2026ApJ..1000L..46B

The eruption of Nova Persei 1901 (GK Per) occurred 125 yr ago; remarkably, it still holds major surprises. Using data from the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), we find that it has a bipolar molecular hydrogen shell. This shell, which has dimensions $18^{\prime} \times 10^{\prime} $, is cospatial with the Hα nebulosity surrounding the nova, which has been suggested to be an ancient planetary nebula (PN). The shell is detected most strongly in the 0-0 S(9) 4.6947 μm line. A filament of emission in the S(9) 4.6947 μm line is seen ≃45″ southwest of GK Per. This coincides, over much of its length, with the site of X-ray and nonthermal radio emission where the 1901 nova ejecta impinges on the ambient medium. We propose that the H₂ emission from the filament arises from the predicted neutral zone between the forward and reverse shocks. Since it is common for bipolar PNe to be accompanied by H₂ envelopes, it ostensibly suggests that the $18^{\prime} \times 10^{\prime} $ nebulosity is a conventional PN with a luminous, ionizing central source. We show this is not the case, and that the Hα nebulosity may be surrounding gas belonging to preexisting material that was ionized during the 1901 eruption. The ionized gas is presently undergoing recombination on a timescale of ≃3000 yr, explaining why the nebulosity is still visible.