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arXiv · 2603.11777

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
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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.

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N. P. Ikonnikova, V. I. Shenavrin, G. V. Komissarova, M. A. Burlak, A. V. Dodin. 2026-03-12. Photometric and Spectral Evolution of the Symbiotic Nova HM Sagittae since 2003. https://arxiv.org/abs/2603.11777

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