2025A&A...704A.251N

Context. Connections between novae with shells and historical observations are crucial for astrophysical understanding of long-term evolution of shells and cataclysmic variables.
Aims. Three of five previously considered links are revisited here: extended features in M22 in BC48, Te-11 in 483, and AT Cnc in 1645. We aim to develop a procedure to check whether these links are credible.
Methods. Literal translations of the Chinese texts, historically based arguments, and close readings are combined with astrophysics, (peak brightness, decay time estimate, shell age expansion model calculation, etc.).
Results. (a) Nandou's second star, near which the BC48 'guest star' was reported, is identified as τ Sgr, not λ Sgr, far from the M22 location. A nova in M22 would peak at only m = 6.4 ± 1.4 mag, and thus a description as a 'blue-white' 'melon' does not fit; it was likely a comet. (b) The imprecise position ('Shen['s] east') of the 'guest star' in 483, its extended (dipper-like) radiance, and the context speak for a bolide. Considering the new (larger) Gaia distance and small extinction towards Te-11 (outside a cloud), its bi-polar morphology and current expansion velocity point to a planetary nebula; as a nova, the shell expansion age is ∼1100-2000 yr from detailed supersonic expansion calculations. (c) Most certainly, Mars was meant when the source for 1645 reported 'a large star entered Yugui'; the verb implies motion. AT Cnc lies neither in Yugui's asterism box nor in the eponymous lunar mansion range. The fluid drag expansion age of AT Cancri's ejecta is ∼128-631 yr.
Conclusions. All three exact ages are unsubstantiated. True novae or nova shells can be connected to historical records only if the position and object type are plausible. Duration, brightness (light curve), and color (evolution) should fit and could provide more astrophysical insight. Then, shell ages are sufficiently precise for properly studying secular evolution of novae, shell sizes, Hα luminosities, long-term decay, etc.