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We have already established in Chapter 3 that the SBK2 fast rotators appear to be overlumi- nous compared to normal field main-sequence stars. To verify this for our NYMG candidates, we create color-absolute magnitude diagrams using the magnitudes, colors, and parallaxes from GAIA. Figure 4.5 shows these diagrams, where the fast rotators which are members of a specific cluster/group are shown as color symbols. The black points represent 10,000 randomly selected SUPERBLINK stars, which act as comparison sample, representing the normal (older) main-sequence stars in the field.

Figure 4.5 plots from left to right (1) Upper and Lower Sco star forming regions (10 Myr). (2) Candidate members of the Pleiades cluster (100 Myr). The diagram shows a full sequence of FGKM dwarfs. The FG dwarfs lie on the ZAMS while the K dwarfs lie slightly above it. The M dwarfs are the most “elevated” of the cluster. As we explained in

Figure 4.5 Color-magnitude diagram utilizingGAIA distances of all SBK2 stars overplotted with the subset of NYMG members, including the new candidate associates.

Section 3.4, this most likely is not because the stars are still contracting (pre-main sequence stars) but more probably because these stars are “puffed up” due to magnetic inflation, and thus shifted red in the diagram compared to normal field stars. Interestingly, the figure also suggests that only the highest mass (FG) and lowest mass (M) stars in these clusters and NYMGs are rapid rotators with very few K dwarfs detected as rapid rotators. The highest mass stars are rapid rotators because they do not experience magnetic braking and never lost their rapid rotation. The lowest mass stars have now begun to slow down (see Figure 4.4) but because they experience weaker braking or are stars of a faster initial spin rate, we find them still being registered as fast rotators at that age. We are likely missing the K dwarfs in the fast rotator sample because those have already slowed down significantly from magnetic braking, and are no longer registering as fast rotators, now having rotation rates slower than 3.5 days, even at the relatively young ages of these clusters and moving groups. This suggests that K dwarfs in NYMGs may not be easy to find based on rotation alone, or at least that they will not stand out as being very fast rotators.

Finally, Figure 4.6 plots the GAIA derived color-magnitude diagram for the four can- didate groups that we identified in Section 4.2. Interestingly, each group shows similar characteristics to each other and to the Beehive and Hyades clusters. Each group has a small number (if any) high mass stars that are rapid rotators. Each group shows a majority of low mass stars that are overluminous compared to the SBK2 field population. And, as we saw in the right panel of Figure 4.5, there are almost no K dwarfs that register as rapid rotators. We saw in Figure 4.4 that the candidate groups shared similar characteristics to

the Pleiades in rotation-color space. However, the candidate groups look more similar to the older Hyades and Beehive clusters in color-magnitude space, which this time suggests that these groups might in fact be somewhat older than the Pleiades. It is however possible that the difference may simply be due to the smaller number of stars identified in these groups, and that the identification of larger numbers of these stars may better reveal the full morphology of the main-sequences in these moving groups.

Figure 4.6 Color-magnitude diagram utilizingGAIA distances of all SBK2 stars overplotted with the subset of NYMG members, including the new candidate associates.

WHITE LIGHT FLARES IN FAST ROTATORS

5.1 Introduction

Chromospheric emission for Sun-like stars results, in part, from the heating of the stellar atmosphere magnetically through the αΩ magnetic dynamo process. The process occurs at the tacholine which is the boundary between the radiative core and convective envelope of these intermediate mass stars. The Ω effect occurs due to the winding of magnetic fields lines by the differential rotation of the star. Then, theαeffect is the twisting of the magnetic field lines (Newton et al. 2017). Eventually the magnetic field lines reconnect. This reconnection process fuels the observed high-energy flares.

However, we also observe high-energy chromospheric emission in low-mass stars across the entire electromagnetic spectrum. The origins for this process cannot be the same as the αΩ process as late-type M dwarfs are fully convective (M <0.35M). We also observe complicated, randomly occurring flare events in the white-light continuum of low-mass stars. The connection between these white light flare events and high energy flares is not currently understood for M dwarfs. (Davenport et al. 2014).

In any case, these events are an indicator of magnetic activity which is generally assumed to be an indicator of youth. Because magnetic activity is driven by the rotation of a star, one expects stars with fast rotations rates, i.e. relatively young stars, to be significantly more active. We therefore wish to examine the occurrence of white light flare events in the SBK2 fast rotator subsample. Our main purpose to further confirm that the stars we

have identified are active stars, and look for any pattern in the flaring behavior of these fast rotators.