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Appendix A: Comparison with published time series

Appendix A.1: Variability

Compared to other published time-series, we measured lower dispersions for all M dwarfs apart from Gl 846 and known planet-host stars. Gl 1 is not found to be variable in E06 and Z09 but their dispersion is limited by a higher photon noise (~2.6 m/s, against σ_e = 1.9 m/s in our case). We report some variability of Gl 229 but at a level of <1.9 m/s, while the variability reported in E06 and Z09 implies a jitter of 3.9–4.7 m/s. The slightly smaller dispersion we observe for Gl 357 (σ_e = 3.2 m/s) compared to 3.7 m/s and 5.3 m/s for E06 and Z09, respectively, might not be significant given our small number of observations (6). For Gl 551, we measured only a dispersion slightly smaller (3.3 against 3.6 m/s). We observe significantly lower variability for Gl 682 (1.8 against 3.6 m/s) and Gl 699 (1.7 against 3.4 and 3.3 m/s), and a larger dispersion for Gl 846 (5.6 against 3.0 m/s). Although different time-spans, epochs of observations and activity levels at those epochs could explain the different dispersions for individual stars (as is certainly the case for Gl 846 – see Sect. 5.2), that we measure a smaller dispersion for most comparison stars most likely reflects the superior performance of the HARPS spectrograph.

Appendix A.2: Trends

As in this paper, Z09 reported non-significant slopes for Gl 357 and Gl 682 and significant slopes for Gl 1, Gl 551, and Gl 699 (although in our case Gl699 is attributed a significant trend only by the F-test). Nonetheless, the slopes that Z09 reported for Gl 551 and Gl 699 seem different and they moreover found a significant trend for Gl 229, whereas we do not. Time series have also been published for Gl 832 and Gl 849 as they were they were identified as likely hosts of orbiting planet (Bailey et al. 2009; Butler et al. 2006). For both stars, the planetary reflex motion clearly dominates the radial velocity signal so we discard them from any quantitative comparison. In Table A.1, we compare the slopes of linear fits to the time series in Z09 and to those of this paper. We note that the significant differences most often reflect a signal more complex than a simple linear drift.

Appendix A.3: Periodicity

Among stars with identified periodicity in RV data, Gl 832, Gl 849, and Gl 876 have time series published to report on detected planets. The periodicities we have found for those three stars are similar to their planetary orbital periods. Only Gl 876d is undetected with our automated procedure because one has to do a full N-body integration to subtract properly the signal induced by planets “b” and “c”. Besides known planet hosts, Z09 also report an absence of periodicities for Gl 229, Gl 357, Gl 433, and Gl 682, and significant periodicities for Gl 551 and Gl 699. Our results and Z09 are therefore in contradiction for three stars: Gl 433, Gl 551 and Gl 699. We noted in Sect. 5.1 that, for Gl 433, the RVs reported by Z09 and in this paper are not incompatible provided that the merged data set is fitted by a model composed of one planet plus a quadratic drift. The about one-year periodicity found for Gl 551 by Z09 and Endl & Kürster (2008) led these authors to attribute the signal to an alias of a low frequency signal with the typical one-year sampling. After Endl & Kürster (2008), the low frequency signal is believed to be caused by a clustering of points that are both blue-shifted and have a higher Hα index than other points in the time series. This putative activity signal might not be seen in our time series because it represented by only 24 measurements, against 229 in Z09. Finally, the periodicity found for Gl 699 is also attributed to activity, with a clear counterpart in Hα filling factor. In addition, if that activity signal is not seen in our time series, it is likely because it is represented by only 22 measurements, compared to 226 for Z09.

© ESO, 2013