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Register a new userAnalysis of The Hipparcos Measurements of Upsilon Andromedae - A Mass Estimate of Its Outermost Known Planetary Companion
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Authors
Tsevi Mazeh
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We present an analysis of Hipparcos astrometric measurements of Upsilon Andromedae, a nearby main-sequence star around which three planet candidates have recently been discovered by means of radial-velocity measurements. The stellar orbit associated with the outermost candidate has a period of 1269 +/- 9 days and a minimum semi-major axis of 0.6 milli-arc-sec (mas). Using the Hipparcos data together with the spectroscopic elements we found a semi-major axis of 1.4 +/- 0.6 mas. This implies a mass of 10.1 (+4.7, -4.6) Jupiter masses for that planet of Upsilon Andromedae.
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Upsilon Andromedae is an F8V star known to have an extrasolar system of at least 3 planets in orbit around it. Here we report the discovery of a low-mass stellar companion to this system. The companion shares common proper motion, lies at a projected separation of ~750 AU, and has a spectral type of M4.5V. The effect of this star on the radial velocity of the brighter primary is negligible, but this system provides an interesting testbed for stellar planetary formation theory and understanding dynamical stability since it is the first multiple planetary system known in a multiple stellar system.
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We evaluate the orbital evolution and several plausible origins scenarios for the mutually inclined orbits of Upsilon Andromedae c and d. These two planets have orbital elements that oscillate with large amplitudes and lie close to the stability boundary. This configuration, and in particular the observed mutual inclination, demands an explanation. The planetary system may be influenced by a nearby low-mass star, Upsilon And B, which could perturb the planetary orbits, but we find it cannot modify two coplanar orbits into the observed mutual inclination of ~30 deg. However, it could incite ejections or collisions between planetary companions that subsequently raise the mutual inclination to >30 deg. Our simulated systems with large mutual inclinations tend to be further from the stability boundary than Upsilon And, but we are able to produce similar systems. We conclude that scattering is a plausible mechanism to explain the observed orbits of Upsilon And c and d, but we cannot determine whether the scattering was caused by instabilities among the planets themselves or by perturbations from Upsilon And B. We also develop a procedure to quantitatively compare numerous properties of the observed system to our numerical models. Although we only implement this procedure to Upsilon And, it may be applied to any exoplanetary system.
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