## Does the Sun have a Companion

As we have seen the Sun is, as a single star, apparently in a minority amongst the stars in the local neighbour-

 Table 6.2. Notes on double and multiple stars within 5 parsecs of the Sun. Gleise no Other names Notes Gl 551 Proxima Cen; V645 Cen variable 0.01m; double - P = 80 days Gl 559A a Cen A; a = 17'.' 515, P = 79.920 yr Gl 559B a Cen B; Gl 411 Lalande 21185; planetary companion? Gl 244A a CMa; Sirius a = 7'.'500, P = 50.090 yr Gl 244B a CMa B. Gl 65A BL Cet; V(AB) = 11.99, Am = 0.45 Gl 65B UV Cet; a = 1'.' 95, P = 26.52 yr Gl 144 £ Eri; dust ring; planetary companion P = 2502 days Gl 866A EZ Aqr; A is SB with Am = 0, a = 0'.' 34, P = 2.25 yr Gl 866B The system Gl 866 is SB3 Gl 280A a CMi; Procyon, a = 4'.' 271, P = 40.82 yr Gl 280B a CMi B; V = 10.75 (HST) Gl 820A 61 Cyg ; V1803 Cyg. Gl 820B a = 24'.' 4, P = 659 yr Gl 725A BD+59°1915; NSV 11288 Gl 725B G227-047; a = 13'.' 88, P = 408 yr Gl 15A GX And; Gl 15B GQ And; sep (AB) 39", 60° Gl 860A Kr 60; V(AB) = 9.59, Am = 1. Gl 860B DO Cep; a = 2'.' 412, P = 44.6 yr Gl 234A Ross 614; V(AB) = 11.09, Am = 3.09 Gl 234B V577 Mon; a = 1'.'009, P = 16.60 yr Gl 473A Wolf 424; V(AB) = 12.44, Am = -0.01. Gl 473B FL Vir; a = 0'.' 715, P = 15.643 yr Gl 687AB BD+68° 946; sep 0'.' 307, 1°2 (1984.4) GJ 1245A V1581 Cyg; V(AC) = 13.41, Am = 3.31 GJ 1245B G208-045; sep (AB) 7'.' 969, 98. 04 GJ 1245C a(AC) = 0'.' 28, P = 15.22 yr Gl 876 IL Aqr; 2 planetary companions P = 30.1 and 61.0 days Gl 412A BD +44° 2051; Gl 412B WX UMa; sep(AB) = 28", PA = 133° Gl 388 A D Leo; resolved by speckle (?) Heintz: no companion. Gl166A o2 Eri Gl 166B 40 Eri; P(BC) = 252.1 yr Gl 166C DY Eri; Gl 702A 70 Oph A; Gl 702B 70 Oph B; V(AB) = 4.02, Am = 1, a = 4'.'545, P = 88.13 yr
 Table 6.3. Distribution of single and multiple stars near the Sun Distance Volume Single Binaries Triples Multiples Planets Binary freq. 0-5 pc 0.8% 31 31 3 1 50% 5-10 pc 5.6% 152 53 13 6 45% 10-15 pc 15.2% 417 127 13 1 7 39% 15-20 pc 29.6% 662 172 17 1 8 35% 20-25 pc 48.8% 793 210 25 5 3 36%

hood. As more very faint companions to nearby stars are found this will make it even more unusual, but do we really live in a Solar System with a single Sun?

In 1984 Raup and Sepkowski1 reported evidence for a 26 million year (Myr) periodicity in the occurrence of mass extinctions based on a study of marine fossils. Such impacts included the one 65 million years ago that produced the Chicxulub crater in Yucatan and killed the dinosaurs. Steel2 refers to later work by Sepkowski which indicates ten such events over the last 260 million years which strongly correlate with a 26 Myr cycle.

This produced a flurry of interest from astronomers who came up with several ideas on how this could be linked to astronomical events. One idea related to the rotation of the Solar System around the Galaxy. It is well established that one rotation around the Galactic centre takes about 250 Myr but during this time the Sun also moves perpendicular to the Galactic plane in a sinusoidal fashion and crosses the plane every 30 Myr or so, reaching a distance of about 100 pc from the plane at the ends of the cycle. During the plane passage, it is surmised, the Earth's biosphere can be exposed to increased levels of radiation. (A recent theory speculates that another intense source of radiation may emanate from supernovae which tend to occur in the Galactic plane.) Rampino and Stothers in Nature3 argued that the original Rapp and Sepkowski data could be interpreted as having a period of 30 Myr rather than 26, and then stated that this is in better agreement with the periodic Galactic-plane crossing period of 33 Myr. With the Sun spending more than two-thirds of its time within 60 pc of the Galactic plane there was ample opportunity for encounters with passing giant molecular clouds to disturb comets from the Oort cloud. Rampino and Stothers also found a periodic term of 31 Myr in the occurrence of large craters on the Earth.

In the same edition of Nature the American astronomers Whitmire and Jackson,4 and, independently, Davis, Hut and Muller5 came up with a theory to try and explain the apparent 26 Myr periodicity. Whitmire and Jackson postulated a star with mass between 0.0002 and 0.07 M0 (M0 is the mass of the Sun), with an orbit of eccentricity 0.9 and semimajor axis of 88,000 AU. The companion postulated by Davis et al. was similar but at apastron such an orbit would take it out to a distance of about 3 light years after which the companion would then approach the Sun, skirt the Oort cloud, disrupting comets into the inner Solar System and return again to the depths of space. This companion star was named Nemesis to reflect the catastrophes that its appearances would trigger. Detractors from the theory argued that when at apastron passing stars would have more effect on Nemesis than the Sun, but work by the Dutch astronomer Piet Hut argued that Nemesis could survive such encounters for about a billion years. Today it is difficult to explain binary orbits on this scale. None out of the thousand or so binary orbits which have been catalogued have aphelion distances on this scale.

The main argument against the Nemesis theory is that the projected orbit is too large and too eccentric to allow the star to stay bound to the Sun after more than a few passages through the Galactic plane

Recent studies of wide binaries6 conclude that some wide pairs have separations in excess of 10,000 AU. To give an idea of this scale, Pluto is about 30 AU away and a Centauri is about 280,000 AU distant.

If Nemesis exists then clearly it is not a twin of the Sun because even at apastron it would have apparent magnitude +3 and its parallax of well over 1'' would have marked it out many years ago. Nemesis must be at least a faint red dwarf, perhaps even a brown dwarf whose apparent magnitude is likely to be at least +15. The proper motion of such a star will be very small and this will be a distinguishing feature as many very faint nearby stars have large proper motions. So a survey such as the Sloan Digital Survey could pick it up, providing the star lies in the 25% of the sky which the survey will cover. Any suitable candidates could then be observed individually by ground-based telescopes since the parallax will be large.

Could the extinction in the late Eocene period be due to a passing star? One possibility of resolving this question may come with data from the projected GAIA

mission. The expected accuracy of the proper motion and parallax determination for the stars in the solar neighbourhood will allow a more accurate backward interpolation to determine the history of close stellar approaches to the Solar System.

References

1 Raup, D.M. and Sepkowski, J.J., 1984, Proc. Natl. Acad. Sci. USA, 81, 801.

2 Steel, D., 1995, Rogue Asteroids and Doomsday Comets, Wiley.

3 Rampino, M.R. and Stothers, R.B., 1984, Nature, 308, 709.

4 Whitmire, D.P. and Jackson, A.A., 1984, Nature, 308, 713.

5 Davis, M., Hut, P. and Muller, P.A., 1984, Nature, 308, 715

6 Allen, C., Herrera, M.A. and Poveda, A., 1999, Astrophys. Space Sci., 265, 233.

\ Chapter 7