Observational studies of galactic cataclysmic variables

Cataclysmic variables (CVs) are binary stars consisting of a white dwarf accreting from a main sequence donor. They are important laboratories for studying accretion and binary evolution in multiple contexts. In this thesis I describe my work to build volume-limited samples and spectroscopically-identified samples of CVs thereby reducing the observational bias and enabling robust estimates of population density that can be reconciled with theoretical evolutionary models. As a foundation I have produced a set of reliable volume-limited samples to aid future targeting of spectroscopic surveys. These have already been used to inform the targeting of SDSS-V. The first 20 years of CVs observed by SDSS have also been analysed; with data from the last 10 years published for the first time. The sub-type of each CV, and hence its place in the evolutionary timeline, has been established. Light curves and radial velocities of CVs have also been used where available to find the orbital period. This analysis has enabled me to estimate the space density of each of the major subtypes of CV. The most recent SDSS survey, SDSS-V, has for the first time systematically targeted white dwarfs and CVs. Significantly the targeted nature of the survey has enabled me to put an upper limit on the space density of accreting period bouncers. This is an important result providing input to the long running debate regarding the ultimate fate of CVs. In the course of my analysis I also made some other discoveries. Firstly the analysis of ASAS J071404+7004.3 spectroscopy yielded a new insight into disc winds from nova-likes and, by using computer simulation, I was able to show that the Balmer emission lines could originate entirely in the wind far from the CV thereby offering a potential explanation for the single-peaked emission lines in SWSex nova-likes. Secondly whilst analysing the ZTF light curves of SDSS-identified CVs a small number stood out because of their unusual state changes. I found seven examples of such systems that exhibit significant state changes on timescales that indicate the cause is a change in mass transfer rate, rather than storage in the disc. This phenomenon is most unexpected and further investigation is needed

A catalogue of cataclysmic variables from 20 years of the Sloan Digital Sky Survey with new classifications, periods, trends and oddities

We present a catalogue of 507 cataclysmic variables (CVs) observed in SDSS I to IV including 70 new classifications collated from multiple archival data sets. This represents the largest sample of CVs with high-quality and homogeneous optical spectroscopy. We have used this sample to derive unbiased space densities and period distributions for the major sub-types of CVs. We also report on some peculiar CVs, period bouncers and also CVs exhibiting large changes in accretion rates. We report 70 new CVs, 59 new periods, 178 unpublished spectra and 262 new or updated classifications. From the SDSS spectroscopy, we also identified 18 systems incorrectly identified as CVs in the literature. We discuss the observed properties of 13 peculiar CVS, and we identify a small set of eight CVs that defy the standard classification scheme. We use this sample to investigate the distribution of different CV sub-types, and we estimate their individual space densities, as well as that of the entire CV population. The SDSS I to IV sample includes 14 period bounce CVs or candidates. We discuss the variability of CVs across the Hertzsprung-Russell diagram, highlighting selection biases of variability-based CV detection. Finally, we searched for, and found eight tertiary companions to the SDSS CVs. We anticipate that this catalogue and the extensive material included in the Supplementary Data will be useful for a range of observational population studies of CVs.Comment: Submitted to MNRAS. Supplementary information available at https://warwick.ac.uk/fac/sci/physics/research/astro/people/keithinight/sdss_paper_supplementary_informatio

DAHe white dwarfs from the DESI survey

A new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer lines in emission has recently emerged. However, the physical origin of these emission lines remains unclear. We present here a sample of 21 newly identified DAHe systems and determine magnetic field strengths and (for a subset) periods which span the ranges of ~ 6.5 -- 147 MG and ~ 0.4 -- 36 h respectively. All but four of these systems were identified from the Dark Energy Spectroscopic Instrument (DESI) survey sample of more than 47000 white dwarf candidates observed during its first year of observations. We present detailed analysis of the new DAHe WDJ161634.36+541011.51 with a spin period of 95.3 min, which exhibits an anti-correlation between broadband flux and Balmer line strength that is typically observed for this class of systems. All DAHe systems cluster closely on the Gaia Hertzsprung-Russell diagram where they represent ~ 1 per cent of white dwarfs within that region. This grouping further solidifies their unexplained emergence at relatively late cooling times and we discuss this in context of current formation theories. Nine of the new DAHe systems are identifiable from SDSS spectra of white dwarfs that had been previously classified as featureless DC-type systems. We suggest high S/N, unbiased observations of DCs as a possible route for discovering additional DAHe systems.Comment: 19 pages, 10 Figures, accepted for publication in MNRA

A 99 minute double-lined white dwarf binary from SDSS-V

We report the discovery of SDSS J133725.26+395237.7 (hereafter SDSS J1337+3952), a double-lined white dwarf (WD+WD) binary identified in early data from the fifth-generation Sloan Digital Sky Survey (SDSS-V). The double-lined nature of the system enables us to fully determine its orbital and stellar parameters with follow-up Gemini spectroscopy and Swift UVOT ultraviolet fluxes. The system is nearby (d = 113 pc), and consists of a 0.51 M⊙ primary and a 0.32 M⊙ secondary. SDSS J1337+3952 is a powerful source of gravitational waves in the millihertz regime, and will be detectable by future space-based interferometers. Due to this gravitational wave emission, the binary orbit will shrink down to the point of interaction in ≈220 Myr. The inferred stellar masses indicate that SDSS J1337+3952 will likely not explode as a Type Ia supernova (SN Ia). Instead, the system will probably merge and evolve into a rapidly rotating helium star and could produce an underluminous thermonuclear supernova along the way. The continuing search for similar systems in SDSS-V will grow the statistical sample of double-degenerate binaries across parameter space, constraining models of binary evolution and SNe Ia