The physics of star formation remains an outstanding problem within astrophysics. This problem is particularly acute for the production of OB stars (with masses in excess of 20 times that of the Sun); where the apparent brevity of the process (<100,000 yrs) and the presence of significant extinction (>>30 visual magnitudes) due to their birth clouds, make observations of high mass protostars highly challenging.
An alternative approach is to utilise the current properties of ensembles of OB stars to constrain their formation mechanisms; one such constraint is the binary fraction of the resultant population. Since dynamical evolution of individual stars in star clusters is unable to readily form binaries, any present must result from the initial fragmentation of their progenitor molecular clouds (e.g. Goodwin & Kroupa 2005). Hence knowledge of the initial distribution of binary properties (in particular of periods and mass ratios) provides a particularly stringent test for the physics of this process, a vital stage in star formation.
While numerous observational results (summarised in Goodwin & Kroupa 2005) suggest that most, if not all, low mass stars are found in multiple systems, the observational constraints on high mass stars are much less compelling. Given that it is an open question as to whether massive star formation is simply a `scaled up' version of the low mass process, or whether alternative modes such as coalescence dominate (effectively the cannibalism of low mass stars by their higher mass brethren), determination of the OB star binary fraction will provide important and much needed constraints on the competing hypothesis.
We propose to use the unique capabilities of the Faulkes Telescopes to investigate these problems by searching for and identifying binaries via modulation of their light-curves, either due to eclipse of one star by another in systems seen ‘edge on’, or by the distortion of the stellar surface of one star by the gravitation field of the other (so called ‘ellipsoidal modulation’). This work in part will be accomplished in collaboration with the FLAMES massive star consortium - which has been awarded >100hours of time on the Very Large Telescope (VLT) to study OB stars in the galaxy and in the Magellanic Clouds (MCs).
This project addresses the problem of calibrating the mass-luminosity relationship for binary stars. In effect we cannot determine the accurate masses of such stars from the combination of their luminosities (derived from temperature and radius) – as we may for low mass stars such as the Sun. Hence this project will investigate binary systems containing such stars, since by the application of Kepler’s Laws we may effectively weigh both stars and hence constrain the so called mass-luminosity relationship. Moreover, we intend to search for such systems in dense young stellar clusters, in turn determining the fraction of massive stars born in binaries – a key constraint on theories of star formation.
As well as essential for mass measurement amongst the eclipsing systems, such observations will enable the complete properties of the binary populations (percentage binarity, period, orbital eccentricity and mass ratio distribution) to be determined via comparison to monte carlo simulations (c.f. Kobulnicky et al. 2006), for which the code has already been written.