Superflares and Variability in Solar-Type Stars with $\it TESS$ in the Southern Hemisphere (2003.14410v1)

Abstract: Superflares on solar-type stars has been a rapidly developing field ever since the launch of $\it Kepler$. Over the years, there have been several studies investigating the statistics of these explosive events. In this study, we present a statistical analysis of stellar flares on solar-type stars made using photometric data in 2-min cadence from $\it TESS$ of the whole southern hemisphere (sectors 1 - 13). We derive rotational periods for all stars in our sample from rotational modulations present in the lightcurve as a result of large starspot(s) on the surface. We identify 1980 stellar flares from 209 solar-type stars with energies in the range of $10^{31} - 10^{36}$erg (using the solar flare classification, this corresponds to X1 - X100,000) and conduct an analysis into their properties. We investigate the rotational phase of the flares and find no preference for any phase suggesting the flares are randomly distributed. As a benchmark, we use GOES data of solar flares to detail the close relationship between solar flares and sunspots. In addition, we also calculate approximate spot areas for each of our stars and compare this to flare number, rotational phase, and flare energy. Additionally, two of our stars were observed in the continuous viewing zone with lightcurves spanning one year, as a result, we examine the stellar variability of these stars in more detail.

• The paper applies high-cadence TESS photometry to 209 stars, identifying 1980 flares with 92% qualifying as superflares (energy >10^33 erg).
• The paper correlates stellar rotation periods (0.24–11.16 days) with decreasing flare activity, suggesting complex starspot interactions.
• The paper demonstrates no significant phase-flare correlation, challenging solar analogies and pointing to a more distributed magnetic energy release.

Superflares and Variability in Solar-Type Stars in the Southern Hemisphere

The investigation into superflares on solar-type stars offers valuable insights into stellar activity that extends beyond our own Sun. This paper conducts a detailed statistical analysis of stellar flares from 209 solar-type stars using 2-minute cadence photometric data. The data spans the entire southern hemisphere, covering sectors 1 through 13, which provides a comprehensive view of flaring activity on stars classified as F7 to K2 by means of the SIMBAD catalog. The paper identifies and characterizes 1980 stellar flares with energies ranging from $10^{31}$ to $10^{36}$ erg.

Key Findings

1. Flare Properties and Energies: Through its detailed analysis, the paper finds that approximately 92% of the identified flares qualify as superflares (energies over $10^{33}$ erg). The synchronicity in observational cadence significantly contributes to detecting both high and low energy flares. A noteworthy observation is the random distribution of flares over the stellar rotational phase.
2. Rotational Modulation and Spot Effects: The rotational period aspect of these flaring stars is of particular interest. With periods ranging from 0.24 to 11.16 days, the paper notes a decline in flare activity with an increase in rotation period, albeit with a wide scatter, indicating complexities in how starspots influence flare production.
3. Lack of Phase-Flare Correlation: In stark contrast to solar physics, the paper finds no significant correlation between flare occurrence and the rotational phase of these stars, suggesting a potential divergence from the Sun's more understood flare-spot relationship. This could imply a more complex distribution of magnetic energy across these stars, independent of the visible starspots.
4. Solar Analogies: By comparing its findings to solar phenomena, the paper highlights that lower energy solar flares seem to display anti-correlative behavior with sunspots during solar minima. This may suggest that lower energy flares on solar-type stars are not primarily associated with large starspots.

Implications and Future Directions

The implications of these findings are multifaceted. On a practical level, understanding the behavior of superflares on solar-type stars enriches our knowledge of stellar atmospheres and can influence how we estimate the magnetic activity cycles of stars similar to our Sun. From a theoretical perspective, the paper hints at the need for revised models of starspot and flare interactions in stellar environments distinct from the solar model, considering the randomized nature of flare occurrences observed over rotation phases.

Future directions could involve conducting longitudinal studies that extend observation beyond the current temporal limitations, providing deeper insights into the long-term cycling activity of stars, similar to what is achieved with the Sun. Moreover, incorporating data from other instruments (such as those providing spectroscopic measurements) might help explicate the possible mechanisms of flare genesis and interaction with starspots.

Overall, this paper on superflares and solar-type stellar variability enhances our comprehension of stellar magnetic activity and serves as a springboard for further research into the magnetic and photometric variability of main-sequence stars.