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Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae since 2003

N. P. Ikonnikova, V. I. Shenavrin, G. V. Komissarova, M. A. Burlak, A. V. Dodin

Abstract

We present photometric and spectral monitoring of the symbiotic system HM Sge, which consists of a Mira variable with a dust shell and a hot white dwarf ionizing the surrounding gaseous nebula. The system underwent a nova-like outburst in 1975 and experienced a high-activity episode during 2018-2021. UBV photometry from 2003 to 2025 shows monotonic fading at about 0.05 mag/yr until 2018, followed by a 0.3 mag brightening peaking in 2021 and a decline by 2025 to the faintest level in five decades of monitoring. Near-infrared observations (JHKLM, 2009-2025), combined with archival data, reveal Mira pulsations with a period of 532 d and long-term variability driven by changes in the optical depth of the dust shell. Spectral monitoring (2016-2025) reveals a substantial evolution in the emission spectrum of the gaseous envelope. The 2018-2021 high-activity episode was accompanied by enhanced fluxes in recombination lines (H I, He I, He II) and forbidden transitions ([O I], [Ar V], [Fe VI], [Ca VII]), together with a 17-fold brightening of the Raman-scattered O VI $λ$6725 line. We report the first detection of the [Fe X] $λ$6374 line in HM Sge. Monitoring this line from 2007 to 2025 shows its equivalent width growing through 2017 - indicating gradual coronal heating - then declining by 2021, likely reflecting altered accretion conditions and/or hot-component properties during the high-activity episode. We propose that both the 1975 outburst and the 2018-2021 high-activity episode may be linked to periastron passage of the binary components; if so, the about 46-year interval would constrain the system's orbital period.

Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae since 2003

Abstract

We present photometric and spectral monitoring of the symbiotic system HM Sge, which consists of a Mira variable with a dust shell and a hot white dwarf ionizing the surrounding gaseous nebula. The system underwent a nova-like outburst in 1975 and experienced a high-activity episode during 2018-2021. UBV photometry from 2003 to 2025 shows monotonic fading at about 0.05 mag/yr until 2018, followed by a 0.3 mag brightening peaking in 2021 and a decline by 2025 to the faintest level in five decades of monitoring. Near-infrared observations (JHKLM, 2009-2025), combined with archival data, reveal Mira pulsations with a period of 532 d and long-term variability driven by changes in the optical depth of the dust shell. Spectral monitoring (2016-2025) reveals a substantial evolution in the emission spectrum of the gaseous envelope. The 2018-2021 high-activity episode was accompanied by enhanced fluxes in recombination lines (H I, He I, He II) and forbidden transitions ([O I], [Ar V], [Fe VI], [Ca VII]), together with a 17-fold brightening of the Raman-scattered O VI 6725 line. We report the first detection of the [Fe X] 6374 line in HM Sge. Monitoring this line from 2007 to 2025 shows its equivalent width growing through 2017 - indicating gradual coronal heating - then declining by 2021, likely reflecting altered accretion conditions and/or hot-component properties during the high-activity episode. We propose that both the 1975 outburst and the 2018-2021 high-activity episode may be linked to periastron passage of the binary components; if so, the about 46-year interval would constrain the system's orbital period.
Paper Structure (14 sections, 11 figures, 3 tables)

This paper contains 14 sections, 11 figures, 3 tables.

Table of Contents

  1. Introduction
  2. $UBV$ observations
  3. $JHKLM$ observations
  4. Spectroscopic observations
  5. Photometric evolution
  6. Optical range
  7. Near-IR range
  8. Spectral evolution
  9. Variation of absolute emission-line fluxes
  10. Raman-scattered O vi lines
  11. [Fe X] line
  12. [O iii] line formation region: gas diagnostics
  13. Discussion
  14. Conclusion

Figures (11)

  • Figure 1: Light and color curves of HM Sge from 1976 to 2025. New data are shown to the right of the vertical dashed line.
  • Figure 2: Motion of HM Sge in the two-color diagram. Annual mean values are shown, corrected for interstellar extinction with $E(B-V) = 0.6$. Years are indicated by numbers. Data from 1976--2002 are shown as stars, and from 2003 to 2025 as circles. Gray lines show the loci of blackbody radiation (bb), main-sequence stars (V), and supergiants (I).
  • Figure 3: Light curves in the $JHKLM$ bands from 1978 to 2025. New data are shown to the right of the vertical dashed line. The period of optical brightening is highlighted in gray.
  • Figure 4: Periodogram of the $J$-band time series (1) and the corresponding phase-folded light curve with $P = 532.1$ d (2), based on the complete 1978--2025 dataset.
  • Figure 5: Light and color curves from 2008 to 2025. The dashed line shows a second-order polynomial fit.
  • ...and 6 more figures