arXiv subjects

C. Hellier

Publications and source records attributed to C. Hellier.

At least 91 records · Page 5Linked to original sources

arXiv2016

WASP-157b, a Transiting Hot Jupiter Observed with K2

We announce the discovery of the transiting hot Jupiter WASP-157b in a 3.95-d orbit around a V = 12.9 G2 main-sequence star. This moderately inflated planet has a Saturn-like density with a mass of $0.57 \pm 0.10$ M$_{\rm Jup}$ and a radius of $1.06 \pm 0.05$ R$_{\rm Jup}$. We do not detect any rotational or phase-curve modulations, nor the secondary eclipse, with conservative semi-amplitude upper limits of 250 and 20 ppm, respectively.

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arXiv2016

Absolute parameters for AI Phoenicis using WASP photometry

AI Phe is a double-lined, detached binary, in which a K-type sub-giant star totally eclipses its main-sequence companion every 24.6 days. This configuration makes AI Phe ideal for testing stellar evolutionary models. Difficulties in obtaining a complete lightcurve mean the precision of existing radii measurements could be improved. Our aim is to improve the precision of the radius measurements for the stars in AI Phe using high-precision photometry from WASP, and use these improved radius measurements together with estimates of the masses, temperatures and composition of the stars to place constraints on the mixing length, helium abundance and age of the system. A best-fit ebop model is used to obtain lightcurve parameters, with their standard errors calculated using a prayer-bead algorithm. These were combined with previously published spectroscopic orbit results, to obtain masses and radii. A Bayesian method is used to estimate the age of the system for model grids with different mixing lengths and helium abundances. The radii are found to be $R_1=1.835\pm0.014$ R$_{\odot}$, $R_2=2.912\pm0.014$ R$_{\odot}$ and the masses $M_1=1.1973\pm0.0037$ M$_{\odot}$, $M_2=1.2473\pm0.0039$ M$_{\odot}$. From the best-fit stellar models we infer a mixing length of 1.78, a helium abundance of $Y_{AI}=0.26^{+0.02}_{-0.01}$ and an age of $4.39\pm0.32$ Gyr. Times of primary minimum show the period of AI Phe is not constant. Currently, there are insufficient data to determine the cause of this variation. Improved precision in the masses and radii have improved the age estimate, and allowed the mixing length and helium abundance to be constrained. The eccentricity is now the largest source of uncertainty in calculating the masses. More binaries with parameters measured to a similar level of precision would allow us to test for relationships between helium abundance and mixing length.

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arXiv2016

Five transiting hot Jupiters discovered using WASP-South, Euler and TRAPPIST: WASP-119 b, WASP-124 b, WASP-126 b, WASP-129 b and WASP-133 b

We have used photometry from the WASP-South instrument to identify 5 stars showing planet-like transits in their light curves. The planetary nature of the companions to these stars has been confirmed using photometry from the EulerCam instrument on the Swiss Euler 1.2-m telescope and the TRAPPIST telescope, and spectroscopy obtained with the CORALIE spectrograph. The planets discovered are hot Jupiter systems with orbital periods in the range 2.17 to 5.75 days, masses from 0.3M$_{\rm Jup}$ to 1.2M$_{\rm Jup}$ and with radii from 1R$_{\rm Jup}$ to 1.5R$_{\rm Jup}$. These planets orbit bright stars (V = 11-13) with spectral types in the range F9 to G4. WASP-126 is the brightest planetary system in this sample and hosts a low-mass planet with a large radius (0.3 M$_{\rm Jup}$ , 0.95R$_{\rm Jup}$), making it a good target for transmission spectroscopy. The high density of WASP-129 A suggests that it is a helium-rich star similar to HAT-P-11 A. WASP-133 has an enhanced surface lithium abundance compared to other old G-type stars, particularly other planet host stars. These planetary systems are good targets for follow-up observations with ground-based and space-based facilities to study their atmospheric and dynamical properties.

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arXiv2016

Starspots on WASP-85

By analysing K2 short-cadence observations we detect starspots on WASP-85A, the host star of the hot Jupiter WASP-85Ab. The detection of recurring starspot occultation events indicates that the planet's orbit is aligned with the star's rotational axis ($λ<14^{\circ}$) and suggests a stellar rotational period of $15.1 \pm 0.6$ d. The K2 lightcurve reveals a rotational modulation with a period of $13.6 \pm 1.6$ d, consistent with the period determined from starspots. 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.

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arXiv2016

WASP-120b, WASP-122b and WASP-123b: Three newly discovered planets from the WASP-South survey

We present the discovery by the WASP-South survey of three planets transiting moderately bright stars (V ~ 11). WASP-120b is a massive (5.0MJup) planet in a 3.6-day orbit that we find likely to be eccentric (e = 0.059+0.025-0.018) around an F5 star. WASP-122b is a hot-Jupiter (1.37MJup, 1.79RJup) in a 1.7-day orbit about a G4 star. Our predicted transit depth variation cause by the atmosphere of WASP-122b suggests it is well suited to characterisation. WASP-123b is a hot-Jupiter (0.92MJup, 1.33RJup) in a 3.0-day orbit around an old (~ 7 Gyr) G5 star.

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arXiv2016

FORS2 observes a multi-epoch transmission spectrum of the hot Saturn-mass exoplanet WASP-49b

Context: Transmission spectroscopy has proven to be a useful tool for the study of exoplanet atmospheres, and has lead to the detection of a small number of elements and molecules (Na, K, H$_2$O), but also revealed that many planets show flat transmission spectra consistent with the presence of opaque high-altitude hazes or clouds. Aims: We apply this technique to the $M_P=0.38 M_{jup}$, $R_p=1.12 R_{jup}$, $P=2.78d$ planet WASP-49b, aiming to characterize its transmission spectrum between 0.73 and 1 $\mathrmμ$m and search for the features of K and H$_2$O. Methods: Three transits of WASP-49b have been observed with the FORS2 instrument installed at the VLT/UT1 telescope at the ESO Paranal site. We used FORS2 in MXU mode with grism GRIS_600z, producing simultaneous multiwavelength transit lightcurves throughout the i' and z' bands. We combined these data with independent broadband photometry from the Euler and TRAPPIST telescopes to obtain a good measurement of the transit shape. Strong correlated noise structures are present in the FORS2 lightcurves, which are due to rotating flat-field structures that are introduced by inhomogeneities of the linear atmospheric dispersion corrector's transparency. We accounted for these structures by constructing common noise models from the residuals of lightcurves bearing the same noise structures, and used them together with simple parametric models to infer the transmission spectrum. Results: We present three independent transmission spectra of WASP-49b between 0.73 and 1.02 $μm$, as well as a transmission spectrum between 0.65 and 1.02 $μm$ from the combined analysis of FORS2 and broadband data. The results obtained from the three individual epochs agree well. The transmission spectrum of WASP-49b is best fit by atmospheric models containing a cloud deck at pressure levels of 1 mbar or lower.

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arXiv2015

Hot Jupiters with relatives: discovery of additional planets in orbit around WASP-41 and WASP-47

We report the discovery of two additional planetary companions to WASP-41 and WASP-47. WASP-41 c is a planet of minimum mass 3.18 $\pm$ 0.20 M$_{\rm Jup}$ and eccentricity 0.29 $\pm$ 0.02, and it orbits in 421 $\pm$ 2 days. WASP-47 c is a planet of minimum mass 1.24 $\pm$ 0.22 M$_{\rm Jup}$ and eccentricity 0.13 $\pm$ 0.10, and it orbits in 572 $\pm$ 7 days. Unlike most of the planetary systems that include a hot Jupiter, these two systems with a hot Jupiter have a long-period planet located at only $\sim$1 au from their host star. WASP-41 is a rather young star known to be chromospherically active. To differentiate its magnetic cycle from the radial velocity effect induced by the second planet, we used the emission in the H$α$ line and find this indicator well suited to detecting the stellar activity pattern and the magnetic cycle. The analysis of the Rossiter-McLaughlin effect induced by WASP-41 b suggests that the planet could be misaligned, though an aligned orbit cannot be excluded. WASP-47 has recently been found to host two additional transiting super Earths. With such an unprecedented architecture, the WASP-47 system will be very important for understanding planetary migration.

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arXiv2015

WASP-121 b: a hot Jupiter in a polar orbit and close to tidal disruption

We present the discovery by the WASP-South survey, in close collaboration with the Euler and TRAPPIST telescopes, of WASP-121 b, a new remarkable short-period transiting hot Jupiter, whose planetary nature has been statistically validated by the PASTIS software. The planet has a mass of $1.183_{-0.062}^{+0.064}$ $M_{\mathrm{Jup}}$, a radius of 1.865 $\pm$ 0.044 $R_{\mathrm{Jup}}$, and transits every $1.2749255_{-0.0000025}^{+0.0000020}$ days an active F6-type main-sequence star ($V$=10.4, $1.353_{-0.079}^{+0.080}$ $M_{\odot}$, 1.458 $\pm$ 0.030 $R_{\odot}$, $T_{\mathrm{eff}}$ = 6460 $\pm$ 140 K). A notable property of WASP-121 b is that its orbital semi-major axis is only $\sim$1.15 times larger than its Roche limit, which suggests that the planet might be close to tidal disruption. Furthermore, its large size and extreme irradiation ($\sim$$7.1\:10^{9}$ erg $\mathrm{s}^{-1} \mathrm{cm}^{-2}$) make it an excellent target for atmospheric studies via secondary eclipse observations. Using the TRAPPIST telescope, we indeed detect its emission in the $z'$-band at better than $\sim$4$σ$, the measured occultation depth being 603 $\pm$ 130 ppm. Finally, from a measurement of the Rossiter-McLaughlin effect with the CORALIE spectrograph, we infer a sky-projected spin-orbit angle of $257.8_{-5.5}^{+5.3}$ deg. This result indicates a significant misalignment between the spin axis of the host star and the orbital plane of the planet, the planet being in a nearly polar orbit. Such a high misalignment suggests a migration of the planet involving strong dynamical events with a third body.

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arXiv2014

WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system

We report the discovery of two hot-Jupiter planets, each orbiting one of the stars of a wide binary system. WASP-94A (2MASS 20550794-3408079) is an F8 type star hosting a transiting planet with a radius of 1.72 +/- 0.06 R_Jup, a mass of 0.452 +/- 0.034 M_Jup, and an orbital period of 3.95 days. The Rossiter-McLaughlin effect is clearly detected, and the measured projected spin-orbit angle indicates that the planet occupies a retrograde orbit. WASP-94B (2MASS 20550915-3408078) is an F9 stellar companion at an angular separation of 15" (projected separation 2700 au), hosting a gas giant with a minimum mass of 0.618 +/- 0.028 M_Jup with a period of 2.008 days, detected by Doppler measurements. The orbital planes of the two planets are inclined relative to each other, indicating that at least one of them is inclined relative to the plane of the stellar binary. These hot Jupiters in a binary system bring new insights into the formation of close-in giant planets and the role of stellar multiplicity.

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arXiv2014

Six newly-discovered hot Jupiters transiting F/G stars: WASP-87b, WASP-108b, WASP-109b, WASP-110b, WASP-111b & WASP-112b

We present the discoveries of six transiting hot Jupiters: WASP-87b, WASP-108b, WASP-109b, WASP-110b, WASP-111b and WASP-112b. The planets have masses of 0.51--2.2 $M_{\rm Jup}$ and radii of 1.19--1.44 $R_{\rm Jup}$ and are in orbits of 1.68--3.78 d around stars with masses 0.81--1.50 $M_{\rm \odot}$. WASP-111b is in a prograde, near-aligned ($λ= -5 \pm 16^\circ$), near-circular ($e < 0.10$ at 2 $σ$) orbit around a mid-F star. As tidal alignment around such a hot star is thought to be inefficient, this suggests that either the planet migrated inwards through the protoplanetary disc or that scattering processes happened to leave it in a near-aligned orbit. WASP-111 appears to have transitioned from an active to a quiescent state between the 2012 and 2013 seasons, which makes the system a candidate for studying the effects of variable activity on a hot-Jupiter atmosphere. We find evidence that the mid-F star WASP-87 is a visual binary with a mid-G star. Two host stars are metal poor: WASP-112 has [Fe/H] = $-0.64 \pm 0.15$ and WASP-87 has [Fe/H] = $-0.41 \pm 0.10$. The low density of WASP-112 (0.81 $M_{\rm \odot}$, $0.80 \pm 0.04$ $ρ_{\rm \odot}$) cannot be matched by standard models for any reasonable value of the age of the star, suggesting it to be affected by the "radius anomaly".

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arXiv2014

The well-aligned orbit of WASP-84b: evidence for disc migration

We report the sky-projected orbital obliquity (spin-orbit angle) of WASP-84b, a 0.70-$M_{\rm Jup}$ planet in a 8.52-day orbit around a G9V/K0V star, to be $λ= 0.3 \pm 1.7^\circ$. We obtain a true obliquity of $ψ= 14.8 \pm 8.0^\circ$ from a measurement of the inclination of the stellar spin axis with respect to the sky plane. Due to the young age and the weak tidal forcing of the system, we suggest that the orbit of WASP-84b is unlikely to have both realigned and circularised from the misaligned and/or eccentric orbit likely to have arisen from high-eccentricity migration. Therefore we conclude that the planet probably migrated via interaction with the protoplanetary disc. This would make it the first short-orbit, giant planet to have been shown to have migrated via this pathway. Further, we argue that the distribution of obliquities for planets orbiting cool stars ($T_{\rm eff}$ < 6250 K) suggests that high-eccentricity migration is an important pathway for the formation of short-orbit, giant planets.

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arXiv2014

WASP-117b: a 10-day-period Saturn in an eccentric and misaligned orbit

We report the discovery of WASP-117b, the first planet with a period beyond 10 days found by the WASP survey. The planet has a mass of $M_p= 0.2755 \pm 0.0089 \, M_{J}$, a radius of $R_p= 1.021_{-0.065}^{+0.076}\, R_{J}$ and is in an eccentric ($ e= 0.302 \pm 0.023 $), $ 10.02165 \pm 0.00055 $~d orbit around a main-sequence F9 star. The host star's brightness (V=10.15 mag) makes WASP-117 a good target for follow-up observations, and with a periastron planetary equilibrium temperature of $T_{eq}= 1225_{-39}^{+36}$ K and a low planetary mean density ($ρ_p= 0.259_{-0.048}^{+0.054} \, ρ_{J}$) it is one of the best targets for transmission spectroscopy among planets with periods around 10 days. From a measurement of the Rossiter-McLaughlin effect, we infer a projected angle between the planetary orbit and stellar spin axes of $β= -44 \pm 11$ deg, and we further derive an orbital obliquity of $ψ= 69.6 ^{+4.7}_{-4.1}$ deg. Owing to the large orbital separation, tidal forces causing orbital circularization and realignment of the planetary orbit with the stellar plane are weak, having had little impact on the planetary orbit over the system lifetime. WASP-117b joins a small sample of transiting giant planets with well characterized orbits at periods above ~8 days.

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arXiv2014

Period and amplitude variations in post-common-envelope eclipsing binaries observed with SuperWASP

Period or amplitude variations in eclipsing binaries may reveal the presence of additional massive bodies in the system, such as circumbinary planets. Here, we have studied twelve previously-known eclipsing post-common-envelope binaries for evidence of such light curve variations, on the basis of multi-year observations in the SuperWASP archive. The results for HW Vir provided strong evidence for period changes consistent with those measured by previous studies, and help support a two-planet model for the system. ASAS J102322-3737.0 exhibited plausible evidence for a period increase not previously suggested; while NY Vir, QS Vir and NSVS 14256825 afforded less significant support for period change, providing some confirmation to earlier claims. In other cases, period change was not convincingly observed; for AA Dor and NSVS 07826147, previous findings of constant period were confirmed. This study allows us to present hundreds of new primary eclipse timings for these systems, and further demonstrates the value of wide-field high-cadence surveys like SuperWASP for the investigation of variable stars.

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arXiv2014

A Window on Exoplanet Dynamical Histories: Rossiter-McLaughlin Observations of WASP-13b and WASP-32b

We present Rossiter-McLaughlin observations of WASP-13b and WASP-32b and determine the sky-projected angle between the normal of the planetary orbit and the stellar rotation axis ($λ$). WASP-13b and WASP-32b both have prograde orbits and are consistent with alignment with measured sky-projected angles of $λ={8^{\circ}}^{+13}_{-12}$ and $λ={-2^{\circ}}^{+17}_{-19}$, respectively. Both WASP-13 and WASP-32 have $T_{\mathrm{eff}}<6250$K and therefore these systems support the general trend that aligned planetary systems are preferentially found orbiting cool host stars. A Lomb-Scargle periodogram analysis was carried out on archival SuperWASP data for both systems. A statistically significant stellar rotation period detection (above 99.9\% confidence) was identified for the WASP-32 system with $P_{\mathrm{rot}}=11.6 \pm 1.0 $ days. This rotation period is in agreement with the predicted stellar rotation period calculated from the stellar radius, $R_{\star}$, and $v \sin i$ if a stellar inclination of $i_{\star}=90^{\circ}$ is assumed. With the determined rotation period, the true 3D angle between the stellar rotation axis and the planetary orbit, $ψ$, was found to be $ψ=11^{\circ} \pm 14$. We conclude with a discussion on the alignment of systems around cool host stars with $T_{\mathrm{eff}}<6150$K by calculating the tidal dissipation timescale. We find that systems with short tidal dissipation timescales are preferentially aligned and systems with long tidal dissipation timescales have a broad range of obliquities.

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arXiv2014

On the incidence of eclipsing Am binary systems in the SuperWASP survey

The results of a search for eclipsing Am star binaries using photometry from the SuperWASP survey are presented. The light curves of 1742 Am stars fainter than V = 8.0 were analysed for the presences of eclipses. A total of 70 stars were found to exhibit eclipses, with 66 having sufficient observations to enable orbital periods to be determined and 28 of which are newly identified eclipsing systems. Also presented are spectroscopic orbits for 5 of the systems. The number of systems and the period distribution is found to be consistent with that identified in previous radial velocity surveys of `classical' Am stars.

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arXiv2014

WASP-20b and WASP-28b: a hot Saturn and a hot Jupiter in near-aligned orbits around solar-type stars

We report the discovery of the planets WASP-20b and WASP-28b along with measurements of their sky-projected orbital obliquities. WASP-20b is an inflated, Saturn-mass planet (0.31 $M_{\rm Jup}$; 1.46 $R_{\rm Jup}$) in a 4.9-day, near-aligned ($λ= 8.1 \pm 3.6^\circ$) orbit around CD-24 102 ($V$=10.7; F9). WASP-28b is an inflated, Jupiter-mass planet (0.91 $M_{\rm Jup}$; 1.21 $R_{\rm Jup}$) in a 3.4-day, near-aligned ($λ= 8 \pm 18^\circ$) orbit around a $V$=12, F8 star. As intermediate-mass planets in short orbits around aged, cool stars ($7^{+2}_{-1}$ Gyr for WASP-20 and $5^{+3}_{-2}$ Gyr for WASP-28; both with $T_{\rm eff}$ < 6250 K), their orbital alignment is consistent with the hypothesis that close-in giant planets are scattered into eccentric orbits with random alignments, which are then circularised and aligned with their stars' spins via tidal dissipation.

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arXiv2014

High frequency A-type pulsators discovered using SuperWASP

We present the results of a survey using the WASP archive to search for high frequency pulsations in F-, A- and B-type stars. Over 1.5 million targets have been searched for pulsations with amplitudes greater than 0.5 millimagnitude. We identify over 350 stars which pulsate with periods less than 30 min. Spectroscopic follow-up of selected targets has enabled us to confirm 10 new rapidly oscillating Ap stars, 13 pulsating Am stars and the fastest known $δ$ Scuti star. We also observe stars which show pulsations in both the high-frequency domain and in the low-frequency $δ$ Scuti range. This work shows the power of the WASP photometric survey to find variable stars with amplitudes well below the nominal photometric precision per observation.

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arXiv2014

WASP-103b: a new planet at the edge of tidal disruption

We report the discovery of WASP-103b, a new ultra-short-period planet (P=22.2 hr) transiting a 12.1 V-magnitude F8-type main-sequence star (1.22+-0.04 Msun, 1.44-0.03+0.05 Rsun, Teff = 6110+-160 K). WASP-103b is significantly more massive (1.49+-0.09 Mjup) and larger (1.53-0.07+0.05 Rjup) than Jupiter. Its large size and extreme irradiation (around 9 10^9 erg/s/cm^2) make it an exquisite target for a thorough atmospheric characterization with existing facilities. Furthermore, its orbital distance is less than 20% larger than its Roche radius, meaning that it might be significantly distorted by tides and might experience mass loss through Roche-lobe overflow. It thus represents a new key object for understanding the last stage of the tidal evolution of hot Jupiters.

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