Mapping the emission lines from accreting compact objects
Cataclysmic Variables (CVs) are semidetached interacting binary systems containing a white dwarf (WD) primary and a late-type main sequence secondary which transfers mass onto the primary through Roche Lobe over flow. The mass-transfer process in CVs means that line emission is a common observational feature of the majority of the CVs, especially in te Balmer series but relatively unexplored features remain, such as the Ca II triplet lines near 860nm, in the i-band.
A subset of the CVs are the WZ Sge type Dwarf Novae (DN) systems which have very short orbital periods (around 80 minutes) and exhibit only rare super outbursts where the system brightens by more than 6 magnitudes. Few of these systems are known and even fewer have solved system parameters. Where the mass of the primaries gives inside into the mass distribution of WDs in binary systems, the mass of the secondary reveals whether the system has passed through the period bounce at which point the secondary star has lost enough mass to become a brown dwarf and is sustained by degeneracy pressure and therefore the period will increase with further mass loss as the radius will increase. Studies show that about 70 percent of all known CVs should have reached this point but only few systems have been found so far and to increase their numbers or find a true deficiency it is vital to determine the mass ratio of the shortest period CVs.
The commonly used emission lines are dominated by the features from the accretion disc as they outshine the much dimmer donor star. The Ca II triplet offers advantages that more than make up for its relative weakness compared to the Balmer lines, namely it has a lower ionisation energy than either hydrogen and helium and is thus capable of being excited even by cool sources of radiation, and its thermal width and pressure broadening are much smaller than for hydrogen leading to sharper, more easily detected spikes of emission. Ca II is therefore a promising, but so far neglected, avenue for emission line studies of CVs.
For my PhD, firstly we will look at spectroscopic data of several short orbital period dwarf novae systems to find signatures of the donor star in the Ca II triplet and use these to determine the system parameters. This will give a better insight in our binary evolution models at the period bounce. Next, we will explore the emission lines arising from the disc in these different elements.
My publications are listed here.