Today I discovered what I think (and hope) is a new eclipsing cataclysmic variable (CV). Here is the light curve.
I’ve mentioned a few times in previous posts that searching for new eclipsing CVs would be my next substantial science project for my PhD. After the loss of the mystery transient paper (and corresponding thesis chapter), this project became the top priority, and at the moment is the only science contribution to my thesis plan.
So, this discovery is very exciting news, as it now feels like the project is going somewhere. Until now, I’d studied tens of known CV systems and found no eclipses. This is hopefully the first of many to come.
But what are cataclysmic variables? Well, the wikipedia page is a bit rubbish on this particular astro topic, but there are a few more detailed explanations online, e.g. this one. For those too lazy to click, here’s a brief explanation…
Cataclysmic variable systems consist of two stars orbiting each other in a close binary. One of the stars, originally of higher mass than the other, has evolved to become a white dwarf (having stopped burning hydrogen and lost it’s outer envelope, it becomes a hot, dense, degenerate star). Our Sun will become one of these in around 5 billion years time. The other star in the binary could be anything of significantly less mass than the other star was originally, but the most common are cool M-dwarf stars.
In CV systems, the stars are so close that the white dwarf is literally stealing gas from the companion star. Even though it has lost it’s envelope, the white dwarf is still more massive than the companion, and so it pulls gas away from the lighter star with gravity alone. The gas moves in a stream towards the white dwarf, and enters an accretion disk – a wide disk of hot gas orbiting the white dwarf. The gas slowly makes its way through the disk and accretes onto the white dwarf.
These systems were all the rage in the 1990’s, but seem to be much less of interest to most astronomers these days. However, they are particularly interesting because they are thought to perhaps be the progenitors of Type-Ia supernovae (SNe-Ia). It’s still unclear whether or not they could actually accrete enough mass to explode in a supernova, which is why they are interesting! SNe-Ia are extensively used to measure distances to far away galaxies in cosmological studies. The better we understand them, the more accurately we can understand the fundamental physics in the early universe, so they are pretty important!
Why are eclipsing systems important? My fellow PhD student Martin is conducting in depth studies of eclipsing systems. Using complex models for the two stars, the accretion disk and the bright spot (where the in-falling gas hits the disk), he can determine the masses of the two stars. A fundamental question for the theory of CVs becoming SNe-Ia would be solved if we could measure an overall increase in mass as systems evolve. He is studying a range of systems at different stages of evolution, and searching for a statistically significant difference in mass. The problem is, there aren’t quite enough long-period systems known yet, so he needs some more.
The plan is for me to study many more recently discovered CV systems, and search for eclipses. If I can find 10 or more over the next year, I should have a solid science chapter for my thesis, and at least one published paper. So far I’ve only been studying systems that have been discovered this year, via what are known as ‘dwarf novae outbursts’ (this is how most CVs are discovered). I’m now beginning to go back through the records to pick out systems which were discovered in the last 5 years, and following up any which don’t appear to have been studied yet. Hopefully this growing database of systems will provide me with plenty of targets over the next year, and I can discover more eclipsing systems. I’ll keep you posted!