Thread by @realscientists

Right, drum role please, let's talk about...

...BINARY STARS!

So that's a pretty cool gif as I think it's the binary star Algol and I've got a feeling the data is from @astroprofhoff although she might tell me different.

But yes so, binary stars, these are systems where two stars are gravitationally bound to one another and orbit each other. Much like you can think of the Earth-Moon system being a binary planet, or imagine taking away the Earth and replacing it with another Sun.

They're useful because if they're like the stars in this gif and are eclipsing - so the light dips a little twice each orbit as one passes in front of the other - and we measure their velocities we know everything about the stars. Their mass, radius, surface temperature and...

...luminosity. The only star we know better is the Sun! Or those which have been studied with asteroseismology. So binary stars can be useful tools to understand and calibrate our stellar models. So here in this plot form my paper arxiv.org/abs/1710.02154 you can see eclipsing...

..binaries plotted in the triangles against a contour of where we'd expect to see stars in the Galaxy as well as stellar tracks of different initial masses. We see most point lie on the darkest contours and the colours of the observations match the colours of the tracks at the...

...expected luminosity and temperature from the models. So yay! We matched observations. :o) But what else are binaries good for? Well they can get in each other's way!

So Algol in the annimation at the top really started it all. When people first tried to understand it, it was confusing. More massive stars normally evolve faster and should be brighter than less massive stars but in Algol the less massive star is brighter!

The solution is that the two stars have interacted and transferred material from the initially more massive the the initially less massive and so the mass ratio reverse but the initially more massive star is still effectively "older" or more evolved than the other star.

We end up seeing the core of the more massive star and in low mass stars these will evolve on to become white dwarfs. In massive stars instead of seeing a hydrogen-rich type II supernova we'll end up seeing a type Ib/c supernova which has no hydrogen. So these binary interactions

...are important for understanding the evolution and ages of stars and supernovae.

The problem is that when the cores of the stars are revealed after the interaction they tend to be hotter than they were. This means if we're looking at a population of stars with a number of binaries that have interacted it will look bluer than we'd expect if all stars...

...were single. This is basically the main theme of my research, working out how interacting binaries change the appearance of stellar populations, i.e. entire galaxies. The thing is for decades people have typically assumed all stars are single or binary stars don't matter.

This is despite astronomers seeing binaries everywhere, it's just making single star models is "easy". We only have to worry about the initial mass of the star to determine it's evolution over it's lifetime. In a binary, we have the mass of a second star to worry about as well...

...as the initial period. (If we're worrying about it we should think about their initial rotation rates, whether they're in a triple or higher order system with more companion stars, and other things...)

So to make a grid of models assuming they're all single I have to make a few hundred models, other groups make fewer than that! If I want to model binary stars accurately I need to calculate 250,000 or so.

Some might say "well that's not so many", it's not I guess but it's still difficult enough. We're talking about terabytes of data, many many CPU days, and a lot of human input. I had to edit my stellar evolution code so that it does as much automatically as possible.

What is unique about my binary models compared to others though is that I use detailed evolution models so I model the full structure. Most other binary codes for making large populations use rapid and approximate models, which is fine but you lose some detail when stars...

...interact as you don't have the structure. Also you can't explode the stars like I mentioned a few days ago. So there are advantage in having the detail but the code is more difficult to run and takes much longer.

Anyway so the result is I have my large set of binary stellar evolution models, I then weigh each model so that my synthetic population of stars matches that we observe in the Universe. We can then "observe" my models to see what we see if it matches "real" stars. :o)

The other thing is I make my models publicly available so other astronomers can download and use them here: bpass.auckland.ac.nz. The great thing is some interesting results have come from using my BPASS without any input from me. :o)

Btw BPASS = Binary Population And Spectral Synthesis. The "spectral" synthesis being where I attach atmosphere models to the stellar models so we know what it would look like. We can then combine the spectra of all the stars and work out what entire galaxies would look like!

The funny thing Assoc. Prof. Elizabeth Stanway from @WarwickAstro did this to study high redshift galaxies. Turns out to explain specific lines in these galaxies you have to include binary stars. Other people are also finding more and more cases where stellar populations based...

...only single stars do a bad job of matching observed galaxies and stars. The binary populations seem to include a natural complexity that we see in stellar populations. This has lots of implications for estimating ages of stars.

Elizabeth's most recent paper: arxiv.org/abs/1805.08784

Showed this in a big way. We compared the ages we derive with our latest models to those from other people's single star models. The results were significant to say the least. In this plot we plot BPASS ages vs single-star

ages as you can see the single-star ages are all close to 14 billion years, the age of the Universe. Where the BPASS ages give a spread of answers. In some cases the difference is about 8 billion years (1Gyr=1billion years). That's quite a big difference!

We're still working on which is the correct answer and there is still a lot of work to do but we do seem to get more sensible results with BPASS. For example the older galaxies appear to be more metal poor so enriched by less material from stars. While younger galaxies are more..

...metal rich.

Anyway what all got me into this game in the first place was trying to reproduce the number of hydrogen rich to hydrogen free supernovae. You can only reproduce the observed rate if you have lots of binary stars. About 1 single star for 1 binary.

So yes, binaries are cool and we really need to rewrite some textbooks to include them. Single star evolution is the exception for stars rather than the rule.

So as one final point you can see the difference here in a Hertzsprung-Russell diagram for hydrogen-free stars. On the left we have single stars on the right binary stars (examples if I put on all the lines it'd be a mess). Here we plot a stars luminosity vs it's temperature.

The contours show where we expect Wolf-Rayet stars from the models. We see on the single stars there are many that are lower luminosity than possible from single stars. While for binaries we get all the stars. Normally stars that would explode as red supergiants lose all...

...hydrogen so explode as hot dense helium stars which is what the Wolf-Rayet stars are. And only binaries can make the full range of those Wolf-Rayet stars we see.

Btw the colours coding here is surface hydrogen abundance, red=none. So note the agreement isn't perfect.

That means my models are too hot compared to the observations but the atmospheres of these stars are complicated and while the luminosity is correct we're still all trying to work out the atmospheres of Wolf-Rayet stars. Which is exciting new science. :oD

Right, that was a rather large thread but I hope you all found it interesting. And there is much, much more I could say about binaries but I'll leave it here. :oD