Bound states, resonances, and formation of the H$_3^-$ anion*

Accurate potential energy surface and permanent dipole moments of the H$_3^-$ molecule are calculated. The obtained potential energy surface is used to obtain bound states, rotational constants, predissociated vibrational resonances and their lifetimes of the four isotopologues, H$_3^-$, H$_2$D$^-$, D$_2$H$^-$, and D$_3^-$, of the ion. We have also calculated the cross-section for the formation of H$_3^-$ by radiative association: H$_2+$H$^-\to$H$_3^- +\hbar\omega$. The obtained results suggest that the H$_3^-$ ions can be formed in the interstellar medium (ISM). An eventual detection of H$_3^-$ in the ISM would also be a proof that the H$^-$ ion is present in the ISM.