Through complex modeling, researchers have painted a timely and striking image of quark star mergers. This exciting area of astrophysics is still a mystery in many ways. Led by Zhiqiang Miao, this team made a history-making calculation. Their work culminated in a successful parametric fit of the non-equilibrium equation of state for decompressed quark matter at finite temperatures. They used this model to predict the outcomes of quark star mergers. Consequently, they discovered three plausible scenarios for the ejecta from such cosmic occurrences.
To rigorously validate their model, the research compared their approach to standard, realistic merger simulations. It tracked the evolution of more than 1,000 distinct fluid elements as they evolved post-merger. The research team went to great lengths to examine the many gravitational, electromagnetic, and particle interactions involved in the phenomenon. This modeling showed for the first time how the ejecta compositions would vary from those produced in neutron star mergers.
The Model and Its Mechanisms
The team’s innovative model tracks three crucial physical processes during the merger: quark nugget evaporation, nugget cooling, and weak interactions. Weak interactions are especially relevant because they are responsible for converting neutrons to protons (and vice versa). According to Miao, “Because protons are charged, their absorption is strongly suppressed.” This suppression is a critical factor in deciding the final makeup of the ejecta.
At temperatures on the order of 10 MeV, their model predicts that neutrons are reabsorbed much more effectively than protons. The fate of the ejecta critically hinges on one parameter: the binding energy of quark matter. Binding energy is the energy required to unbind one neutron from bulk, uniform quark matter. This is an important element to understanding why these mergers have such consistently disastrous outcomes.
For the first time, researchers noted that saturation happens in as little as 10-11 seconds in the very saturated conditions produced by the merger. This rapid saturation happens an order of magnitude faster than the ejecta expansion timescale. Indeed, the expansion may take 10^{-3} seconds and more. Consequently, the evolution of the ejecta demonstrates a delicate competition between each of these processes.
Three Possible Outcomes
This study validated a three-outcome framework independent of the initial conditions and temperature in the mergers of quark stars. The possible compositions of ejecta can be categorized into two main types:
- A gas of quark nuggets with a small fraction of nucleons.
- A gas predominantly composed of nucleons.
Miao elaborated on these findings, stating, “For relatively large binding energies of quark matter, the merger ejecta are composed mainly of massive quark nuggets plus a small fraction of nucleons.” This structure is in sharp contrast to the nucleon gas created in neutron star mergers.
The impact of these results reach far beyond academic arithmetic. Miao noted that “because the ejecta are dominated by nuggets, they cannot efficiently undergo nucleosynthesis to form heavy elements.” This means that quark star mergers will not produce the kilonova emissions typically produced by heavy element decay. These emissions are often observed following neutron star mergers.
Implications for Astrophysics
This study provides new insights on the consequences of observing kilonovae for the case of quark stars. Miao explained, “The implications of kilonova observations regarding quark stars consist of two complementary perspectives.” On one hand, detecting a kilonova signal—if attributed to a quark star merger—could help constrain the properties of quark matter.
The non-detection of kilonovae for sufficiently nearby neutron star mergers could potentially serve as evidence supporting the existence of quark stars. This duality underscores the need for additional geospatial observational studies to improve comprehension in this space.
Miao emphasized the challenges faced during this research, stating, “In fact, the main challenge is not the technical calculation of the non-equilibrium equation of state itself but rather in constructing the right physical picture.” Once such a picture is established, he assured, “the calculations become relatively straightforward.”