|Submitter:||Thijs Coenen, Joeri van Leeuwen|
|Description:|| A sub-luminous B dwarf (sdB) is a small, about 0.5 solar mass, core helium burning star with a thin hydrogen envelope. Although sdBs are faint blue objects they are amongst the most numerous such objects, contributing a large fraction of the UV light in for instance early type galaxies. Many of the sdB stars are in binaries so binary formation scenarios seem likely and would provide a mechanism to dissipate the hydrogen envelope. |
Observationally sdBs are known in binaries with a zoo of companions; planets, brown dwarfs, main sequence stars, several types of white dwarfs, neutron-star mass objects, while one system even contains a black hole. Neutron stars have not been observed directly in binaries with an sdB, but are inferred from sdB radial motion and rotation. Finding pulsed radio emission would for the first time positively identify a neutron star companion to an sdB. Such a system could then be used for pulsar timing (possibly allowing a precise mass determination of the sub-luminous B dwarf). By observing a sample of candidate sub-luminous B dwarf plus neutron star systems extra constraints can be put on the formation mechanisms of sub-luminous B dwarfs.
Using the Green Bank Telescope we searched for such millisecond pulsars in 4 binary systems containing sub-luminous B dwarfs. We could only place upper limits. In the above graph, we show where these upper limits lie on the cumulative luminosity distribution of known millisecond pulsars. As one can see on the right-hand axis, for 3 sources we are over 95% confident that we would have detected any millisecond pulsar, if present. The full results were published last week as Coenen et al., A&A 2011 (see http://adsabs.harvard.edu/abs/2011A%26A...531A.125C ). We continue our search, stay tuned for more results from the Green Bank Telescope and Westerbork.