Where does double-star observation go next? In the immediate future it will be from the ground where a number of specially built optical arrays will be operating over the next few years.
At Cambridge in the UK, the COAST (Cambridge Optical Aperture Synthesis Telescope) five-mirror interferometer has been working for some years with a current baseline of 48 metres and there are plans to extend this to 100 metres. This is an extension of the Michelson instrument at Mount Wilson. By using more mirrors and using the Earth's spin to rotate the instrument with respect to the star, astronomers have used phase closure, a technique first used in radio astronomy, to effectively image the structure of stars such as Betelgeuse.
It has easily resolved the bright spectroscopic binary Capella, whose components are about 50 milliarc-seconds (mas) apart. Another such instrument, the NPOI (Navy Prototype Optical Interferometer) using 50-metre baselines in Arizona, has resolved spectro-scopic binaries such as the brighter component of Mizar. Long known to have a period of 20.5 days, the NPOI can detect and measure the individual stars even though at closest approach they are only 4 mas apart (see Figure 9.1). The combination of the NPOI data and the spectroscopic data can give very accurate values for the size of the orbit, the parallax of the system and the individual masses, and the radius of each component.
Soon the CHARA (Centre for High Resolution Astronomy, Georgia State University) array in Arizona which employs 1-metre telescopes will be operating with a 350-metre baseline, and the Sydney University Stellar Interferometer (SUSI) instrument, currently working at a baseline of 160 metres, is eventually planned to operate at 640 metres. This will ultimately give a resolution of 75 microarcseconds (0'.'000075) and will allow binaries with periods of hours to be observed directly.
Peter Lawson's website6 covers all the current interferometer projects and has links to the historical ones.
Two planned satellites, DIVA and GAIA, will certainly make a significant contribution to our knowledge of binary stars. DIVA (Double Interferometer for Visual Interferometry) is a small Fizeau interferometer planned to be mounted on an Earth-orbiting satellite and it is planned to fly in the new few years. If it flies, and at the time of writing this is highly uncertain, it will operate with a scanning law similar to Hipparcos and carry out an astrometric and photometric survey down to V = 15. High-precision positions, parallaxes, proper motion and photometry will be done for 35 million stars. With a resolution of about 0.5'', and a spectral capability it is expected that this survey will reveal several hundred thousand new binaries by either direct resolution, astro-metric shift, anomalous spectral signals or eclipsing systems detected from the photometry.
GAIA is not due to fly until about 2010, but it is estimated that tens of millions of new double stars will de detected. For the resolved pairs, the magnitude difference is important. Equally bright pairs (<15th magnitude) will probably be completely resolved at 10 mas, while a 20th magnitude companion would be seen only at some 50 mas. Closer pairs will be observed by their photocentres but, in the "favourable" period range 1-10 years, a large proportion of them will have their astrometric orbits determined. This will be possible for photocentric orbit sizes below 1 mas, at least for the brighter systems. Bright (again <15th magnitude) shorter-period systems (days/months) will be observed by the radial-velocity instrument (at 0.1 mas separation), and millions of (mainly even shorter-period) eclipsing binaries will be observed photometrically.
1 Toomer, G.J., 1984, Ptolemy's Almagest, Duckworth.
2 Hartkopf, W. I. and Mason, B.D., 2001, http://ad.usno. navy.mil/wds/orb6.html
3 Tokovinin, A.A., 1997, Astron. Astrophys. Suppl., 124, 75.
4 Original article by U. Fedele, 1949, Coelum, 17, 65, but see the web page of Leos Ondra at http://leo.astronomy.cz/ mizar/article.htm
5 Pease, F.G., 1927, Publ. Astron. Soc. Pacific, 39, 313.
6 Lawson, Peter http://olbin.jpl.nasa.gov/links
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