R. Paul Butler
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.
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.
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.
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.
|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).|
more on the AFOE's planet detection program.
Last modified: Fri Apr 9 12:00:08 1999