Detection of a Molecular Hydrogen Envelope around Nova GK Persei

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>18</mml:mn> <mml:mo accent="false">′</mml:mo> <mml:mo>×</mml:mo> <mml:mn>10</mml:mn> <mml:mo accent="false">′</mml:mo> </mml:math> , is cospatial with the H α 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 μ m line. A filament of emission in the S (9) 4.6947 μ m 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 H 2 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 H 2 envelopes, it ostensibly suggests that the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>18</mml:mn> <mml:mo accent="false">′</mml:mo> <mml:mo>×</mml:mo> <mml:mn>10</mml:mn> <mml:mo accent="false">′</mml:mo> </mml:math> nebulosity is a conventional PN with a luminous, ionizing central source. We show this is not the case, and that the H α 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.