Stellar Radial-Velocity and Orbital-Element Distributions

Roger Griffin (The Observatories, University of Cambridge)

Abstract:

Historical development of radial-velocity studies

• Development of photographic spectrographs largely free from flexure and thermal drifts, notably at Lick (Campbell 1898, Campbell & Moore 1928)

• The Radial Velocity Catalogue (Wilson 1953); stagnation of photographic efforts

• Development of the photoelectric method at Cambridge (Griffin 1967) and Palomar (Griffin & Gunn 1974), and with Coravel at Geneva (Baranne et al. 1979).

• Spectrometers employing multi-channel detectors with direct cross-correlation of the stellar spectrum with a numerical mask (Latham 1985)

• Development of techniques to measure radial velocities of high precision such as are needed to look for the re ex motions caused to stars by orbiting planets (Griffin 1973; every journal you look at nowadays).

All high- precision instruments until comparatively recently have relied on imposing additional absorption lines on the stellar spectrum before it enters the spectrograph, in order to provide sufficiently reliable fiducial wavelengths (Young 1978, Smith 1982, telluric lines; McMillan & Smith 1987, F-P interferometer; Campbell, Walker & Yang 1988, HF; Marcy & Butler 1992, Cochran & Hatzes 1994, I2 ). Through the use of image- and aperture- scrambling fibres, however, Baranne et al. (1996) have been able to dispense with the extra absorption lines and to return to the classical method of using an emission source for wavelength reference

Orbital-element distributions

Previous compilations, e.g. Observatory 103, 273, 1983; 111, 291, 1991; 120, 195, 2000, have graphically illustrated how the distribution of the periods known for spectroscopic binaries is hopelessly skewed by observational selection, which has been reduced in the speaker's own work. The matter is taken a little further in the talk. It is of great importance, because it is only now that there is becoming a significant overlap between the domains of spectroscopic and of `visual' binaries, and without information from both techniques the masses of the stars concerned are in general indeterminate. The speaker cannot hope to duplicate the distribution of the periods of visual binaries, many of which are of centuries or millennia, but it seems encouraging to have shown already that the (logarithmically) most popular periods are at least several years, rather than the several days that they appeared to be only twenty years ago. Examples will be shown of spectroscopic orbits of long period, including two that are close to a century and one that is more. Currently continuing observations of certain other objects may be described.

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