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Michel Mayor - One of the best experts on this subject based on the ideXlab platform.
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search for exoplanets with the radial velocity technique quantitative diagnostics of stellar activity
Astronomy and Astrophysics, 2007Co-Authors: Morgan Desort, Stéphane Udry, Anne-marie Lagrange, Franck Galland, Michel MayorAbstract:Aims. It is known that stellar activity may complicate the analysis of high-precision radial-velocity spectroscopic data when looking for exoplanets signatures. We aim at quantifying the impact of stellar spots on stars with various spectral types and rotational velocities and comparing the simulations with data obtained with the HARPS spectrograph. Methods. We have developed detailed simulations of stellar spots and estimated their effects on a number of observables commonly used in the analysis of radial-velocity data when looking for extrasolar planets, such as radial-velocity curves, cross-correlation functions, Bisector velocity spans, and photometric curves. Stellar and spot properties are taken into account, as well as the characteristics of the spectrograph used (generally HARPS). The computed stellar spectra are then analyzed in the same way as when searching for exoplanets. Results. 1) A first grid of simulation results (radial-velocity amplitudes, Bisector velocity-span amplitudes and shapes, and photometry) is built for F-K type stars, with different stellar and spot properties. 2) It is shown quantitatively that star spots with typical sizes of 1% can mimic both radial-velocity curves and the Bisector behavior of short-period giant planets around G-K type stars with a v sini lower than the spectrograph resolution. For stars with intermediate v sini, smaller spots may produce similar features. Such spots may complicate the search for low-mass planets on short-period orbits. In these cases, additional observables (e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the presence of short-period planets. We discuss these possibilities and show that, in some cases, photometric variations may not be enough to clearly rule out spots as explanations of the observed radial-velocity variations. This is particularly important when searching for super-Earth planets. 3) It is also stressed that quantitative values obtained for radial-velocity and Bisector velocity-span amplitudes depend strongly on the detailed star properties, on the spectrograph used, on the line or set of lines used, and on the way they are measured. High-resolution spectrographs will help in distinguishing between spots and planets.
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Search for exoplanets with the radial-velocity technique: quantitative diagnostics of stellar activity
2007Co-Authors: Morgan Desort, Stéphane Udry, Anne-marie Lagrange, Franck Galland, Michel MayorAbstract:Aims: Stellar activity may complicate the analysis of high-precision radial-velocity spectroscopic data when looking for exoplanets signatures. We aim at quantifying the impact of stellar spots on stars with various spectral types and rotational velocities and comparing the simulations with data obtained with the HARPS spectrograph. Methods: We have developed detailed simulations of stellar spots and estimated their effects on a number of observables commonly used in the analysis of radial-velocity data when looking for extrasolar planets, such as radial-velocity curves, cross-correlation functions, Bisector spans and photometric curves. The computed stellar spectra are then analyzed in the same way as when searching for exoplanets. Results: 1) A first grid of simulation results is built for F-K type stars, with different stellar and spot properties. 2) It is shown quantitatively that star spots with typical sizes of 1% can mimic both radial-velocity curves and the Bisector behavior of short-period giant planets around G-K type stars with a vsini lower than the spectrograph resolution. For stars with intermediate vsini, smaller spots may produce similar features. In these cases, additional observables (e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the presence of short-period planets. We show that, in some cases, photometric variations may not be enough to clearly rule out spots as explanations of the observed radial-velocity variations. This is particularly important when searching for super-Earth planets. 3) It is also stressed that quantitative values obtained for radial-velocity and Bisector span amplitudes depend strongly on the detailed star properties, on the spectrograph used, on the set of lines used, and on the way they are measured.
N C Santos - One of the best experts on this subject based on the ideXlab platform.
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line profile variations in radial velocity measurements two alternative indicators for planetary searches
Astronomy and Astrophysics, 2013Co-Authors: P Figueira, N C Santos, F Pepe, C Lovis, N NardettoAbstract:Aims. We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting (and monitoring of the parameters derived from it), and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. We assess the usefulness of each of these methods by comparing their results with those delivered by current indicators. Methods. We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions that are created with the program SOAP and which are a ected by the presence of stellar spots. We consider di erent spot properties on stars with di erent rotation profiles and simulate observations as obtained with high-resolution spectrographs. We then apply our methodology to real cross-correlation functions, which are computed from HARPS spectra, for stars with a signal originating in activity (thus spots) and for those with a signal rooted on a planet. Results. We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard Bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the Bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the Bisector inverse slope is shown to be more e cient on high-signal-to-noise data than both the standard Bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more e ective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the Bisector, showing significant correlations with RV for active stars for which Bisector analysis is inconclusive. Conclusions. We show that the two indicators discussed here should be considered as standard tests to check for the planetary nature of a radial-velocity signal. We encourage the usage of di erent diagnosis as a way of characterizing the often elusive line profile deformations.
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line profile variations in radial velocity measurements two alternative indicators for planetary searches
arXiv: Earth and Planetary Astrophysics, 2013Co-Authors: P Figueira, N C Santos, F Pepe, C Lovis, N NardettoAbstract:Aims. We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting, and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. Methods. We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions created with the program SOAP and which are affected by the presence of stellar spots, and to real cross-correlation functions, calculated from HARPS spectra, for stars with a signal originating both in activity and created by a planet. Results. We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard Bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the Bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the Bisector inverse slope is shown to be more efficient on high-signal-to-noise data than both the standard Bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more effective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the Bisector, showing significant correlations with RV for active stars for which Bisector analysis is inconclusive. (abridged)
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
Astronomy and Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract:In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-mag fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measure- ments. Since HD 41004 A and B are separated by only 0.5 00 , 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 eect 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 plane- tary 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 of0.003-0.004 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 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.
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
arXiv: Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract: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.
C Moutou - One of the best experts on this subject based on the ideXlab platform.
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pastis bayesian extrasolar planet validation ii constraining exoplanet blend scenarios using spectroscopic diagnoses
arXiv: Earth and Planetary Astrophysics, 2015Co-Authors: A Santerne, P Figueira, R F Diaz, J M Almenara, F Bouchy, M Deleuil, G Hebrard, C MoutouAbstract:The statistical validation of transiting exoplanets proved to be an efficient technique to secure the nature of small exoplanet signals which cannot be established by purely spectroscopic means. However, the spectroscopic diagnoses are providing us with useful constraints on the presence of blended stellar contaminants. In this paper, we present how a contaminating star affects the measurements of the various spectroscopic diagnoses as function of the parameters of the target and contaminating stars using the model implemented into the PASTIS planet-validation software. We find particular cases for which a blend might produce a large radial velocity signal but no Bisector variation. It might also produce a Bisector variation anti-correlated with the radial velocity one, as in the case of stellar spots. In those cases, the full width half maximum variation provides complementary constraints. These results can be used to constrain blend scenarios for transiting planet candidates or radial velocity planets. We review all the spectroscopic diagnoses reported in the literature so far, especially the ones to monitor the line asymmetry. We estimate their uncertainty and compare their sensitivity to blends. Based on that, we recommend the use of BiGauss which is the most sensitive diagnosis to monitor line-profile asymmetry. In this paper, we also investigate the sensitivity of the radial velocities to constrain blend scenarios and develop a formalism to estimate the level of dilution of a blended signal. Finally, we apply our blend model to re-analyse the spectroscopic diagnoses of HD16702, an unresolved face-on binary which exhibits Bisector variations.
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detecting planets around active stars impact of magnetic fields on radial velocities and line Bisectors
Monthly Notices of the Royal Astronomical Society, 2014Co-Authors: E M Hebrard, J F Donati, X Delfosse, J Morin, I Boisse, C MoutouAbstract:Although technically challenging, detecting Earth-like planets around very low mass stars is in principle accessible to the existing velocimeters of highest radial-velocity (RV) precision. However, low-mass stars being active, they often feature dark spots and magnetic regions at their surfaces generating a noise level in RV curves (called activity jitter) that can severely limit our practical ability at detecting Earth-like planets. Whereas the impact of dark spots on RV data has been extensively studied in the literature, that of magnetic features only received little attention up to now. In this paper, we aim at quantifying the impact of magnetic fields (and the Zeeman broadening they induce) on line profiles, line Bisectors and RV data. With a simple model, we quantitatively study the RV signals and Bisector distortions that small magnetic regions or global magnetic dipoles can generate, especially at infrared wavelengths where the Zeeman broadening is much larger than that in the visible. We report in particular that the impact of magnetic features on line Bisectors can be different from that of cool spots when the rotational broadening is comparable to or larger than the Zeeman broadening; more specifically, we find in this case that the top and bottom sections of the Bisectors are anticorrelated, i.e. the opposite behaviour of what is observed for cool spots. We finally suggest new options to show and ultimately filter the impact of the magnetic activity on RV curves.
E H Olsen - One of the best experts on this subject based on the ideXlab platform.
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
Astronomy and Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract:In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-mag fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measure- ments. Since HD 41004 A and B are separated by only 0.5 00 , 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 eect 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 plane- tary 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 of0.003-0.004 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 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.
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
arXiv: Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract: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.
F Pepe - One of the best experts on this subject based on the ideXlab platform.
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line profile variations in radial velocity measurements two alternative indicators for planetary searches
Astronomy and Astrophysics, 2013Co-Authors: P Figueira, N C Santos, F Pepe, C Lovis, N NardettoAbstract:Aims. We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting (and monitoring of the parameters derived from it), and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. We assess the usefulness of each of these methods by comparing their results with those delivered by current indicators. Methods. We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions that are created with the program SOAP and which are a ected by the presence of stellar spots. We consider di erent spot properties on stars with di erent rotation profiles and simulate observations as obtained with high-resolution spectrographs. We then apply our methodology to real cross-correlation functions, which are computed from HARPS spectra, for stars with a signal originating in activity (thus spots) and for those with a signal rooted on a planet. Results. We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard Bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the Bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the Bisector inverse slope is shown to be more e cient on high-signal-to-noise data than both the standard Bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more e ective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the Bisector, showing significant correlations with RV for active stars for which Bisector analysis is inconclusive. Conclusions. We show that the two indicators discussed here should be considered as standard tests to check for the planetary nature of a radial-velocity signal. We encourage the usage of di erent diagnosis as a way of characterizing the often elusive line profile deformations.
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line profile variations in radial velocity measurements two alternative indicators for planetary searches
arXiv: Earth and Planetary Astrophysics, 2013Co-Authors: P Figueira, N C Santos, F Pepe, C Lovis, N NardettoAbstract:Aims. We introduce two methods to identify false-positive planetary signals in the context of radial-velocity exoplanet searches. The first is the bi-Gaussian cross-correlation function fitting, and the second is the measurement of asymmetry in radial-velocity spectral line information content, Vasy. Methods. We make a systematic analysis of the most used common line profile diagnosis, Bisector Inverse Slope and Velocity Span, along with the two proposed ones. We evaluate all these diagnosis methods following a set of well-defined common criteria and using both simulated and real data. We apply them to simulated cross-correlation functions created with the program SOAP and which are affected by the presence of stellar spots, and to real cross-correlation functions, calculated from HARPS spectra, for stars with a signal originating both in activity and created by a planet. Results. We demonstrate that the bi-Gaussian method allows a more precise characterization of the deformation of line profiles than the standard Bisector inverse slope. The calculation of the deformation indicator is simpler and its interpretation more straightforward. More importantly, its amplitude can be up to 30% larger than that of the Bisector span, allowing the detection of smaller-amplitude correlations with radial-velocity variations. However, a particular parametrization of the Bisector inverse slope is shown to be more efficient on high-signal-to-noise data than both the standard Bisector and the bi-Gaussian. The results of the Vasy method show that this indicator is more effective than any of the previous ones, being correlated with the radial-velocity with more significance for signals resulting from a line deformation. Moreover, it provides a qualitative advantage over the Bisector, showing significant correlations with RV for active stars for which Bisector analysis is inconclusive. (abridged)
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
Astronomy and Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract:In this article we present the case of HD 41004 AB, a system composed of a K0V star and a 3.7-mag fainter M-dwarf companion. We have obtained 86 CORALIE spectra of this system with the goal of obtaining precise radial-velocity measure- ments. Since HD 41004 A and B are separated by only 0.5 00 , 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 eect 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 plane- tary 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 of0.003-0.004 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 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.
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the coralie survey for southern extra solar planets ix a 1 3 day period brown dwarf disguised as a planet
arXiv: Astrophysics, 2002Co-Authors: N C Santos, F Pepe, M Mayor, D Naef, D Queloz, S Udry, M Burnet, J V Clausen, B E Helt, E H OlsenAbstract: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.