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HAT-P-2
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Spitzer Hears Stellar 'Heartbeat' from Planetary Companion

NASA's Spitzer Space Telescope has detected unusual pulsations in the outer shell of a star called HAT-P-2. Scientists' best guess is that a closely orbiting planet, called HAT-P-2b, causes these vibrations each time it gets close to the star in its orbit.
The star's pulsations are the most subtle variations of light from any source that Spitzer has ever measured. A similar effect had been observed in binary systems called "heartbeat stars" in the past, but never before between a star and a planet.

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The heart of a far-off star beats for its planet

For the first time, astronomers from MIT and elsewhere have observed a star pulsing in response to its orbiting planet.
The star, which goes by the name HAT-P-2, is about 400 light years from Earth and is circled by a gas giant measuring eight times the mass of Jupiter - one of the most massive exoplanets known today. The planet, named HAT-P-2b, tracks its star in a highly eccentric orbit, flying extremely close to and around the star, then hurtling far out before eventually circling back around.

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Title: Planet-Induced Stellar Pulsations in HAT-P-2's Eccentric System
Author: J. de Wit, N.K. Lewis, H.A. Knutson, J. Fuller, V. Antoci, B.J. Fulton, G. Laughlin, D. Deming, A. Shporer, K. Batygin, N.B. Cowan, E. Agol, A.S. Burrows, J.J. Fortney, J. Langton, A.P. Showman

Extrasolar planets on eccentric short-period orbits provide a laboratory in which to study radiative and tidal interactions between a planet and its host star under extreme forcing conditions. Studying such systems probes how the planet's atmosphere redistributes the time-varying heat flux from its host and how the host star responds to transient tidal distortion. Here, we report the insights into the planet-star interactions in HAT-P-2's eccentric planetary system gained from the analysis of 350 hr of 4.5 micron observations with the Spitzer Space Telescope. The observations show no sign of orbit-to-orbit variability nor of orbital evolution of the eccentric planetary companion, HAT-P-2 b. The extensive coverage allows us to better differentiate instrumental systematics from the transient heating of HAT-P-2 b's 4.5 micron photosphere and yields the detection of stellar pulsations with an amplitude of approximately 40 ppm. These pulsation modes correspond to exact harmonics of the planet's orbital frequency, indicative of a tidal origin. Transient tidal effects can excite pulsation modes in the envelope of a star, but, to date, such pulsations had only been detected in highly eccentric stellar binaries. Current stellar models are unable to reproduce HAT-P-2's pulsations, suggesting that our understanding of the interactions at play in this system is incomplete.

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Title: Atmospheric Circulation of Eccentric Hot Jupiter HAT-P-2b
Author: Nikole K. Lewis, Adam P. Showman, Jonathan J. Fortney, Heather A. Knutson, Mark S. Marley

The hot-Jupiter HAT-P-2b has become a prime target for Spitzer Space Telescope observations aimed at understanding the atmospheric response of exoplanets on highly eccentric orbits. Here we present a suite of three-dimensional atmospheric circulation models for HAT-P-2b that investigate the effects of assumed atmospheric composition and rotation rate on global scale winds and thermal patterns. We compare and contrast atmospheric models for HAT-P-2b, which assume one and five times solar metallicity, both with and without TiO/VO as atmospheric constituents. Additionally we compare models that assume a rotation period of half, one, and two times the nominal pseudo-synchronous rotation period. We find that changes in assumed atmospheric metallicity and rotation rate do not significantly affect model predictions of the planetary flux as a function of orbital phase. However, models in which TiO/VO are present in the atmosphere develop a transient temperature inversion between the transit and secondary eclipse events that results in significant variations in the timing and magnitude of the peak of the planetary flux compared with models in which TiO/VO are omitted from the opacity tables. We find that no one single atmospheric model can reproduce the recently observed full orbit phase curves at 3.6, 4.5 and 8.0 microns, which is likely due to a chemical process not captured by our current atmospheric models for HAT-P-2b. Further modelling and observational efforts focused on understanding the chemistry of HAT-P-2b's atmosphere are needed and could provide key insights into the interplay between radiative, dynamical, and chemical processes in a wide range of exoplanet atmospheres.

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Title: HATS-2b: A transiting extrasolar planet orbiting a K-type star showing starspot activity
Authors: M. Mohler-Fischer, L. Mancini, J. D. Hartman, G. B. Bakos, K. Penev, D. Bayliss, A. Jordan, Z. Csubry, G. Zhou, M. Rabus, N. Nikolov, R. Brahm, N. Espinoza, L. A. Buchhave, B. Beky, V. Suc, B. Csak, T. Henning, D. J. Wright, C. G. Tinney, B. C. Addison, B. Schmidt, R. W. Noyes, I. Papp, J. Lazar, P. Sari, P. Conroy

We report the discovery of HATS-2b, the second transiting extrasolar planet detected by the HATSouth survey. HATS-2b is moving on a circular orbit around a V=13.6 mag, K-type dwarf star (GSC 6665-00236), at a separation of 0.0230 ± 0.0003 AU and with a period of 1.3541 days. The planetary parameters have been robustly determined using a simultaneous fit of the HATSouth, MPG/ESO 2.2 m /GROND, Faulkes Telescope South/Spectral transit photometry and MPG/ESO 2.2 m /FEROS, Euler 1.2 m /CORALIE, AAT 3.9m /CYCLOPS radial-velocity measurements. HATS-2b has a mass of 1.37 ± 0.16 Jupiter masses, a radius of 1.14 ± 0.03 Jupiter radii and an equilibrium temperature of 1567 ± 30 K. The host star has a mass of 0.88 ± 0.04 solar masses, radius of 0.89 ± 0.02 solar radii and shows starspot activity. We characterised the stellar activity by analysing two photometric follow-up transit light curves taken with the GROND instrument, both obtained simultaneously in four optical bands (covering the wavelength range of 3860-9520 \AA). The two light curves contain anomalies compatible with starspots on the photosphere of the parent star along the same transit chord.

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Title: Orbital Phase Variations of the Eccentric Giant Planet HAT-P-2b
Authors: Nikole K. Lewis, Heather A. Knutson, Adam P. Showman, Nicolas B. Cowan, Gregory Laughlin, Adam Burrows, Drake Deming, Justin R. Crepp, Kenneth J. Mighell, Eric Agol, Gáspár Á. Bakos, David Charbonneau, Jean-Michel Désert, Debra A. Fischer, Jonathan J. Fortney, Joel D. Hartman, Sasha Hinkley, Andrew W. Howard, John Asher Johnson, Melodie Kao, Jonathan Langton, Geoffrey W. Marcy, Joshua N. Winn

We present the first secondary eclipse and phase curve observations for the highly eccentric hot Jupiter HAT-P-2b in the 3.6, 4.5, 5.8, and 8.0 µm bands of the Spitzer Space Telescope. The 3.6 and 4.5 µm data sets span an entire orbital period of HAT-P-2b, making them the longest continuous phase curve observations obtained to date and the first full-orbit observations of a planet with an eccentricity exceeding 0.2. We present an improved non-parametric method for removing the intrapixel sensitivity variations in Spitzer data at 3.6 and 4.5 µm that robustly maps position-dependent flux variations. We find that the peak in planetary flux occurs at 4.39 ±0.28, 5.84 ±0.39, and 4.68 ±0.37 hours after periapse passage with corresponding maxima in the planet/star flux ratio of 0.1138% ±0.0089%, 0.1162% ±0.0080%, and 0.1888% ±0.0072% in the 3.6, 4.5, and 8.0 µm bands respectively. We compare our measured secondary eclipse depths to the predictions from a one-dimensional radiative transfer model, which suggests the possible presence of a transient day side inversion in HAT-P-2b's atmosphere near periapse. We also derive improved estimates for the system parameters, including its mass, radius, and orbital ephemeris. Our simultaneous fit to the transit, secondary eclipse, and radial velocity data allows us to determine the eccentricity and argument of periapse of HAT-P-2b's orbit with a greater precision than has been achieved for any other eccentric extrasolar planet. We also find evidence for a long-term linear trend in the radial velocity data. This trend suggests the presence of another substellar companion in the HAT-P-2 system, which could have caused HAT-P-2b to migrate inward to its present-day orbit via the Kozai mechanism.

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