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Detection of a molecular hydrogen envelope around nova GK Persei

D. P. K. Banerjee, A. Evans, T. Liimets, C. E. Woodward, T. R. Geballe, V. Joshi, S. Starrfield

Abstract

The eruption of Nova Persei 1901 (GK Per) occurred 125 yrs 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 it has a bipolar molecular hydrogen shell. This shell, which has dimensions 18'x10', is co-spatial with the Halpha nebulosity surrounding the nova, which is purported to be an ancient planetary nebula (PN). The shell is detected most strongly in the 0--0 S(9) 4.6947 micron line. A filament of emission in the S(9) 4.6947 micron line is seen 45" SW of GKPer. This coincides, over much of its length, with the site of X-ray and non-thermal 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 18'x10' 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 pre-existing material that was ionized during the 1901 eruption. The ionized gas is presently undergoing recombination on a timescale of ~3000 years, explaining why the nebulosity is still visible.

Detection of a molecular hydrogen envelope around nova GK Persei

Abstract

The eruption of Nova Persei 1901 (GK Per) occurred 125 yrs 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 it has a bipolar molecular hydrogen shell. This shell, which has dimensions 18'x10', is co-spatial with the Halpha nebulosity surrounding the nova, which is purported to be an ancient planetary nebula (PN). The shell is detected most strongly in the 0--0 S(9) 4.6947 micron line. A filament of emission in the S(9) 4.6947 micron line is seen 45" SW of GKPer. This coincides, over much of its length, with the site of X-ray and non-thermal 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 18'x10' nebulosity is a conventional PN with a luminous, ionizing central source. We show this is not the case, and that the H nebulosity may be surrounding gas belonging to pre-existing material that was ionized during the 1901 eruption. The ionized gas is presently undergoing recombination on a timescale of ~3000 years, explaining why the nebulosity is still visible.
Paper Structure (3 sections, 3 figures)

This paper contains 3 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Observations and Results
  3. Discussion

Figures (3)

  • Figure 1: The top panel shows a deep image of GK Per and its surroundings obtained by the Deep Sky Collective (DSC). The total exposure time was 265h 50m. Images were obtained in various filters. The per-filter exposure times were 13h 00m (Lum/clear), 14h 20m (R), 16h 15m (G), 15h 45m (B), 91h 55m ([O iii], 5 nm bandpass), 14h 35m (H$\alpha$, 5 nm bandpass). For the color palette, the broadband is RGB with luminance being used for details and signal (the infrared cirrus hence appears grey). H$\alpha$ is red and [O iii] is mapped to R and G, giving it a cyan-blueish look. The field of view is $50\arcmin \times 43\arcmin.$ North is up, and East is to the left. The bottom panel shows contours at surface brightness levels of 0.72, 1.05 and 1.20 MJy/sr in the 4.6947 H$_{2}$ emission line (discussed in Section \ref{['sec:obsresults']}) observed by SPHEREx superposed on the optical image.
  • Figure 2: SPHEREx images of GK Per in the 1--0 $S$(1) 2.1218 (panel A), the combined 1--0 $S$(0) 2.2235 and 2--1 $S$(1) 2.2477 (panel B), 1--0 $Q$(1) 2.4066 (panel C) and 0--0 $S$(9) 4.6947 lines (panel D). The first three afore-mentioned images are presented with the same color and stretch settings. Panels A through D have a field of view $\approx 24\farcm0 \times 18\farcm5.$ The weakness of the combined 1--0 $S$(0) and 2--1 $S$(1) lines compared to that of 1--0 $S$(1) (2.1218 , indicates that shock excitation rather than UV-fluorescence is the excitation mechanism (see text). The 4.6947 image (middle right) shows a filament of H$_{2}$ emission about 44 south-west (PA = $145^\circ$) of the central star. This filament is magnified in panel E (North to the top) and shown between markers in cyan. Overlaid on this image, but now shown in red in panel F, is an H$\alpha$ image taken by us in 2023 showing the nova ejecta of 1901 in blue. The SW rim of the nova ejecta coincides with the filament over a large extent. The position of the filament was the site of intense X-ray and non-thermal radio emission. The size scale of 10 valid for panel E and F only is inset in the lower right of panel F.
  • Figure 3: SPHEREx spectra at selected positions on GK Per whose image in the H$_{2}$ 0--0 $S$(9) 4.6947 line is shown at center. Markers on the image indicate positions where spectra are shown here and whose spatial location on the nebula and corresponding RA and Dec (2000) positions (in degrees) are indicated at the top left in each panel. The top left panel shows a representative spectrum covering the entire 0.75--5 region measured at position 1; the rest of the spectra focus on the region covering the 0--0 $S$(9) 4.6947 line which was the strongest line in the spectrum and which is seen at almost all the positions. More details are given in the text (Section \ref{['sec:disc-results']}.) The upward rise of some the spectra near 5.0 is likely due to the 0--0 $S$(8) 5.0529 line.