Evidence for a System of planets orbiting upsilon Andromedae

S.G. Korzennik, R.W. Noyes, P. Nisenson, M.J. Holman, A. Contos
Harvard-Smithsonian Center for Astrophysics
T.M. Brown
High Altitude Observatory


Using the Advanced Fiber Optic Echelle (AFOE) spectrograph at SAO's Whipple Observatory, we have monitored the radial velocity of Upsilon Andromedae since September 1994. Similar observations were made by the "Lick" group (P. Butler, G. Marcy, D. Fischer; see Paper 14.02).The AFOE data show, in addition to the already known close-in ``hot Jupiter'' in a 4.6-day circular orbit, two additional companions. The middle companion has a well-defined orbit, with semi-major axis about 0.83 AU, period 243.5 days, eccentricity 0.22, and minimum mass (M sin i) of 2 Jupiter masses. This is in very close agreement with independent findings by the Lick group. The AFOE data alone do not yield well-determined orbital parameters for the outer companion, because the total observing span encompasses only about one period of its orbit. However, the data are consistent with parameters derived for that companion by the Lick group from data with a longer time span, and when combined with the Lick data yield a semi-major axis of 2.5 AU, orbital period of 1267 days, eccentricity of 0.41, and minimum mass about 4.6 Jupiter masses. These results, independently obtained by two different groups with different instruments and analysis methodologies, together give strong indications that a true planetary ``system'' has now been discovered around a star like our own.

We have carried out numerical integrations which show that this system can be stable, but only for certain combinations of periods, masses, and eccentricities of the outer two companions. The stability requirement thus provides a prediction that can be tested as the orbital elements of the outer companion are refined. In addition, it imposes an upper limit on the actual planetary masses, and on the difference in orbital inclination of the two outer planets. Finally, the numerical integrations imply that the longitudes of periastron of the two outer companions are locked to nearly the same value, in accord with the present observations.

This work was supported by NASA, NSF, and the Smithsonian Institution.

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Sylvain G. Korzennik  (skorzennik@cfa.harvard.edu)
Last modified: Thu Jun 3 18:21:13 1999