Object 13: BV Centauri
Podcast release date: 27 January 2020
Right ascension: 13:31:19.5
Corresponding Earth location: 2500 km southeast of New Zealand in the South Pacific Ocean
BV Centauri (or BV Cen for short) is a system located in the constellation of, unsurprisingly, Centaurus. It was first noted in a paper by W. F. H Waterfield in 1929 as one of 25 new systems in Centaurus which showed variability . Since then, it has been confirmed to belong to the U Gem subclass of Cataclysmic Variables . There's a few phrases in there which are likely unfamiliar, so let's start with cataclysmic variables.
These are binary star systems which have white dwarf primaries that are stealing matter from a nearby companion star. They are often subdivided into further classes based on what the matter is doing. For example, the behaviour of systems whose white dwarfs have very strong magnetic fields is very different to those whose white dwarfs have negligible magnetic fields. There are over 1,000 systems known to belong to the cataclysmic variable class , with the majority of them being classified as a U Gem system.
U Gem stars are named after the archetypical system U Geminorum. In these systems, the material which the white dwarf steals from the companion forms a disc of material around the white dwarf. Once this disc has built up enough material, a thermal instability is triggered which causes the disc to dump a lot of matter down onto the white dwarf. This results in the disc becoming very hot, meaning we see the system brighten by up 3 magnitudes at optical wavelengths . These brightenings are called dwarf nova outbursts, and are very different from nova outbursts, during which thermonuclear runaway occurs on the surface of the white dwarf and the system brightens by many more than 3 magnitudes.
BV Cen is one of these systems. Typically, it varies between an optical magnitude of 12.9 and 13.3 over a time scale of 14.6 hours . This variability is caused by the movement of the accretion disc and the companion star around the white dwarf over the orbital period. On top of this variability, the system regular brightens up to a magnitude of 10.7 every 150 days or so, which is due to the previously mentioned instability in the accretion disc.
So, out of the many known cataclysmic variable systems that are known, why is BV Cen interesting? It's white dwarf has a mass around 0.8 that of our Sun's . This is a completely bog standard value for the mass of a white dwarf of a cataclysmic variable , so it's not that. What about it's orbital period? Since the separation between the primary and secondary star in a binary system is dependant on the orbital period, we would expect that systems with short orbital periods (meaning the two stars are very close) are the most likely to interact and produce dwarf novae outbursts. This turns out to be true, with the majority of cataclysmic variables having an orbital period that is less than 8 hours .
At 14.6 hours, BV Cen has such a large binary separation that we would not expect the white dwarf to be stealing material from its companion, meaning it should not should outbursts. And yet it does. This has led some researchers to believe that thermonuclear runaway occurred on the surface of the white dwarf some time in the early 1900s. . This eruption would have led to the companion star being subjected to an intense amount of radiation, causing it to inflate and begin the process of mass transfer . As such, the very rare long period cataclysmic variables such as BV Cen are key in showing us how nova eruptions and dwarf nova systems are related.
There is also another reason BV Cen is an interesting object. It is one of a few systems which researchers have been able to accurately reconstruct the surface of the companion star. This has been done by acquiring optical spectra of the system, isolating the features of that spectrum which we believe come from the companion star, and then, much like facial reconstruction, these features were used to build an image of the stars surface . The results are quite striking, as they show that a large fraction of the surface of the star is covered by star spots. These observations have helped re-enforce the idea that the companions in cataclysmic variables are magnetically active and, more importantly, that this magnetic activity will have a role to play in the systems future evolution. This is due to the fact that the number and location of these spots on the stellar surface is likely to change as the companion star goes through its magnetic cycle, which will directly impact how mass is transferred from the companion to the white dwarf.
 Waterfield, W. F. H., Twenty-five New Variable Stars in Centaurus, 1929, Harvard College Observatory Bulletin, 863, 6
 Turner, H. H., U Geminorum, Pogson's observations of, edited, 1906, Monthly Notices of the Royal Astronomical Society, 67, 119
 Ritter, Hans and Kolb, Ulrich, Catalogue of Cataclysmic Binaries, Low-Mass X-Ray Binaries and Related Objects, RKcat Edition 7.24, 2015
 Osaki, Yoji, Dwarf-Nova Outbursts, 1996, Publications of the Astronomical Society of the Pacific, 108, 39
 Vogt, N. and Breysacher, J., The dwarf nova BV CEN : a spectroscopic binary., 1980, Astrophysical Journal, 235, 945
 Gilliland, R. L., A time-resolved spectroscopic study and modeling of the dwarf nova BVCen., 1982, Astrophysical Journal, 263, 302
 Zorotovic, M. and Schreiber, M. R., The origin of single low-mass WDs: another problem that consequential angular momentum loss in CVs might solve, 2017, Monthly Notices of the Royal Astronomical Society, 466, L63
 Menzies, J. W. et al., BV-Centauri - Dwarf of Classical Nova, 1986, Astrophysics and Space Science, 122, 73
 Hillman, Yael et al., A unified theory of cataclysmic variables from self-consistent numerical simulations, 2020, arXiv e-prints, arXiv:2001.05025
 Watson, C. A. et al., Roche tomography of cataclysmic variables - IV. Star-spots and slingshot prominences on BV Cen, 2007, Monthly Notices of the Royal Astronomical Society, 382, 1105