The CORALIE survey for southern extra-solar planets IX. A 1.3-day period brown dwarf disguised as a planet

TL;DR

This study reveals that a 1.3-day radial-velocity variation initially attributed to a planet around HD 41004 A is actually caused by a brown dwarf orbiting the M-dwarf companion HD 41004 B, highlighting the importance of bisector analysis.

Contribution

The paper demonstrates that short-period radial-velocity signals can be caused by brown dwarfs around companion stars, not planets, emphasizing the need for bisector analysis in planet detection.

Findings

Radial-velocity variation caused by a brown dwarf around HD 41004 B.

No significant photometric variations detected.

Bisector analysis revealed line profile variations in phase with RV signals.

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 separated by only 0.5 arcsec. An analysis of CORALIE radial-velocity measurements has revealed a variation with an amplitude of about 50m/s and a periodicity of 1.3days. This radial-velocity signal is consistent with the expected variation induced by the presence 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. 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.