  We investigate the string solutions and cosmological implications of the
gauge ${\rm U(1)_Z}\,\times$ global ${\rm U(1)_{PQ}}$ model. With two
hierarchical symmetry-breaking scales, the model exhibits three distinct string
solutions: a conventional global string, a global string with a heavy core, and
a gauge string as a bound state of the two global strings. This model reveals
rich phenomenological implications in cosmology. During the evolution of the
universe, these three types of strings can form a Y-junction configuration.
Intriguingly, when incorporating this model with the QCD axion framework, the
heavy-core global strings emit more axion particles compared to conventional
axion cosmic strings due to their higher tension. This radiation significantly
enhances the QCD axion dark matter abundance, thereby opening up the QCD axion
mass window. Consequently, axions with masses exceeding $\sim 10^{-5}\, {\rm
eV}$ have the potential to constitute the whole dark matter abundance.
Furthermore, in contrast to conventional gauge strings, the gauge strings in
this model exhibit a distinctive behavior by radiating axions.
