The 2021 Hot-type Symbiotic Outburst of YY Herculis: An Optical Follow-up Study

Abstract We have followed up on the hot-type classical symbiotic outburst reported in YY Her using the Himalayan Chandra Telescope. The outburst coincides with the secondary minima of the system. Approximately 12 similar brightening events have been reported between 1890 and 2020, with only the 1993 outburst being studied spectroscopically. In our study, we monitored the system from 2021 to 2023, covering ∼1.5 orbital cycles, providing an opportunity to understand the spectral evolution of the outburst over a complete orbital period of YY Her. We found that the temperature and luminosity estimations based on emission line fluxes exhibit orbital phase dependence. The values estimated at phase 0.5, corresponding to the secondary minimum, were the most reliable. The temperature of the hot component is ≈1.41 × 10 5 K, and the luminosity is ≈1020 L ⊙ during the outburst, reduced to ≈1.3 × 10 5 K and ≈830 L ⊙ after one orbital cycle at phase 0.5. Temperature estimations during the outbursts suggest that YY Her exhibits both hot-type (2021) and cool-type (1993) behavior, similar to another symbiotic star, AG Dra. Using variations of the Ca II absorption lines, we confirmed the contribution of the ellipsoidal effect in secondary minima in the YY Her light curve.