Susanne M. Hoffmann | Searcharxiv

arXiv2026

Two Predicted, Concentric Nova Shells Surround the Prototype Dwarf Nova U Geminorum

In the 170 years since its discovery, U Gem has been intensively studied as the prototypical cataclysmic binary star. Its massive white dwarf (WD) (~ 1.2 Msun) is cannibalizing its ~ 0.42 Msun red dwarf companion. The WD's resulting accreted hydrogen-rich envelope has previously undergone thermonuclear runaways seen as nova eruptions. These weekslong transient events brighten U Gem to m ~ -3 and eject the accreted envelope (~ 0.00001 Msun) at thousands of km/s. Ultraviolet observations show that the accreted envelope of U Gem's WD is greatly enhanced in nitrogen and depleted in carbon relative to solar composition, and that the WD's effective temperature is much hotter than that of most cataclysmic binary WDs. These suggest that U Gem underwent a classical nova eruption quite recently and should therefore still be surrounded by the ejecta of that event. Modeling of U Gem predicts a nova event ~ 1000 years ago, and the existence of two concentric shells, each of order 1 degree in size, surrounding U Gem. We obtained deep narrowband H-alpha imaging of U Gem with the Condor Array Telescope. The two concentric H-alpha-bright shells that we find, centered on U Gem, may be the first-ever predicted old nova ejecta. A transient "guest star" in the asterism Shuiwei, recorded by Chinese imperial astrologers in November 829 CE is consistent with the timing and location of U Gem's last predicted nova eruption, but the transient's absence in Japanese and Korean records weighs against this suggestion.

astro-ph.SR↗

arXiv2019

On the possibilities of classical nova identifications among historical Far Eastern guest star observations

More than 100 guest star observations have been obtained by Chinese, Korean, Japanese and Vietnamese astronomers between ~600 BCE and ~1690 CE. Comparing the coordinates from the information given in old texts for eight supernova recoveries with modern supernova remnant positions, we estimate a typical positional accuracy of the order of 0.3 to 7 deg for these supernovae. These values turn out to be also a start for the expected deviation angle between a classical nova observed as a guest star and its modern counterpart among known cataclysmic variables (CVs). However, there are considerable disagreements among modern authors in the interpretation of ancient Far Eastern texts, emphasizing the need to consult again the original historic sources, in order to improve the positioning reliability. We also discuss the typical amplitudes of well observed classical novae and find that modern counterparts of nova guest stars should be V = 18 mag and thus easily observable. In this context we also consider the "hibernation scenario" and conclude that it is impossible to decide from currently-available observations whether hibernation is common. In addition to the limiting magnitude around 2 mag for ancient guest star detections mentioned in the literature, we consider the possibility that also fainter guest stars (4-5 mag) could have been detected by ancient observers and give arguments in favor of this possibility. For these limits we compare the expected nova detection rate of ancient naked-eye observers with that during modern times, and conclude that they coincide in order of magnitude, which implies that, indeed, a considerable number of classical nova remnants should be hidden among the Far Eastern guest star reports. Finally, we present a statistical analysis of the probability of casual misidentifications based on frequency and galactic distribution of CVs in the AAVSO-VSX catalogue.

astro-ph.SR↗