Planetary Orbital Period Research Articles

Disentangling between stellar activity and planetary signals

Photospheric stellar activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in stellar radial-velocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of stellar activities to differentiate them from planetary signals. We simulate dark spots on a rotating stellar photosphere. The variations in the photometry, RV, and spectral line shapes are characterized and analyzed according to the stellar inclination, the latitude, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requires a good sampling to be resolved when there are several spots on the photosphere. The Lomb-Scargle periodograms of the RV variations induced by activity present power at the rotational period P rot of the star and its two first harmonics P rot /2 and P rot /3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow us to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planet-host stars: HD 189733, GJ 674, CoRoT-7, and ι Hor. We succeed in fitting simultaneously activity and planetary signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly higher masses of CoRoT-7b and c of respectively, 5.7 ± 2.5 M Earth and 13.2 ± 4.1 M Earth . The larger uncertainties properly take into account the stellar active jitter. We exclude short-period low-mass exoplanets around ι Hor. For data with realistic time-sampling and white Gaussian noise, we use simulations to show that our approach is effective in distinguishing reflex-motion due to a planetary companion and stellar-activity-induced RV variations provided that 1) the planetary orbital period is not close to that of the stellar rotation or one of its two first harmonics; 2) the semi-amplitude of the planet exceeds ~ 30% of the semi-amplitude of the active signal; 3) the rotational period of the star is accurately known, and 4) the data cover more than one stellar rotational period.

Quantization of Planetary Systems and its Dependency on Stellar Rotation

AbstractWith the discovery of now more than 500 exoplanets, we present a statistical analysis of the planetary orbital periods and their relationship to the rotation periods of their parent stars. We test whether the structural variables of planetary orbits, i.e. planetary angular momentum and orbital period, are ‘quantized’ in integer or half-integer multiples of the parent star's rotation period. The Solar System is first shown to exhibit quantized planetary orbits that correlate with the Sun's rotation period. The analysis is then expanded over 443 exoplanets to statistically validate this quantization and its association with stellar rotation. The results imply that the exoplanetary orbital periods are highly correlated with the parent star's rotation periods and follow a discrete half-integer relationship with orbital ranks n = 0.5, 1.0, 1.5, 2.0, 2.5, etc. The probability of obtaining these results by pure chance is p < 0.024. We discuss various mechanisms that could justify this planetary quantization, such as the hybrid gravitational instability models of planet formation, along with possible physical mechanisms such as the inner disc's magnetospheric truncation, tidal dissipation, and resonance trapping. In conclusion, we statistically demonstrate that a quantized orbital structure should emerge from the formation processes of planetary systems and that this orbital quantization is highly dependent on the parent star's rotation period.

Primary and secondary eclipse spectroscopy with JWST: exploring the exoplanet parameter space

Eclipse exoplanet spectroscopy has yielded detection of H_2O, CH_4, CO_2 and CO in the atmosphere of hot jupiters and neptunes. About 40 large terrestrial planets are announced or confirmed, two of which are transiting, and another deemed habitable. Hence the potential for eclipse spectroscopy of terrestrial planets with James Webb Space Telescope (JWST) has become an active field of study. We explore the parameter space (type of stars, planet orbital periods and types, and instruments/wavelengths) in terms of the signal-to-noise ratio (S/N) achievable on the detection of spectroscopic features. We use analytic formula and model data for both the astrophysical scene and the instrument, to plot S/N contour maps, while indicating how the S/N scales with the fixed parameters. We systematically compare stellar photon noise-only figures with ones including detailed instrumental and zodiacal noises. Likelihood of occurring targets is based both on model and catalog star population of the solar neighborhood. The 9.6 micron ozone band is detectable (S/N = 3) with JWST, for a warm super-earth 6.7 pc away, using ~2% of the 5-year nominal mission time (summing observations, M4V and lighter host star for primary eclipses, M5V for secondary). If every star up to this mass limit and distance were to host a habitable planet, there should be statistically ~1 eclipsing case. Investigation of systematic noises in the co-addition of 5 years worth-, tens of days separated-, hours-long observations is critical, complemented by dedicated characterisation of the instruments, currently in integration phase. The census of nearby transiting habitable planets must be complete before the beginning of science operations.

Revisiting the eccentricities of hot Jupiters

AbstractMost short period transiting exoplanets have circular orbits, as expected from an estimation of the circularisation timescale using classical tidal theory. Interestingly, a small number of short period transiting exoplanets seem to have orbits with a small eccentricity. Such systems are valuable as they may indicate that some key physics is missing from formation and evolution models. We have analysed the results of a campaign of radial velocity measurements of known transiting planets with the SOPHIE and HARPS spectrographs using Bayesian methods and obtained new constraints on the orbital elements of 12 known transiting exoplanets. We also reanalysed the radial velocity data for another 42 transiting systems and show that some of the eccentric orbits reported in the Literature are compatible with a circular orbit. As a result, we show that the systems with circular and eccentric orbits are clearly separated on a plot of the planetary mass versus orbital period. We also show that planets following the trend where heavier hot Jupiters have shorter orbital periods (the “mass-period relation” of hot Jupiters), also tend to have circular orbits, with no confirmed exception to this rule so far.

Statistical patterns in ground-based transit surveys

AbstractAs the number of known transiting planets from ground-based surveys passes the 100 mark, it is becoming possible to perform meaningful statistical analyses on their physical properties. Caution is needed in their interpretation, because subtle differences in survey strategy can lead to surprising selection effects affecting the distributions of planetary orbital periods and radii, and of host-star metallicity. Despite these difficulties, the planetary mass-radius relation appears to conform more or less to theoretical expectations in the mass range from Saturns to super-Jupiters. The inflated radii of many hot Jupiters indicate that environmental factors can have a dramatic effect on planetary structure, and may even lead to catastrophic loss of the planetary envelope under extreme irradiation. High-precision radial velocities and secondary-eclipse timing are yielding eccentricity measurements of exquisite precision. They show some hot Jupiters to be in almost perfectly circular orbits, while others remain slightly but significantly eccentric.

THE CALIFORNIA PLANET SURVEY. I. FOUR NEW GIANT EXOPLANETS

We present precise Doppler measurements of four stars obtained during the past decade at Keck Observatory by the California Planet Survey (CPS). These stars, namely, HD 34445, HD 126614, HD 13931, and Gl 179, all show evidence for a single planet in Keplerian motion. We also present Doppler measurements from the Hobby-Eberly Telescope (HET) for two of the stars, HD 34445 and Gl 179, that confirm the Keck detections and significantly refine the orbital parameters. These planets add to the statistical properties of giant planets orbiting near or beyond the ice line, and merit follow-up by astrometry, imaging, and space-borne spectroscopy. Their orbital parameters span wide ranges of planetary minimum mass (M sin i = 0.38-1.9 M_Jup), orbital period (P = 2.87-11.5 yr), semi-major axis (a = 2.1-5.2 AU), and eccentricity (e = 0.02-0.41). HD 34445b (P = 2.87 yr, M sin i = 0.79 M_Jup, e = 0.27) is a massive planet orbiting an old, G-type star. We announce a planet, HD 126614Ab, and an M dwarf, HD 126614B, orbiting the metal-rich star HD 126614 (which we now refer to as HD 126614A). The planet, HD 126614Ab, has minimum mass M sin i = 0.38 M_Jup and orbits the stellar primary with period P = 3.41 yr and orbital separation a = 2.3 AU. The faint M dwarf companion, HD 126614B, is separated from the stellar primary by 489 mas (33 AU) and was discovered with direct observations using adaptive optics and the PHARO camera at Palomar Observatory. The stellar primary in this new system, HD 126614 A, has the highest measured metallicity (\feh = +0.56) of any known planet-bearing star. HD 13931b (P = 11.5 yr, M sin i = 1.88 M_Jup, e = 0.02) is a Jupiter analog orbiting a near solar twin. Gl 179b (P = 6.3 yr, M sin i = 0.82 M_Jup, e = 0.21) is a massive planet orbiting a faint M dwarf. The high metallicity of Gl 179 is consistent with the planet-metallicity correlation among M dwarfs, as documented recently by Johnson and Apps.

Disentangling between stellar activity and planetary signals

AbstractPhotospheric stellar activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in the radial-velocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of stellar activities to disentangle it from planetary signals.We simulate dark spots on a rotating stellar photosphere. The variations of the RV are characterized and analyzed according to the stellar inclination, the latitude and the number of spots. The Lomb-Scargle periodograms of the RV variations induced by activity present power at the rotational period Prot of the star and its two-first harmonics Prot/2 and Prot/3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planet-host stars: HD 189733, GJ 674, CoRoT-7 and ι Hor. We succeed in fitting simultaneously activity and planetary signals on GJ674 and CoRoT-7. We excluded short-period low-mass exoplanets around ι Hor. Our approach is efficient to disentangle reflex-motion due to a planetary companion and stellar-activity induced-RV variations provided that 1) the planetary orbital period is not close to that of the stellar rotation or one of its two-first harmonics, 2) the rotational period of the star is accurately known, 3) the data cover more than one stellar rotational period.

Evidence against the young hot-Jupiter around BD +20 1790

Context. The young active star BD +20 1790 is believed to host a substellar companion, revealed by radial-velocity measurements that detected the reflex motion induced on the parent star. Aims. A complete characterisation of the radial-velocity signal is necessary in order to assess its nature. Methods. We used CORALIE spectrograph to obtain precise (~10 m/s) velocity measurements on this active star, while characterizing the bisector span variations. Particular attention was given to correctly sample both the proposed planetary orbital period, of 7.8 days, and the stellar rotation period, of 2.4 days. Results. A smaller radial-velocity signal (with peak-to-peak variations <500 m/s) than had been reported previously was detected, with different amplitude on two different campaigns. A periodicity similar to the rotational period is found on the data, as well as a clear correlation between radial-velocities and bisector span. This evidence points towards a stellar origin of the radial-velocity variations of the star instead of a barycentric movement of the star, and repudiates the reported detection of a hot-Jupiter.

Hot Jupiters and the evolution of stellar angular momentum

Giant planets orbiting main-sequence stars closer than 0.1 AU are called hot Jupiters. They interact with their stars affecting their angular momentum. Recent observations provide suggestive evidence of excess angular momentum in stars with hot Jupiters in comparison to stars with distant and less massive planets. This has been attributed to tidal interaction, but needs to be investigated in more detail considering also other possible explanations because in several cases the tidal synchronization time scales are much longer than the ages of the stars. We select stars harbouring transiting hot Jupiters to study their rotation and find that those with an effective temperature greater than 6000 K and a rotation period shorter than 10 days are synchronized with the orbital motion of their planets or have a rotation period approximately twice that of the planetary orbital period. Stars with an effective temperature lower than 6000 K and a rotation period longer than 10 days show a general trend toward synchronization with increasing effective temperature or decreasing orbital period. We propose a model for the angular momentum evolution of stars with hot Jupiters to interpret these observations. It is based on the hypothesis that a close-in giant planet affects the coronal field of its host star leading to a topology with predominantly closed field lines. Our model can be tested observationally and has relevant consequences for the relationship between stellar rotation and close-in giant planets as well as for the application of gyrochronology to estimate the age of planet-hosting stars.

EMPIRICAL CONSTRAINTS ON THE OBLATENESS OF AN EXOPLANET

We show that the gas giant exoplanet HD 189733b is less oblate than Saturn, based on Spitzer Space Telescope photometry of seven transits. The observable manifestations of oblatenesswould have been slight anomalies during the ingress and egress phases, as well as variations in the transit depth due to spin precession. Our nondetection of these effects gives the first empirical constraints on the shape of an exoplanet. The results are consistent with the theoretical expectation that the planetary rotation period and orbital period are synchronized, in which case the oblateness would be an order of magnitude smaller than our upper limits. Conversely, if HD 189733b is assumed to be in a synchronous, zero-obliquity state, then the data give an upper bound on the quadrupole moment of the planet (J2 < 0.068 with 95% confidence) that is too weak to constrain the interior structure of the planet. An Appendix describes a fast algorithm for computing the transit light curve of an oblate planet, which was necessary for our analysis.

MASSIVE SATELLITES OF CLOSE-IN GAS GIANT EXOPLANETS

We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature. We argue that even Earth-size satellites around hot Jupiters can be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization. The star's gravity induces significant periodic eccentricity in the satellite's orbit. The resulting tidal heating rates, per unit mass, are far in excess of Io's and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal Q. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite. Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellite's atmosphere is thin and vapor pressure is negligible. Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.

On the MOND external field effect in the solar system

In the framework of the MOdified Newtonian Dynamics (MOND), the internal dynamics of a gravitating system s embedded in a larger one S is affected by the external background field E of S even if it is constant and uniform, thus implying a violation of the Strong Equivalence Principle: it is the so-called External Field Effect (EFE). In the case of the solar system, E would be Acen≈10−10 m s−2 because of its motion through the Milky Way: it is orders of magnitude smaller than the main Newtonian monopole terms for the planets. We address here the following questions in a purely phenomenological manner: are the Sun’s planets affected by an EFE as large as 10−10 m s−2? Can it be assumed that its effect is negligible for them because of its relatively small size? Does E induce vanishing net orbital effects because of its constancy over typical solar system’s planetary orbital periods? It turns out that a constant and uniform acceleration, treated perturbatively, does induce non-vanishing long-period orbital effects on the longitude of the pericenter ϖ of a test particle. In the case of the inner planets of the solar system and with E≈10−10 m s−2, they are 4–6 orders of magnitude larger than the present-day upper bounds on the non-standard perihelion precessions \(\Delta\dot{\varpi}\) recently obtained with by E.V. Pitjeva with the EPM ephemerides in the Solar System Barycentric frame. The upper limits on the components of E are Ex≤1×10−15 m s−2, Ey≤2×10−16 m s−2, Ez≤3×10−14 m s−2. This result is in agreement with the violation of the Strong Equivalence Principle by MOND. Our analysis also holds for any other exotic modification of the current laws of gravity yielding a constant and uniform extra-acceleration. If and when other corrections \(\Delta\dot{\varpi}\) to the usual perihelion precessions will be independently estimated with different ephemerides it will be possible to repeat such a test.

CoRoT-2a Magnetic Activity: Hints for Possible Star–Planet Interaction

CoRoT-2a is a young (≈0.5 Gyr) G7V star accompanied by a transiting hot-Jupiter, discovered by the CoRoT satellite (Alonso et al. Astron Astrophys 482:L21, 2008; Bouchy et al. Astron Astrophys 482:L25, 2008). An analysis of its photospheric activity, based on spot modelling techniques previously developed by our group for the analysis of the Sun as a star, shows that the active regions on CoRoT-2a arised within two active longitudes separated by about 180° and rotating with periods of 4.5221 and 4.5543 days, respectively, at epoch of CoRoT observations (112 continous days centered at ≈2007.6). We show that the total spotted area oscillates with a period of about 28.9 days, a value close to 10 times the synodic period of the planet with respect to the active longitude pattern rotating in 4.5221 days. Moreover, the variance of the stellar flux is modulated in phase with the planet orbital period. This suggests a possible star–planet magnetic interaction, a phenomenon already seen in other extrasolar planetary systems hosting hot-Jupiters.

DEEP MMT TRANSIT SURVEY OF THE OPEN CLUSTER M37 IV: LIMIT ON THE FRACTION OF STARS WITH PLANETS AS SMALL AS 0.3RJ

We present the results of a deep (15 r 23), 20 night survey for transiting planets in the intermediate-age open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. We do not detect any transiting planets among the ~1450 observed cluster members. We do, however, identify a ~1RJ candidate planet transiting a ~0.8 M ? Galactic field star with a period of 0.77 days. The source is faint (V = 19.85 mag) and has an expected velocity semiamplitude of K ~ 220 m s-1(M/MJ ). We conduct Monte Carlo transit injection and recovery simulations to calculate the 95% confidence upper limit on the fraction of cluster members and field stars with planets as a function of planetary radius and orbital period. Assuming a uniform logarithmic distribution in the orbital period, we find that <1.1%, <2.7%, and <8.3% of cluster members have 1.0RJ planets within extremely hot Jupiter (EHJ; 0.4 < P < 1.0 day), very hot Jupiter (VHJ; 1.0 < P < 3.0 day), and hot Jupiter (HJ; 3.0 < P < 5.0 day) period ranges, respectively. For 0.5RJ planets, the limits are less than 3.2% and less than 21% for EHJ and VHJ period ranges, respectively, while for 0.35RJ planets we can only place an upper limit of less than 25% on the EHJ period range. For a sample of 7814 Galactic field stars, consisting primarily of FGKM dwarfs, we place 95% upper limits of <0.3%, <0.8%, and <2.7% on the fraction of stars with a 1.0RJ EHJ, VHJ, and HJ, respectively, assuming that the candidate planet is not genuine. If the candidate is genuine, the frequency of ~1.0RJ planets in the EHJ period range is 0.002% < f EHJ < 0.5% with 95% confidence. We place limits of <1.4%, <8.8%, and <47% for 0.5RJ planets, and a limit of <16% on 0.3RJ planets in the EHJ period range. This is the first transit survey to place limits on the fraction of stars with planets as small as Neptune.

Planet migration and disc destruction due to magneto-centrifugal stellar winds

This paper investigates the influence of magneto-centrifugally driven or simply magnetic winds of rapidly-rotating, strongly-magnetized T Tauri stars in causing the inward or outward migration of close-in giant planets. The azimuthal ram pressure of the magnetized wind acting on the planet tends to increase the planet's angular momentum and cause outward migration if the star's rotation period $P_*$ is less than the planet's orbital period $P_p$. In the opposite case, $P_* > P_p$, the planet migrates inward. Thus, planets orbiting at distances larger (smaller) than $0.06 {\rm AU}(P_*/5{\rm d})^{2/3}$ tend to be pushed outward (inward), where $P_*$ is the rotation period of the star assumed to have the mass of the sun. The magnetic winds are likely to occur in T Tauri stars where the thermal speed of the gas close to the star is small, where the star's magnetic field is strong, and where the star rotates rapidly. The time-scale for appreciable radial motion of the planet is estimated as $\sim 2 - 20$ Myr. A sufficiently massive close-in planet may cause tidal locking and once this happens the radial migration due to the magnetic wind ceases. The magnetic winds are expected to be important for planet migration for the case of a multipolar magnetic field rather than a dipole field where the wind is directed away from the equatorial plane and where a magnetospheric cavity forms. The influence of the magnetic wind in eroding and eventually destroying the accretion disk is analyzed. A momentum integral is derived for the turbulent wind/disk boundary layer and this is used to estimate the disk erosion time-scale as $\sim 1-10^2$ Myr, with the lower value favored.

The On/Off Nature of Star‐Planet Interactions

Evidence suggesting an observable magnetic interaction between a star and its hot Jupiter appears as a cyclic variation of stellar activity synchronized to the planet's orbit. In this study, we monitored the chromospheric activity of 7 stars with hot Jupiters using new high-resolution echelle spectra collected with ESPaDOnS over a few nights in 2005 and 2006 from the CFHT. We searched for variability in several stellar activity indicators (Ca II H, K, the Ca II infrared triplet, Halpha, and He I). HD 179949 has been observed almost every year since 2001. Synchronicity of the Ca II H & K emission with the orbit is clearly seen in four out of six epochs, while rotational modulation with P_rot=7 days is apparent in the other two seasons. We observe a similar phenomenon on upsilon And, which displays rotational modulation (P_rot=12 days) in September 2005, in 2002 and 2003 variations appear to correlate with the planet's orbital period. This on/off nature of star-planet interaction (SPI) in the two systems is likely a function of the changing stellar magnetic field structure throughout its activity cycle. Variability in the transiting system HD 189733 is likely associated with an active region rotating with the star, however, the flaring in excess of the rotational modulation may be associated with its hot Jupiter. As for HD 179949, the peak variability as measured by the mean absolute deviation for both HD 189733 and tau Boo leads the sub-planetary longitude by 70 degrees. The tentative correlation between this activity and the ratio of Mpsini to the planet's rotation period, a quantity proportional to the hot Jupiter's magnetic moment, first presented in Shkolnik et al. 2005 remains viable. This work furthers the characterization of SPI, improving its potential as a probe of extrasolar planetary magnetic fields.

MOST detects variability on τ Bootis A possibly induced by its planetary companion

Context. There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems. Aims. We shall demonstrate from MOST satellite photometry and Call K line emission that there has been a persistent, variable region on the surface of r Boo A, which tracked its giant planetary companion for some 440 planetary revolutions and lies ∼68° (Φ = 0.8) in advance of the sub-planetary point. Methods. The light curves are folded on a range of periods centered on the planetary orbital period, and phase-dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Call K line reversals. Results. The region varies in brightness on the time scale of a rotation by ∼1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. The 2004 light curve gives a spot rotation period of 3.5 ± 0.7 d compared to the known planetary orbital period of 3.3125 d. The amplitude spectrum of the 2005 light curve shows no marked peak at the orbital period but the mean absolute deviation (MAD) of the light curve has a well-defined maximum (half width ∼0.15 d) centered on the orbital frequency. Over the 123 planetary orbits spanned by the photometry, the variable region detected in 2004 and in 2005 are synchronized to the planetary orbital period within 0.0015 d. The Call K line in 2001, 2002, and 2003 also shows enhanced K-line variability centered on Φ = 0.8, extending coverage to some 440 planetary revolutions. Conclusions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at Φ = 0.3 and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Call K line reversal results make a good case for an active region induced magnetically on the surface of T Boo A by its planetary companion.

HAT-P-4b: A Metal-rich Low-Density Transiting Hot Jupiter

We describe the discovery of HAT-P-4b, a low-density extrasolar planet transiting BD+36 2593, a V = 11.2 mag slightly evolved metal-rich late F star. The planet's orbital period is 3.056536+/-0.000057 d with a mid-transit epoch of 2,454,245.8154 +/- 0.0003 (HJD). Based on high-precision photometric and spectroscopic data, and by using transit light curve modeling, spectrum analysis and evolutionary models, we derive the following planet parameters: Mp= 0.68 +/- 0.04 MJ, Rp= 1.27 +/- 0.05 RJ, rho = 0.41 +/- 0.06 g cm-3 and a = 0.0446 +/- 0.0012 AU. Because of its relatively large radius, together with its assumed high metallicity of that of its parent star, this planet adds to the theoretical challenges to explain inflated extrasolar planets.

The On/Off nature of star-planet interactions in the HD 179949 and υ And systems

AbstractEvidence suggesting an observable magnetic interaction between a star and its hot Jupiter (Porb&lt; 7 days,a&lt; 0.1 AU,Mpsini&gt; 0.2MJ) appears as a cyclic variation of stellar activity synchronized to the planet's orbit. HD 179949 has been observed almost every year since 2001. Synchronicity of the Ca II H &amp; K emission with the orbit is clearly seen in four out of six epochs, while rotational modulation withProt=7 days is apparent in the other two seasons. We observe a similar phenomenon on υ And, which displays rotational modulation (Prot=12 days) in September 2005, while in 2002 and 2003 variations appear to correlate with the planet's orbital period. This on/off nature of star-planet interaction (SPI) in the two systems is likely a function of the changing stellar magnetic field structure throughout its activity cycle. The tentative correlation between this activity in the 13 stars we have observed to date and the ratio ofMpsinito the planet's rotation period, a quantity proportional to the hot Jupiter's magnetic moment, first presented in Shkolniket al. (2005) remains viable. This work furthers the characterization of SPI, improving its potential as a probe of extrasolar planetary magnetic fields.

Spectropolarimetric observations of the transiting planetary system of the K dwarf HD 189733

With a Jupiter-mass planet orbiting at a distance of only 0.031 AU, the active K2 dwarf HD 189733 is a potential candidate in which to study the magnetospheric interactions of a cool star with its recently-discovered close-orbiting giant planet. We decided to explore the strength and topology of the large-scale magnetosphere of HD 189733, as a future benchmark for quantitative studies for models of the star/planet magnetic interactions. To this end, we used ESPaDOnS, the new generation spectropolarimeter at the Canada-France-Hawaii 3.6m telescope, to look for Zeeman circular polarisation signatures in the line profiles of HD 189733 in 2006 June and August. Zeeman signatures in the line profiles of HD 189733 are clearly detected in all spectra, demonstrating that a field is indeed present at the surface of the star. The Zeeman signatures are not modulated with the planet's orbital period but apparently vary with the stellar rotation cycle. The reconstructed large-scale magnetic field, whose strength reaches a few tens of G, is significantly more complex than that of the Sun; it involves in particular a significant toroidal component and contributions from magnetic multipoles of order up to 5. The CaII H & K lines clearly feature core emission, whose intensity is apparently varying mostly with rotation phase. Our data suggest that the photosphere and magnetic field of HD 189733 are sheared by a significant amount of differential rotation. Our initial study confirms that HD 189733 is an optimal target for investigating activity enhancements induced by closely orbiting planets. More data are needed, densely covering both the orbital and rotation cycles, to investigate whether and how much the planet contributes to the overall activity level of HD 189733.