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"Hints of the fourth
planet in the υ Andromedae system" R.V. Baluev (Central (Pulkovo) Astronomical Observatory of RAS) Abstract: We analyse a recently published
array of 288 refined and new radial velocity (RV) measurements of υ And. This star is currently
believed to host 3 giant planets having orbital periods about 4.6 days, 240
days è 1280 days [1].
These data cover about 15 years and have intrinsic (instrumental) precision
near 5-10 m/s (starting approximately from 1997). Periodogram analysis of
these data reveals two extra, previously unknown, RV variations for this
star, both have about 10 m/s semi-amplitude and high statistical
significance. The first oscillation has annual
character, and it can be treated as having period of either 360 days or 180
days. This ambiguity is a consequence of the aliasing effect, caused by
periodic seasonic gaps in the observation sequence. Due to the dynamical
stability argument, this oscillation cannot be treated as a hint of an extra
planet with the corresponding orbital period. Therefore, this seasonic
periodicity is caused, most probably, by systematic errors. The source of
these errors can lie in some instrumental drifts or in imperfections of the
algorithm of the spectrum analysis and deriving of the actual barycentric
radial velocity “measurement”. Such seasonic errors are frequently met in the
exoplanet search practice [2]. The second variation has period of
roughly 12 years, if we consider it is sinusoidal. Under assumption of the
planetary nature of this RV variation, more long orbital periods become
admissible as well (from the viewpoint of pure data), up to parabolic orbits
(with pericentric distance of 3-6 AU). Taking into account the requirement of
the dynamical stability of the system allows us to mark a few regions in the
parameter space, where the orbit of the putative fourth planet (e) can be
located. These may be moderately eccentric orbits in the 3:1, 4:1, 5:1, 6:1
mean-motion resonances with the known planet d, or highly eccentric orbits
with pericentric distance of 5.5 AU (but such orbits are less probable from
the viewpoint of exoplanetary statistics). Nonetheless, given the small
amplitude of the variation with respect to the errors background, other
(non-planetary) interpretations of the long-term RV variation of υ And are possible. These include:
instrumental systematic errors, hints of the stellar magnetic activity cycle
(an 11-years Solar cycle analog). In recent years several different authors
have already conducted long-term observation series, which can clarify this
question in future (e.g., monitoring of the activity of this star based on
the emission level in H and K Ca II lines, paper [3]). However, currently
available data can neither confirm nor reject the planetary interpretation of
the long-period RV oscillation of υ Andromedae. It is necessary to continue
observations of this star by means of heterogeneous methods (high-precision
radial velocity measurements at different observatories, magnetic activity
monitoring, high-precision astrometric observation) for a few extra years. 1. Wright J.T., Upadhyay S., Marcy
G.W., Fischer D.A., Ford E.B., Johnson J.A. “Ten new and updated multi-planet
systems, and a survey of exoplanetary systems”, ApJ, 2009, 693, 1084-1099 2. Baluev R.V. “Accounting for
velocity jitter in planet search surveys”, MNRAS, 2009, 393, 969-978 3. Shkolnik E., Bohlender D.A.,
Walker G.A.H., Cameron A.C. “The on/off nature of star-planet interactions”, ApJ, 2008, 676, 628-638 |
