The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert

The discovery of WASP-151b, WASP-153b, WASP-156b: Insights on giant planet migration and the upper boundary of the Neptunian desert
Authors:

Demangeon et al 

Abstract:
To investigate the origin of the features discovered in the exoplanet population, the knowledge of exoplanets' mass and radius with a good precision is essential. In this paper, we report the discovery of three transiting exoplanets by the SuperWASP survey and the SOPHIE spectrograph with mass and radius determined with a precision better than 15 %. WASP-151b and WASP-153b are two hot Saturns with masses, radii, densities and equilibrium temperatures of 0.31^{+0.04}_{-0.03} MJ, 1.13^{+0.03}_{-0.03} RJ, 0.22^{-0.03}_{-0.02} rhoJ and 1, 290^{+20}_{-10} K, and 0.39^{+0.02}_{-0.02} MJ, 1.55^{+0.10}_{-0.08} RJ, 0.11^{+0.02}_{-0.02} rhoJ and 1, 700^{+40}_{-40} K, respectively. Their host stars are early G type stars (with magV ~ 13) and their orbital periods are 4.53 and 3.33 days, respectively. WASP-156b is a Super-Neptune orbiting a K type star (magV = 11.6) . It has a mass of 0.128^{+0.010}_{-0.009} MJ, a radius of 0.51^{+0.02}_{-0.02} RJ, a density of 1.0^{+0.1}_{-0.1} rhoJ, an equilibrium temperature of 970^{+30}_{-20} K and an orbital period of 3.83 days. WASP-151b is slightly inflated, while WASP-153b presents a significant radius anomaly. WASP-156b, being one of the few well characterised Super-Neptunes, will help to constrain the formation of Neptune size planets and the transition between gas and ice giants. The estimates of the age of these three stars confirms the tendency for some stars to have gyrochronological ages significantly lower than their isochronal ages. We propose that high eccentricity migration could partially explain this behaviour for stars hosting a short period planet. Finally, these three planets also lie close to (WASP-151b and WASP-153b) or below (WASP-156b) the upper boundary of the Neptunian desert. Their characteristics support that the ultra-violet irradiation plays an important role in this depletion of planets observed in the exoplanet population.

Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b

Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b 

Authors:
Cubillos et al

Abstract:
The strong, nearly wavelength-independent absorption cross section of aerosols produces featureless exoplanet transmission spectra, limiting our ability to characterize their atmospheres. Here we show that even in the presence of featureless spectra, we can still characterize certain atmospheric properties. Specifically, we constrain the upper and lower pressure boundaries of aerosol layers, and present plausible composition candidates. We study the case of the bloated Saturn-mass planet WASP-49b, where near-infrared observations reveal a flat transmission spectrum between 0.7 and 1.0 {\microns}. First, we use a hydrodynamic upper-atmosphere code to estimate the pressure reached by the ionizing stellar high-energy photons at 10−8 bar, setting the upper pressure boundary where aerosols could exist. Then, we combine HELIOS and Pyrat Bay radiative-transfer models to constrain the temperature and photospheric pressure of atmospheric aerosols, in a Bayesian framework. For WASP-49b, we constrain the transmission photosphere (hence, the aerosol deck boundaries) to pressures above 10−5 bar (100× solar metallicity), 10−4 bar (solar), and 10−3 bar (0.1× solar) as lower boundary, and below 10−7 bar as upper boundary. Lastly, we compare condensation curves of aerosol compounds with the planet's pressure-temperature profile to identify plausible condensates responsible for the absorption. Under these circumstances, we find as candidates: Na2S (at 100× solar metallicity); Cr and MnS (at solar and 0.1× solar); and forsterite, enstatite, and alabandite (at 0.1× solar).

A hot Saturn on an eccentric orbit around the giant star EPIC228754001

A hot Saturn on an eccentric orbit around the giant star EPIC228754001

Authors:
Jones et al

Abstract:

Although the majority of radial velocity detected planets have been found orbiting solar-type stars, a fraction of them have been discovered around giant stars. These planetary systems have revealed different orbital properties when compared to solar-type stars companions. In particular, radial velocity surveys have shown that there is a lack of giant planets in close-in orbits around giant stars, in contrast to the known population of hot-Jupiters orbiting solar-type stars. The reason of this distinctive feature in the semimajor-axis distribution has been theorized to be the result of the stellar evolution and/or due to the effect of a different formation/evolution scenario for planets around intermediate-mass stars. However, in the past few years, a handful of transiting short-period planets (P≲ 10 days) have been found around giant stars, thanks to the high precision photometric data obtained initially by the Kepler mission, and later by its two-wheels extension K2. These new discoveries, have allowed us for the first time to study the orbital properties and physical parameters of these intriguing and elusive sub-stellar companions. In this paper we report on an independent discovery of a transiting planet in field 10 of the K2 mission, also reported recently by Grunblatt et al. (2017). The main orbital parameters of EPIC\,228754001\,b, obtained with all the available data for the system, are the following: P = 9.1708 ± 0.0025 d, e = 0.290 ± 0.049, Mp = 0.495 ± 0.007 Mjup \,and Rp = 1.089 ± 0.006 Rjup. This is the fifth known planet orbiting any giant star with a less than 0.1, and the most eccentric one among them, making EPIC\,228754001\,b a very interesting object.

K2-60b & EPIC 216468514b: a hot Jupiter and hot Saturn

K2-60b and epic 216468514b. A sub-jovian and a jovian planet from the k2 mission

Authors:

Eigmüller et al

Abstract:

We report the characterization and independant detection of K2-60b, as well as the detection and characterization of EPIC 216468514b, two transiting hot gaseous planets from the K2 space mission. We confirm the planetary nature of the two systems and determine their fundamental parameters combining the K2 time-series data with FIES@NOT and HARPS-N@TNG spectroscopic observations. K2-60b has a radius of 0.683 +/- 0.037 RJup and a mass of 0.426 +/- 0.037 MJup and orbits a G4V star with an orbital period of 3.00267 +/- 0.00006 days. EPIC 216468514b has a radius of 1.44 +/- 0.15RJup and a mass of 0.84 +/- 0.08 MJup and orbits an F9 IV star every 3.31392 +/- 0.00002 days. K2-60b is among the few planets at the edge of the so-called "desert" of short-period sub Jovian planets. EPIC 216468514b is a highly inflated Jovian planet orbiting an evolved star about to leave the main sequence.

KELT-11b/HD 93396b: a Highly Inflated hot Saturn Orbiting a subGiant Host Star

KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V=8 Subgiant HD 93396

Authors:

Pepper et al

Abstract:

We report the discovery of a transiting exoplanet, KELT-11b, orbiting the bright (V=8.0) subgiant HD 93396. A global analysis of the system shows that the host star is an evolved subgiant star with Teff=5370±51 K, M∗=1.438+0.061−0.052M⊙, R∗=2.72+0.21−0.17R⊙, log g∗=3.727+0.040−0.046, and [Fe/H]=0.180±0.075. The planet is a low-mass gas giant in a P=4.736529±0.00006 day orbit, with MP=0.195±0.018MJ, RP=1.37+0.15−0.12RJ, ρP=0.093+0.028−0.024 g cm−3, surface gravity log gP=2.407+0.080−0.086, and equilibrium temperature Teq=1712+51−46 K. KELT-11 is the brightest known transiting exoplanet host in the southern hemisphere by more than a magnitude, and is the 6th brightest transit host to date. The planet is one of the most inflated planets known, with an exceptionally large atmospheric scale height (2763 km), and an associated size of the expected atmospheric transmission signal of 5.6%. These attributes make the KELT-11 system a valuable target for follow-up and atmospheric characterization, and it promises to become one of the benchmark systems for the study of inflated exoplanets.

HAT-P-47b AND HAT-P-48b: Two Low Density Sub-Saturn-Mass Transiting Planets on the Edge of the Period--Mass Desert

HAT-P-47b AND HAT-P-48b: Two Low Density Sub-Saturn-Mass Transiting Planets on the Edge of the Period--Mass Desert

Authors:

Bakos et al

Abstract:

We report the discovery of two new transiting extrasolar planets orbiting moderately bright (V = 10.7 and 12.2 mag) F stars (masses of 1.39 Msun and 1.10 Msun, respectively). The planets have periods of P = 4.7322 d and 4.4087 d, and masses of 0.21 MJ and 0.17 MJ which are almost half-way between those of Neptune and Saturn. With radii of 1.31 RJ and 1.13 RJ, these very low density planets are the two lowest mass planets with radii in excess that of Jupiter. Comparing with other recent planet discoveries, we find that sub-Saturns (0.18MJ < Mp < 0.3MJ) and super-Neptunes (0.05MJ < Mp < 0.18MJ) exhibit a wide range of radii, and their radii exhibit a weaker correlation with irradiation than higher mass planets. The two planets are both suitable for measuring the Rossiter-McLaughlin effect and for atmospheric characterization. Measuring the former effect would allow an interesting test of the theory that star-planet tidal interactions are responsible for the tendency of close-in giant planets around convective envelope stars to be on low obliquity orbits. Both planets fall on the edge of the short period Neptunian desert in the semi-major axis-mass plane.

A Cloudiness Index for Transiting hot Jupiters Based on the Sodium and Potassium Lines

A Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines: Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths

Authors:

Heng et al

Abstract:

We present a dimensionless index that quantifies the degree of cloudiness of the atmosphere of a transiting exoplanet. Our cloudiness index is based on measuring the transit radii associated with the line center and wing of the sodium or potassium line. In deriving this index, we revisited the algebraic formulae for inferring the isothermal pressure scale height from transit measurements. We demonstrate that the formulae of Lecavelier et al. and Benneke & Seager are identical: the former is inferring the temperature while assuming a value for the mean molecular mass and the latter is inferring the mean molecular mass while assuming a value for the temperature. More importantly, these formulae cannot be used to distinguish between cloudy and cloudfree atmospheres. We derive values of our cloudiness index for a small sample of 7 hot Saturns/Jupiters taken from Sing et al. We show that WASP-17b, WASP-31b and HAT-P-1b are nearly cloudfree at visible wavelengths. We find the tentative trend that more irradiated atmospheres tend to have less clouds consisting of sub-micron-sized particles. We also derive absolute sodium and/or potassium abundances ∼102 cm−3 for WASP-17b, WASP-31b and HAT-P-1b (and upper limits for the other objects). Higher-resolution measurements of both the sodium and potassium lines, for a larger sample of exoplanetary atmospheres, are needed to confirm or refute this trend.

Tidal Decay and Disruption of hot gas Giants

Tidal Decay and Disruption of Short-Period Gaseous Exoplanets

Authors:

Jackson et al

Abstract:

Many gaseous exoplanets in short-period orbits are on the verge or are in the process of tidal disruption. Moreover, orbital stability analysis shows tides can drive many hot Jupiters to spiral toward their host stars. Thus, the coupled processes of orbital evolution and tidal disruption likely shape the observed distribution of close-in exoplanets and may even be responsible for producing some of the short-period rocky planets. However, the exact outcome for a disrupting planet depends on its internal response to mass loss, and the accompanying orbital evolution can act to enhance or inhibit the disruption process. In this study, we apply the fully-featured and robust Modules for Experiments in Stellar Astrophysics (MESA) suite to model Roche-lobe overflow (RLO) of short-period gaseous planets. We show that, although the detailed evolution may depend on several properties of the planetary system, it is largely determined by the core mass of the disrupting gas giant. In particular, we find that the orbital expansion that accompanies RLO often stops and reverses at a specific maximum period that depends on the core mass. We suggest that RLO may often strand the remnant of a disrupted gas giant near this orbital period, which provides an observational prediction that can corroborate the hypothesis that short period gas giants undergo RLO. We conduct a preliminary comparison of this prediction to the observed population of small, short-period planets and find some planets in orbits that may be consistent with this picture. To the extent that we can establish some short-period planets are indeed the remnants of disrupted gas giants, that population can elucidate the properties of gas giant cores, the properties of which remain largely unconstrained.

Where are the Very Shot Period Hot Neptunes (II)?

On the origin of the sub-Jovian desert in the orbital-period--planetary-mass plane

Authors:

Matsakos et al

Abstract:

Transit and radial velocity observations indicate a dearth of sub-Jupiter--mass planets on short-period orbits, outlined roughly by two oppositely sloped lines in the period--mass plane. We interpret this feature in terms of high-eccentricity migration of planets that arrive in the vicinity of the Roche limit, where their orbits are tidally circularized, long after the dispersal of their natal disk. We demonstrate that the two distinct segments of the boundary are a direct consequence of the different slopes of the empirical mass--radius relation for small and large planets, and show that this relation also fixes the mass coordinate of the intersection point. The period coordinate of this point, as well as the detailed shape of the lower boundary, can be reproduced with a plausible choice of a key parameter in the underlying migration model. The detailed shape of the upper boundary, on the other hand, is determined by the post-circularization tidal exchange of angular momentum with the star and can be reproduced with a stellar tidal quality factor Q′∗∼106.

Hot Saturn WASP-39b Does NOT Have Clouds

HST hot-Jupiter transmission spectral survey: Clear skies for cool Saturn WASP-39b

Authors:

Fischer et al

Abstract:

We present HST STIS optical transmission spectroscopy of the cool (approximately 1116 K) Saturn-mass exoplanet WASP-39b from 0.29-1.025 micron, along with complementary transit observations from Spitzer IRAC at 3.6 and 4.5 micron. The low density and large atmospheric pressure scale height of WASP-39b make it particularly amenable to atmospheric characterization using this technique. We detect a Rayleigh scattering slope as well as sodium and potassium absorption features; this is the first exoplanet in which both alkali features are clearly detected with the extended wings predicted by cloud-free atmosphere models. The full transmission spectrum is well matched by a clear, H2- dominated atmosphere or one containing a weak contribution from haze, in good agreement with the preliminary reduction of these data presented in Sing et al. (2016). WASP-39b is predicted to have a pressure-temperature profile comparable to that of HD 189733b and WASP-6b, making it one of the coolest transiting gas giants observed in our HST STIS survey. Despite this similarity, WASP- 39b appears to be largely cloud-free while the transmission spectra of HD 189733b and WASP-6b both indicate the presence of high altitude clouds or hazes. These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres.

EPIC-203771098b & EPIC-203771098c: Two Transiting hot low Density Sub-Saturns from K2

Two Transiting Low Density Sub-Saturns from K2

Authors:

Petigura et al

Abstract:

We report the discovery and confirmation of two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 ± 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 ± 0.56 Earth-radii and 7.82 ± 0.72 Earth-radii and have orbital periods of 20.8851 ± 0.0003 d and 42.3633±0.0006 d, near to the 2:1 mean-motion resonance. We obtained 32 radial velocities (RVs) with Keck/HIRES and detected the reflex motion due to EPIC-203771098b and c. These planets have masses of 21.0 ± 5.4 Earth-masses and 27.0 ± 6.9 Earth-masses, respectively. With low densities of 0.63 ± 0.25 g/cc and 0.31 ± 0.12 g/cc, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 ± 4.3 Earth-masses and 16.1 ± 4.2 Earth-masses, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offer rich opportunities for TTV follow-up. Finally, the low surface gravities of the EPIC-203771098 planets make them favorable targets for transmission spectroscopy by HST, Spitzer, and JWST.

A Multi-epoch Transmission Spectrum of the hot Saturn WASP-49b

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

Authors:

Lendl et al

Abstract:

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, H2O), 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 MP=0.38Mjup, Rp=1.12Rjup, P=2.78d planet WASP-49b, aiming to characterize its transmission spectrum between 0.73 and 1 μm and search for the features of K and H2O.

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's MXU mode with grism GRIS_600z, producing simultaneous multi-wavelength 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 due to rotating flat field structures that are introduced by inhomogeneities of the linear atmospheric dispersion corrector's transparency. We account for these structures by constructing common noise models from the residuals of lightcurves bearing the same noise structures, and use these together with simple parametric models for the inference of 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 less.

K2-19: A hot SuperSaturn and Neptune in a 3:2 Orbital Resonance

Characterization of the K2-19 Multiple-Transiting Planetary System via High-Dispersion Spectroscopy, AO Imaging, and Transit Timing Variations

Authors:

Narita et al

Abstract:

K2-19 (EPIC201505350) is a unique planetary system in which two transiting planets with radii ~ 7 REarth (inner planet b) and ~ 4 REarth (outer planet c) have orbits that are nearly in a 3:2 mean-motion resonance. Here, we present results of ground-based follow-up observations for the K2-19 planetary system. We have performed high-dispersion spectroscopy and high-contrast adaptive-optics imaging of the host star with the HDS and HiCIAO on the Subaru 8.2m telescope. We find that the host star is relatively old (greater than 8 Gyr) late G-type star (Teff ~ 5350 K, Ms ~ 0.9 MSun, and Rs ~ 0.9 RSun). We do not find any contaminating faint objects near the host star which could be responsible for (or dilute) the transit signals. We have also conducted transit follow-up photometry for the inner planet with KeplerCam on the FLWO 1.2m telescope, TRAPPISTCAM on the TRAPPIST 0.6m telescope, and MuSCAT on the OAO 1.88m telescope. We confirm the presence of transit-timing variations, as previously reported by Armstrong and coworkers. We model the observed transit-timing variations of the inner planet using the synodic chopping formulae given by Deck & Agol (2015). We find two statistically indistinguishable solutions for which the period ratios (Pc/Pb) are located slightly above and below the exact 3:2 commensurability. Despite the degeneracy, we derive the orbital period of the inner planet Pb ~ 7.921 days and the mass of the outer planet Mc ~ 20 MEarth. Additional transit photometry (especially for the outer planet) as well as precise radial-velocity measurements would be helpful to break the degeneracy and to determine the mass of the inner planet.

Kelt-10b: an Inflated hot Saturn Expected to be Destroyed Within a Billion Years

KELT-10b: The First Transiting Exoplanet from the KELT-South Survey -- A Hot Sub-Jupiter Transiting a V = 10.7 Early G-Star

Authors:

Kuhn et al

Abstract:

We report the discovery of KELT-10b, the first transiting exoplanet discovered using the KELT-South telescope. KELT-10b is a highly inflated sub-Jupiter mass planet transiting a relatively bright V=10.7 star (TYC 8378-64-1), with Teff = 5948±74 K, logg = 4.319+0.020−0.030 and [Fe/H] = 0.09+0.11−0.10, an inferred mass M∗ = 1.112+0.055−0.061 M⊙ and radius R∗ = 1.209+0.047−0.035 R⊙. The planet has a radius RP = 1.399+0.069−0.049 RJ and mass MP = 0.679+0.039−0.038 MJ. The planet has an eccentricity consistent with zero and a semi-major axis a = 0.05250+0.00086−0.00097 AU. The best fitting linear ephemeris is T0 = 2457066.72045±0.00027 BJDTDB and P = 4.1662739±0.0000063 days. This planet joins a group of highly inflated transiting exoplanets with a radius much larger and a mass much less than those of Jupiter. The planet, which boasts deep transits of 1.4%, has a relatively high equilibrium temperature of Teq = 1377+28−23 K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817+0.068−0.054 × 109 erg s−1 cm−2, which places it far above the insolation threshold above which hot Jupiters exhibit increasing amounts of radius inflation. Evolutionary analysis of the host star suggests that KELT-10b is unlikely to survive beyond the current subgiant phase, due to a concomitant in-spiral of the planet over the next ∼1 Gyr. The planet transits a relatively bright star which is accessible to large telescopes and exhibits the third largest transit depth of all transiting exoplanets with V less than 11 in the southern hemisphere, making it a promising candidate for future atmospheric characterization studies.

Hot Saturn Kelt-6b has a Sister Exoplanet Gas Giant

The GAPS Programme with HARPS-N@TNG X. The multi-planet system KELT-6: detection of the planet KELT-6 c and measurement of the Rossiter-McLaughlin effect for KELT-6 b

Authors:

Damasso et al

Abstract:

Aims.

For more than 1.5 years we monitored spectroscopically the star KELT-6 (BD+312447), known to host the transiting hot Saturn KELT-6b, because a previously observed long-term trend in radial velocity time series suggested the existence of an outer companion.

Methods.

We collected a total of 93 new spectra with the HARPS-N and TRES spectrographs. A spectroscopic transit of KELT-6b was observed with HARPS-N, and simultaneous photometry was obtained with the IAC-80 telescope.

Results.

We proved the existence of an outer planet with a mininum mass Mpsini=3.71±0.21 MJup and a moderately eccentric orbit (e=0.21+0.039−0.036) of period P∼3.5 years. We improved the orbital solution of KELT-6b and obtained the first measurement of the Rossiter-McLaughlin effect, showing that the planet has a likely circular, prograde, and slightly misaligned orbit, with a projected spin-orbit angle λ=−36±11 degrees. We improved the KELT-6b transit ephemeris from photometry, and we provided new measurements of the stellar parameters. KELT-6 appears as an interesting case to study the formation and evolution of multi-planet systems.

Ground-based transit observations of the HAT-P-18, HAT-P-19, HAT-P-27/WASP-40 and WASP-21 systems

Ground-based transit observations of the HAT-P-18, HAT-P-19, HAT-P-27/WASP-40 and WASP-21 systems

Authors:

Seeliger et al

Abstract:

As part of our ongoing effort to investigate transit timing variations (TTVs) of known exoplanets, we monitored transits of the four exoplanets HAT-P-18b, HAT-P-19b, HAT-P-27b/WASP-40b and WASP-21b. All of them are suspected to show TTVs due to the known properties of their host systems based on the respective discovery papers. During the past three years 46 transit observations were carried out, mostly using telescopes of the Young Exoplanet Transit Initiative. The analyses are used to refine the systems orbital parameters. In all cases we found no hints for significant TTVs, or changes in the system parameters inclination, fractional stellar radius and planet to star radius ratio. However, comparing our results with those available in the literature shows that we can confirm the already published values.

The Transmission Spectrum of Inflated hot Saturn HAT-P-19b

Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b

Authors:

Mallonn et al

Abstract:

We observed the Saturn-mass and Jupiter-sized exoplanet HAT-P-19b to refine its transit parameters and ephemeris as well as to shed first light on its transmission spectrum. We monitored the host star over one year to quantify its flux variability and to correct the transmission spectrum for a slope caused by starspots. A transit of HAT-P-19b was observed spectroscopically with OSIRIS at the Gran Telescopio Canarias in January 2012. The spectra of the target and the comparison star covered the wavelength range from 5600 to 7600 AA. One high-precision differential light curve was created by integrating the entire spectral flux. This white-light curve was used to derive absolute transit parameters. Furthermore, a set of light curves over wavelength was formed by a flux integration in 41 wavelength channels of 50 AA width. We analyzed these spectral light curves for chromatic variations of transit depth. The transit fit of the combined white-light curve yields a refined value of the planet-to-star radius ratio of 0.1390 pm 0.0012 and an inclination of 88.89 pm 0.32 degrees. After a re-analysis of published data, we refine the orbital period to 4.0087844 pm 0.0000015 days. We obtain a flat transmission spectrum without significant additional absorption at any wavelength or any slope. However, our accuracy is not sufficient to significantly rule out the presence of a pressure-broadened sodium feature. Our photometric monitoring campaign allowed for an estimate of the stellar rotation period of 35.5 pm 2.5 days and an improved age estimate of 5.5^+1.8_-1.3 Gyr by gyrochronology.

HATS-8b: an Inflated Hot Saturn

HATS-8b: A Low-Density Transiting Super-Neptune

Authors:

Bayliss et al

Abstract:

HATS-8b is a low density transiting super-Neptune discovered as part of the HATSouth project. The planet orbits its solar-like G dwarf host (V=14.03 ± 0.10 and Teff =5679 ± 50 K) with a period of 3.5839 d. HATS-8b is the third lowest mass transiting exoplanet to be discovered from a wide-field ground based search, and with a mass of 0.138 ± 0.019 MJ it is approximately half-way between the masses of Neptune and Saturn. However HATS-8b has a radius of 0.873 (+0.123,-0.075) RJ, resulting in a bulk density of just 0.259 ± 0.091 g.cm−3. The metallicity of the host star is super-Solar ([Fe/H]=0.210 ± 0.080), arguing against the idea that low density exoplanets form from metal-poor environments. The low density and large radius of HATS-8b results in an atmospheric scale height of almost 1000 km, and in addition to this there is an excellent reference star of near equal magnitude at just 19 arcsecond separation on the sky. These factors make HATS-8b an exciting target for future atmospheric characterization studies, particularly for long-slit transmission spectroscopy.

UV Driven Evaporation of Close-in Exoplanets

UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows

Authors:

Owen et al

Abstract:

We have investigated the evaporation of close-in exoplanets irradiated by ionizing photons. We find that the properties of the flow are controlled by the ratio of the recombination time to the flow time-scale. When the recombination time-scale is short compared to the flow time-scale the the flow is in approximate local ionization equilibrium with a thin ionization front, where the photon mean free path is short compared to flow scale. In this "recombination limited" flow the mass-loss scales roughly with the square root of the incident flux. When the recombination time is long compared to the flow time-scale the ionization front becomes thick and encompasses the entire flow, with the mass-loss rate scaling linearly with flux. If the planet's potential is deep the flow is approximately "energy-limited"; however, if the planet's potential is shallow we identify a new limiting mass-loss regime, which we term "photon-limited". In this scenario the mass-loss rate is purely limited by the incoming flux of ionizing photons. We have developed a new numerical approach that takes into account the frequency dependence of the incoming ionizing spectrum and performed a large suite of 1D simulations to characterise UV driven mass-loss around low mass planets. We find the flow is "recombination-limited" at high fluxes but becomes "energy-limited" at low fluxes; however, the transition is broad occurring over several order of magnitude in flux. Finally, we point out the transitions between the different flow types does not occur at a single flux value, but depends on the planet's properties, with higher mass planets becoming "energy-limited" at lower fluxes.

HAT-P-50b, HAT-P-51b, HAT-P-52b, and HAT-P-53b: Three Transiting Hot Jupiters and a Transiting Hot Saturn

HAT-P-50b, HAT-P-51b, HAT-P-52b, and HAT-P-53b: Three Transiting Hot Jupiters and a Transiting Hot Saturn From the HATNet Survey

Authors:

Hartman et al

Abstract:

We report the discovery and characterization of four transiting exoplanets by the HATNet survey. The planet HAT-P-50b has a mass of 1.35 M_J and a radius of 1.29 R_J, and orbits a bright (V = 11.8 mag) M = 1.27 M_sun, R = 1.70 R_sun star every P = 3.1220 days. The planet HAT-P-51b has a mass of 0.31 M_J and a radius of 1.29 R_J, and orbits a V = 13.4 mag, M = 0.98 M_sun, R = 1.04 R_sun star with a period of P = 4.2180 days. The planet HAT-P-52b has a mass of 0.82 M_J and a radius of 1.01 R_J, and orbits a V = 14.1 mag, M = 0.89 M_sun, R = 0.89 R_sun star with a period of P = 2.7536 days. The planet HAT-P-53b has a mass of 1.48 M_J and a radius of 1.32 R_J, and orbits a V = 13.7 mag, M = 1.09 M_sun, R = 1.21 R_sun star with a period of P = 1.9616 days. All four planets are consistent with having circular orbits and have masses and radii measured to better than 10% precision. The low stellar jitter and favorable R_P/R_star ratio for HAT-P-51 make it a promising target for measuring the Rossiter-McLaughlin effect for a Saturn-mass planet.