GK Per (Nova Persei 1901): HST Imagery and Spectroscopy of the Ejecta, and First Spectrum of the Jet-Like Feature

TL;DR

This study uses HST imagery and spectroscopy to analyze the ejecta of GK Persei, revealing complex structures, variable physical conditions, and a jet-like feature, providing new insights into nova remnants and their interactions with the environment.

Contribution

First detailed imaging and spectral analysis of GK Per's ejecta and jet-like feature, revealing their morphology, physical conditions, and mass estimates with high spatial resolution.

Findings

Ejecta contain hundreds of cometary-like structures with varied morphologies.

The ejecta's physical conditions vary strongly on small spatial scales.

The jet-like feature is likely a shock with a mass of a few times 10^{-6} solar masses.

Abstract

We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), revealing hundreds of cometary-like structures. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with the Helix nebula). Some of the structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. A detailed expansion map of the nova's ejecta was created by comparing HST images taken in successive years. WFPC2 narrowband images and STIS spectra demonstrate that the physical conditions in the ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor the ejecta is < 1%, and the nova ejecta mass must be less than $10^{-4} \Msun$. A few of the nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [NII], [OII], [OIII], and [SII], this feature is probably a shock due to ejected material running into stationary ISM, slowly moving ejecta from a previous nova episode, or circum-binary matter present before 1901. An upper limit to the mass of the jet is of order a few times $10^{-6} \Msun$. The jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.