Fine Structures in the Main Sequence Revealed by Gaia Data Release 2

The Hertzsprung–Russell diagram (HRD) is fully examined using the Fourier analysis. This work shows more stars are above the gap in the lower main sequence than below it, and this implies that stars spend more time above the gap while they undergo variability associated with the ³He instability. The enhanced HRD also shows the width of the gap is not linear and depends on the $G_{BP}-G_{RP}$ color up until $G_{BP}-G_{RP}=2.7$. Beyond this color limit, the gap is hardly seen. Besides, a new low density region is revealed for the first time centered at M _G ≈ 10.7 and G _BP − G _RP ≈ 2.8, which is below the lower right corner of the gap. This work also shows that the main sequence appears to have fine stripes where stellar densities are relatively low or high compared to their adjacent regions on the main sequence. These stripes can be seen throughout the main sequence of stars redder than G _BP − G _RP = 0.8 and are not limited to any specific color or spectral type. Slopes of these features are different from the main sequence, but are pretty consistent throughout the main sequence, with a few exceptions. We are perplexed by these new features, but the complexities of stellar atmospheric features and opacities of dwarfs may have caused these patterns.