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Neutron stars and stellar-mass black holes are the remnants of massive stars, which ended their lives in supernova explosions. These exotic objects can only be studied in relatively rare cases. If they are interacting with close companions they become bright X-ray sources. If they are neutron stars, they may be detected as pulsars. Only a few hundred such systems are presently known in the Galaxy. However, there should be many more binaries with basically invisible compact objects in non-interacting binaries. Here we report the discovery of unseen compact companions to hot subdwarfs in close binary systems. Hot subdwarfs are evolved helium-core-burning stars that have lost most of their hydrogen envelopes, often due to binary interactions. Using high-resolution spectra and assuming tidal synchronisation of the subdwarfs, we were able to constrain the companion masses of 32 binaries. While most hot subdwarf binaries have white-dwarf or late-type main sequence companions, as predicted by binary evolution models, at least 5% of the observed subdwarfs must have very massive companions: unusually heavy white dwarfs, neutron stars and, in some cases, even black holes. We present evolutionary models which show that such binaries can indeed form if the system has evolved through two common-envelope phases. This new connection between hot subdwarfs, which are numerous in the Galaxy, and massive compact objects may lead to a tremendous increase in the number of known neutron stars and black holes and shed some light on this dark population and its evolutionary link to the X-ray binary population.
Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their entire hydrogen envelope in the red-giant phase. Since a high fraction of those stars are in close binary systems, common envelope ejection is an important formation channel.
We started a new project which aims to find compact hot subdwarf binaries at low Galactic latitudes. Targets are selected from several photometric surveys and a spectroscopic follow-up campaign to find radial velocity variations on timescales as shor
We present the results of a study of the late-type companions in hot subdwarf composite spectrum binaries. The exact nature of these late-type companions has been disputed in the literature -- some argue that they are main sequence stars, and others
We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that beside
The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding sdBs with compact companions like supermassive white dwarfs (M>1.0 Msun), neutron stars or black holes. The existence of such systems is predi