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The CORALIE Survey for Southern Extra-Solar Planets IX. a 1.3-Day

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The CORALIE Survey for Southern Extra-Solar Planets IX. a 1.3-Day Astronomy & Astrophysics manuscript no. (will be inserted by hand later) The CORALIE survey for southern extra-solar planets⋆ IX. A 1.3-day period brown dwarf disguised as a planet N.C. Santos1, M. Mayor1, D. Naef1, F. Pepe1, D. Queloz1, S. Udry1, M. Burnet1, J.V. Clausen2, B.E. Helt2, E.H. Olsen2, and J.D. Pritchard3 1 Observatoire de Gen`eve, 51 ch. des Maillettes, CH–1290 Sauverny, Switzerland 2 Niels Bohr Institute for Astronomy, Physics, and Geophysics; Astronomical Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark 3 European Southern Observatory, Santiago, Chile Received / Accepted Abstract. In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7- magnitude fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measurements. Since HD 41004 A and B are separated by only 0.5′′, in every spectrum taken for the radial-velocity measurement, we are observing the blended spectra of the two stars. An analysis of the measurements has revealed a velocity variation with an amplitude of about 50 m s−1 and a periodicity of 1.3 days. This radial-velocity signal is consistent with the expected variation induced by the presence of a companion to either HD 41004 A or HD 41004 B, or to some other effect due to e.g. activity related phenomena. In particular, such a small velocity amplitude could be the signature of the presence of a very low mass giant planetary companion to HD 41004 A, whose light dominates the spectra. The radial-velocity measurements were then complemented with a photometric campaign and with the analysis of the bisector of the CORALIE Cross- Correlation Function (CCF). While the former revealed no significant variations within the observational precision of ∼0.003-0.004 mag (except for an observed flare event), the bisector analysis showed that the line profiles are varying in phase with the radial-velocity. This latter result, complemented with a series of simulations, has shown that we can explain the observations by considering that HD 41004 B has a brown-dwarf companion orbiting with the observed 1.3-day period. As the spectrum of the fainter HD 41004 B “moves” relative to the one of HD 41004 A (with an amplitude of a few km s−1), the relative position of the spectral lines of the two spectra changes, thus changing the blended line-profiles. This variation is large enough to explain the observed radial-velocity and arXiv:astro-ph/0206213v1 13 Jun 2002 bisector variations, and is compatible with the absence of any photometric signal. If confirmed, this detection represents the first discovery of a brown dwarf in a very short period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as a serious warning about the importance of analyzing the bisector when looking for planets using radial-velocity techniques. Key words. techniques: radial velocities – binaries: visual – binaries: spectroscopic – stars: brown dwarfs – stars: exoplanets – stars: individual: HD 41004 1. Introduction tion of the line-of-sight with a precision of the order of 2- 3ms−1 (e.g. Queloz et al. 2001a; Butler et al. 2001; Pepe Radial-velocity techniques have so far unveiled about 80 et al. 2002), but even higher precision is expected from 1 planetary companions around solar type dwarfs . The instruments available in the near future (e.g. HARPS – most precise instruments currently available for planet Pepe et al. 2000a). This will definitely allow the discov- searches can measure the velocity of a star in the direc- ery of lower mass and longer period planets, that remained undetected up to now due to the low amplitude of the in- Send offprint requests to: Nuno C. Santos, e-mail: duced radial-velocity variation. [email protected] ⋆ Based on observations collected at the La Silla Observatory, The gain in precision will, however, bring to light some ESO (Chile), with the CORALIE spectrograph at the 1.2- of the limitations of the radial-velocity method. It is well m Euler Swiss telescope and with the Str¨omgren Automatic known, for example, that radial-velocity “jitter” with am- − Telescope (SAT). plitudes up to a few tens of m s 1 is expected to result from 1 See e.g. obswww.unige.ch/∼udry/planet/planet.html the presence of strong photospheric features like spots 2 N.C. Santos et al.: A brown dwarf around HD 41004 B Table 1. Stellar parameters for HD41004A Parameter Value Reference Spectral type K1V/K2V Hipparcos (ESA 1997)/ uvby (see Sect 2.4) Parallax [mas] 23.24 ± 1.02 Hipparcos (ESA 1997) Distance [pc] 43 Hipparcos (ESA 1997) mv 8.65 Hipparcos (ESA 1997) B − V 0.887 Hipparcos (ESA 1997) Teff [K] 5010 See text log g [cgs] 4.42 See text Mv 5.48 – Luminosity [L⊙] 0.65 Flower (1996) Mass [M⊙] ∼0.7 – ′ log RHK −4.66 Henry et al. (1996) Age [Gyr] 1.6 Donahue (1993) Prot [days] ∼27 Noyes et al. (1984) v sin i [km s−1] 1.22 CORALIE [Fe/H] −0.09/+0.10 uvby/CORALIE Fig. 1. Upper panel: Added CORALIE spectrum in the or convective inhomogeneities, associated with chromo- Lithium line region for HD 41004. The plot reveals no clear spheric activity phenomena (Saar & Donahue 1997; Saar Li detection for this star. Lower panel: Added CORALIE et al. 1998; Santos et al. 2000). The presence of spots can spectrum in the Ca ii H line central region for HD 41004. even induce a periodic radial-velocity signal similar to the The strong emission near line center suggests that the star one expected from the presence of a planet. This is the case is chromospherically active for HD166435 (Queloz et al. 2001b), a star presenting a radial-velocity signal with a period of about 3.8-days, but showing both photometric and bisector variations with the same periodicity. 2. The case of HD 41004 The case of HD166435 illustrates very well the need to confirm, at least for the shortest period cases, that the 2.1. Stellar characteristics radial-velocity signature is indeed due to the presence of a low mass companion, and not due to some kind of intrinsic In the Hipparcos catalogue HD41004 (HIP28393, phenomena. As shown by Queloz et al. (2001b), the use CD 482083) is presented as the A component of a double system,− where the 3.68mag fainter companion is at a sep- of photometric data and bisector analysis was crucial to ′′ clarify the origin of the radial-velocity variations observed aration of about 0.5 . From the magnitude difference, and on this star. considering that both stars are at the same distance (i.e. In this paper we present the case of HD41004, a visual that they constitute a physical double system) we can de- double system consisting of a K1V-M2V pair (A and B duce that HD 41004 B is probably a M2 dwarf. In the rest components). This system was found to present a radial- of this paper we will denote by HD 41004 AB the system velocity signature similar to the one expected as if the composed by HD41004A and HD41004B. K1 dwarf had a very low mass planetary companion in a The small angular separation between the two com- 1.3-day period orbit. Although the photometric data re- ponents implies that we cannot separate (with the in- vealed no significant photometric variations, an analysis of strumentation used) the light coming from the two stars. the Bisector Inverse Slope (BIS) of the CORALIE Cross- However, given the relatively “large” magnitude differ- Correlation Function (CCF) (Queloz et al. 2001b) revealed ence, we do not expect the flux coming from HD41004B a periodic variation in phase with the radial-velocity sig- (about 3% of that coming from the A component) to affect nal. In the following sections we will show that the radial- significantly the determination of the stellar parameters of velocity variation is in fact not a result of the periodic HD41004A. All the parameters described below are thus, motion of the A component, but of the Doppler motion in a good approximation, those of HD41004A (the star of the spectrum of the B component due to the presence whose light dominates). of a brown-dwarf companion. The results strongly caution At a distance of 43 parsec – π=23.24 1.02mas – about the need to use methods capable of detecting line ESA (1997), HD 41004∼ A is a K1 dwarf shining± with a asymmetries, like the bisector analysis, when dealing with visual magnitude V=8.65 in the southern constellation high-precision planet searches with radial-velocity tech- Pictoris (the Painter’s Easel). Its colour index is B V = niques. 0.887, as listed by the Hipparcos catalogue and its− abso- N.C. Santos et al.: A brown dwarf around HD 41004 B 3 Table 2. Elements of the fitted orbit P 1.3298 ± 0.0002 d a1 sin i 0.0009 ± 0.0000 Gm T 2452200.43 ± 0.13 d e† 0.07 ± 0.04 −1 Vr 42.532 ± 0.002 km s ω† 23 ± 35 degr −1 K1 50 ± 2 m s −10 −10 f1(m) 0.1741 · 10 ± 0.0224 · 10 M⊙ σ(O − C) 14 m s−1 N 86 † Consistent with a circular orbit according to the Lucy & Sweeney (1971) test. lute magnitude Mv=5.48. The basic stellar parameters are summarized in Table 1. There are not many references in the literature on HD 41004 A, and no precise spectroscopic analysis is avail- able for this star.
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