Most of the exoplanet science is dependent on the stellar knowledge. One of them that has to be understood is the magnetic activity when we search for planets with radial velocity or photometry measurements. The main shape of stellar activity and spots properties have to be understood, for example, to choose the best targets to search for low-mass planets in the habitable zone or to derive the accurate parameters of a planetary system. With that aim, we show in this presentation how these studies lead to give clues on spots latitudes and on the long term variation of stellar activity. The properties of magnetic activity on the low rotators solar-type stars are not easily reachable by other techniques (spectropolarimetry or Doppler imaging) and these studies should be used to constrain theories of stellar dynam

Boisse, I.

Bonfils, X.

Boué, G.

Dumusque, X.

Lovis, C.

Montalto, M.

Oshagh, M.

Santos, N. C.

RERO DOC Digital Library

Solar and Astrophysical Dynamos and Magnetic ActivityProceedings IAU Symposium No. 294, 2012A.G. Kosovichev, E.M. de Gouveia Dal Pino, & Y. Yan, eds.c© International Astronomical Union 2013doi:10.1017/S1743921313002937How the planetary research helps to thestellar dynamo understandingI. Boisse1, M. Oshagh1,2, C. Lovis3, N. C. Santos1,2, X. Dumusque2,3,X. Bonﬁls4, M. Montalto1 and G. Boue´11Centro de Astrof´ısica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugalemail: Isabelle.Boisse@astro.up.pt2Departamento de Fı´sica e Astronomia, Faculdade de Cieˆncias, Universidade do Porto,4169-007 Porto, Portugal3Observatoire de Gene`ve, Universite´ de Gene`ve, 51 Ch. des Maillettes, 1290 Sauverny,Switzerland4UJF-Grenoble1/CNRS-INSU, Institut de Plane´tologie et d’Astrophysique de Grenoble, UMR5274, Grenoble, F-38041, FranceAbstract. Most of the exoplanet science is dependent on the stellar knowledge. One of themthat has to be understood is the magnetic activity when we search for planets with radial velocityor photometry measurements. The main shape of stellar activity and spots properties have tobe understood, for example, to choose the best targets to search for low-mass planets in thehabitable zone or to derive the accurate parameters of a planetary system. With that aim, weshow in this presentation how these studies lead to give clues on spots latitudes and on thelong term variation of stellar activity. The properties of magnetic activity on the low rotatorssolar-type stars are not easily reachable by other techniques (spectropolarimetry or Dopplerimaging) and these studies should be used to constrain theories of stellar dynamo.Keywords. Techniques: radial velocities, photometry, Stars: activity, Dynamo, Planetarysystem1. Why planetary researchers are worried about stellar activity?Up to now, more than 800 planets were discovered and characterized, most of themthanks to the radial-velocity (RV) technique. However, this eﬃcient method is indirectas well as the photometric transit and astrometry techniques. One of the problems withthis is the fact that RV variations can in some cases be caused by other mechanismsthat are not related to the presence of low-mass companions. Even if RV surveys couldreject very active targets, all stars present variability at diﬀerent level (from few cm−1 sto hundreds of m−1 s). Moreover, mainly RV is needed to establish the planetary natureand measure the mass of a transiting candidate (and stellar activity is less determinantfor transit detection).RV values are derived from the measurement of position in wavelength of thousandsof spectral lines. Unfortunately, the measurement is not only sensitive to Doppler shiftdue to gravitationally bounded companion but also to line proﬁle deformations. Espe-cially, apparition and disappearance of spots from the observer view due to the stellarrotation, or change of the convection pattern with magnetic cycle induce a distortion ofthe line that changes with time. The jitter that results in RV measurement may hamperplanet discovery or degrade the orbital parameters estimation. They may also give falsecandidates, if they produce a stable periodic signal (e.g. Queloz et al. 2001). An essentialwork is then needed to understand all phenomena caused by stellar variability, and to471at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313002937Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 19:09:06, subject to the Cambridge Core terms of use, available472 I. Boisse et al.characterize their impact on RV and photometry to be able to distinguish between theseand real planetary signatures.2. How planetary researchers track stellar activity ?Phenomena such as stellar pulsation or inhomogeneous convection could be averagedby speciﬁc observational strategy (Dumusque et al. 2011). Because the typical timescaleof those oscillations is less than ﬁfteen minutes, an exposure time of ﬁfteen minutes orlarger reduces greatly the oscillation noise. In the same way, the timescale of granulation isless than 30 hours, therefore taking several measurements per night allows a reduction ofthe RV contribution of granulation. These methods have already been used by Pepe et al.(2011) and showed good eﬃciency (e.g. Santos et al. 2004, that averaged asteroseismologicdata to detect one of the ﬁrst low-mass planet).In the following, we focus on stellar activity, mainly dark spots and bright plages onthe stellar surface, and on magnetic cycle. These phenomenon are assumed to be part ofthe stellar dynamo process, that generates stellar magnetic ﬁeld. To identify stellar ac-tivity, we can monitor the bisector span (BIS), the width and the depth of the mean lineof the spectrum, and also calculate the chromospheric emission ratio, logR′HK from theCaII H&K lines and monitor other lines sensitive to active features: Hα, HeI, NaI. How-ever the behavior of these parameters is not yet understood properly, moreover becausethey depend on stellar parameters (Desort et al. 2007, Boisse et al. 2011, Boisse et al.2012). For example, Bonﬁls et al. (2007) has to compared RV and stellar activity index,because the BIS becomes ineﬃcient as an indicator of activity when the star rotates toolow.SOAP: a tool for the fast computation of the stellar spots impact†SOAP is an online software that simulates the eﬀect of stellar spots and plages on RVand photometry (Boisse et al. 2012). This numerical model centers the visible stellardisk on a grid of several tens of thousands of resolution elements. The typical line of theemerging spectrum of the star is modeled by a Gaussian for each grid cell (equivalentto the spectrum cross-correlation function that can be seen as a weighted average ofall the spectral lines convolved with a rectangular function). Each Gaussian (in a givenstellar position) is Doppler-shifted according to the projected rotational velocity andweighted by a linear limb-darkening law. SOAP calculates the position of the surfaceinhomogeneities deﬁned by their latitudes, longitudes, and sizes. The cells inside thespots are modeled by the same Gaussian as for the stellar disk, but are weighted by theirbrightness. The code then removes (for dark spot) or adds (for bright plage) the Gaussianand ﬂux of the inhomogeneities to those of the non-spotted star. Finally, SOAP deliversthe integrated spectral line, the ﬂux, the RV, and the bisector span as a function of thestellar rotational phase.SOAP-T is an improved version that includes the impact of a transiting planet in RVand photometry (Oshagh et al. 2012).3. What we learnt about stellar activity?Monitoring and determining properties of active regionsBoisse et al. (2009) monitored HD189733 simultaneously in RV with SOPHIE and inphotometry with the MOST satellite. They showed the presence of an active region on† The tool is available at http://www.astro.up.pt/soapat https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313002937Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 19:09:06, subject to the Cambridge Core terms of use, availablePlanetary research and stellar dynamo 473the stellar surface. The plage was tracked by Hα emission, but in photometry the darkspot dominates the signal. The spots temperature were constrained from the crossing ofthe transiting planet observed by HST (Pont et al. 2007). The ratio of the covered surfaceby dark spots and bright plages, and their contrast in temperature with the stellar pho-tosphere, are key parameters to model activity, mainly to analyze photometric rotationalmodulations from Kepler and CoRoT lightcurves (e.g. Lanza et al. 2011, Kipping 2012).Long-term RV and logR′HK variations: Characterizing photospheric modiﬁcations withmagnetic ﬁeld evolutionRV surveys searching for planets monitor primarily main-sequence stars of spectral-typefrom A-F to M. Some database reach now a time coverage of more than 15 years. Evenif the sampling of the observation and the target selection are not optimized for activitymonitoring, results can be drawn about long term variations.Lovis et al. (2011) studied 304 FGK stars, mainly dwarfs, from the HARPS sample.They searched for long term variability with period greater than 2 years. They computedthe activity index, logR′HK , from the CaII H&K lines. They compared its variability withthe one of the RV, BIS and parameters of the mean spectral line. We would like to pointout three of their results. They detected variability periods shorter than the 11 yrs ofthe Sun cycle. It may be a sign of multiperiodism in other stars. They found few F starspresent periodicity. But we could also argue that magnetic cycle measured in F stars(Tau Boo Fare`s et al. 2009, Iota Hor Metcalfe et al. 2010) present periods shorter than 2years. The last point is the dependence of the correlation slope between RV and R′HK , asa function of eﬀective temperature (see Fig. 1). This indicate that for a same variationin the logR′HK , the impact in RV will be 2-3 times greater for a G dwarf than for a lateK star. Which phenomena (or composition of phenomena) is responsible for this is stillan open question. It could be due to a diﬀerent modiﬁcation of the convection patternwith activity level or to a distinct ﬁlling factor of plages compared to spots with spectraltypes.Figure 1. Correlation slope between RV and R′HK , as a function of eﬀective temperature(from Lovis et al. 2011).at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313002937Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 19:09:06, subject to the Cambridge Core terms of use, available474 I. Boisse et al.Track the spots with planetary transitsA planet transiting its host-star could overlap with active zones on the stellar surface. Itwill generate anomalies (bumps) inside the transit light-curve (see Fig. 2). The evolutionof these anomalies can be tracked if long-term and continuous observations are accessible(e.g. CoRoT and Kepler). It could allow to determine the spin-orbit angle, the inclinationof the stellar spin axis, and the conﬁguration of the spots on the stellar surface (e.g.Nutzman et al. 2011, De´sert et al. 2011, Sanchis-Ojeda et al. 2011.Sanchis-Ojeda & Winn (2011) detected two bumps in the folded Kepler light-curveof 26 transits of HAT-P-11. They interpreted these peaks as evidence of long lived spotbelt regions, and derived two possible scenario for the planet orbit, stellar spin axis andspots positions. Using SOAP-T and the out-of-transit light curve, Oshagh et al. (2012)showed that the most probable scenario of the previous study is the one that presentstwo active belts at ∼ ± 20◦, comparable to what is observed on the Sun but at lowerlatitude. Kepler will continue to monitor the star during several years. In analogy towhat is seen on the Sun, we could expect to observe a change in the spots latitude withtime if HAT-P-11 has a magnetic cycle.Figure 2. Left: Kepler observation of HAT-P-11 (green) and SOAP-T ﬁt (red) of HAT-P-11during one stellar rotation. Blue are the residuals. Middle and right: Zooms on two transitsof the left ﬁgure. The ﬂux values are normalized to one. Figures from Oshagh et al. (2012).4. FutureSOAP simulates the photometric eﬀect due to inhomogeneities on the stellar surface.However, it does not consider the eﬀect that they also block the convection pattern.Moreover, spots and plages are not only areas darker or brighter, but have a diﬀerenttemperature (then a diﬀerent spectra) than the stellar photosphere.The wavelength dependency of spots must be taken into account because, if mostof the accurate spectrographs are now in the optical, soon near-infrared instrumentswill be available, a wavelength domain in which the amplitude of RV variation due toactivity is smaller (e.g. Hue´lamo et al. 2008). Stellar magnetic ﬁelds might be moni-tored by polarimetry of the stellar lines induced by Zeeman eﬀect with instruments suchas ESPADONS@CFHT, Narval@TBL, HARPSPol@ESO, or soon SPIRou@CFHT alsodedicated to the search of planets around low-mass stars.Dedicated observations of active stars are therefore needed, if possible simultaneously,in RV, photometry, and polarimetry to characterize the diﬀerent level of stellar vari-ability, to understand the correlation between the diﬀerent proxies, and to constrain thesimulations. The release of thousands of light curves from the space missions CoRoT andKepler dedicated to asteroseismology and the search for planets via transit will help usto understand statistically the photometric variability of these stars.at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313002937Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 19:09:06, subject to the Cambridge Core terms of use, availablePlanetary research and stellar dynamo 475AcknowledgementsIB, MO, NCS, MM, and GB acknowledge the support of the European Research Coun-cil/ European Community under the FP7 through a Starting Grant, as well from Fun-daccao para a Cieˆncia e a Tecnologia (FCT), Portugal, through SFRH/BPD/81084/2011and the project PTDC/CTE-AST/098528/2008.ReferencesBoisse, I., Moutou, C., Vidal-Madjar, A. et al. 2009, A&A, 495, 959Boisse, I., Bouchy, F., He´brard, G. et al. 2011, A&A, 528, A4Boisse, I., Bonﬁls, X., & Santos, N. C. 2012, A&A, 545, 109Bonﬁls, X., Mayor, M., Delfosse, X. et al. 2007, A&A, 474, 293De´sert, J.-M., Charbonneau, D., Demory, B.-O. et al. 2011, ApJS, 197, 14Desort, M., Lagrange, A;-M., Galland, F., Udry, S.,& Mayor, M. 2007, A&A, 473, 983Dumusque, X., Udry, S., Lovis, C., Santos, N. C., & Monteiro, M. 2011, A&A, 525, 140Fare`s, R., Donati, J.-F., & Moutou, C. et al. 2009, MNRAS, 398, 1383Hue´lamo, N., Figueira, P., Bonﬁls, X. et al. 2008, A&A, 489, L9Kipping 2012, arXiv: 1209.2985Lanza, A. F., Boisse, I., Bouchy, F., & Bonomo, A. S. and Moutou, C. 2011, A&A, 533, 44Lovis, C., Dumusque, X., & Santos, N. C. et al. 2011, arXiv:1107.5325Metcalfe, T. S., Basu, S., Henry, T. J. et al. 2010, ApJ, 723, 213Nutzman, Fabrycky & Fortney 2011, ApJ, 740, L10Oshagh, M., Boisse, I., Boue´, G. et al. 2012, A&A, in pressPepe, F., Lovis, C., Se´gransan, D. et al. 2011, A&A, 534A, 58P1Pont, F., Gilliland, R. L., Moutou, C. et al. 2007, A&A, 476, 1347Queloz, D., Henry, G. W., Sivan, J.-P. et al. 2001, A&A, 379, 279Sanchis-Ojeda, R., Winn, J. N., Holman, M. et al. 2011, ApJ, 733, 127Sanchis-Ojeda & Winn 2011, ApJ, 743, 61Santos, N. C., Bouchy, F., Mayor, M. et al. 2004, A&A, 426L, 19at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313002937Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 19:09:06, subject to the Cambridge Core terms of use, available

How the planetary research helps to the stellar dynamo understanding

How the planetary research helps to the stellar dynamo understanding

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