Flares and superflares on the southern active binary CC Eri

TL;DR

This study presents a comprehensive optical spectroscopic census of flares on CC Eri, a nearby, very active SB2, using 70 PUCHEROS+ Echelle nights and complementary photometry. Thirty-one flares are identified spectroscopically, eight with coordinated g' photometry and two reaching bolometric flare energies well above $10^{33}$ erg, i.e., superflares; continuum enhancements (white-light) are detected for the two strongest events. The spectra reveal rich excess emission across dozens of lines, along with pronounced Hα line asymmetries, including blue wings that indicate potential prominence eruptions, and red-wing signatures consistent with coronal condensation. The results support the interpretation that superflares on CC Eri are scaled-up versions of normal flares, though some limb- or high-energy events may involve somewhat distinct effects or geometries. Overall, the work demonstrates the valuable role of time-resolved spectroscopy in linking flare energetics to line formation, plasma motions, and potential eruptive phenomena in active binary stars.

Abstract

Flares and CMEs are known to be the dominating high-energy phenomena on cool stars. Superflares were thoroughly investigated using broadband photometry predominantly from Kepler, K2, and TESS. Here we present a spectroscopic investigation of superflares on the very active spectroscopic binary CC~Eri. We focus on spectroscopic signatures of (super)-flares and line asymmetries with the goal to characterize superflares spectroscopically. In 70 nights of spectroscopic observations obtained at the ESO~1.52m telescope with the Echelle spectrograph PUCHEROS+ hosted by the PLATOSpec consortium we identified 31 flares from which two are superflares already from the deduced g'-band energies. We also find a broad blue-wing asymmetry occurring in the impulsive phase of another superflare which shows great potential to be a prominence eruption. For the second most luminous flare we find the largest number of excess emissions during the impulsive and gradual flare. We identify the flare to be occurring close to the stellar limb which indicates that the flare was even more energetic than derived from its g'-band and spectral line energies. We identify more than sixty spectral lines in the spectral range of 4100 and 7200Å showing excess emission during this flare. We detect continuum enhancements as well as photospheric line fillings during the flare. Generally we find that depending on the flare energy the number of spectral lines revealing excess emission increases, especially for the more energetic superflares. We therefore conclude that superflares are likely scaled-up versions of less energetic normal flares.

Figures (32)

• Figure 1: Only TESS light curve (sector 3) of CC Eri. Flares are indicated by blue arrows.
• Figure 2: Histograms of timing and energy information of the flares. Upper left panel: Histogram of the length of the H$\alpha$ impulsive phase. Upper right panel: Histogram of the length of the H$\alpha$ gradual phase. Lower left panel: Histogram of the H$\alpha$ full flare duration. Lower right panel: Histogram of H$\alpha$ flare energies.
• Figure 3: Cumulative flare frequency distribution of the H$\alpha$ flares of CC Eri of the present study.
• Figure 4: H$\alpha$, H$\beta$, H$\gamma$, H$\delta$, HeI D3 (5876Å), NaD2+D1 and g'-band (from left to right and top to bottom) light curves of CC Eri in the night of 2023-11-14 (flare no. 4) where the most energetic H$\alpha$ flare occurred during the monitoring of CC Eri. The shaded area denotes the part of the EW light curve belonging to the flare.
• Figure 5: SA200 spectrum of CCEri obtained from all spectra of the night of 2023-11-28.