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.