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Variability observed in photometric lightcurves of late-type stars (on timescales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) lightcurves of active G2, K0, M0 and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of lightcurve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density and active band latitudes is explored. For our lightcurve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates 2-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, lightcurves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte-Carlo simulations, calculating lightcurve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of 1%, the typical variability of a solar-type star is around 2 parts-per-thousand. The presence of faculae clearly affects the mean brightness and lightcurve shape, but has relatively little influence on the variability., Comment: 17 pages, 16 figures. Accepted for publication in MNRAS 22 April 2021

Previous studies have related surface temperature maps, obtained with the Doppler imaging (DI) technique, of LQ Hya with long-term photometry. We compare surface magnetic field maps, obtained with the Zeeman Doppler imaging (ZDI) technique, with contemporaneous photometry, with the aim of quantifying the star's magnetic cycle characteristics. We inverted Stokes IV spectropolarimetry into magnetic field and surface brightness maps using a tomographic inversion code that models high signal-to-noise ratio mean line profiles produced by the least squares deconvolution (LSD) technique. The magnetic field and surface brightness maps reveal similar patterns to previous DI and ZDI studies: non-axisymmetric polar magnetic field structure, void of fields at mid-latitudes, and a complex structure in the equatorial regions. There is a weak but clear tendency of the polar structures to be linked with strong radial field and the equatorial ones with the azimuthal. We find a polarity reversal in the radial field between 2016 and 2017 coincident with an activity minimum seen in the long-term photometry. The inverted field strengths cannot easily be related with the observed spottedness, but we find that they are partially connected with the retrieved field complexity. Comparing to global magnetoconvection models for rapidly rotating young Suns, this field topology and dominance of the poloidal field component could be explained by a turbulent dynamo, where differential rotation does not play a major role (so called alpha^2 Omega or alpha^2 dynamos), and axi- and non-axisymmetric modes are excited simultaneously. The complex equatorial magnetic field structure could arise from the twisted (helical) wreaths often seen in these simulations, while the polar feature would be connected to the mostly poloidal non-axisymmetric component having a smooth spatial structure., 13 pages, 8 figures, 7 tables

International audience; In order to estimate fundamental parameters (effective temperature, surface gravity and metallicity) of the large amount of stars in the PolarBase data base, we need a fast and reliable algorithm. With this aim, we developed a convolutional neural network able to derive this parameter triplet. Our neural network was trained on observed spectra from the PolarBase and Elodie data bases (M to F stars). We used the spectral region between 6095 and 6185 Angströms which has proved its efficiency in a number of previous studies. We analyzed the outcome of our approach for a sample of spectra from the same data bases. We discuss the accuracy and reliability of the neural network depending on the parameter domain, size and quality of the training data set.

Observations of Sun-like stars over the last half-century have improved our understanding of how magnetic dynamos, like that responsible for the 11-year solar cycle, change with rotation, mass and age. Here we show for the first time how metallicity can affect a stellar dynamo. Using the most complete set of observations of a stellar cycle ever obtained for a Sun-like star, we show how the solar analog HD 173701 exhibits solar-like differential rotation and a 7.4-year activity cycle. While the duration of the cycle is comparable to that generated by the solar dynamo, the amplitude of the brightness variability is substantially stronger. The only significant difference between HD 173701 and the Sun is its metallicity, which is twice the solar value. Therefore, this provides a unique opportunity to study the effect of the higher metallicity on the dynamo acting in this star and to obtain a comprehensive understanding of the physical mechanisms responsible for the observed photometric variability. The observations can be explained by the higher metallicity of the star, which is predicted to foster a deeper outer convection zone and a higher facular contrast, resulting in stronger variability., Submitted to ApJ

New detections of debris discs at submillimetre wavelengths present highly valuable complementary information to prior observations of these sources at shorter wavelengths. Characterization of discs through spectral energy distribution modelling including the submillimetre fluxes is essential for our basic understanding of disc mass and temperature, and presents a starting point for further studies using millimetre interferometric observations. In the framework of the ongoing SCUBA-2 Observations of Nearby Stars, the instrument SCUBA-2 on the James Clerk Maxwell Telescope was used to provide measurements of 450 and 850 mu m fluxes towards a large sample of nearby main-sequence stars with debris discs detected previously at shorter wavelengths. We present the first results from the ongoing survey, concerning 850 mu m detections and 450 mu m upper limits towards 10 stars, the majority of which are detected at submillimetre wavelengths for the first time. One, or possibly two, of these new detections is likely a background source. We fit the spectral energy distributions of the star+disc systems with a blackbody emission approach and derive characteristic disc temperatures. We use these temperatures to convert the observed fluxes to disc masses. We obtain a range of disc masses from 0.001 to 0.1 M-circle plus, values similar to the prior dust mass measurements towards debris discs. There is no evidence for evolution in dust mass with age on the main sequence, and indeed the upper envelope remains relatively flat at approximate to 0.5 M-circle plus at all ages. The inferred disc masses are lower than those from disc detections around pre-main-sequence stars, which may indicate a depletion of solid mass. This may also be due to a change in disc opacity, though limited sensitivity means that it is not yet known what fraction of pre-main-sequence stars have discs with dust masses similar to debris disc levels. New, high-sensitivity detections are a path towards investigating the trends in dust mass evolution.

There are two possible mechanisms to imprint planet signatures in the chemical composition of Sun-like stars: i) dust condensation at the early stages of planet formation, causing a depletion of refractory elements in the gas accreted by the star in the late stages of its formation; ii) planet engulfment, enriching the host star in lithium and refractory elements. We discuss both planet signatures, the influence of galactic chemical evolution, and the importance of binaries composed of stellar twins as laboratories to verify abundance anomalies imprinted by planets., Comment: The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun. Edited by G. A. Feiden

The discovery of lithium-rich giants contradicts expectations from canonical stellar evolution. Here we report on the serendipitous discovery of 20 Li-rich giants observed during the Gaia-ESO Survey, which includes the first nine Li-rich giant stars known towards the CoRoT fields. Most of our Li-rich giants have near-solar metallicities, and stellar parameters consistent with being before the luminosity bump. This is difficult to reconcile with deep mixing models proposed to explain lithium enrichment, because these models can only operate at later evolutionary stages: at or past the luminosity bump. In an effort to shed light on the Li-rich phenomenon, we highlight recent evidence of the tidal destruction of close-in hot Jupiters at the sub-giant phase. We note that when coupled with models of planet accretion, the observed destruction of hot Jupiters actually predicts the existence of Li-rich giant stars, and suggests Li-rich stars should be found early on the giant branch and occur more frequently with increasing metallicity. A comprehensive review of all known Li-rich giant stars reveals that this scenario is consistent with the data. However more evolved or metal-poor stars are less likely to host close-in giant planets, implying that their Li-rich origin requires an alternative explanation, likely related to mixing scenarios rather than external phenomena., This work is based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the $\textit{Gaia}$-ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and the Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR) in the form of the grant Premiale VLT 2012 and the Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies (prot. 2010LY5N2T). The results presented here benefit from discussions held during the $\textit{Gaia}$-ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. GR acknowledges support from the project grant ‘The New Milky Way’ from the Knut and Alice Wallenberg Foundation. GMK is supported by the Royal Society as a Royal Society University Research Fellow. LS acknowledges support provided by the Chilean Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to the Millennium Institute of Astrophysics, MAS. Funding for the Stellar Astrophysics Centre is provided by the Danish National Research Foundation. The research is supported by the ASTERISK project (ASTERoseismic Investigations with SONG and Kepler) funded by the European Research Council (Grant agreement no. 267864). VSA acknowledges support from VILLUM FONDEN (research grant 10118). SLM acknowledges support from the Australian Research Council through DECRA fellowship DE140100598. TM acknowledges financial support from Belspo for contract PRODEX Gaia-DPAC. JM acknowledges the support from the European Research Council Consolidator Grant funding scheme (project STARKEY, G.A. no. 615604). This research has made use of the ExoDat Database, operated at LAM-OAMP, Marseille, France, on behalf of the CoRoT/Exoplanet programme. This research made use of astropy, a community-developed core python package for Astronomy (Astropy Collaboration et al. 2013). This research has made use of NASA's Astrophysics Data System Bibliographic Services., This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw1512

With the present state of exoplanet detection techniques, none of the rocky planets of the Solar System would be discovered, yet their presence is a very strong constraint on the scenarios of formation of planetary systems. Astrometry, by measuring the reflex effect of planets on their central host stars, lead us to the mass of planets and to their orbit determination. This technique is used frequently and is very successful to determine the masses and the orbits of binary stars. From space, it is possible to use differential astrometry around nearby Solar-type stars to detect exoplanets down to one Earth mass in habitable zone, where the sensitivity of the technique is optimal. Finding habitable Earths in the Solar neighborhood would be a major step forward for exoplanet detection and these planets would be prime targets for attempting to find life outside of the Solar System, by searching for bio-markers in their atmospheres.A scientific consortium has formed to promote this kind of astrometric space mission. A mission called NEAT (Nearby Earth Astrometric Telescope) has been proposed to ESA in 2010. A laboratory testbed called NEAT-demo was assembled at IPAG, its main goal is to demonstrate CCD detector calibration to the required accuracy. During my PhD, my activities were related to astrophysical aspects as well as instrumental aspects of the mission. Regarding the scientific case, I compiled a catalog of mission target stars and reference stars (needed for the differential astrometric measurements) and I estimated the scientific return of NEAT-like missions in terms of number of detected exoplanets and their parameter distributions. The second aspect of the PhD is relative to the testbed, which mimics the NEAT telescope configuration. I am going to present the testbed itself, the data analysis methods and the results. An accuracy of 3e-4 pixel was obtained for the relative positions of artificial stars and we have determined that measures of pixel positions by the metrology is currently limited by stray light., Dans l’état actuel des techniques de détection des exoplanètes, aucune planète tellurique du Système Solaire ne pourrait être découverte et pourtant leur présence est une contrainte très forte sur les scénarios de formation des systèmes planétaires. L’astrométrie, en mesurant l’effet réflexe des planètes sur leur étoile centrale, permet de remonter à la masse des planètes et aux paramètres orbitaux. C'est une technique très utilisée pour la détermination des masses et des orbites des étoiles binaires et couronnée de succès. Depuis l'espace, il est possible d'utiliser l'astrométrie différentielle autour des étoiles proches de type solaire pour détecter leurs exoplanètes à partir d'une masse terrestre en zone habitable, là ou la sensibilité de cette technique est optimale. Trouver des Terres habitables au voisinage du système solaire constituerait une avancée majeure en terme de détection d'exoplanètes et ces planètes seraient des cibles de 1er choix pour tenter de détecter de la vie au delà du système solaire, en cherchant des bio-marqueurs dans leur atmosphère.Un consortium scientifique s'est formé pour promouvoir une telle mission spatiale astrométrique. Une mission appelée NEAT (Nearby Earth Astrometric Telescope) a été proposée à l'ESA en 2010. Un banc de test appelé NEAT-demo a été assemblé à l'IPAG, son but principal est de démontrer la calibration du détecteur CCD à la précision requise. Mes activités pendant la thèse se sont portées à la fois sur les aspects astrophysiques et instrumentaux de la mission. En ce qui concerne le cas scientifique, j'ai établi un catalogue des étoiles cibles de la mission et des étoiles de référence (pour la mesure astrométrique différentielle), puis estimé le retour scientifique d'une mission spatiale telle que NEAT en terme de nombre d'exoplanètes détectées et de la distribution de leurs paramètres. Le second volet de la thèse est centré autour du banc de test, qui imite la configuration du télescope spatial NEAT. Je vais présenter le banc lui-même, les méthodes d'analyse des données et les résultats obtenus. Une précision de 3e-4 pixel a été obtenue sur les positions relatives des étoiles artificielles et nous avons déterminé que la mesure de position des pixels par la métrologie est limité par la lumière parasite.

Astronomy and Astrophysics 555 (2013): A11 reproduced with permission from Astronomy & Astrophysics, Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 μm were obtained, complemented in some cases with observations at 70 μm, and at 250, 350 and 500 μm using SPIRE. The observing strategy was to integrate as deep as possible at 100 μm to detect the stellar photosphere. Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of ∼12.1% ± 5% before Herschel to ∼20.2% ± 2%. A significant fraction (∼52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70–160 μm range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age., This work was supported by the Spanish grants AYA2008-01727 and AYA2011-26202, the Marie Curie Actions of the European Commision (FT7-COFUND), AYA2008-02038 and AYA2011-30147-C03-03

We present Herschel spatially resolved images of the debris disc orbiting the subgiant Kappa CrB. Not only are these the first resolved images of a debris disc orbiting a subgiant, but Kappa CrB is a rare example of an intermediate mass star where a detailed study of the structure of the planetary system can be made, including both planets and planetesimal belt(s). The only way to discover planets around such stars using the radial velocity technique is to observe 'retired' A stars, which are cooler and slower rotators compared to their main-sequence counterparts. A planetary companion has already been detected orbiting the subgiant Kappa CrB, with revised parameters of m sin i = 2.1MJ and apl = 2.8AU (Johnson et al. 2008a). We present additional Keck I HIRES radial velocity measurements that provide evidence for a second planetary companion, alongside Keck II AO imaging that places an upper limit on the mass of this companion. Modelling of our Herschel images shows that the dust is broadly distributed, but cannot distinguish between a single wide belt (from 20 to 220AU) or two narrow dust belts (at around 40 and 165AU). Given the existence of a second planetary companion beyond approximately 3AU it is possible that the absence of dust within approximately 20AU is caused by dynamical depletion, although the observations are not inconsistent with depletion of these regions by collisional erosion, which occurs at higher rates closer to the star., Updated abstract

We identify a set of 18 solar analogs among the seismic sample of solar-like stars observed by the Kepler satellite rotating between 10 and 40 days. This set is constructed using the asteroseismic stellar properties derived using either the global oscillation properties or the individual acoustic frequencies. We measure the magnetic activity properties of these stars using observations collected by the photometric Kepler satellite and by the ground-based, high-resolution Hermes spectrograph mounted on the Mercator telescope. The photospheric (Sph) and chromospheric (S index) magnetic activity levels of these seismic solar analogs are estimated and compared in relation to the solar activity. We show that the activity of the Sun is comparable to the activity of the seismic solar analogs, within the maximum-to-minimum temporal variations of the 11-year solar activity cycle 23. In agreement with previous studies, the youngest stars and fastest rotators in our sample are actually the most active. The activity of stars older than the Sun seems to not evolve much with age. Furthermore, the comparison of the photospheric, Sph, with the well-established chromospheric, S index, indicates that the Sph index can be used to provide a suitable magnetic activity proxy which can be easily estimated for a large number of stars from space photometric observations., Comment: Accepted for publication in A&A

In order to detect and characterise cold extended circumstellar dust originating from collisions of planetesimal bodies in disks, belts, or rings at Kuiper-Belt distances (30-50 AU or beyond) sensitive submillimetre observations are essential. Measurements of the flux densities at these wavelengths will extend existing IR photometry and permit more detailed modelling of the Rayleigh-Jeans tail of the disks spectral energy distribution (SED), effectively constraining dust properties and disk extensions. By observing stars spanning from a few up to several hundred Myr, the evolution of debris disks during crucial phases of planet formation can be studied. We have performed 870-micron observations of 22 exo-Kuiper-Belt candidates, as part of a Large Programme with the LABOCA bolometer at the APEX telescope. Dust masses (or upper limits) were calculated from integrated 870-micron fluxes, and fits to the SED of detected sources revealed the fractional dust luminosities f_dust, dust temperatures T_dust, and power-law exponents beta of the opacity law. A total of 10 detections with at least 3-sigma significance were made, out of which five (HD 95086, HD 131835, HD 161868, HD 170773, and HD 207129) have previously never been detected at submillimetre wavelengths. Three additional sources are marginally detected with >2.5-sigma significance. The best-fit beta parameters all lie between 0.1 and 0.8, in agreement with previous results indicating the presence of grains that are significantly larger than those in the ISM. From our relatively small sample we estimate f_dust proportional to t^(-alpha), with alpha~0.8-2.0, and identify an evolution of the characteristic radial dust distance R_dust that is consistent with the t^(1/3) increase predicted from models of self-stirred collisions in debris disks., 15 pages, 4 figures, accepted for publication in Astronomy & Astrophysics

We measure planet occurrence rates using the planet candidates discovered by the Q1-Q16 Kepler pipeline search. This study examines planet occurrence rates for the Kepler GK dwarf target sample for planet radii, 0.75, Comment: 19 Pages, 17 Figures, Submitted ApJ. Python source to support Kepler pipeline completeness estimates available at http://github.com/christopherburke/KeplerPORTs/