We explore the possibility that ( Bose-Einstein ) condensation of scalar fields from string compactifications can lead to long-lived compact objects .
Depending on the type of scalar fields we find different realisations of star-like and solitonic objects .
We illustrate in the framework of type IIB string compactifications that closed string moduli can lead to heavy microscopic stars with masses of order \mathcal { V } ^ { \alpha } M _ { Planck } , \alpha = 1 , 3 / 2 , 5 / 3 where \mathcal { V } is the volume of the extra dimensions .
Macroscopic compact objects from ultra-light string axions are realised with masses of order e ^ { \mathcal { V } ^ { 2 / 3 } } M _ { Planck } . Q-ball configurations can be obtained from open string moduli whereas the closed string sector gives rise to a new class of solutions , named PQ-balls , that arise in the two-field axion-modulus system .
The stability , the potential for the formation , and the observability of moduli stars through gravitational waves are discussed .
In particular we point out that during the early matter phase given by moduli domination , density perturbations grow by a factor \mathcal { V } ^ { \beta } with \beta > 2 and non-linear effects can not be neglected .