Modelling and phenomenology of boson stars as gravitational sources for future ground- and space-based interferometers

Boson stars are a new class of astrophysical sources which could form and cohabitate in the Universe along with other families of compact objects. They can bind in close binaries and emit gravitational waves mimicking the evolution of ordinary neutron stars and black holes. The formation channels of boson stars are associated with different physics scenarios, being in general related to the existence of new fundamental fields which can couple to the gravity sector, allowing for solutions within a wide spectrum of masses, from stellar to supermassive scales. The search of gravitational signals from these sources provides hence a way to explore a variety of fundamental, and cross-fields, physics problems, from the dark matter puzzle, to the uniqueness of black holes solutions in General Relativity. Inspired by these motivations, in this work we develop a comprehensive treatment of fast spinning boson stars, modelling their structure, and their gravitational emission in coalescing binaries. Our study builds on the lesson taught by neutron stars, in which a precise characterization of the stellar multipolar structure maps to the features of the nuclear equation of state. Here we exploit this similarity, providing a detailed investigation of how different properties of the boson star structure depend on the fundamental properties of the bosonic field, namely its mass and interactions. We focus in particular on the spin induced multipole moments and on the tidal deformabilities, which affect the binary orbital evolution during the last stage of the coalescence, and leave detectable imprints on the emitted gravitational wave signals. We construct the first coherent waveform model for the inspiral of boson star binaries and exploit such template to forecast the ability of current and future interferometers to infer the stellar properties. Our results show that future instruments such as the Einstein Telescope and the Laser Interferometer Space Antenna can shed new light on the existence and the dynamcis of boson stars, providing strong and complementary measurements of the properties of their underlying scalar field theory.