Last Thursday, one of the most important movers and shakers behind the entire Gaia project, Lennart Lindegren, visited our local astronomy club, ASTB, and gave an informal speech on the science of Gaia. He spent some time talking about a discovery he almost made, but missed, because he used too large pixels in his depiction of the main sequence. A very narrow gap in the lower main sequence!
Wei-Chun Jao et al wrote:
We present the discovery of a gap near MG ≈10 in the main sequence on the Hertzsprung-Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with MG representing luminosity and GBP − GRP representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the Gaia photometry — it also appears when using near-IR photometry (J − Ks vs MKs). The gap is very narrow (∼0.05 mag) and is near the luminosity-temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the H-R Diagram that hints at an underlying astrophysical cause and we propose that it is linked to the onset of full convection in M dwarfs.
The narrow gap in the lower main sequence may have to do with how heat is transported inside these small stars. All main sequence stars produce their energy at their very centers. Then this energy is transported to the surface of the star by the means of either radiation or convection, or both. Think of convection as "boiling". It is a slower means of transmitting energy than radiation.
Below a mass of 0.35 M⊕, a star is fully convective. All the energy is transported from the center to the surface of the star through the means of convection. But above 0.35 M⊕, a small radiative zone is formed at the very center of the star. Here energy is transported more efficiently, which leads to a tiny, tiny jump in luminosity. Hence the gap in luminosity between stars above and below the mass of 0.35 M⊕!
with a second main sequence of binaries located above the "normal" one.
Another truly fascinating tidbit was that the main sequences of open clusters typically show a second main sequence located above the "normal" one. The stars in the second main sequence are the same temperature as the stars in the "normal" main sequence, but twice as bright. These are unresolved binaries!
globular clusters. Source: https://arxiv.org/pdf/1806.07792.pdf
Lennart Lindegren didn't actually talk about globular clusters, but I can't resist showing you this picture from the arxiv paper by Wei-Jun Jao et al. It shows how metallicity affects the colors of the stars in globular clusters. Metal-poor clusters have blue horizontal branches and only moderately red RGB and AGB branches. Metal-rich clusters, by contrast, have short red horizontal branches and very red RGB and AGB branches. What a difference metallicity makes!
Finally, the chairman of our astronomy club, who is obsessed with finding life in space, asked Lennart Lindegren if Gaia can't be used to find Dyson spheres in space. These would be tremendously large structures made by super-advanced civilizations to trap and make use of the energy produced by their sun. Couldn't Gaia spot these humongous constructs?
Lennart Lindegren answered that Gaia has looked at millions of stars already, and trying to find Dyson spheres around some of them based on their spectrum alone would be like looking for a needle in a haystack. Well, objected our chairman, couldn't Gaia look more closely at the stars whose spectrum is weird?
Every tenth star we look at is weird, said Lindegren. Every tenth star - imagine!
There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy. Hamlet (1.5.167-8).