Hydrogen shell detected around Nova Persei 1901 may be a planetary nebula

Using NASA's SPHEREx space telescope, astronomers have observed remnants of the eruption of Nova Persei 1901. As a result, they detected a bipolar molecular hydrogen shell around this nova, which may be a large planetary nebula. The finding was detailed in a paper published March 13 on the arXiv pre-print server.

The mystery of a century-old nova eruption

Nova Persei 1901 (or GK Persei) was the first bright classical nova of the 20th century. It has a round overall morphology consisting of almost a thousand cometary-like structures, and currently extends to a radius of about 1.0 arcminute.

Although Nova Persei 1901 has been extensively studied for over a century, it still holds many surprises. Previous observations have found that the remnant of this explosion may be best described as a thick spherical shell or an equatorial barrel feature with polar cones.

One of the studies detected a larger (about 20 arcminutes in size) bipolar structure, compared to the nova remnant, and it was suggested that it may be a planetary nebula (PN)—an expanding shell of gas and dust that have been ejected from a star during the process of its evolution from a main sequence star into a red giant or white dwarf.

New findings from SPHEREx

Now, new SPHEREx observations of Nova Persei 1901 conducted by a team of astronomers Dipankar Banerjee of the Physical Research Laboratory (PRL) in Ahmedabad, India, provide more evidence supporting the PN hypothesis.

"SPHEREx obtained 114 spectrophotometric images of GK Persei and its environs, covering all 102 of its wavelength bands. The exposure time for each image was 113.58 seconds. Emission was detected in about 30 of the images," the researchers explain.

The observations found that Nova Persei 1901 has a bipolar molecular hydrogen shell with a dimension of 18 x 10 arcminutes. This shell is co-spatial with a hydrogen-alpha nebulosity surrounding the nova. The mass of the shell is estimated to be within the range of 0.28–0.38 solar masses.

Planetary nebula or something else?

Given that planetary nebulae, especially the bipolar ones, often host molecular hydrogen shells, the astronomers therefore assume that the detected shell is a PN. Moreover, the mass of the shell further supports this scenario, as PNe generally have masses between 0.1 and 1.0 solar masses.

However, the authors of the paper note that the bipolar shape requires a dense equatorial constriction, for instance a torus, to shape the nebula, but such a torus is not seen in the collected SPHEREx images.

Furthermore, the results suggest that the PN would have faded below detection level by the time the nova erupted in 1901, so it should not be possible to observe the PN and the nova remnant at the same time.

The other possibility considered by the researchers is that the shell is actually a material that existed before the eruption in 1901. Afterward, it was ionized by the nova outburst and is presently in the recombination phase.

These uncertainties therefore underscore the need for further investigation of Nova Persei 1901.

"GK Persei and its environment remain partially understood and the more we discover about it, the more we realize how incomplete is our understanding of it. There is a need for a spatio-kinematic study of the nebulosity to further test its classification as a PN, and thence understand its relation to the 1901 outburst," the scientists conclude.

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