, 2009

Pulsars and Gravitational Waves Research2 references3 citations

TL;DR

This paper investigates how rotation-driven phase transitions in dense baryonic matter—such as quark-hadron transitions or condensate formation—affect the thermal evolution of neutron stars. Using coupled general relativistic stellar rotation and thermal evolution codes, it shows that latent heat release and changing cooling channels during spin-up/down alter surface temperatures, offering observable signatures of exotic core states.

ABSTRACT

New astrophysical instruments such as skA (square kilometer Array) and IXO (formerly Constellation X) promise the discovery of tens of thousands of new isolated rotating neutron stars (pulsars), neutron stars in low-mass X-ray binaries (LMXBs), anomalous X-ray pulsars (AXPs), and soft gamma repeaters (SGRs). Many of these neutron stars will experience dramatic density changes over their active lifetimes, driven by either stellar spin-up or spin-down, which may trigger phase transitions in their dense baryonic cores. More than that, accretion of matter onto neutron stars in LMXBs is believed to cause pycno-nuclear fusion reactions in the inner crusts of neutron stars. The associated reaction rates may be drastically altered if strange quark matter would be absolutely stable. This paper outlines the investigative steps that need to be performed in order to explore the thermal response of neutron stars to rotationally-driven phase transitions in their cores as well as to nuclear burning scenarios in their crusts. Such research complements the exploration of the phase diagram of dense baryonic matter through particle collider experiments, as performed at RHIC in the USA and as planned at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany.

Motivation & Objective

• To understand how rotational changes in neutron stars—such as spin-up in LMXBs or spin-down in millisecond pulsars—trigger phase transitions in dense baryonic matter.
• To investigate the thermal consequences of latent heat release during quark-hadron or condensate phase transitions in rotating neutron star cores.
• To assess how pycnonuclear burning rates in the crust are altered by the presence of strange quark matter nuggets, affecting thermal evolution.
• To model the thermal response of accreting neutron stars during accretion-quiescence cycles, particularly in soft X-ray transients like SAX J1808.4–3658.
• To explore the role of core-crust boundary instabilities and crystallization in post-protoneutron star evolution.

Proposed method

• Use a 2D general relativistic stellar rotation code to compute metric functions, frame-dragging frequency, density gradients, and particle compositions as functions of rotational frequency.
• Couple the rotation code output to a 2D general relativistic thermal evolution code solving the energy conservation equation with neutrino emissivities, heat capacities, and thermal conductivities.
• Implement the thermal evolution equation involving temperature gradients, rotation-induced terms (e.g., ∂rΩ, ∂θΩ), and anisotropic heat flux with relativistic corrections.
• Model phase transitions via latent heat release and changing cooling channels (e.g., hyperon, quark, or condensate contributions) as a function of density and rotation.
• Simulate thermal response during spin-up/down cycles in isolated pulsars and accreting systems, including transient accretion phases.
• Explore the impact of strange quark matter nuggets on pycnonuclear reaction rates in the crust using modified reaction rate calculations (Fig. 6).

Experimental results

Research questions

• RQ1 How does rotational energy loss or gain induce phase transitions in the dense core of neutron stars, and what is the resulting thermal signature?
• RQ2 What observable changes in surface temperature arise from latent heat release during quark-hadron or condensate phase transitions in rotating neutron stars?
• RQ3 How do pycnonuclear reaction rates in the inner crust change if strange quark matter nuggets are present, and what is the thermal impact?
• RQ4 Can the thermal evolution of soft X-ray transients like SAX J1808.4–3658 reveal core composition changes due to rotational spin-up and subsequent decompression?
• RQ5 What role do thermoelectric instabilities and core-crust crystallization play in the early thermal evolution of proto-neutron stars?

Key findings

• Rotationally driven density changes of up to 60% in neutron stars can trigger phase transitions, altering core composition and cooling behavior.
• Latent heat release during quark-hadron phase transitions significantly modifies the thermal evolution of rotating neutron stars, potentially producing observable temperature spikes.
• The presence of strange quark matter nuggets in the crust can drastically alter pycnonuclear reaction rates, as shown by simulations of 56Fe and 118Kr reactions (Fig. 6).
• Accreting neutron stars in LMXBs undergoing spin-up may experience core decompression, leading to the destruction of exotic phases (e.g., K− condensates, quark matter), which alters their thermal evolution.
• The quiescent thermal emission of soft X-ray transients like SAX J1808.4–3658 provides a probe of core composition, especially when rotational changes alter neutrino emission channels.
• Thermoelectric instabilities at the core–crust boundary may play a significant role in the thermal evolution of proto-neutron stars, particularly during early cooling stages.