2 Overlooked unusual features of KT Mon

In addition to the unusually long distance estimate (Payne-Gaposchkin 1977), which originally questioned the classical nova-nature of KT Mon, we further noted a previously overlooked peculiarity from the available material.

The early presence of high excitation lines, particularly HeII $\lambda$4686 and NIII $\lambda$4640, is unusual for typical development of nova spectra (Payne-Gaposchkin 1957; Bode & Evans 1989; Williams 1992), since strong appearance of NIII $\lambda$4640 Bowen brend requires high-energy photons (van Paradijs & McClintock 1995). Such a condition is hard to achieve during the early stage of a classical nova outburst when thermal emissions from a lower ($\sim$10⁴ K) temperature expanded atmosphere of the white dwarf dominates. High-excitation emission lines can appear when the contracting nova (pseudo)photosphere reaches $T_{\rm eff} \sim 2.5\times 10^4$ K (see e.g. Bode & Evans 1989 Chap. 5), which then produces sufficient high-energy UV photons. This condition is expected to be achieved at $\sim$2.5 mag below the optical maximum, which agrees with the general observational records (Williams et al. 1994). From an examination of photographic records of past nova spectra, these lines were reported to appear at around 4 mag below the optical maximum. Only a few very fast novae (e.g. V2487 Oph, Filippenko et al. 1998) and fast recurrent novae (e.g. U Sco, Sect. 3.1) are known to exhibit these high-excitation lines soon after their maxima, which are presumably caused by an unusually thin ejecta. Since the decline rate of KT Mon was not particularly fast, this object apparently did not achieve this condition at the time of Vyssotsky's observation.

A relatively large ($\sim$1.5) color index (blue-red) inferred by Gaposchkin (1954) also needs to be reexamined. Taking into the recent observation of H$\alpha$ flux of post-outburst novae (Ciardullo et al. 1990) into account, the contribution of the H$\alpha$line to the R band is estimated to $\sim$1.0 mag. Although much of the originally reported color index was tentativity attributed to interstellar reddening in interpreting KT Mon as being a classical nova (Payne-Gaposchkin 1977; Shafter 1997), a more recent estimate of the maximum reddening in this direction E(B-V) = 0.545 or A_V = 1.806 (Schlegel et al. 1998) suggests that the observed color more strongly reflects the contribution of the H$\alpha$ emission line. If the ejecta was indeed unusually thin as in recurrent novae, the contribution of the H$\alpha$ line, however, might be insufficient (cf. Sekiguchi et al. 1988) to explain the overall color index.