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Line-profile tomography of exoplanet transits -- II. A gas-giant planet transiting a rapidly-rotating A5 star
A. Collier Cameron, E. Guenther, B. Smalley, I. McDonald, L. Hebb, J. Andersen, Th. Augusteijn, S. C. C. Barros, D. J. A. Brown, W. D. Cochran, M. Endl, S. J. Fossey, M. Hartmann, P. F. L. Maxted, D. Pollacco, I. Skillen, J. Telting, I. P. Waldmann, R. G. West
TL;DR
The paper addresses the limited evidence for planets around hot, rapidly rotating stars, where conventional radial-velocity confirmation is difficult. Using follow-up photometry and time-series spectroscopy with Doppler imaging, it confirms a gas-giant planet around HD 15082 and characterizes its unusual orbit, while pulsations constrain some derived parameters.
Problem
Planet searches have concentrated on solar- and later-type, sharp-lined stars, leaving planetary companions around hot, fast-rotating stars poorly explored.
Method
The study combines transit photometry, radial velocities, and time-resolved spectral-line-profile modelling with MCMC to infer transit and orbital properties.
Results
The observations confirm a gas-giant planet around HD 15082 with a radius of 1.46 RJup and a retrograde, strongly misaligned orbit.
Takeaways & Limitations
HD 15082b demonstrates that A-type main-sequence stars can host close-in planets and extends exoplanet studies to different formation environments.
Takeaways & Limitations
Non-radial pulsations distort line profiles and introduce substantial uncertainty into parameters including the impact parameter, v sin i, and obliquity λ.
Abstract
from arXiv · showhide
Most of our knowledge of extrasolar planets rests on precise radial-velocity measurements, either for direct detection or for confirmation of the planetary origin of photometric transit signals. This has limited our exploration of the parameter space of exoplanet hosts to solar- and later-type, sharp-lined stars. Here we extend the realm of stars with known planetary companions to include hot, fast-rotating stars. Planet-like transits have previously been reported in the lightcurve obtained by the SuperWASP survey of the A5 star HD15082 (WASP-33; V=8.3, v sin i = 86 km/sec). Here we report further photometry and time-series spectroscopy through three separate transits, which we use to confirm the existence of a gas giant planet with an orbital period of 1.22d in orbit around HD15082. From the photometry and the properties of the planet signal travelling through the spectral line profiles during the transit we directly derive the size of the planet, the inclination and obliquity of its orbital plane, and its retrograde orbital motion relative to the spin of the star. This kind of analysis opens the way to studying the formation of planets around a whole new class of young, early-type stars, hence under different physical conditions and generally in an earlier stage of formation than in sharp-lined late-type stars. The reflex orbital motion of the star caused by the transiting planet is small, yielding an upper mass limit of 4.1 Jupiter masses on the planet. We also find evidence of a third body of sub-stellar mass in the system, which may explain the unusual orbit of the transiting planet. In HD 15082, the stellar line profiles also show evidence of non-radial pulsations, clearly distinct from the planetary transit signal. This raises the intriguing possibility that tides raised by the close-in planet may excite or amplify the pulsations in such stars.
1 INTRODUCTION
Exoplanet searches have largely focused on solar- and later-type, sharp-lined stars, leaving planets around intermediate-mass main-sequence stars poorly constrained. The paper addresses this gap by using Doppler imaging to confirm a transiting planet around rapidly rotating A5 star HD 15082 and derive orbital geometry from its travelling spectral signature.
- More than 400 extrasolar planets were known around solar- and later-type stars, while planets around intermediate-mass main-sequence stars remained poorly understood.
- Gas-giant formation models suggest that formation may peak among late B stars, but radial-velocity searches generally become feasible only after stellar evolution into late-type giants.Close-orbiting planets may already have been engulfed by that stage.
- Doppler imaging confirms a transiting planet around bright, rapidly rotating A5 star HD 15082, where conventional precise radial-velocity confirmation is difficult.HD 15082 has V = 8.3 and v sin i = 86 km s−1.
- The planet’s travelling spectral signature complements photometry by constraining its size, retrograde orbit, and misalignment between stellar and orbital spin axes.
2 DISCOVERY OF TRANSITS AND FOLLOW-UP PHOTOMETRY
HD 15082 showed recurring planet-like transits every 1.22 days, but its rapid rotation complicated conventional confirmation. Follow-up observations across multiple telescopes and bands refined the transit light curve.
- Planet-like transits of HD 15082 (WASP-33) recur every 1.22 days, but rapid stellar rotation prevents straightforward radial-velocity confirmation.
- Dedicated photometry followed the WASP discovery with partial and complete transits observed at JGT, Keele, and Mill Hill.The observations covered R-band and I-band light curves between 2006 and 2008.
- 16 minutes is the duration of ingress and egress, while the transit depth is 0.015 mag in both R and I bands.
3 EXPLORATORY SPECTROSCOPY
Exploratory spectroscopy sought the planet’s mass through the host star’s reflex motion, but extreme rotational broadening limited radial-velocity precision. The data constrained the planet to planetary rather than stellar mass while leaving an outer-companion interpretation uncertain.
- Preliminary iodine-cell spectroscopy measured HD 15082 with a resolving power of 67,000 across 4700–7400 Å to search for orbital reflex motion.
- Extreme rotational broadening, v sin i ≃ 90 ± 10 km s−1, precluded direct precise radial velocities relative to the iodine spectrum.Relative velocities were instead obtained by crosscorrelation with an external stellar template.
- −18 ± 3 m s−1 d−1 is the measured long-term radial acceleration over the 95-day observation span.The authors were cautious about attributing the trend to another body because of stellar non-radial pulsations.
- A 99.9 percent upper limit of 4.1 MJup on companion b’s mass supports a planetary rather than stellar transit source.The reflex motion itself was not significantly detected from these data alone.
4 STELLAR PROPERTIES OF HD 15082
The paper characterizes HD 15082 as an early A-type star using photometry and spectral synthesis, while radial-velocity measurements reveal both long-term acceleration and an orbital signal.
- HD 15082 is classified as A5mA8F4, with A5 from Ca II K, A8 from the H lines, and F4 from metal lines.
- Strömgren-Crawford photometry gives Teff = 7430 K, log g = 4.21, and [M/H] = 0.21.
- The radial-velocity figure separates long-term acceleration from phased velocities after subtracting a 0.018 km s−1 d−1 linear trend.The dashed curve shows the best-fitting circular-orbit solution, with arbitrary velocity zero-point.
- Spectral synthesis provides Teff = 7400 ± 200 K, log g = 4.3 ± 0.2, and [M/H] = 0.1 ± 0.2, broadly agreeing with photometry.
5 TIME-RESOLVED TRANSIT SPECTRA
Time-resolved spectroscopy combines least-squares deconvolved line profiles with transit photometry to track the planet’s Doppler signature across HD 15082. Its motion reveals a retrograde, misaligned orbit, while the signal’s width and depth constrain transit geometry and planet size.
- Planetary signature: The planetary signature appears as a resolved bump within the rotationally broadened stellar line profile.This makes the transit detectable even when the stellar reflex motion cannot be measured precisely.
- Spectral extraction: Least-Squares Deconvolution combines information from all spectral lines into a single high signal-to-noise composite profile.The velocity scale is tied to telluric absorption lines, compensating for instrumental-profile drift to approximately 10–20 m s−1.
- Planetary signature: The bump migrates from approximately 5 km s−1 to −40 km s−1 as time increases through the transit.The trailed profiles show the signal moving across the stellar rotation profile during the observed event.
- Orbital geometry: Ingress occurs over the receding stellar limb and egress over the approaching limb, demonstrating that the planet’s orbit is retrograde relative to stellar rotation.The direction of motion is opposite to the star’s rotational direction.
- Orbital geometry: The signal’s velocity width measures the transit chord relative to the stellar diameter, constraining the impact parameter and orbital inclination.Ingress closer to the stellar spin axis than egress also shows that the orbit is inclined relative to the stellar equator and line of sight.
- Planet properties: The planet trace’s width and depth directly indicate the planet’s radius and surface area after accounting for instrumental and exposure-broadening effects.The relevant corrections include the point-spread function, natural line width, and planetary acceleration during each exposure.
6 COMBINED ANALYSIS
The combined analysis integrates photometry, radial velocities, and transit spectroscopy to infer the system parameters of HD 15082 and its planet. It finds a young, near-main-sequence host, while pulsations and radial-velocity trends limit some inferences.
- Combined data and modelling: 3658 additional WASP observations were combined with earlier data to model the transit light curves and orbital parameters.The fit uses MCMC with transit epoch, period, radius ratio, scaled stellar radius, and impact parameter.
- Combined data and modelling: The stellar density was derived from the scaled stellar radius and orbital period using Kepler’s third law, while stellar and planetary radii followed from an empirical stellar-mass calibration.The stellar mass is estimated at each MCMC step before deriving the stellar and planetary radii.
- Radial velocities: −18 ± 3 m s−1 d−1 is the fitted secular radial-velocity trend, but attributing it to an outer companion is cautioned because pulsations can create spurious shifts.Adding a linear trend reduces the RMS scatter from 1.30 km s−1 to 0.99 km s−1, while an F-test gives a 17 percent probability that the residual variances differ significantly.
- Combined data and modelling: The line-profile analysis models the stellar rotation profile and the planet’s travelling Doppler shadow using three time-resolved transit-spectroscopy sequences.The fit determines the stellar rotation speed and the projected orientation of the stellar and orbital spin axes.
- Uncertainties: Pulsations distort the line profiles at amplitudes approaching the planet’s Doppler shadow, causing several-sigma disagreement among fitted b, v sin i, λ, and vFWHM values.The scatter among separate solutions is therefore a better indicator of the true uncertainty in orbital inclination than the individual formal MCMC errors.
- Stellar properties: The star is inferred to be early in its main-sequence life, with an age no greater than about 400 Myr after allowing for 0.2 dex metallicity uncertainty.The observed density is inconsistent with the upper part of the quoted metallicity range and is too high for metallicities greater than solar on or above the main sequence.
- Planet properties: 2710 ± 50 K is the planet’s equilibrium temperature under the assumed zero albedo and uniform redistribution of incident flux.
7 NON-RADIAL PULSATIONS IN THE HOST STAR
The host star’s line profiles reveal non-radial pulsations distinct from the planet’s Doppler shadow, with characteristics suggesting a γ Dor-type variable.
- Removing the planet’s Doppler shadow exposes the star’s underlying non-radial pulsation pattern.The radial pulsation component vanishes near the line-profile centre, indicative of g-modes.
- The pulsation pattern appears consistent with a sectoral non-radial mode of ℓ = 4 ± 2, suggesting γ Dor-type variability.Its amplitude is greater in the blue than the red wing of the profile.
- HD 49434, a similar star, shows pulsation frequencies near the ∼4 cycles/day tidal forcing frequency imposed on HD 15082 by its planet.The paper notes that resonant tidal forcing has been proposed as a mechanism for exciting stellar pulsations.
8 SUMMARY AND CONCLUSIONS
Time-series spectroscopy confirms a gas giant transiting the rapidly rotating A-type star HD 15082 and reveals an unusually large, strongly irradiated, retrograde planet. Its unknown mass and density limit structural and orbital inferences, while the system’s misalignment and stellar pulsations motivate further study.
- Summary and conclusions: Time-series spectroscopy provided the first confirmation of a gas-giant planet transiting a rapidly rotating A-type main-sequence star.The result establishes that A stars occasionally harbour close-in planets.
- Summary and conclusions: A radius of 1.46 RJup makes HD 15082b one of the more bloated exoplanets discovered.Its large radius remains relevant to unresolved theories of planetary structure.
- Summary and conclusions: Without a measurable radial-velocity orbit, the planet’s mass and density remain unknown, limiting estimates of its core mass and orbital eccentricity.The planet’s equilibrium temperature is approximately 2710 K under isotropic re-radiation and low albedo assumptions.
- Summary and conclusions: HD 15082b is retrograde and strongly misaligned, posing a challenge to disc-migration theories and motivating searches for additional exterior bodies.The paper identifies planet–planet scattering, Kozai oscillations, and tidal damping as alternative explanations.
- Summary and conclusions: The host star is apparently a γ Dor-type non-radial pulsator.A future full pulsation-mode study could independently confirm the stellar density inferred from transit geometry.