Stellar noise and planet detection. II. Radial-velocity noise induced by magnetic cycles

Dumusque, X.; Lovis, C.; Udry, S.; Santos, N. C.

doi:10.1017/s1743921311021090

Cited by 5 publications

(10 citation statements)

References 4 publications

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“…This model allows removal of the effect of magnetic cycles (Meunier & Lagrange 2013;Dumusque et al 2011b) and any long-term drift in the RVs due to binary companions. In Fig.…”

Section: Comparing the Results Obtained By The Different Teamsmentioning

confidence: 99%

“…Pre-treatment phase: removing long-term trends of stellar origin: When generating the data of the RV fitting challenge, Dumusque (2016) considered magnetic cycles and their effect on the different observables, i.e., log(R HK ), BIS SPAN and full width at half maximum (FWHM) of the CCF. In this case, strong correlation between log(R HK ) and FWHM, and between log(R HK ) and RV are expected (Lovis et al 2011;Dumusque et al 2011b;Lindegren & Dravins 2003). To test those correlations, the Torino team calculated Spearman's rank correlation coefficients and found, in most cases, a very strong correlation between these observables, i.e., ρ > 0.9.…”

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confidence: 99%

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Radial-velocity fitting challenge

Dumusque

Borsa

Damasso

et al. 2017

A&A

121

126

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Context. Radial-velocity (RV) signals arising from stellar photospheric phenomena are the main limitation for precise RV measurements Those signals induce RV variations an order of magnitude larger than the signal created by the orbit of Earth-twins, thus preventing their detection. Aims. Different methods have been developed to mitigate the impact of stellar RV signals. The goal of this paper is to compare the efficiency of these different methods to recover extremely low-mass planets despite stellar RV signals. However, because observed RV variations at the meter-per-second precision level or below is a combination of signals induced by unresolved orbiting planets, by the star, and by the instrument, performing such a comparison using real data is extremely challenging. Methods. To circumvent this problem, we generated simulated RV measurements including realistic stellar and planetary signals. Different teams analyzed blindly those simulated RV measurements, using their own method to recover planetary signals despite stellar RV signals. By comparing the results obtained by the different teams with the planetary and stellar parameters used to generate the simulated RVs, it is therefore possible to compare the efficiency of these different methods. Results. The most efficient methods to recover planetary signals take into account the different activity indicators, use red-noise models to account for stellar RV signals and a Bayesian framework to provide model comparison in a robust statistical approach. Using the most efficient methodology, planets can be found down to K/N = K pl /RV rms × √ N obs = 5 with a threshold of K/N = 7.5 at the level of 80-90% recovery rate found for a number of methods. These recovery rates drop dramatically for K/N smaller than this threshold. In addition, for the best teams, no false positives with K/N > 7.5 were detected, while a non-negligible fraction of them appear for smaller K/N. A limit of K/N = 7.5 seems therefore a safe threshold to attest the veracity of planetary signals for RV measurements with similar properties to those of the different RV fitting challenge systems.

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“…This model allows removal of the effect of magnetic cycles (Meunier & Lagrange 2013;Dumusque et al 2011b) and any long-term drift in the RVs due to binary companions. In Fig.…”

Section: Comparing the Results Obtained By The Different Teamsmentioning

confidence: 99%

mentioning

confidence: 99%

Radial-velocity fitting challenge

Dumusque

Borsa

Damasso

et al. 2017

A&A

121

126

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“…It has to be pointed out that stellar magnetic cycles (the solar cycle length is 2 × 11 yr) may cause long-term RV variations which might be mistaken as companions. The CaII H&K lines are an appropriate indicator to check this (Santos et al 2010;Dumusque et al 2011a;Lovis et al 2011). Indeed, some stars, like e.g.…”

Section: Discussionmentioning

confidence: 99%

The planet search programme at the ESO CES and HARPS

Zechmeister

Kürster

Endl³

et al. 2013

A&A

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Context. In 1992 we began a precision radial velocity survey for planets around solar-like stars with the Coudé Echelle Spectrograph and the Long Camera (CES LC) at the 1.4 m telescope in La Silla (Chile) resulting in the discovery of the planet ι Hor b. We have continued the survey with the upgraded CES Very Long Camera (VLC) and the HARPS spectrographs, both at the 3.6 m telescope, until 2007. Aims. In this paper we present additional radial velocities for 31 stars of the original sample with higher precision. The observations cover a time span of up to 15 years and permit a search for Jupiter analogues. Methods. The survey was carried out with three different instruments/instrument configurations using the iodine absorption cell and the ThAr methods for wavelength calibration. We combine the data sets and perform a joint analysis for variability, trends, and periodicities. We compute Keplerian orbits for companions and detection limits in case of non-detections. Moreover, the HARPS radial velocities are analysed for correlations with activity indicators (CaII H&K and cross-correlation function shape). Results. We achieve a long-term RV precision of 15 m/s (CES+LC, 1992-1998, 9 m/s (CES+VLC, 1999, and 2.8 m/s (HARPS, 2003(HARPS, -2009, including archive data), respectively. This enables us to confirm the known planetary signals in ι Hor and HR 506 as well as the three known planets around HR 3259. A steady RV trend for Ind A can be explained by a planetary companion and calls for direct imaging campaigns. On the other hand, we find previously reported trends to be smaller for β Hyi and not present for α Men. The candidate planet Eri b was not detected despite our better precision. Also the planet announced for HR 4523 cannot be confirmed. Long-term trends in several of our stars are compatible with known stellar companions. We provide a spectroscopic orbital solution for the binary HR 2400 and refined solutions for the planets around HR 506 and ι Hor. For some other stars the variations could be attributed to stellar activity, as e.g. the magnetic cycle in the case of HR 8323. Conclusions. The occurrence of two Jupiter-mass planets in our sample is in line with the estimate of 10% for the frequency of giant planets with periods smaller than 10 yr around solar-like stars. We have not detected a Jupiter analogue, while the detections limits for circular orbits indicate at 5 AU a sensitivity for minimum mass of at least 1M Jup (2M Jup ) for 13% (61%) of the stars.

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“…2 in Dumusque et al 2011b). Therefore, apart from the two most obvious signals in the S-indices (long-period trend and 320-day period), the other tentative periodicities are unlikely to produce detectable effects on the RVs.…”

Section: Analysis Of the S-indexmentioning

confidence: 93%

“…Several studies have demonstrated a clear correlation of the S-index with observed RV offsets. For example, Dumusque et al (2011b) performed a comprehensive analysis of simultaneous HARPS RVs and chromospheric emission measurements of G and K dwarfs, showing that the correlation between the chromospheric emission and RV offsets can be clearly traced and quantified as a function of spectral type. Pepe et al (2011) -Escudé & Butler (2012) showed that there is a correlation between the S-indices and RV measurements of HD 69830 (G8 V) and that an apparent long-period trend in the RVs completely disappeared when one obtains the velocities using the redmost half of the HARPS spectra only.…”

Section: Analysis Of the S-indexmentioning

confidence: 99%

Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307

Tuomi

Anglada‐Escudé

Gerlach

et al. 2012

A&A

100

118

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Context. The K2.5 dwarf HD 40307 has been reported to host three super-Earths. The system lacks massive planets and is therefore a potential candidate for having additional low-mass planetary companions. Aims. We re-derive Doppler measurements from public HARPS spectra of HD 40307 to confirm the significance of the reported signals using independent data analysis methods. We also investigate these measurements for additional low-amplitude signals. Methods. We used Bayesian analysis of our radial velocities to estimate the probability densities of different model parameters. We also estimated the relative probabilities of models with differing numbers of Keplerian signals and verified their significance using periodogram analyses. We investigated the relation of the detected signals with the chromospheric emission of the star. As previously reported for other objects, we found that radial velocity signals correlated with the S-index are strongly wavelength dependent. Results. We identify two additional clear signals with periods of 34 and 51 days, both corresponding to planet candidates with minimum masses a few times that of the Earth. An additional sixth candidate is initially found at a period of 320 days. However, this signal correlates strongly with the chromospheric emission from the star and is also strongly wavelength dependent. When analysing the red half of the spectra only, the five putative planetary signals are recovered together with a very significant periodicity at about 200 days. This signal has a similar amplitude as the other new signals reported in the current work and corresponds to a planet candidate with M sin i ∼ 7 M ⊕ (HD 40307 g). Conclusions. We show that Doppler measurements can be filtered for activity-induced signals if enough photons and a sufficient wavelength interval are available. If the signal corresponding to HD 40307 g is a genuine Doppler signal of planetary origin, this candidate planet might be capable of supporting liquid water on its surface according to the current definition of the liquid water habitable zone around a star and is not likely to suffer from tidal locking. Also, at an angular separation of ∼46 mas, HD 40307 g would be a primary target for a future space-based direct-imaging mission.

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Stellar noise and planet detection. II. Radial-velocity noise induced by magnetic cycles

Cited by 5 publications

References 4 publications

Radial-velocity fitting challenge

Radial-velocity fitting challenge

The planet search programme at the ESO CES and HARPS

Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307

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