No planet for HD 166435

Abstract

Abstract. The G0 V star HD 166435 has been observed by the fiber-fed spectrograph ELODIE as one of the targets in the large extra-solar planet survey that we are conducting at the Observatory of Haute-Provence. We detected coherent, low-amplitude, radial-velocity variations with a period of 3.7987 days, suggesting a possible close-in planetary companion. Subsequently, we initiated a series of high-precision photometric observations to search for possible planetary transits and an additional series of Ca II H and K observations to measure the level of surface magnetic activity and to look for possible rotational modulation. Surprisingly, we found the star to be photometrically variable and magnetically active. A detailed study of the phase stability of the radial-velocity signal revealed that the radial-velocity variability remains coherent only for durations of about 30 days. Analysis of the time variation of the spectroscopic line profiles using line bisectors revealed a correlation between radial velocity and line-bisector orientation. All of these observations, along with a one-quarter cycle phase shift between the photometric and the radial-velocity variations, are well explained by the presence of dark photospheric spots on HD 166435. We conclude that the radial-velocity variations are not due to gravitational interaction with an orbiting planet but, instead, originate from line-profile changes stemming from star spots on the surface of the star. The quasi-coherence of the radial-velocity signal over more than two years, which allowed a fair fit with a binary model, makes the stability of this star unusual among other active stars. It suggests a stable magnetic field orientation where spots are always generated at about the same location on the surface of the star.

Binarity, activity and metallicity among late-type stars

“…Unfortunately, our measurements are too sparse to allow a definite conclusion. It is worth noting that Queloz et al (2001) found a negative correlation between BIS and RV in the case of HD 166435, which allowed the authors to conclude that the RV variation was induced by activity. On the other hand, we find a positive correlation between BIS and RV, as did Santos et al (2002) in the case of the visual binary HD 41004, which was ascribed to a brown dwarf orbiting the fainter B component.…”

“…While the measurements are consistent with the period, it is clear that more measurements are needed. To investigate whether this variation may be due to a low-mass companion, we measured the line bisectors using the procedure of Queloz et al (2001). We measured the bisector inverse slope (BIS) of the mean CCF of the spectra, and compared with the measured RV to check for correlations (Fig.…”

The HARPS search for southern extra-solar planets

“…Observationally, these inhomogeneities translate into flux variations, as well as into changes of both the shape and the centroid of spectral lines (Saar & Donahue 1997;Queloz et al 2001). Spots also typically impact spectral indices, whether designed to probe the chromosphere (to which photospheric spots have strong magnetic connections) or the photosphere (because spots have cooler spectra).…”

A Campaign for the Detection of Earth-Mass Planets in the Habitable Zone of Alpha Centauri

“…For active stars, this type of intrinsic variability can mimic the radial-velocity signature of a short-period planet (e.g. Queloz et al 2001). Another type of jitter is stellar pulsation, both radial as well as non-radial.…”