  Recent ultrafast magnetic-sensitive measurements [Phys. Rev. B 92, 184429
(2015) and Phys. Rev. B 96, 184414 (2017)] have revealed a delayed melting of
the long-range cycloid spin-order in TbMnO$_3$ following photoexcitation across
the fundamental Mott-Hubbard gap. The microscopic mechanism behind this slow
transfer of energy from the photoexcited carriers to the spin degrees of
freedom is still elusive and not understood. Here, we address this problem by
combining spectroscopic ellipsometry, ultrafast broadband optical spectroscopy
and ab initio calculations. Upon photoexcitation, we observe the emergence of a
complex collective response, which is due to high-energy coherent optical
phonons coupled to the out-of-equilibrium charge density. This response
precedes the magnetic order melting and is interpreted as the fingerprint of
the formation of anti-Jahn Teller polarons. We propose that the charge
localization in a long-lived self-trapped state hinders the emission of magnons
and other spin-flip mechanisms, causing the energy transfer from the charge to
the spin system to be mediated by the reorganization of the lattice.
Furthermore, we provide evidence for the coherent excitation of a phonon mode
associated with the ferroelectric phase transition.
