Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis

Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis

Authors:

Bourrier et al

Abstract:

Probing the trajectory of a transiting planet across the disk of its star through the analysis of its Rossiter-McLaughlin effect can be used to measure the differential rotation of the host star and the true obliquity of the system. Highly misaligned systems could be particularly conducive to these mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This approach allows us to isolate the local stellar CCF emitted by the planet-occulted regions. As a result we identified a ∼35% variation in the local CCF contrast along the transit chord, which might trace a deepening of the stellar lines from the equator to the poles. Whatever its origin, such an effect cannot be detected when analyzing the RV centroids of the disk-integrated CCFs through a traditional velocimetric analysis of the RM effect. Consequently it injected a significant bias into the results obtained by Queloz et al. (2010) for the projected rotational velocity veqsini⋆ (1.59−0.09+0.08 km/s) and the sky-projected obliquity λ (-123.0−4.4+3.4∘). Using our technique, we measured these values to be veqsini⋆ = 1.90±0.05 km/s and λ = -143.0−1.5+1.6∘. We found no compelling evidence for differential rotation of the star, although there are hints that WASP-8 is pointing away from us with the stellar poles rotating about 25% slower than the equator. Measurements at higher accuracy during ingress/egress will be required to confirm this result. In contrast to the traditional analysis of the RM effect, the reloaded RM technique directly extracts the local stellar CCFs, allowing us to analyze their shape and to measure their RV centroids, unbiased by variations in their contrast or FWHM.

Rossiter-McLaughlin models and their effect on estimates of stellar rotation, illustrated using six WASP systems

Rossiter-McLaughlin models and their effect on estimates of stellar rotation, illustrated using six WASP systems

Authors:

brown et al

Abstract:

We present new measurements of the projected spin–orbit angle λ for six WASP hot Jupiters, four of which are new to the literature (WASP-61, -62, -76, and -78), and two of which are new analyses of previously measured systems using new data (WASP-71, and -79). We use three different models based on two different techniques: radial velocity measurements of the Rossiter-McLaughlin effect, and Doppler tomography. Our comparison of the different models reveals that they produce projected stellar rotation velocities (vsin Is) measurements often in disagreement with each other and with estimates obtained from spectral line broadening. The Boué model for the Rossiter-McLaughlin effect consistently underestimates the value of vsin Is compared to the Hirano model. Although vsin Is differed, the effect on λ was small for our sample, with all three methods producing values in agreement with each other. Using Doppler tomography, we find that WASP-61 b (λ=4.0∘+17.1−18.4), WASP-71 b (λ=−1.9∘+7.1−7.5), and WASP-78 b (λ = −6.4° ± 5.9) are aligned. WASP-62 b (λ=19.4∘+5.1−4.9) is found to be slightly misaligned, while WASP-79 b (λ=−95.2∘+0.9−1.0) is confirmed to be strongly misaligned and has a retrograde orbit. We explore a range of possibilities for the orbit of WASP-76 b, finding that the orbit is likely to be strongly misaligned in the positive λ direction.

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.

Prospects for detecting the Rossiter-McLaughlin effect of Earth-like planets: the test case of TRAPPIST-1b and c

Prospects for detecting the Rossiter-McLaughlin effect of Earth-like planets: the test case of TRAPPIST-1b and c

Authors:

Cloutier et al

Abstract:

The Rossiter-McLaughlin effect is the principal method of determining the sky-projected spin--orbit angle (β) of transiting planets. Taking the example of the recently discovered TRAPPIST-1 system, we explore how ultracool dwarfs facilitate the measurement of the spin--orbit angle for Earth-sized planets by creating an effect that can be an order of magnitude more ample than the Doppler reflex motion caused by the planet if the star is undergoing rapid rotation. In TRAPPIST-1's case we expect the semi-amplitudes of the Rossiter-McLaughlin effect to be 40−50 m/s for the known transiting planets. Accounting for stellar jitter expected for ultracool dwarfs, instrumental noise, and assuming radial velocity precisions both demonstrated and anticipated for upcoming near-infrared spectrographs, we quantify the observational effort required to measure the planets' masses and spin--orbit angles. We conclude that if the planetary system is well-aligned then β can be measured to a precision of ≲10∘ if the spectrograph is stable at the level of 2 m/s. We also investigate the measure of Δβ, the mutual inclination, when multiple transiting planets are present in the system. Lastly, we note that the rapid rotation rate of many late M-dwarfs will amplify the Rossiter-McLaughlin signal to the point where variations in the chromatic Rossiter-McLaughlin effect from atmospheric absorbers should be detectable.

HATS-25b through HATS-30b: A Half-dozen New Inflated Transiting Hot Jupiters

HATS-25b through HATS-30b: A Half-dozen New Inflated Transiting Hot Jupiters from the HATSouth Survey

Authors:

Espinoza et al

Abstract:

We report six new inflated hot Jupiters (HATS-25b through HATS-30b) discovered using the HATSouth global network of automated telescopes. The planets orbit stars with V magnitudes in the range ∼12−14 and have masses in the largely populated 0.5MJ−0.7MJ region of parameter space but span a wide variety of radii, from 1.17RJ to 1.75RJ. HATS-25b, HATS-28b, HATS-29b and HATS-30b are typical inflated hot Jupiters (Rp=1.17−1.26RJ) orbiting G-type stars in short period (P=3.2−4.6 days) orbits. However, HATS-26b (Rp=1.75RJ, P=3.3024 days) and HATS-27b (Rp=1.50RJ, P=4.6370 days) stand out as highly inflated planets orbiting slightly evolved F stars just after and in the turn-off points, respectively, which are among the least dense hot Jupiters, with densities of 0.153 g cm−3 and 0.180 g cm−3, respectively. All the presented exoplanets but HATS-27b are good targets for future atmospheric characterization studies, while HATS-27b is a prime target for Rossiter-McLaughlin monitoring in order to determine its spin-orbit alignment given the brightness (V=12.8) and stellar rotational velocity (vsini≈9.3 km/s) of the host star. These discoveries significantly increase the number of inflated hot Jupiters known, contributing to our understanding of the mechanism(s) responsible for hot Jupiter inflation.

The Rossiter-McLaughlin effect reloaded: Probing the 3D spin-orbit geometry, differential stellar rotation, and the spatially-resolved stellar spectrum of star-planet systems

The Rossiter-McLaughlin effect reloaded: Probing the 3D spin-orbit geometry, differential stellar rotation, and the spatially-resolved stellar spectrum of star-planet systems

Authors:

Cegla et al

Abstract:

When a planet transits its host star, it blocks regions of the stellar surface from view; this causes a distortion of the spectral lines and a change in the line-of-sight (LOS) velocities, known as the Rossiter-McLaughlin (RM) effect. Since the LOS velocities depend, in part, on the stellar rotation, the RM waveform is sensitive to the star-planet alignment (which provides information on the system's dynamical history). We present a new RM modelling technique that directly measures the spatially-resolved stellar spectrum behind the planet. This is done by scaling the continuum flux of the (HARPS) spectra by the transit light curve, and then subtracting the in- from the out-of-transit spectra to isolate the starlight behind the planet. This technique does not assume any shape for the intrinsic local profiles. In it, we also allow for differential stellar rotation and centre-to-limb variations in the convective blueshift. We apply this technique to HD189733 and compare to 3D magnetohydrodynamic (MHD) simulations. We reject rigid body rotation with high confidence (greater than 99% probability), which allows us to determine the occulted stellar latitudes and measure the stellar inclination. In turn, we determine both the sky-projected (lambda ~ -0.4 +/- 0.2 degrees) and true 3D obliquity (psi ~ 7^+12_-4 degrees). We also find good agreement with the MHD simulations, with no significant centre-to-limb variations detectable in the local profiles. Hence, this technique provides a new powerful tool that can probe stellar photospheres, differential rotation, determine 3D obliquities, and remove sky-projection biases in planet migration theories. This technique can be implemented with existing instrumentation, but will become even more powerful with the next generation of high-precision radial velocity spectrographs.

DEdicated MONitor of EXotransits (DEMONEX) Observes Seven Transits of Hot Jupiter XO-4b

The DEdicated MONitor of EXotransits (DEMONEX): Seven Transits of XO-4b

Authors:

Villaneuva et al

Abstract:

The DEdicated MONitor of EXotransits (DEMONEX) was a 20 inch robotic and automated telescope to monitor bright stars hosting transiting exoplanets to discover new planets and improve constraints on the properties of known transiting planetary systems. We present results for the misaligned hot Jupiter XO-4b containing 7 new transits from the DEMONEX telescope, including 3 full and 4 partial transits. We combine these data with archival light curves and archival radial velocity measurements to derive the host star mass M∗=1.293+0.030−0.029M⊙ and radius R∗=1.554+0.042−0.030R⊙ as well as the planet mass MP=1.615+0.10−0.099MJ and radius RP=1.317+0.040−0.029RJ and a refined ephemeris of P=4.1250687±0.0000024 days and T0=2454758.18978±0.00024BJDTDB. We include archival Rossiter-McLaughlin measurements of XO-4 to infer the stellar spin-planetary orbit alignment λ=−40.0+8.8−7.5 degrees.

We test the effects of including various detrend parameters, theoretical and empirical mass-radius relations, and Rossiter-McLaughlin models. We infer that detrending against CCD position and time or airmass can improve data quality, but can have significant effects on the inferred values of many parameters --- most significantly RP/R∗ and the observed central transit times TC. In the case of RP/R∗ we find that the systematic uncertainty due to detrending can be three times that of the quoted statistical uncertainties. The choice of mass-radius relation has little effect on our inferred values of the system parameters. The choice of Rossiter-McLaughlin models can have significant effects of the inferred values of vsinI∗ and the stellar spin-planet orbit angle λ.

Detecting Exoplanet Rotation Rate and Obliquity Through Secondary Eclipses

Radial velocity eclipse mapping of exoplanets

Authors:

Nikolov et al

Abstract:

Planetary rotation rates and obliquities provide information regarding the history of planet formation, but have not yet been measured for evolved extrasolar planets. Here we investigate the theoretical and observational perspective of the Rossiter-McLauglin effect during secondary eclipse (RMse) ingress and egress for transiting exoplanets. Near secondary eclipse, when the planet passes behind the parent star, the star sequentially obscures light from the approaching and receding parts of the rotating planetary surface. The temporal block of light emerging from the approaching (blue-shifted) or receding (red-shifted) parts of the planet causes a temporal distortion in the planet's spectral line profiles resulting in an anomaly in the planet's radial velocity curve. We demonstrate that the shape and the ratio of the ingress-to-egress radial velocity amplitudes depends on the planetary rotational rate, axial tilt and impact factor (i.e. sky-projected planet spin-orbital alignment). In addition, line asymmetries originating from different layers in the atmosphere of the planet could provide information regarding zonal atmospheric winds and constraints on the hot spot shape for giant irradiated exoplanets. The effect is expected to be most-pronounced at near-infrared wavelengths, where the planet-to-star contrasts are large. We create synthetic near-infrared, high-dispersion spectroscopic data and demonstrate how the sky-projected spin axis orientation and equatorial velocity of the planet can be estimated. We conclude that the RMse effect could be a powerful method to measure exoplanet spins.