Astron. Astrophys. 354, 99-102 (2000)

2. A planet orbiting Gliese 86

Gliese 86 (HD13445, HIC 10138) is a bright () early K dwarf (, , , Flynn & Morell 1997) from the southern hemisphere, in the Eridanus (River) constellation. It is a close star, 10.9 pc away from our Sun ( mas, measured by the Hipparcos satellite). Its absolute magnitude is 6.257, yielding (with ) a luminosity . It is a high proper motion star, slightly metal poor ([Fe/H] , Flynn & Morell 1997). It has low chromospheric activity (, Saar & Osten 1997). No rotational broadening has been detected (Saar & Osten 1997) and there is only an upper limit on the Li content in its atmosphere (N(Li), Favata et al. 1997). From Hipparcos photometry, the star is stable (). In summary, Gliese 86 bears all the characteristics of a few billion year old K dwarf from the old disk population. In the H-R diagram, Gliese 86 lies slightly below the ZAMS. However we believe that there are enough uncertainties in the temperature and bolometric correction estimates of Gliese 86 - stemming from its low metal content - to believe that its location in the HR diagram below the ZAMS is not significant.

A 15.8 day period radial velocity variation has been detected from CORALIE measurements (Fig. 1). In Table 1 are listed the orbital elements of the best fit solution (least square) for an orbital motion after correction of a 0.36 m s^-1 d^-1 linear drift (see below). Assuming a 0.8 for the primary and that the radial velocity effect is caused by the orbital motion of the star, we conclude that a 4 M_J companion (minimum mass) is orbiting Gliese 86.

Fig. 1. Phased orbital motion of Gliese 86 corrected from the long term drift. The solid line is the best fit orbit. See orbital elements in Table 1

Table 1. Orbital elements of Gliese 86 after correction of the 0.36 m s^-1 d^-1 linear drift of the -point.

The planetary companion to Gliese 86 is close to its host star with a 0.11 AU semi-major orbital axis. It has a low, although 99% significant non-zero, eccentricity (Lucy & Sweeney 1971). The 7 m s^-1 residual from the fit indicates very low intrinsic intrumental errors from night to night, taking into account that each measurement has approximately 5 m s^-1 photon noise error and could be affected as well by some low level radial velocity variations intrinsic to the stellar atmosphere. Such low instrumental error agrees with the instrumental error measured by analysis of all the stars of our sample so far observed (about 300). See Duquennoy (1991) for a detailed description of the instrument error estimate by the statistic.

A long term drift of the radial velocity (0.5 m s^-1 d^-1) is observed from 20 years of CORAVEL measurements (Fig. 2). With the 300 m s^-1 typical precision of CORAVEL radial velocities, the short orbit is marginally detected in the last measurements. Interestingly, with the recent CORALIE measurements a smaller 0.36 m s^-1 d^-1 drift is observed (Fig. 3). A statistical analysis of the reliability of the drift correction shows that an orbital solution without drift correction has 0.0001% chance to occur (). The probability jumps to 40% when the linear drift correction is taken into account. A conservative 7 m s^-1 instrumental error is assumed for this calculation.

Fig. 2. Filled dots : radial velocity drift observed with CORAVEL. A mean 0.5 m s-1 d-1 variation of the radial velocity of Gliese 86 is measured (dotted line ). The 15.8 day reflex motion from the planet is marginaly seen as an extra scattering in the last CORAVEL measurements. In the right lower box is displayed for comparison the -point drift measured with CORALIE. (Note that the time scale is artificially extended for the sake of a better display). Open dots: previous measurements found in the literature. No error bars are displayed but a typical 2-5 km s-1 error may be assumed for these measurements

Fig. 3. Top: CORALIE observed -point drift (residuals from the short orbit fit). The dashed line is the best linear fit after correction of the short period from the planetary companion. It corresponds to a linear drift of 0.36 m s-1 d-1. The dotted line is the 0.5 m s-1 d-1 drift measured over 20 yr CORAVEL data. Bottom: Set of 3 non-variable stars measured at the same period of time (open dots: HD10700, stars: HD39091, filled dots: HD67199). No instrument zero point drift is observed

The period measurement of the short period planetary companion is still not accurate enough to correct the old CORAVEL data from their extra scattering and obtain a precise drift estimate from these measurements. Thus, the difference in drift slope between the old CORAVEL measurements and the recent CORALIE measurements is perhaps significant, but remains to be confirmed by further measurements during the course of the next season.

The long term radial velocity variation is the signature of a remote and more massive companion. The use of historical radial velocity data together with the CORAVEL and the CORALIE observed drifts suggest a stellar companion with a period longer than 100 yr (semi-major axis larger than 20 AU). A direct detection would be worth attempting since the star is close to us.

Alternative explanations to a low mass companion to explain the observed 15.8 day period radial velocity change of Gliese 86 would be activity related phenomena (Saar & Donahue 1997). However Gliese 86 doesn't exhibit any of the classical activity signatures seen on young stars, as for exemple HD166435 (Queloz et al. in prep). Gliese 86 has no chromospheric activity. No rotational broadening is detected either, and its photometry is very stable. Therefore, the planetary hypothesis is most likely the correct interpretation for the observed periodic radial velocity changes.

© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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