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SkyMapper Protected Science Project

Compact and Ultracompact binaries in the Milky Way Galaxy

Paul Groot (Radboud), Danny Steeghs (Warwick), Lilia Ferrario (ANU), Tom Marsh (Warwick), Philipp Podsiadlowski (Oxford), Gijs Nelemans (Radboud), Lars Bildsten (UCSB), Patrick Woudt (UCT), David Buckley (SAAO), Steven Bloemen (Radboud), Gavin Ramsay (Armagh), Ben Stappers (Manchester), P. Jonker (SRON/Radboud), Brian Warner (UCT), Boris Gäsicke (Warwick), Stephane Vennes (CAS), Adela Kawka (CAS)

Compact and Ultracompact binary systems consist of at least one stellar remnant in a very close orbit, with periods less than ~1-6 hours. They can either be detached or semi-detached, and the primary star can be a white dwarf or a neutron star. Depending on the exact binary constellation they are called double degenerates, post common-envelope binaries, AM CVn stars, Cataclysmic Variables, Ultracompact X-ray binaries or recycled pulsars.
The astrophysical significance of these systems is multifold:

In the last decade our teams have opened up the study of these objects, both theoretically as well as observationally. The known source populations have doubled to tripled over this period, in particular through the availability of large scale (Northern) optical photometric surveys, radio surveys and the combination of the two. As an example, in the transition radio pulsar systems the combination of having optical and radio data at the same time has been vital in understanding the evolution of these objects. Population numbers are often still low due to their intrinsic rarity, severely limiting population studies. The Southern Hemisphere is virtually unexplored. SkyMapper offers unprecedented opportunities for the detection of new systems in the South. Our aim is to, at least, double the number of known systems and then use them to understand the Galactic population of (ultra)compact binaries as well as the final stages of binary stellar evolution. The SkyMapper sample will complement the study of these systems using the VPHAS+ ESO Public Survey and the GBS Survey, both led by our team. Selection of candidates from SkyMapper will be based on source colours, as well as variability (outbursts/eclipses).
Detailed (high-speed) photometric and spectroscopic optical and radio follow-up is, and will be, obtained with the suite of observational resources to our disposal (ESO, SAAO, MeerKAT+MeerLICHT, Parkes, La Palma and AAO). We will combine these observational studies with detailed population synthesis modelling (Nelemans; Podsiadlowski) as well as in depth modelling of individual systems (e.g. using the MESA code developed by Bildsten et al.).