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

TL;DR

This study monitors the photometric and spectral evolution of the symbiotic nova HM Sge from 2003 to 2025, revealing activity episodes, spectral changes, and potential orbital period constraints through multi-wavelength observations.

Contribution

It provides the first detection of the [Fe X] line in HM Sge and links nova outbursts to binary periastron passages, offering new insights into the system's activity cycles.

Findings

Monotonic fading until 2018, then brightening peaking in 2021.

Detection of the [Fe X] λ6374 line and its evolution.

Correlation of activity episodes with binary orbital dynamics.

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 $\lambda$6725 line. We report the first detection of the [Fe X] $\lambda$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.