The HCO⁺/N₂H⁺ Flux Ratio as a Novel Tracer of Accretion Burst Activity in Protostars

Abstract Massive young stellar objects (MYSOs) accrete mass primarily through episodic accretion events, yet detecting these events remains challenging, relying chiefly on infrared and maser variability. We investigated environmental changes during accretion burst and post-burst stages in the MYSO G36.115+0.552 using multiepoch 3 mm molecular lines observed with the Purple Mountain Observatory 13.7 m telescope during 2022–2023. Significant variability was detected in the HCO + (1–0)/N 2 H + (1–0) flux ratio. Radiative transfer analysis indicates this ratio variability likely results from temperature fluctuations driven by an accretion burst, as chemical processes operate on timescales exceeding our observational window. Correlation with a 6.7 GHz methanol maser light curve suggests the HCO + /N 2 H + ratio serves as a novel tracer of accretion bursts in MYSOs. This method complements infrared and maser techniques, providing a valuable tool for assessing the impact of accretion bursts on MYSO evolution.