3. A planet around Gl 876
Since planet detection was not initially emphasized in the observing program, its sampling strategy is not optimal for detection of low amplitude variations on timescales shorter than a few years. Gl 876 was observed once at each observing seasons in 1995 and 1996 and its velocity variations became apparent from the three observations obtained in late 1997. It was then marked in our program lists as a variable. This low declination source however became unobservable from OHP before we could gather more data and determine its orbit. The commissioning of the swiss 1.2 m telescope at la Silla and its CORALIE spectrograph in June 1998 provided the first opportunity to obtain 3 additional measurements of this southerly source, which allowed to finally determine its orbit. These observations were obtained within two weeks of the first light of this telescope, providing an encouraging indication on its potential for planet discovery. An end of night measurement from OHP at a large airmass provided a confirmation on June 22, just in time to confidently announce the discovery at the IAU "Precise Stellar Radial Velocities" conference (Victoria, Canada, June 21st to 26th). At this conference we learned from G. Marcy that his group independently identified the orbit of Gl 876, with orbital elements compatible with our own determination. Weather permitting, we have since then attempted to observe Gl 876 at most every three nights, and often every night. The 1998 data therefore dominate the orbital solution.
The orbital solution is given in Table 1. Preliminary solutions included a velocity zero point offset between the northern (ELODIE) and southern (CORALIE) datasets as a free parameter. The two systems were found to be entirely consistent, and this parameter was thus held fixed to zero for the final solution. Fig. 1 shows the individual radial velocity measurements as a function of orbital phase (the 16 orbital periods elapsed since the first measurement make unpractical a display as a function of time; we however have essentially continuous coverage of one period in June and July 1998, excluding any possible spectral alias). The orbital period is two months and the velocity semi-amplitude is 250 m.s-1, over 10 times the standard error of one radial velocity measurement. The radial velocity curve implies a moderate but highly significant eccentricity of e=0.34.
Table 1. Orbital elements of GL 876.
The large amplitude and moderate period of the radial velocity variation argue strongly for orbital motion as its cause. An integration of the radial velocity curve implies a minimum physical motion of 0.5 . This variation is about twice the radius of an M4 dwarf (Baraffe & Chabrier, 1996; Chabrier & Baraffe, 1997), excluding pulsation as a possible explanation. Gl 876 in addition only has low level photometric variability, and indeed happens to be one of the standard stars of the original UBV system (Johnson & Harris, 1954). It is actually variable, but with low rms amplitudes of 13 mmag at V, 9 mmag at R and 6 mmag at I (Weis, 1994). The photometric variations don't phase well at 61 days and appear consistent with a BY Dra type variability (Sasselov and Cody, private communication). Since Gl 876 is a very slow rotator (V2km.s-1, Delfosse et al., 1998a), rotational modulation of such low level surface inhomogeneities cannot explain its large radial velocity variations. Densely sampled photometry would on the other hand be of considerable interest to establish the stellar rotation period.
The mass of Gl 876 unfortunately contributes some uncertainty to the minimum mass of its companion. As a consequence of H2 recombination in the photosphere and the deepening convection for lower mass stars (Kroupa et al. 1990), the luminosity does not drop nearly as quickly per unit mass for mid-M dwarfs as it does for both higher and lower mass stars (Henry & Mc Carthy, 1993), and it has a stronger metallicity dependence. Between 0.50 and 0.18, Mass-Luminosity relations therefore have both shallow slopes and large intrinsic dispersions (Henry & Mc Carthy, 1993). Gl 876 thus belongs to a spectral type range where the mass of a single star is poorly constrained by its observable characteristics. Taking at face value either the solar neighborhood observational mass-luminosity relation of Henry & Mc Carthy (1993) or the solar metallicity models of Baraffe et al. (1998), the absolute magnitudes of Gl 876 (MV=11.81, MJ=7.56, MH=6.96, MK=6.70, Leggett (1992) and ESA (1997)) imply a mass of 0.300.05 for Gl 876. As an illustration of possible uncertainties however, Delfosse et al (1998c) measure M = 0.432 0.001 and MV = 11.70.2 (MV=11.81 for Gl 876) for the brighter star in GJ 2069A, an M3.5V eclipsing binary which is probably super-metal-rich ([M/H]+0.5). From its position in colour-colour diagrams (Leggett, 1992), and from the relative depth of its cross-correlation dips with several binary templates (Delfosse et al., in preparation), Gl 876 is probably more metallic than the sun, though not as much as GJ 2069A. We adopt a mass of 0.3 for the rest of the discussion but warn that it is uncertain by perhaps 30%. The minimum semi-major axis and planetary mass which result from the orbital solution are then a = 0.20 AU and M = 2.0MJ. They respectively scale as and .
© European Southern Observatory (ESO) 1998
Online publication: September 14, 1998