The Upsilon Andromedae Planetary System
R. Paul Butler
Anglo-Australian Observatory
Geoffrey W. Marcy and Debra A. Fischer
San Francisco State University and University of California,
Berkeley
Sylvain G. Korzennik, Peter Nisenson,
Robert W. Noyes and Adam R. Contos
Harvard-Smithsonian Center for Astrophysics
Timothy W. Brown
High Altitude Observatory,
National Center for Atmospheric Research
The bright F8 V star 
Andromedae
was previously reported to have a 4.6 day Doppler velocity periodicity,
consistent with a Jupiter-mass companion orbiting at 0.059 Astronomical
Units ( or AU, which is the distance from the Earth to the Sun). Follow-up
observations by both the Lick and AFOE planet survey programs confirm this
periodicity, and reveal additional periodicities at 242 and 1269 days, and
imply two additional companions orbiting at 0.83 and 2.5 AU, with minimum
masses of 2.0 and 4.1 Jupiter-masses respectively.
Content
Radial Velocity Observations
Precision Doppler velocity measurements of Andromedae have been independently carried out by the Lick
and AFOE planet search programs.
The Lick measurements have been made with the 3-m
``Shane'' and 0.6-m ``CAT'' telescopes. The AFOE observations have been
carried out with the 1.5-m
Whipple telescope, which feeds the ``Advanced Fiber-Optic Echelle''
(AFOE) instrument.
Figure 1: Lick Observed Doppler measurements for Andromedae. The velocity zero-point is arbitrary
and independent.
 |
Figure 2: AFOE Observed Doppler measurements for Andromedae. The velocity zero-point is arbitrary
and independent.
 |
The table of orbital
elements gives the parameters for the best fit to the combined Lick and
AFOE data sets. Figure 3 shows velocity residuals of the combined data to this
fit, and figures 4-6 show the corresponding phased residuals for each
companion in turn, after subtracting the velocity fit for the other two
companions.
Figure 3: Velocity residuals of the combined data to the
fit that contains three companions. Circles represent the Lick data and
diamonds the AFOE data.
 |
Figure 4: Residuals after subtraction
of the combined velocities for companions c and d, plotted vs. orbital phase
of companion b, phased with a period of 4.617 d.
 |
Figure 5: Residuals after subtraction
of the combined velocities for companions b and d, plotted vs. orbital phase
of companion c, phased with a period of 241 d.
 |
Figure 6: Residuals after subtraction
of the combined velocities for companions b and c, plotted vs. orbital phase
of companion d, phased with a period of 1267 d.
 |
Formation and Evolution of the System
The possibility of three Jupiter-mass planets orbiting within 2.5 AU
of each other immediately raises questions about the dynamic stability of
this system. Greg Laughlin has carried out a numerical simulation of the
system of 3 planets described in the table. In this simulation, the orbits
have proven stable over the main sequence lifetime of the central star.
Althought the innermost companion has a nearly circular orbit, the outer
two companions both have significantly non-zero eccentricity. This continues
the observational trend: all of the extrasolar planets orbiting beyond 0.2
AU have eccentricity greater than 0.1. This unexpected result suggests that
the theories of planetary formation and evolution must provide an explanation
for these eccentricities as a common by-product. Assuming that the three
companions are real, planet-planet interactions constitute a plausible explanation
for the orbital evolution of this system.
Recent improvements in computer speed and computational algorithms make
possible increasingly realistic simulations of evolving planetary systems.
For example, Levison et al. (1998) have constructed models that begin with
thousands of protoplanets embedded in a gas/dust disk. These systems evolve
over several Gyrs, until orbital stability is achieved. Planet-planet interactions
typically dominate in forming the final stable architecture of a given system.
Many of the model systems formed in this way resemble the Andromedae system.
Assuming the albedo of all three companions in the Andromedae system are similar to Jupiter (~ 0.35), the equilibrium temperature
for the companions, in order of increasing distance from the central star,
are 1370, 365, and 210 K (Guillot et al. 1996). Internal heating probably
raises the temperature of the middle (c) companion above the boiling point
of water, while the the outer (d) companion is probably pushed up to near
the water freezing point at a pressure of 1 atm.
The formation of three Jupiter-mass companions having these apparent
orbital sizes and eccentricities is difficult to explain in the context
of current theories of the origin of planetary systems. Most such theories
suppose that gas giants form beyond a distance where water condenses into
ice, typically ~ 4 AU from the star. However all three companions around
Andromedae reside closer than this ice
boundary.
Two possibilities can be considered for their formation. They may have
formed outside the 4 AU ice-boundary. If so, some dynamical interaction
with yet a fourth body would be required to drain orbital energy from the
three observed Jupiter-mass companions to bring them closer to the star.
Plausible candidates for the fourth body are a giant planet, a passing star,
or the protoplanetary disk. Detailed models are required to determine the
physical plausibility of these three candidates.
Alternatively, the three Jupiter-mass companions may have formed within
4 AU. If so, it is tempting to wonder if they formed in situ. Typical
models of protoplanetary disks do not contain enough mass at 0.05 AU within
a Hill sphere (tidal reach) of a Jupiter-mass planet. Thus it is difficult
to explain in situ formation for the inner-most companion, unless
either the surface mass density of the disk is higher than expected at 0.05
AU or the gas drains onto the planet from the outskirts of the disk.
More on the star Andromedae
The physical parameters of the star Andromedae are
(from the scientific literature):
| R.A.: |
01:36:48.527 |
| Dec.: |
+41:24:38.71 (2000.0) |
| Vis Mag.: |
4.09 |
| aka: |
HD9826, HR458, HIP7513 |
| Spectral Type: |
F8 V |
| T(eff): |
~ 6100 K |
| Parallax: |
~ 74.25 mas |
| Distance: |
~ 43.93 light years (13.5 pc) |
| Luminosity: |
~ 3.0 Lsun |
| Age: |
~ 2.6 Gyr |
| Mass: |
~ 1.3 Msun |
| P(rotation): |
< 12 days |
|
Research with the AFOE is supported by the Smithsonian Institution
(SI),
the National Aeronautics & Space Administration
(NASA),
and the National Science Foundation
(NSF).
|
There is
more on the AFOE's planet detection program.
__________________________________________________
Adam R. Contos (acontos@cfa.harvard.edu)
Sylvain G. Korzennik (skorzennik@cfa.harvard.edu)
Last modified: Fri Apr 9 12:00:08 1999