2 A long-period companion to HD28185 in a circular orbit

2.1 Stellar characteristics

At a distance of $\sim$40 parsec ( $\pi=25.28\pm1.08$mas, from Hipparcos data - ESA 1997), HD28185 (HIP20723, BD-10 919) is a G5 dwarf shining with a visual magnitude V=7.8 in the constellation Eridanus (the River). Its colour index is $(B-V)\,=\,0.75$ as listed by the Hipparcos catalogue, and its absolute magnitude M_v=4.81. The basic stellar parameters are summarized in Table1.

   

Table 1: Observed and inferred stellar parameters for HD28185 and HD213240 with the corresponding references.

$$\begin{tabular}{l@{}lccl} \hline \multicolumn{2}{l}{Paramete... ...& Donahue (\cite{Don93})/Geneva models \\ \hline \end{tabular}$$

Recently, we have derived atmospheric parameters for this star using a detailed spectroscopic analysis (Santos et al. 2001). The obtained $T_{{\rm eff}}$, $\log{g}$, microturbulence parameter ( $\xi_{\rm t}$), and iron abundance ([Fe/H]) are 5705K, 4.59dex, 1.09kms^-1, and +0.24dex, respectively, typical parameters for a metal-rich G dwarf. The high [Fe/H] value is compatible with the one listed by Laughlin et al. (2000) who used ubvy-photometry calibrations. A similar value of +0.15 is obtained from a calibration of the CORALIE cross-correlation dip. The stellar mass and age, determined from the Geneva theoretical isochrones of Schaerer et al. (1993) using M_v and the obtained $T_{{\rm eff}}$, are $0.99\pm0.07\,M_{\odot}$ and $\sim$7.5Gyr, respectively. This age is compatible with the fact that only an upper value for the Li abundance was found (Israelian et al., in preparation).

Strassmeier et al. (2000) have derived a value of -4.82 for the chromospheric activity index $\log{R'_{\rm HK}}$. This corresponds to a non active dwarf (see e.g. Henry et al. 1996), with an age of $\sim$3Gyr, as derived from the calibration of Donahue (1993) - also presented in Henry et al. (1993).

2.2 Planetary signature of HD28185

Between October 1999 and September 2001 we obtained 40 high-precision radial-velocity measurements of HD28185 - see Fig.1. An analysis of the data revealed a periodic variation; the best Keplerian solution gives a period of 383 days, an amplitude of 161ms^-1 and an eccentricity of 0.07. The obtained value for the eccentricity is compatible with a circular orbit according to the Lucy & Sweeney (1971) test. In Table2 we present the derived orbital and planetary parameters.

Since the period is close to one year, some problems arise when trying to completely cover the orbital phase. Even if the orbital parameters are quite well constrained, some uncertainties are present; we hope to eliminate them when more data will be added during the next season.

Given the mass for the star, the observed radial-velocity variation is best interpreted as the signature of a planetary companion with a minimum mass of 5.7 $M_{{\rm Jup}}$. The combined CORALIE and old CORAVEL measurements show sign of a possible (but not clear) radial-velocity drift over 10 years. On the other hand, the residuals around the CORALIE orbital solution suggest the presence of a second longer period companion, but the detection is, up to now, only marginal. HD28185 is noted as "single'' in the Hipparcos catalogue (ESA 1997).

With a minimum mass of 5.7 $M_{{\rm Jup}}$, some questions might arise concerning the real nature of the discovered companion. We note, however, that the probability that its mass is in the Brown-Dwarf regime is of the order of 10%, and thus we think it is reasonable to refer to this companion as a planet.

In fact, further support to this comes from geometrical considerations. The $v\,\sin{i}=2.54\,\pm\,1.02$kms^-1 derived for HD213240 (from the CORAVEL cross-correlation function - Benz & Mayor 1984) together with the rotational period estimated from the activity level of the star (30 days from the calibration of Noyes et al. 1984), strongly suggest a value for $\sin{i}$ close to unity, i.e., the star seen "equator-on''. Assuming that the orbital plane is perpendicular to the rotation axis (this is approximately true for the case of the Solar System, and verified for HD209458 - Queloz et al. 2000), we conclude that the measured minimum mass is most probably a good estimate of the real mass of the planet.

Activity related phenomena may induce radial velocity "jitter'' (Saar & Donahue 1997; Santos et al. 2000a). For HD28185, the scatter around the Keplerian orbital solution is around 10ms^-1, while the mean photon-noise error of the measurements is $\sim$6ms^-1. Subtracting quadratically we obtain a "jitter'' of 8ms^-1, lower than (but compatible to) the $\sim$10ms^-1 expected (in average) for a G dwarf with $\log{R'_{\rm HK}}=-4.82$ (Santos et al. 2000a).

A look at the Hipparcos catalogue (ESA 1997) shows that the scatter in magnitude is about 0.012 (in 117 measurements). Although relatively high, this value is typical for an 8th magnitude star; HD28185 is labeled constant in the catalogue.

Figure 1: Top: phase-folded radial-velocity measurements for HD28185. Bottom: radial-velocity measurements as a function of time for the same star. The 40 measurements span about 2 years. Error-bars represent the photon noise error.

Bisector changes can be used as a diagnostic of activity induced radial-velocity variations. Although we did not expect that activity related phenomena could be able to produce the observed radial-velocity signature, we have analyzed the inverse slope of the bisector of the CORALIE cross-correlation function (Queloz et al. 2001). The analysis revealed no significant bisector variations. Given the long orbital period, and the lack of any significant signature in the Hipparcos astrometry (F. Arenou, private communication), it is quite difficult to imagine that another cause, besides the presence of a planet, could induce the observed radial-velocity variation.

Figure 2: Same as Fig.1 for HD213240. The 72 measurements span about 2 years.