Boson Stars and Oscillatons: A Review

Abstract

Compact objects occupy a pivotal role in the exploration of Nature. The interest spans from the role of compact objects in astrophysics to their detection through various methods (gravitational waves interferometry, microlensing, imaging). While the existence of compact objects made of fermions (neutron stars and white dwarfs) has been assessed, a parallel search for localized solitons made of bosons is ongoing, stemming from Wheeler's original proposal of electromagnetic "geons". Boson fields can clump up and form compact objects such as boson stars (for complex scalar fields), oscillons and oscillatons (for real scalar fields), or Proca stars (for massive vector fields), which can show up in searches as black hole mimickers, dark matter sources, and a variety of other phenomena. I review some crucial properties of these bosonic systems, including recent progress in the field.

Soliton boson stars, Q-balls and the causal Buchdahl bound

“…SBSs in the thin-wall regime are a physical example of objects with a LinEoS 6 , because in the bulk of the star ϕ ≈ 1 and hence ϕ ≈ V ≈ 0, making in turn P rad ≈ ρ. This argument implies that the maximal compactness of SBS is C max < ∼ C B+C , which is consistent with our numerical results and the values reported in the previous work [23,30,33,81]. Now we address two possible loopholes in the previous argument.…”

“…As the matter density is given by ρ m ∼ µ 2 |Φ| 2 (with µ the scalar mass), the equation of state is P ∼ λ µ 4 ρ 2 m . SIBSs thus have the same mass-radius scaling as fermionic pressure-supported objects [scaling (30)], and the maximum mass is given by the Chandrasekhar limit…”

“…configurations made from complex scalars Φ minimally coupled to gravity and admitting a U (1) symmetry. These objects have been thoroughly studied as potential ECOs [14,[28][29][30]. Among the different models of boson stars, the most investigated are mini-boson stars [31] (MBSs; which only have a mass term in the potential), self-interacting boson stars [32] (SIBSs; whose potential also includes a repulsive λΦ 4 term) and solitonic boson stars [33] (SBSs), which present the potential…”

Gravitational waves and kicks from the merger of unequal mass, highly compact boson stars

“…In this work we study BSs, which are solutions of the Einstein equations coupled to a complex scalar field with a harmonic time dependence describing a macroscopic wave-function of a Bose-Einstein condensate (see Ref. [17][18][19] for reviews). BSs are particularly promising as possible astrophysical objects because: (i) a formation mechanism for BSs has been identified, known as gravitational cooling [20,21], whereby BSs can be produced from arbitrary scalar field configurations, (ii) their stability properties resemble those of NSs so that static BSs below a critical mass are radially stable [18,22,23], and finally (iii) BSs have been invoked in open problems in cosmological and particle physics, such as the nature of the dark matter and the possibility of early Universe remnants.…”

Solar mass black holes from neutron stars and bosonic dark matter

“…To this end we adopt a simple version of the variational ansatz [57,72,80] ignoring order-one numerical coefficients wherever possible. We will separately study the solitonic Q-ball [81] stabilized by the attractive self-interactions and the gravitationally bound Bose star [82] in the case of self-repulsion.…”