Planetary Occultations Research Articles

X-ray variability of HD 189733 across eight years of XMM-Newton observations

The characterization of exoplanets, their formation, evolution, and chemical changes is tightly linked to our knowledge of their host stars. In particular, stellar X-rays and UV emission have a strong impact on the dynamical and chemical evolution of planetary atmospheres. We analyzed 25 XMM-Newton observations encompassing about eight years and totaling about 958 ks in order to study the X-ray emission of HD 189733 A. We find that the corona of HD 189733 A has an average temperature of 0.4 keV and it is only during flares that the mean temperature increases to 0.9 keV. Apart from the flares, there is no significant change in the flux and hardness of the coronal emission on a timescale of several months to years. Thus, we conclude that there is no detectable activity cycle on such timescales. We identified the flares and built their energy distribution. The number of flares observed around the phases of the planetary eclipses is not statistically different from the number of flares during transit phases. However, we do find a hint of a difference in the flare-energy distributions, as the flares observed around the planetary eclipses tend to be more energetic than the flares observed around the primary transits of the planet. We modeled the distribution of the number of flares per day with a power law, showing that it is steeper than the one observed in the Sun and in other Main Sequence stars. The steepness hints at a significant fraction of undetected micro-flares. Altogether, the plasma temperatures below 1 keV observed during the flares, along with the slightly larger fraction of energetic flares seen at the secondary transits highlight the peculiarity of the corona of HD 189733 A and points to star-planet interaction as the plausible origin of part of its X-ray emission. However, more observational and modeling efforts are required to confirm or disprove this scenario.

The Ariel Instrument Control Unit

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission (Tinetti 2019; Puig et al. 2018; Pascale et al. 2018), has been selected in March 2018 by ESA for the fourth medium-class mission (M4) launch opportunity of the Cosmic Vision Program, with an expected lift off in late 2028. It is the first mission dedicated to measuring the chemical composition and thermal structures of the atmospheres of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of our own Solar System. Its Payload (P/L) (Eccleston and Tinetti 2018; Eccleston et al. 2017; Middleton et al. 2019), has been designed to perform transit spectroscopy from space during primary and secondary planetary eclipses in order to achieve a large unbiased survey concerning the nature of exoplanets atmospheres and their interiors, to determine the key factors affecting the formation and evolution of planetary systems (Tinetti et al. 2017, 2018). Ariel will observe hundreds of warm and hot transiting gas giants, Neptunes and super-Earths around a wide range of host star types, targeting planets hotter than $\sim $ 600 K to take advantage of their well-mixed atmospheres. It will exploit primary and secondary transit spectroscopy in the 1.10 to 7.80 μm spectral range and broad-band photometry in the optical (0.50 - 0.80 μm) and Near IR (0.80 - 1.10 μm) . One of the two instruments of the Ariel Payload is the Fine Guidance System (FGS), including three photometric channels (two used for guiding as well as science) between 0.5-1.1 μm plus a low resolution NIR spectrometer for 1.1-1.95 μm range. Along with FGS an IR Spectrometer (AIRS) (Amiaux et al. 2017) is foreseen, providing low-resolution spectroscopy in two IR channels: Channel 0 (CH0) for the 1.95 − 3.90 μm band and Channel 1 (CH1) for the 3.90 − 7.80 μm range. Finally, an Active Cooler System (ACS) including a Ne Joule-Thomson cooler is adopted to provide active cooling capability to the AIRS detectors working at cryogenic temperatures. AIRS is located at the intermediate focal plane of the telescope and common optical system and it hosts two HgCdTe-based hybrid IR detectors and two cold front-end electronics (CFEE) for detectors control and readout. Each CFEE is driven by a Detector Control Unit (DCU) part of AIRS but hosted within and managed by the Instrument Control Unit (ICU) of the Payload (Focardi et al. 2018). ICU is a warm unit residing into the S/C Service Module (SVM) and it is based on a cold redundant configuration involving the Power Supply Unit (PSU) and the Commanding and Data Processing Unit (CDPU) boards; both DCUs are instead cross-strapped and can be managed by the nominal or the redundant (PSU+CDPU) chain. ICU is in charge of AIRS management, collecting scientific and housekeeping (HK) telemetries from the spectrometer and HK from the telescope (temperatures readings), the P/L Optical Bench (OB) and other Subsystems (SS), thanks to a warm slave unit (TCU, Telescope Control Unit) interfaced to the ICU. Science and HK telemetries are then forwarded to the S/C, for temporary storage, before sending them to Ground. Here we describe the status of the ICU design at the end of B1 Phase, prior to the Mission Adoption Review (MAR) by ESA, with some still open architectural choices to be addressed and finalised once selected the ICU industrial Prime contractor.

TRAPPIST-1: Global results of theSpitzerExploration Science Program Red Worlds

Context.With more than 1000 h of observation from Feb. 2016 to Oct. 2019, theSpitzerExploration Program Red Worlds (ID: 13067, 13175 and 14223) exclusively targeted TRAPPIST-1, a nearby (12 pc) ultracool dwarf star, finding that it is orbited by seven transiting Earth-sized planets. At least three of these planets orbit within the classical habitable zone of the star, and all of them are well-suited for a detailed atmospheric characterization with the upcoming JWST.Aims.The main goals of theSpitzerRed Worlds program were (1) to explore the system for new transiting planets, (2) to intensively monitor the planets’ transits to yield the strongest possible constraints on their masses, sizes, compositions, and dynamics, and (3) to assess the infrared variability of the host star. In this paper, we present the global results of the project.Methods.We analyzed 88 new transits and combined them with 100 previously analyzed transits, for a total of 188 transits observed at 3.6 or 4.5μm. For a comprehensive study, we analyzed all light curves both individually and globally. We also analyzed 29 occultations (secondary eclipses) of planet b and eight occultations of planet c observed at 4.5μm to constrain the brightness temperatures of their daysides.Results.We identify several orphan transit-like structures in ourSpitzerphotometry, but all of them are of low significance. We do not confirm any new transiting planets. We do not detect any significant variation of the transit depths of the planets throughout the different campaigns. Comparing our individual and global analyses of the transits, we estimate for TRAPPIST-1 transit depth measurements mean noise floors of ~35 and 25 ppm in channels 1 and 2 ofSpitzer/IRAC, respectively. We estimate that most of this noise floor is of instrumental origins and due to the large inter-pixel inhomogeneity of IRAC InSb arrays, and that the much better interpixel homogeneity of JWST instruments should result in noise floors as low as 10 ppm, which is low enough to enable the atmospheric characterization of the planets by transit transmission spectroscopy. Our analysis reveals a few outlier transits, but we cannot conclude whether or not they correspond to spot or faculae crossing events. We construct updated broadband transmission spectra for all seven planets which show consistent transit depths between the twoSpitzerchannels. Although we are limited by instrumental precision, the combined transmission spectrum of planet b to g tells us that their atmospheres seem unlikely to be CH4-dominated. We identify and model five distinct high energy flares in the whole dataset, and discuss our results in the context of habitability. Finally, we fail to detect occultation signals of planets b and c at 4.5μm, and can only set 3-σupper limits on their dayside brightness temperatures (611 K for b 586 K for c).

Occultation of Planets by the Moon in European Narrative Medieval Sources

Existing research dealing with astronomical observations from medieval Europe have extensively covered topics such as solar and lunar eclipses and sightings of comets and meteors, but no compilation of occultations of planets by the Moon has been carried out and, till now, the data have remained scattered in different publications. The main reasons for this are the small number of observations that has reached us, their limited use for calculation of parameters associated with the rotation of the Earth, and the fact that between the fifth and fifteenth centuries, the period that we consider, almost none of these observations were made scientifically, since they usually appear in narrative texts, be they chronicles or annals. Our purpose is to make a compilation of these phenomena, trying to shed light on some of the most controversial observations after examining them in their historical context. We will examine European sources, but, occasionally, we will also consider reports from other parts of the world to make comparisons, when necessary.

Traces of exomoons in computed flux and polarization phase curves of starlight reflected by exoplanets

Context.Detecting moons around exoplanets is a major goal of current and future observatories. Moons are suspected to influence rocky exoplanet habitability, and gaseous exoplanets in stellar habitable zones could harbor abundant and diverse moons to target in the search for extraterrestrial habitats. Exomoons contribute to exoplanetary signals but are virtually undetectable with current methods.Aims.We identify and analyze traces of exomoons in the temporal variation of total and polarized fluxes of starlight reflected by an Earth-like exoplanet and its spatially unresolved moon across all phase angles, with both orbits viewed in an edge-on geometry.Methods.We compute the total and linearly polarized fluxes, and the degree of linear polarizationPof starlight that is reflected by the exoplanet with its moon along their orbits, accounting for the temporal variation of the visibility of the planetary and lunar disks, and including the effects of mutual transits and mutual eclipses. Our computations pertain to a wavelength of 450 nm.Results.Total fluxFshows regular dips due to planetary and lunar transits and eclipses. PolarizationPshows regular peaks due to planetary transits and lunar eclipses, andPcan increase and/or slightly decrease during lunar transits and planetary eclipses. Changes inFandPwill depend on the radii of the planet and moon, on their reflective properties, and their orbits, and are about one magnitude smaller than the smooth background signals. The typical duration of a transit or an eclipse is a few hours.Conclusions.Traces of an exomoon due to planetary and lunar transits and eclipses show up in theFandPof sunlight reflected by planet–moon systems and could be searched for in exoplanet flux and/or polarization phase functions.

Edward Gresham, Copernican Cosmology, and Planetary Occultations in Pre-Telescopic Astronomy

This article introduces an understudied source in the history of astronomy, the Astrostereon or the Discourse of the Falling of the Planet (1603). Written by the English astrologer Edward Gresham, this text presents, among other things, the earliest known set of predicted planetary occultations (for 1603–1604) and the use of these phenomena to defend the Copernican cosmology. We analyse those predictions and then briefly survey all known pre-telescopic observations of reported planetary occulations and the motivations for such observations. These data suggest that for early observers, the greater the difference in apparent brightness between the two occulting bodies, the greater the angular separation could be for an occultation nonetheless to be reported. An appendix seeks to explain this finding by considering several factors known from modern experimental analyses of human visual performance.

CTR Wilson Meeting 2016

CTR Wilson Meeting 2016

Planet–Planet Occultations in TRAPPIST-1 and Other Exoplanet Systems

Abstract We explore the occurrence and detectability of planet–planet occultations (PPOs) in exoplanet systems. These are events during which a planet occults the disk of another planet in the same system, imparting a small photometric signal as its thermal or reflected light is blocked. We focus on the planets in TRAPPIST-1, whose orbital planes we show are aligned to at 90% confidence. We present a photodynamical model for predicting and computing PPOs in TRAPPIST-1 and other systems for various assumptions of the planets’ atmospheric states. When marginalizing over the uncertainties on all orbital parameters, we find that the rate of PPOs in TRAPPIST-1 is about 1.4 per day. We investigate the prospects for detection of these events with the James Webb Space Telescope, finding that ∼10–20 occultations per year of b and c should be above the noise level at 12–15 μm. Joint modeling of several of these PPOs could lead to a robust detection. Alternatively, observations with the proposed Origins Space Telescope should be able to detect individual PPOs at high signal-to-noise ratios. We show how PPOs can be used to break transit timing variation degeneracies, imposing strong constraints on the eccentricities and masses of the planets, as well as to constrain the longitudes of nodes and thus the complete three-dimensional structure of the system. We further show how modeling of these events can be used to reveal a planet’s day/night temperature contrast and construct crude surface maps. We make our photodynamical code available on github (https://github.com/rodluger/planetplanet).

Measuring stellar granulation during planet transits

Context. Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal. Aims. We used realistic three-dimensional (3D) radiative hydrodynamical (RHD) simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet. Methods. We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared that cover the range of several ground- and space-based telescopes which observe exoplanet transits. We modeled the transit using synthetic stellar-disk images obtained with a spherical-tile imaging method and emulated the temporal variation of the granulation intensity generating random images covering a granulation time-series of 13.3 h. We measured the contribution of the stellar granulation on the light curves during the planet transit. Results. We identified two types of granulation noise that act simultaneously during the planet transit: (i) the intrinsic change in the granulation pattern with timescale (e.g., 10 min for solar-type stars assumed in this work) is smaller than the usual planet transit (~hours as in our prototype cases); and (ii) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness as a result of convective-related surface structures. First, we showed that our modeling approach returns granulation timescale fluctuations that are comparable with what has been observed for the Sun. Then, our statistical approach shows that the granulation pattern of solar and K-dwarf-type stars have a non-negligible effect of the light curve depth during the transit, and, consequentially on the determination of the planet transit parameters such as the planet radius (up to 0.90% and ~0.47% for terrestrial and gaseous planets, respectively). We also showed that larger (or smaller) orbital inclination angles with respect to values corresponding to transit at the stellar center display a shallower transit depth and longer ingress and egress times, but also granulation fluctuations that are correlated to the center-to-limb variation: they increase (or decrease) the value of the inclination, which amplifies the fluctuations. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions. Conclusions. The prospects for planet detection and characterization with transiting methods are excellent with access to large amounts of data for stars. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations. This can be achieved by performing precise and continuous observations of stellar photometry and radial velocity, as we explained with RHD simulations, before, after, and during the transit periods.

EFFICIENT GEOMETRIC PROBABILITIES OF MULTI-TRANSITING EXOPLANETARY SYSTEMS FROM CORBITS

ABSTRACT NASA’s Kepler Space Telescope has successfully discovered thousands of exoplanet candidates using the transit method, including hundreds of stars with multiple transiting planets. In order to estimate the frequency of these valuable systems, it is essential to account for the unique geometric probabilities of detecting multiple transiting extrasolar planets around the same parent star. In order to improve on previous studies that used numerical methods, we have constructed an efficient, semi-analytical algorithm called the Computed Occurrence of Revolving Bodies for the Investigation of Transiting Systems (CORBITS), which, given a collection of conjectured exoplanets orbiting a star, computes the probability that any particular group of exoplanets can be observed to transit. The algorithm applies theorems of elementary differential geometry to compute the areas bounded by circular curves on the surface of a sphere. The implemented algorithm is more accurate and orders of magnitude faster than previous algorithms, based on comparisons with Monte Carlo simulations. We use CORBITS to show that the present solar system would only show a maximum of three transiting planets, but that this varies over time due to dynamical evolution. We also used CORBITS to geometrically debias the period ratio and mutual Hill sphere distributions of Kepler's multi-transiting planet candidates, which results in shifting these distributions toward slightly larger values. In an Appendix, we present additional semi-analytical methods for determining the frequency of exoplanet mutual events, i.e., the geometric probability that two planets will transit each other (planet–planet occultation, relevant to transiting circumbinary planets) and the probability that this transit occurs simultaneously as they transit their star. The CORBITS algorithms and several worked examples are publicly available.

Earth rotation derived from occultation records

Abstract We determined the values of the Earth's rotation parameter, ΔT = T T − UT, around AD 500 after confirming that the value of the tidal acceleration, $\dot{n}$, of the lunar motion remained unchanged during the period between ancient times and the present. For determining of ΔT, we used contemporaneous occultations of planets by the Moon. In general, occultation records are not useful. However, there are some records that give us a stringent condition for the range of ΔT. Records of the lunar occultations in AD 503 and AD 513 are such examples. In order to assure the usefulness of this occultation data, we used contemporaneous annular and total solar eclipses, which have not been used in the preceding work. This is the first work in which the lunar occultation data have been used as primary data to determine the value of ΔT together with auxiliary contemporaneous annular and total solar eclipses. Our ΔT value is less than a smoothed value (Stephenson 1997) by at least 450 s. The result is consistent with our earlier results obtained from solar eclipses.

Star-Planet Interaction: The Curious Case of the Planet Spoon-feeding Its Host Star (and Other Amenities)

AbstractWe report two cases of Star-Planet Interaction (SPI) in two systems with hot Jupiters: HD 189733 and HD 17156. We used HST-COS to study the FUV variability of HD 189733 after the planetary eclipse. With the support of MHD simulations, we evince that material is likely evaporating from the planet and accreting onto the parent star. This produces a hot spot on the stellar surface, co-moving with the planetary motion and responsible of the X-ray and FUV variability at peculiar planetary phases. In HD 17156, which hosts a hot Jupiter in an eccentric orbit, we observed an enhancement of the X-ray activity at the passage of its planet at the periastron. The origin can be due to magnetic reconnection between the planetary and stellar magnetic fields, or due to material tidally stripped from the planet and accreting onto the star.

Rotation Profile of Kepler-63 from Planetary Transits

AbstractCurrently it is possible to estimate the rotation profile of a star that harbours a planet in an orbit such that it eclipses the star periodically. During one of these transits, the planet may occult a spot on the photosphere of the star, causing small variations in its light curve. By detecting the same spot in a later transit, it is possible to estimate the stellar rotation period. Here we present the results of this model for the case of the star Kepler-63, which has a planet in an orbit with high obliquity. This means that the planetary eclipse occults many latitude bands of the star, from near the equator to the poles. The results show that Kepler-63 has differential rotation of 0.133 rd/d and a relative differential rotation of 11.4%.

TRANSITING THE SUN: THE IMPACT OF STELLAR ACTIVITY ON X-RAY AND ULTRAVIOLET TRANSITS

Transits of hot Jupiters in X-rays and the ultraviolet have been shown to be both deeper and more variable than the corresponding optical transits. This variability has been attributed to hot Jupiters having extended atmospheres at these wavelengths. Using resolved images of the Sun from NASA's Solar Dynamics Observatory spanning 3.5 years of Solar Cycle 24 we simulate transit light curves of a hot Jupiter to investigate the impact of Solar like activity on our ability to reliably recover properties of the planet's atmosphere in soft X-rays (94 {\AA}), the UV (131-1700 {\AA}), and the optical (4500 {\AA}). We find that for stars with similar activity levels to the Sun, the impact of stellar activity results in the derived radius of the planet in soft X-ray/EUV to be underestimated by up-to 25% or overestimated by up-to 50% depending on whether the planet occults active regions. We also find that in up-to 70% of the X-ray light curves the planet transits over bright star spots. In the far ultraviolet (1600 & 1700 {\AA}), we find the mean recovered value of the planet-to-star radius ratio to be over-estimated by up-to 20%. For optical transits we are able to consistently recover the correct planetary radius. We also address the implications of our results for transits of WASP-12b and HD 189733b at short wavelengths.

Evryscope Science: Exploring the Potential of All-Sky Gigapixel-Scale Telescopes

Low-cost mass-produced sensors and optics have recently made it feasible to build telescope arrays which observe the entire accessible sky simultaneously. In this article, we discuss the scientific motivation for these telescopes, including exoplanets, stellar variability, and extragalactic transients. To provide a concrete example we detail the goals and expectations for the Evryscope, an under-construction 780 MPix telescope which covers 8660 sq. deg. in each 2-minute exposure; each night, 18,400 sq. deg. will be continuously observed for an average of ≈6 hr. Despite its small 61 mm aperture, the system’s large field of view provides an étendue which is ∼10% of LSST. The Evryscope, which places 27 separate individual telescopes into a common mount which tracks the entire accessible sky with only one moving part, will return 1%-precision, many-year-length, high-cadence light curves for every accessible star brighter than ∼16th magnitude. The camera readout times are short enough to provide near-continuous observing, with a 97% survey time efficiency. The array telescope will be capable of detecting transiting exoplanets around every solar-type star brighter than mV = 12, providing at least few-millimagnitude photometric precision in long-term light curves. It will be capable of searching for transiting giant planets around the brightest and most nearby stars, where the planets are much easier to characterize; it will also search for small planets nearby M-dwarfs, for planetary occultations of white dwarfs, and will perform comprehensive nearby microlensing and eclipse-timing searches for exoplanets inaccessible to other planet-finding methods. The Evryscope will also provide comprehensive monitoring of outbursting young stars, white dwarf activity, and stellar activity of all types, along with finding a large sample of very-long-period M-dwarf eclipsing binaries. When relatively rare transients events occur, such as gamma-ray bursts (GRBs), nearby supernovae, or even gravitational wave detections from the Advanced LIGO/Virgo network, the array will return minute-by-minute light curves without needing pointing toward the event as it occurs. By coadding images, the system will reach V ∼ 19 in 1-hr integrations, enabling the monitoring of faint objects. Finally, by recording all data, the Evryscope will be able to provide pre-event imaging at 2-minute cadence for bright transients and variable objects, enabling the first high-cadence searches for optical variability before, during and after all-sky events.

ATMOSPHERIC CHARACTERIZATION OF THE HOT JUPITER KEPLER-13Ab

Kepler-13Ab (= KOI-13.01) is a unique transiting hot Jupiter. It is one of very few known short-period planets orbiting a hot A-type star, making it one of the hottest planets currently known. The availability of Kepler data allows us to measure the planet's occultation (secondary eclipse) and phase curve in the optical, which we combine with occultations observed by warm Spitzer at 4.5 mic and 3.6 mic and a ground-based occultation observation in the Ks band (2.1 mic). We derive a day-side hemisphere temperature of 2,750 +- 160 K as the effective temperature of a black body showing the same occultation depths. Comparing the occultation depths with one-dimensional planetary atmosphere models suggests the presence of an atmospheric temperature inversion. Our analysis shows evidence for a relatively high geometric albedo, Ag= 0.33 +0.04 -0.06. While measured with a simplistic method, a high Ag is supported also by the fact that the one-dimensional atmosphere models underestimate the occultation depth in the optical. We use stellar spectra to determine the dilution, in the four wide bands where occultation was measured, due to the visual stellar binary companion 1.15 +- 0.05" away. The revised stellar parameters measured using these spectra are combined with other measurements leading to revised planetary mass and radius estimates of Mp = 4.94 - 8.09 Mjup and Rp = 1.406 +- 0.038 Rjup. Finally, we measure a Kepler mid-occultation time that is 34.0 +- 6.9 s earlier than expected based on the mid-transit time and the delay due to light travel time, and discuss possible scenarios.

Kepler-77b: a very low albedo, Saturn-mass transiting planet around a metal-rich solar-like star

We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry (quarters 1-13) with high-resolution spectroscopy from the Sandiford@McDonald and FIES@NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of Mp=0.430+/-0.032 Mjup, a radius of Rp=0.960+/-0.016 Rjup, and a bulk density of 0.603+/-0.055 g/cm^3. It orbits a slowly rotating (P=36+/-6 days) G5V star with M*=0.95+/-0.04 Msun, R*=0.99+/-0.02 Rsun, Teff=5520+/-60 K, [M/H]=0.20+/-0.05, that has an age of 7.5+/-2.0 Gyr. The lack of detectable planetary occultation with a depth higher than about 10 ppm implies a planet geometric and Bond albedo of Ag<0.087+/-0.008 and Ab<0.058+/-0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of about 0.5 minutes, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transits observed in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet's passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.

Early Chinese Observations of Occultations of Planets by the Moon

1. INTRODUCTIONAmong the numerous observations of celestial phenomena recorded in Chinese history are many allusions to occultations of planets by the Moon. In most cases the exact dates are reported. As is also true for eclipses, the accuracy and reliability of these observations can be readily tested using modern astronomical computations. Chinese records of eclipses have been extensively studied in recent years - notably for their importance in investigating Earth's past rotation.1 However, observations of the Moon's obscuring planets reported in these same sources have so far attracted relatively little attention.The earliest extant Chinese account of an occultation of a planet by the Moon dates from 69 b.c.: in the Former Han dynasty. On this occasion, the Moon was said to have eclipsed Mars. In later Chinese history, numerous similar events involving one or other of the five bright planets (Mercury, Venus, Mars, Jupiter and Saturn) are described. Until the end of the Yuan dynasty (a.D. 1367), the main sources of these observations are the astronomical treatises (usually termed tianwenshi) in the official dynastic histories (zhengshi). However, during the succeeding Ming and Qing dynasties the available sources are more varied. In addition to the dynastic histories, these later compilations include the extensive Ming shilu (Veritable records of the Ming Dynasty), and also many fangzhi or local gazettes.In the present paper we have investigated the reliability and accuracy of the reports of lunar occultations of planets as cited in the astronomical treatises of the various dynastic histories down to the end of the Yuan dynasty. We have concentrated specifically on those reports for which an exact date is given: year of the appropriate imperial reign period, lunar month and day of the sexagenary cycle. (N.B. On one occasion, in a.D. 220, the day of the lunar month is specified rather than the cyclical day.) The observations range in date from 69 b.c. to a.D. 1342.2. AIMS OF THIS STUDYIn translating and analysing the various Chinese texts relating to occultations of planets by the Moon, we have had several goals. Our primary aims have been twofold: to investigate (i) the reliability with which the dates of these events are recorded; and (ii) the precision with which the individual observations were made. This has entailed detailed comparison of the various records with the results of modern retrospective computation.Unlike solar eclipses, which almost invariably occur on the first or last day of the month on the Chinese luni-solar calendar, occultations of the outer planets Mars, Jupiter and Saturn by the Moon can occur at any time in the month. Although the dates of similar events involving Venus are more restricted, they can take place within a period of about a week around the beginning and end a lunar month. (Mercury is never far from the Sun, and occultations of this planet are only rarely recorded.) Hence comparison of the reported dates of occultations with their computed equivalents should provide a useful - although indirect - indication of the likely reliability of the dates of certain other important celestial events noted in the astronomical chapters of the same histories. These latter events, which can also occur at any time of the lunar month, include comets, novae, meteors, aurorae, and sunspots. The dates of phenomena such as these can only rarely be verified by retrospective computation and in most cases they have to be simply taken on trust.In describing an occultation, three separate astronomical terms are almost exclusively used in Chinese texts. We have investigated whether these expressions prove to be consistently precise - implying that the Moon indeed passed in front of the specified planet - or whether they were more loosely applied. An item of further concern in the present paper is the approximate time of night when the calendar date was changed for astronomical purposes. …

Starspot activity and rotation of the planet-hosting star Kepler-17

Context. Kepler-17 is a G2V sun-like star accompanied by a transiting planet with a mass of ~2.5 Jupiter masses and an orbital period of 1.486 d, recently discovered by the Kepler space telescope. This star is highly interesting as a young solar analogue. Aims. We used about 500 days of high-precision, high-duty-cycle optical photometry collected by Kepler to study the rotation of the star and the evolution of its photospheric active regions. Methods. We applied a maximum-entropy light curve inversion technique to model the flux rotational modulation induced by active regions that consist of dark spots and bright solar-like faculae with a fixed area ratio. Their configuration was varied after a fixed time interval to take their evolution into account. Active regions were used as tracers to study stellar differential rotation, and planetary occultations were used to constrain the latitude of some spots. Results. Our modelling approach reproduces the light variations of Kepler-17 with a standard deviation of the residuals comparable with the precision of Kepler photometry. We find several active longitudes where individual active regions appear, evolve, and decay with lifetimes comparable to those observed in the Sun, although the star has a spotted area ~10-15 times larger than the Sun at the maximum of the 11-yr cycle. Kepler-17 shows a solar-like latitudinal differential rotation, but the fast spot evolution prevents a precise determination of its amplitude. Moveover, the star shows a cyclic variation of the starspot area with a period of 47.1 \pm 4.5 d, particularly evident during the last 200 days of the observations, similar to the solar Rieger cycles. Possible effects of the close-in massive planet on stellar photospheric activity cannot be excluded, but require a long-term monitoring to be unambiguously detected.

SOPHIE velocimetry ofKeplertransit candidates

We report the discovery of two new transiting hot Jupiters, KOI-135b and KOI-204b, that were previously identified as planetary candidates by Borucki et al. 2011, and, independently of the Kepler team, confirm the planetary nature of Kepler-17b, recently announced by Desert et al. 2011. Radial-velocity measurements, taken with the SOPHIE spectrograph at the OHP, and Kepler photometry (Q1 and Q2 data) were used to derive the orbital, stellar and planetary parameters. KOI-135b and KOI-204b orbit their parent stars in 3.02 and 3.25 days, respectively. They have approximately the same radius, Rp=1.20+/-0.06 R_jup and 1.24+/-0.07 R_jup, but different masses Mp=3.23+/-0.19 M_jup and 1.02+/-0.07 M_jup. As a consequence, their bulk densities differ by a factor of four, rho_p=2.33+/-0.36 g.cm^-3 (KOI-135b) and 0.65+/-0.12 g.cm-3 (KOI-204b). Our SOPHIE spectra of Kepler-17b, used both to measure the radial-velocity variations and determine the atmospheric parameters of the host star, allow us to refine the characterisation of the planetary system. In particular we found the radial-velocity semi-amplitude and the stellar mass to be respectively slightly smaller and larger than Desert et al. These two quantities, however, compensate and lead to a planetary mass fully consistent with Desert et al.: our analysis gives Mp=2.47+/-0.10 M_jup and Rp=1.33+/-0.04 R_jup. We found evidence for a younger age of this planetary system, t<1.8 Gyr, which is supported by both evolutionary tracks and gyrochronology. Finally, we confirm the detection of the optical secondary eclipse and found also the brightness phase variation with the Q1 and Q2 Kepler data. The latter indicates a low redistribution of stellar heat to the night side (<16% at 1-sigma), if the optical planetary occultation comes entirely from thermal flux. The geometric albedo is A_g<0.12 (1-sigma).