A Quantitative Method to Investigate the Spectral Class Time Evolution of Novae
Abstract
Novae are spectroscopically classified as Fe II or He/N according to the dominance of Fe II, or He and N emission lines in their pre-nebular spectra, while spectra exhibiting both are classified as hybrid. Evidence suggests that these spectral classes represent spectral evolution. We investigate this scenario using a large spectroscopic dataset and introduce a quantitative coefficient, κ, to characterise the relative strength of Fe II and He/N emission. The coefficient is defined as the ratio of the flux of the Fe II (λ5169) line to the mean flux of five He and N emission lines, with κ>1 indicating Fe II-dominated spectra and κ<1 indicating He/N-dominated spectra. We apply this method to 26 novae using spectra from the Astronomical Ring for Amateur Spectroscopy (ARAS) Database and compare the evolution of κ with optical light curves. Fifteen of the 24 classical novae and one of the two recurrent novae exhibit at least one transition between Fe II- and He/N-dominated phases, while temporal trends suggest this fraction could approach 90% with complete spectroscopic coverage. The κ coefficient closely tracks the optical light curves and repeatedly crosses unity in novae exhibiting jitters, revealing non-monotonic spectral evolution linked to brightness fluctuations. These results support the interpretation of Fe II, He/N, and hybrid spectra as evolutionary stages rather than distinct nova types and establish κ as a quantitative tool for tracing nova spectral evolution while reducing biases from incomplete spectroscopic coverage.
Versions
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BibTeX
@misc{kopsacheili2026quantitative,
title = {{A Quantitative Method to Investigate the Spectral Class Time Evolution of Novae}},
author = {Maria Kopsacheili and Martín Guerrero and Diego Alejandro Vasquez Torres},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22432},
eprint = {2609.22432},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22432},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Quantitative Method to Investigate the Spectral Class Time Evolution of Novae
AU - Maria Kopsacheili
AU - Martín Guerrero
AU - Diego Alejandro Vasquez Torres
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.22432
UR - https://arxiv.org/abs/2609.22432
AB - Novae are spectroscopically classified as Fe II or He/N according to the dominance of Fe II, or He and N emission lines in their pre-nebular spectra, while spectra exhibiting both are classified as hybrid. Evidence suggests that these spectral classes represent spectral evolution. We investigate this scenario using a large spectroscopic dataset and introduce a quantitative coefficient, κ, to characterise the relative strength of Fe II and He/N emission. The coefficient is defined as the ratio of the flux of the Fe II (λ5169) line to the mean flux of five He and N emission lines, with κ>1 indicating Fe II-dominated spectra and κ<1 indicating He/N-dominated spectra. We apply this method to 26 novae using spectra from the Astronomical Ring for Amateur Spectroscopy (ARAS) Database and compare the evolution of κ with optical light curves. Fifteen of the 24 classical novae and one of the two recurrent novae exhibit at least one transition between Fe II- and He/N-dominated phases, while temporal trends suggest this fraction could approach 90% with complete spectroscopic coverage. The κ coefficient closely tracks the optical light curves and repeatedly crosses unity in novae exhibiting jitters, revealing non-monotonic spectral evolution linked to brightness fluctuations. These results support the interpretation of Fe II, He/N, and hybrid spectra as evolutionary stages rather than distinct nova types and establish κ as a quantitative tool for tracing nova spectral evolution while reducing biases from incomplete spectroscopic coverage.
ER -