Young stars were identified through a literature search. The Gaia kinematics were run through BANYAN-SIGMA to assign each star to a young moving groups, young open clusters, OB associations, or star forming region. In total, we have a sample of 3200 young stars observed at TESS 2-minute cadence in Sectors 1-19.
To identify flares, we trained a convolutional neural network (CNN), stella on the flare catalog from Gunther+2020. The results of the training are shown left. Accuracy is the fraction of predictions the models got correct. The loss tests how far from the true values the network is labeling examples. Thus, a well performing CNN will have accuracy converge to 1 and loss converge to 0, which is what we see in stella.
We can evaluate the confusion matrix for the stella test set. This highlights examples that are:
With stella we are able to assign a probability that a given cadence is part of a flare. Here is a video demonstration of this: As the CNN moves along the light curve, the two flares light up in yellow, but also in the probability time series, shown in the bottom subplot. By assigning probabilities, which has never been done in other flare detection techniques, we are able to say something truly statistical about flare rates.
Flares are borken down into bins of Teff and by age (teal = tage < 50 Myr; yellow = tage > 50 Myr). Temperature ranges are labeled in each subplot. We note a detection bias towards lower energy flares for cooler stars.
There is a noticeable drop-off in flare rate and energy as the star's temperature increases. M and late K dwarfs (Teff < 4000 K) experience similar flare rates and energies across the entire age range of our sample. The high-energy flare tail extends out to 1035ergs and is most noticeable for tage < 50 Myr stars. Teff > 6200 K corresponds to the Kraft break and thus a change in interal structure, which may be the cause of such a lack of flares.
Here, we plot Gaia color against our measured rotation periods, Prot. Points are colored by the flare rate of that star. There is a noticeable decrease in flare rate at Gaia Bp> - Rp> = 2, which corresponds to a Teff = 4000 K. Most stars with color Bp> - Rp> < 2 do not exhibit any flares, while the cooler stars show a variety of flare rates across all rotation periods. There is a clear artificially induced break at Prot > 12 days, as the result of our rotation period metrics.
We were able to measure rotation periods for 1500 stars in our sample. There is no preference for where flares occur with respect to phase. Here a phase = 0 is the peak of the rotation modulation (i.e. the less spotted hemisphere). Doyle+2018, Doyle+2019, and Doyle+2020 found similar results across smaller samples (30-200) of G and M dwarfs in both K2 and TESS data.
The result presented here is the largest sample of young stars for which such an analysis has been completed.
Modifying the photoevaporative atmospheric mass loss equation from Owen & Wu (2017), to account for flares, we find that: