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 $\sim$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 $\approx 1.41 \times 10^5 \, \text{K}$, and the luminosity is $\approx 1020 \, L_{\odot}$ during the outburst, reduced to $\approx 1.3 \times 10^5 \, \text{K}$ and $\approx 830 \, L_{\odot}$ 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.

Figures (14)

• Figure 1: YY Her outburst light curve shown in multiband photometry available from GAIA, ASAS-SN and ZTF. The 2013 and 2021 outbursts are visible in the light curve. An offset is applied to ASAS-SN V band for clarity.
• Figure 2: Combined V and g band data from ASAS-SN, ZTF, AAVSO and literature after applying appropriate shift (see text for details).
• Figure 3: LSP of YY Her using ASAS-SN g and V band, ZTF g band, AAVSO V band, V data compiled from the literature and combined data is plotted (see text for details). The period corresponding to primary minima and secondary minima is marked with dashed lines.
• Figure 4: YY Her light curve shown in GIT multi-band photometry. Phase estimated based on the ephemeris given in Equation \ref{['eq:ephemeris']}. Offsets are applied for clarity.
• Figure 5: Spectral energy distribution of the YY Her obtained from different photometric bands. (see Section \ref{['sed']} for details)