Formation of the simplest stable negative molecular ion H$_3^-$ in interstellar medium

We present the theory of radiative association of atoms and molecules, and we apply it to the (H$_2$-H$^-$) van der Waals complex. We discuss the possibility for the H$_3^-$ ion to be formed in the interstellar medium in an environment with abundant electrons. The observation of H$_3^-$ would also be a probe for the presence of H$^-$ in the interstellar medium. By computing the electronic structure of the H$_3^-$ ion, we determine its dipole moment, bound states, rotational constants, predissociated vibrational resonances and their lifetimes, and suggest a way to detect the ion in the interstellar medium.