Recurring sets of recurring starspot occultations on exoplanet-host Qatar-2

Recurring sets of recurring starspot occultations on exoplanet-host Qatar-2


Authors:

Močnik et al

Abstract:

We announce the detection of recurring sets of recurring starspot occultation events in the short-cadence K2 lightcurve of Qatar-2, a K dwarf star transited every 1.34 d by a hot Jupiter. In total we detect 34 individual starspot occultation events, caused by five different starspots, occulted in up to five consecutive transits or after a full stellar rotation. The longest recurring set of recurring starspot occultations spans over three stellar rotations, setting a lower limit for the longest starspot lifetime of 58 d. Starspot analysis provided a robust stellar rotational period measurement of 18.0 ± 0.2 d and indicates that the system is aligned, having a sky-projected obliquity of 0 ± 8°. A pronounced rotational modulation in the lightcurve has a period of 18.2 ± 1.6 d, in agreement with the rotational period derived from the starspot occultations. We tentatively detect an ellipsoidal modulation in the phase-curve, with a semi-amplitude of 18 ppm, but cannot exclude the possibility that this is the result of red noise or imperfect removal of the rotational modulation. We detect no transit-timing and transit-duration variations with upper limits of 15 s and 1 min, respectively. We also reject any additional transiting planets with transit depths above 280 ppm in the orbital period region 0.5–30 d.

Recurring sets of recurring starspots on exoplanet-host Qatar-2

Recurring sets of recurring starspots on exoplanet-host Qatar-2

Authors:

Močnik et al

Abstract:

We announce the detection of recurring sets of recurring starspot occultation events in the short-cadence K2 lightcurve of Qatar-2, a K dwarf star transited every 1.34 d by a hot Jupiter. In total we detect 30 individual starspot occultation events, 28 of which form nine sets of recurring events with the longest set consisting of five occultation events in consecutive transits. Moreover, three sets of spot occultations were found to be from the same starspot over two full stellar rotations, setting a minimum starspot lifetime to 40 d. Another starspot reappeared after one stellar rotation. Starspot analysis provided a robust stellar rotational period measurement of 18.0±0.1 d and indicates that the system is aligned, having an obliquity of 0±8∘. A pronounced rotational modulation in the lightcurve has a period of 18.2±1.6 d, in agreement with the rotational period derived from the starspot occultations. We tentatively detect an ellipsoidal modulation in the phase-curve, with a semi-amplitude of 18 ppm, but cannot exclude the possibility that this is the result of red noise or imperfect removal of the rotational modulation. We detect no transit-timing and transit-duration variations with upper limits of 15 s and 1 min, respectively. We also reject any additional transiting planets with transit depths above 330 ppm in the orbital period region 0.5-30 d.

The Interaction of Clouds & Stellar Spots on Brown Dwarf WISEP J004701.06+680352.1

Cloud Atlas: Discovery of Patchy Clouds and High-amplitude Rotational Modulations In a Young, Extremely Red L-type Brown Dwarf

Authors:

Lew et al

Abstract:

Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-Ks=2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hubble Space Telescope (HST) time-resolved grism spectroscopy we find a best-fit rotational period (13.20±0.14 hours) with a larger amplitude at 1.1 micron than at 1.7 micron. This is the third largest near-infrared variability amplitude measured in a brown dwarf, demonstrating that large-amplitude variations are not limited to the L/T transition but are present in some extremely red L-type dwarfs. We report a tentative trend between the wavelength dependence of relative amplitude, possibly proxy for small dust grains lofted in the upper atmosphere, and the likelihood of large-amplitude variability. By assuming forsterite as haze particle, we successfully explain the wavelength dependent amplitude with submicron-sized haze particles sizes of around 0.4 {\mu}m. W0047 links the earlier spectral and later spectral type brown dwarfs in which rotational modulations have been observed, the large amplitude variations in this object make this a benchmark brown dwarf for the study of cloud properties close to the L/T transition.

An Interplay of Starspots & Clouds Likely Drive the Variability of the L3.5 dwarf 2MASS 0036+18

Long-term, Multiwavelength Light Curves of Ultra-cool Dwarfs: I. An Interplay of Starspots & Clouds Likely Drive the Variability of the L3.5 dwarf 2MASS 0036+18

Authors:

Croll et al

Abstract:

We present multi-telescope, ground-based, multiwavelength optical and near-infrared photometry of the variable L3.5 ultra-cool dwarf 2MASSW J0036159+182110. We present 22 nights of photometry of 2MASSW J0036159+182110, including 7 nights of simultaneous, multiwavelength photometry, spread over ~120 days allowing us to determine the rotation period of this ultra-cool dwarf to be 3.080 +/- 0.001 hr. Our many nights of multiwavelength photometry allow us to observe the evolution, or more specifically the lack thereof, of the light curve over a great many rotation periods. The lack of discernible phase shifts in our multiwavelength photometry, and that the amplitude of variability generally decreases as one moves to longer wavelengths for 2MASSW J0036159+182110, is generally consistent with starspots driving the variability on this ultra-cool dwarf, with starspots that are ~100 degrees K hotter or cooler than the ~1700 K photosphere. Also, reasonably thick clouds are required to fit the spectra of 2MASSW J0036159+182110, suggesting there likely exists some complex interplay between the starspots driving the variability of this ultra-cool dwarf and the clouds that appear to envelope this ultra-cool dwarf.

Orbital alignment and starspot properties in the WASP-52 planetary system

Orbital alignment and starspot properties in the WASP-52 planetary system

Authors:

Mancini et al

Abstract:

We report 13 high-precision light curves of eight transits of the exoplanet WASP-52 b, obtained by using four medium-class telescopes, through different filters, and adopting the defocussing technique. One transit was recorded simultaneously from two different observatories and another one from the same site but with two different instruments, including a multi-band camera. Anomalies were clearly detected in five light curves and modelled as starspots occulted by the planet during the transit events. We fitted the clean light curves with the jktebop code, and those with the anomalies with the prism+gemc codes in order to simultaneously model the photometric parameters of the transits and the position, size and contrast of each starspot. We used these new light curves and some from the literature to revise the physical properties of the WASP-52 system. Starspots with similar characteristics were detected in four transits over a period of 43 days. In the hypothesis that we are dealing with the same starspot, periodically occulted by the transiting planet, we estimated the projected orbital obliquity of WASP-52 b to be λ = 3.8° ± 8.4°. We also determined the true orbital obliquity, ψ = 20° ± 50°, which is, although very uncertain, the first measurement of ψ purely from starspot crossings. We finally assembled an optical transmission spectrum of the planet and searched for variations of its radius as a function of wavelength. Our analysis suggests a flat transmission spectrum within the experimental uncertainties.

Orbital obliquities of transiting planets from starspot occultations

Orbital obliquities of transiting planets from starspot occultations

Authors:

Southworth et al

Abstract:

When a planet passes in front of a starspot during a transit of its host star, it causes a small upward blip in the light curve. Modelling the transit with the starspot allows the size, brightness and position of the spot to be measured. If the same spot can be observed in two different transits, it is possible to track the motion of the spot due to the rotation of the star. The rotation period and velocity of the star (Prot and Vsini) and the sky-projected orbital obliquity of the system (lambda) can then be determined. If one has three or more observations of the same spot, the true orbital obliquity (psi) can be measured. We are performing this analysis for a number of cool stars orbited by transiting planets. We present our results so far and compile a catalogue of lambda and psi measurements from spot crossing events. The method is particularly useful for cool stars, and is therefore complementary to studies of the Rossiter-McLaughlin effect, which perform better on hotter and faster-rotating stars.

Are intermediate range periodicities in sunspot area associated with planetary motion?

Are intermediate range periodicities in sunspot area associated with planetary motion?

Author:

Edmonds

Abstract:

Rieger quasi-periodicities have been reported numerous times. However, no accepted explanation of the quasi-periodicities has emerged. We examine the possibility that the some of the reported periodicities are associated with a Mercury to Sun interaction of base period 88 days. To test this idea we filter the daily sunspot area record with band pass filters centred on the 88 day period and 176 day sub harmonic period of Mercury. We observed that the time variation of the amplitude of the components was comprised of episodes that varied in duration from 1.5 to 4 years, with successive episodes usually overlapping in time but, for significant intervals in the record, the episodes were discrete, i.e. not overlapping. The time variation of the filtered components was compared with the time variation of the tidal effect of Mercury. We were able to show that when episodes were discrete the time variation of the component of sunspot area during the episode was either in-phase or in anti-phase with the tidal effect. We interpret this result as an indication of a connection between planetary motion and sunspot emergence. When several discrete episodes of the 88 day or the 176 day period components occurred during a solar cycle the spectrum of sunspot area exhibited periodicities at sidebands to the 88 day or 176 day periods with the periodicity of the sidebands dependent on the duration of the episodes. A model based on amplitude modulation of 88 day and 176 day period sinusoids was able to consistently reproduce periodicities observed in the spectra of sunspot area. It is proposed that the observed connection between planetary motion and sunspot emergence involves magnetic Rossby waves with mode periods close to the sub harmonic periods associated with Mercury and the triggering of sunspot emergence by those Rossby waves.

How Sunspots Influence Timing Transit Variations

How do starspots influence the transit timing variations of exoplanets? Simulations of individual and consecutive transits

Authors:

Ioannidis et al

Abstract:

Transit timing variations (TTVs) of exoplanets are normally interpreted as the consequence of gravitational interaction with additional bodies in the system. However, TTVs can also be caused by deformations of the system transits by starspots, which might thus pose a serious complication in their interpretation. We therefore simulate transit light curves deformed by spot-crossing events for different properties of the stellar surface and the planet, such as starspot position, limb darkening, planetary period, and impact parameter. Mid-transit times determined from these simulations can be significantly shifted with respect to the input values; these shifts cannot be larger than ~1% of the transit duration and depend most strongly on the longitudinal position of the spot during the transit and the transit duration. Consequently, TTVs with amplitudes larger than the above limit are very unlikely to be caused by starspots. We also investigate whether TTVs from sequences of consecutive transits with spot-crossing anomalies can be misinterpreted as the result of an additional body in the system. We use the Generalized Lomb-Scargle periodogram to search for periods in TTVs and conclude that low amplitude TTVs with statistically significant periods around active stars are the most problematic cases. In those cases where the photometric precision is high enough to inspect the transit shapes for deformations, it should be possible to identify TTVs caused by starspots, however, especially for cases with low transit signal to noise light curves (TSNR ≲ 15) it becomes quite difficult to reliably decide whether these periods come from starspots, physical companions in the system or if they are random noise artifacts.

Evidence of a Planetary Influence on Solar Activity

Evidence of a planetary influence on solar activity: Phase coherence of the variation in sunspot area with the tidal effect of Mercury

Authors:

Edmonds et al

Abstract:

There have been numerous reports of quasiperiodicities in solar activity in the intermediate period range. However, no accepted explanation for the episodic occurrence of quasiperiodicities has emerged. This paper examines the possibility that the periodicities are associated with a Mercury Sun interaction of base period 88 days. To test this idea we band pass filter the 140 year long daily sunspot area data to obtain the 88 day period and 176 day sub harmonic period components of the data and compare the time variation of the components with the time variation of the orbital radius of Mercury, or more specifically with the time variation of the tidal effect of Mercury. We were able to show that, when successive episodes of the occurrence of the 88 day period component were discrete and not overlapping in time, the time variation of this component of sunspot area was either exactly in-phase or exactly in anti-phase with the time variation of tidal effect. A similar result was obtained for the 176 day period component. When several discrete episodes of the components occurred during a solar cycle the spectrum of the sunspot area data exhibited strong sidebands with periods dependent on the duration of the episodes. A simple model based on episode modulation and solar cycle modulation of 88 day and sub harmonic period sinusoids reproduced most of the spectral peaks observed in the intermediate range of sunspot area periodicity. This is compelling evidence of a link between the motion of Mercury and the periodic emergence of sunspots. It is proposed that the link involves magnetic surface waves with mode periods close to the sub harmonic periods associated with Mercury and the triggering of sunspot emergence by the waves.

Why do we not see Grazing Exoplanet Transits?

Polar stellar-spots and grazing planetary transits: possible explanation for the low number of discovered grazing planets

Authors:

Oshagh et al

Abstract:

We assess a physically feasible explanation for the low number of discovered (near-)grazing planetary transits through all ground and space based transit surveys. We performed simulations to generate the synthetic distribution of detectable planets based on their impact parameter, and found that a larger number of (near-)grazing planets should have been detected than have been detected. Our explanation for the insufficient number of (near-)grazing planets is based on a simple assumption that a large number of (near-)grazing planets transit host stars which harbor dark giant polar spot, and thus the transit light-curve vanishes due to the occultation of grazing planet and the polar spot. We conclude by evaluating the properties required of polar spots in order to make disappear the grazing transit light-curve, and we conclude that their properties are compatible with the expected properties from observations.

Starspot Distributions on Fully Convective M dwarfs

Starspot distributions on fully convective M dwarfs: implications for radial velocity planet searches

Authors:

Barnes et al

Abstract:

Since M4.5 - M9 dwarfs exhibit equatorial rotation velocities of order 10 km/s on average, radial velocity surveys targeting this stellar population will likely need to find methods to effectively remove starspot jitter. We present the first high resolution Doppler images of the M4.5 dwarf, GJ 791.2A, and the M9 dwarf, LP 944-20. The time series spectra of both objects reveal numerous line profile distortions over the rotation period of each star which we interpret as starspots. The transient distortions are modelled with spot/photosphere contrast ratios that correspond to model atmosphere temperature differences of Tphot-Tspot = 200 and 300 K. GJ 791.2A is a fully convective star with vsini = 35.1 km/s. Although we find more starspot structure at high latitudes, we reconstruct spots at a range of phases and latitudes with a mean spot filling of ~3%. LP 944-20 is one of the brightest known late-M dwarfs, with spectral type M9V and vsini = 30.8 km/s. Its spectral time series exhibits two dominant transient line distortions that are reconstructed as high latitude spots, while a mean spot filling factor of only 1.5% is found. The occurrence of low-contrast spots at predominantly high latitudes, which we see in both targets here, is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. For GJ 791.2A, the radial velocities induced by the starspot features yield an rms velocity variability of 138 m/s, which can be reduced by a factor of 1.9 using our reconstructed surface brightness distributions.

Sunspots Detected on Hot Jupiter WASP-85Ab's Host Star

Starspots on WASP-85

Authors:

Močnik et al

Abstract:

By analysing K2 short-cadence observations we detect starspots on WASP-85A, the host star of the hot Jupiter WASP-85Ab. The star shows a rotational modulation with a period of 13.6 \pm 0.1 d. The absence of repeated occultations of the same spots suggests that the planet's orbit is not aligned with the star's rotational axis (\lambda>10^{\circ}). There are no significant transit-timing variations and thus no evidence of any additional planet in the system. Given the pronounced rotational modulation we are only able to place an upper limit of 100 parts per million for any phase-curve modulations and the secondary eclipse.

Starspot Effects in Microlensing Events

Starspot induced effects in microlensing events with rotating source star

Authors:

Giordano et al

Abstract:

We consider the effects induced by the presence of hot and cold spots on the source star in the light curves of simulated microlensing events due to either single or binary lenses taking into account the rotation of the source star and the orbital motion of the lens system. Our goal is to study the anomalies induced by these effects on simulated microlensing light curves.

Exoplanet Transit Polarimetry

Polarization in exoplanetary systems caused by transits, grazing transits, and starspots

Authors:

Kostogryz et al

Abstract:

We present results of numerical simulations of flux and linear polarization variations in transiting exoplanetary systems, caused by the host star disk symmetry breaking. We consider different configurations of planetary transits depending on orbital parameters. Starspot contribution to the polarized signal is also estimated. Applying the method to known systems and simulating observational conditions, a number of targets is selected where transit polarization effects could be detected. We investigate several principal benefits of the transit polarimetry, particularly, for determining orbital spatial orientation and distinguishing between grazing and near-grazing planets. Simulations show that polarization parameters are also sensitive to starspots, and they can be used to determine spot positions and sizes.

Magnetic Activity and Asteroseismology of τ Bootis A

The GAPS Programme with HARPS-N at TNG. VII. Putting exoplanets in the stellar context: magnetic activity and asteroseismology of τ Bootis A

Authors:

Borsa et al

Abstract:

Aims.

We observed the τ Boo system with the HARPS-N spectrograph to test a new observational strategy aimed at jointly studying asteroseismology, the planetary orbit, and star-planet magnetic interaction.

Methods.

We collected high-cadence observations on 11 nearly consecutive nights and for each night averaged the raw FITS files using a dedicated software. In this way we obtained spectra with a high signal-to-noise ratio, used to study the variation of the CaII H&K lines and to have radial velocity values free from stellar oscillations, without losing the oscillations information. We developed a dedicated software to build a new custom mask that we used to refine the radial velocity determination with the HARPS-N pipeline and perform the spectroscopic analysis.

Results.

We updated the planetary ephemeris and showed the acceleration caused by the stellar binary companion. Our results on the stellar activity variation suggest the presence of a high-latitude plage during the time span of our observations. The correlation between the chromospheric activity and the planetary orbital phase remains unclear. Solar-like oscillations are detected in the radial velocity time series: we estimated asteroseismic quantities and found that they agree well with theoretical predictions. Our stellar model yields an age of 0.9±0.5 Gyr for τ Boo and further constrains the value of the stellar mass to 1.38±0.05 M⊙.

WASP-6b Migrated Into Becoming a Inflated Hot Jupiter

Transits and starspots in the WASP-6 planetary system

Authors:

Tregloan-Reed et al

Abstract:

We present updates to \textsc{prism}, a photometric transit-starspot model, and \textsc{gemc}, a hybrid optimisation code combining MCMC and a genetic algorithm. We then present high-precision photometry of four transits in the WASP-6 planetary system, two of which contain a starspot anomaly. All four transits were modelled using \textsc{prism} and \textsc{gemc}, and the physical properties of the system calculated. We find the mass and radius of the host star to be 0.836±0.063M⊙ and 0.864±0.024R⊙, respectively. For the planet we find a mass of 0.485±0.027MJup, a radius of 1.230±0.035RJup and a density of 0.244±0.014ρJup. These values are consistent with those found in the literature. In the likely hypothesis that the two spot anomalies are caused by the same starspot or starspot complex, we measure the stars rotation period and velocity to be 23.80±0.15\,d and 1.78±0.20\,km\,s−1, respectively, at a co-latitude of 75.8∘. We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is λ=7.2∘±3.7∘, indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect. These results suggest that WASP-6b formed at a much greater distance from its host star and suffered orbital decay through tidal interactions with the protoplanetary disc.

Starspots and Hot Jupiters

Is SuperEarth/Mini Neptune Gliese 176b a Stellar Activity False Positive?

Stellar Activity and its Implications for Exoplanet Detection on GJ 176

Authors:

Robertson et al

Abstract:

We present an in-depth analysis of stellar activity and its effects on radial velocity (RV) for the M2 dwarf GJ 176 based on spectra taken over 10 years from the High Resolution Spectrograph on the Hobby-Eberly Telescope. These data are supplemented with spectra from previous observations with the HIRES and HARPS spectrographs, and V- and R-band photometry taken over 6 years at the Dyer and Fairborn observatories. Previous studies of GJ 176 revealed a super-Earth exoplanet in an 8.8-day orbit. However, the velocities of this star are also known to be contaminated by activity, particularly at the 39-day stellar rotation period. We have examined the magnetic activity of GJ 176 using the sodium I D lines, which have been shown to be a sensitive activity tracer in cool stars. In addition to rotational modulation, we see evidence of a long-term trend in our Na I D index, which may be part of a long-period activity cycle. The sodium index is well correlated with our RVs, and we show that this activity trend drives a corresponding slope in RV. Interestingly, the rotation signal remains in phase in photometry, but not in the spectral activity indicators. We interpret this phenomenon as the result of one or more large spot complexes or active regions which dominate the photometric variability, while the spectral indices are driven by the overall magnetic activity across the stellar surface. In light of these results, we discuss the potential for correcting activity signals in the RVs of M dwarfs.

55 Cancri may Have a 13 Year Sunspot Cycle

Improving robustness of exoplanetary orbital fits through a regularization of the white and red Doppler noise models. Hints of a 13-year star-spot activity cycle of 55 Cancri

Authors:

Baluev et al

Abstract:

We consider the impact of the Doppler noise model on the statistical robustness of the exoplanetary radial-velocity fits. We show that the traditional model of the Doppler noise with an additive jitter can generate large non-linearity effects, decreasing the reliability of the fit, especially in the cases when a correleated Doppler noise is involved. We introduce a regularization of the additive noise model that can gracefully eliminate its singularities together with the associated non-linearity effects.

We apply this approach to Doppler time-series data of several exoplanetary systems. It demonstrates that our new regularized noise model yields orbital fits that have either increased or at least the same statistical robustness, in comparison with the simple additive jitter. Various statistical uncertainties in the parametric estimations are often reduced, while planet detection significance is often increased.

Concerning the 55 Cnc five-planet system, we show that its Doppler data contain significant correlated ("red") noise. Its correlation timescale is in the range from days to months, and its magnitude is much larger than the effect of the planetary N-body perturbations in the radial velocity (these perturbations thus appear undetectable). Characteristics of the red noise depend on the spectrograph/observatory, and also show a cyclic time variation in phase with the public Ca II H&K and photometry measurements. We interpret this modulation as a hint of the long-term activity cycle of 55 Cnc, similar to the Solar 11-year cycle. We estimate the 55 Cnc activity period by 12.6 (+2.5,-1.0) yrs, with the nearest minimum presumably expected in 2014 or 2015.