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See detailConstraints on the structure of 16 Cygni A and 16 Cygni B using inversion techniques
Buldgen, Gaël ULg; Reese, Daniel; Dupret, Marc-Antoine ULg

in Astronomy and Astrophysics (2016)

Abstract Constraining mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution ... [more ▼]

Abstract Constraining mixing processes and chemical composition is a central problem in stellar physics as their impact on stellar age determinations leads to biases in our studies of stellar evolution, galactic history and exoplanetary systems. In two previous papers, we showed how seismic inversion techniques could offer strong constraints on such processes by pointing out weaknesses in theoretical models. We now apply our technique to the solar analogues 16CygA and 16CygB, being amongst the best targets in the Kepler field to test the diagnostic potential of seismic inversions. The combination of various seismic indicators helps to provide more constrained and accurate fundamendal parameters for these stars. We use the latest seismic, spectroscopic and interferometric observational constraints in the litterature for this system to determine reference models independently for both stars. We carry out seismic inversions of the acoustic radius, the mean density and a core conditions indicator. We note that a degeneracy exists for the reference models. Namely, changing the diffusion coefficient or the chemical composition within the observational values leads to 5% changes in mass, 3% changes in radius and up to 8% changes in age. We use acoustic radius and mean density inversions to improve our reference models then carry out inversions for a core conditions indicator. Thanks to its sensitivity to microscopic diffusion and chemical composition mismatches, we are able to reduce the mass dispersion to 2%, namely [0.96, 1.0] M_sun, the radius dispersion to 1%, namely [1.188, 1.200] R_sun and the age dispersion to 3%, namely [7.0, 7.4] Gy, for 16CygA. For 16CygB, we can check the consistency of the models but not reduce independently the age dispersion. Nonetheless, assuming consistency with the age of 16CygA helps to further constrain its mass and radius. [less ▲]

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See detailThe HARPS-N Rocky Planet Search. I. HD 219134 b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun
Motalebi, F.; Udry, S.; Gillon, Michaël ULg et al

in Astronomy and Astrophysics (2015), 584

We know now from radial velocity surveys and transit space missions that planets only a few times more massive than our Earth are frequent around solar-type stars. Fundamental questions about their ... [more ▼]

We know now from radial velocity surveys and transit space missions that planets only a few times more massive than our Earth are frequent around solar-type stars. Fundamental questions about their formation history, physical properties, internal structure, and atmosphere composition are, however, still to be solved. We present here the detection of a system of four low-mass planets around the bright (V = 5.5) and close-by (6.5 pc) star HD 219134. This is the first result of the Rocky Planet Search programme with HARPS-N on the Telescopio Nazionale Galileo in La Palma. The inner planet orbits the star in 3.0935 ± 0.0003 days, on a quasi-circular orbit with a semi-major axis of 0.0382 ± 0.0003 AU. Spitzer observations allowed us to detect the transit of the planet in front of the star making HD 219134 b the nearest known transiting planet to date. From the amplitude of the radial velocity variation (2.25 ± 0.22 ms[SUP]-1[/SUP]) and observed depth of the transit (359 ± 38 ppm), the planet mass and radius are estimated to be 4.36 ± 0.44 M[SUB]⊕[/SUB] and 1.606 ± 0.086 R[SUB]⊕[/SUB], leading to a mean density of 5.76 ± 1.09 g cm[SUP]-3[/SUP], suggesting a rocky composition. One additional planet with minimum-mass of 2.78 ± 0.65 M[SUB]⊕[/SUB] moves on a close-in, quasi-circular orbit with a period of 6.767 ± 0.004 days. The third planet in the system has a period of 46.66 ± 0.08 days and a minimum-mass of 8.94 ± 1.13 M[SUB]⊕[/SUB], at 0.233 ± 0.002 AU from the star. Its eccentricity is 0.46 ± 0.11. The period of this planet is close to the rotational period of the star estimated from variations of activity indicators (42.3 ± 0.1 days). The planetary origin of the signal is, however, thepreferred solution as no indication of variation at the corresponding frequency is observed for activity-sensitive parameters. Finally, a fourth additional longer-period planet of mass of 71 M[SUB]⊕[/SUB] orbits the star in 1842 days, on an eccentric orbit (e = 0.34 ± 0.17) at a distance of 2.56 AU. The photometric time series and radial velocities used in this work are available in electronic form at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/584/A72">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/584/A72</A> [less ▲]

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See detailPolarization microlensing in the quadruply imaged broad absorption line quasar H1413+117
Hutsemekers, Damien ULg; Sluse, Dominique ULg; Braibant, Lorraine ULg et al

in Astronomy and Astrophysics (2015), 584

We have obtained spectropolarimetric observations of the four images of the gravitationally lensed broad absorption line quasar H1413+117. The polarization of the microlensed image D is significantly ... [more ▼]

We have obtained spectropolarimetric observations of the four images of the gravitationally lensed broad absorption line quasar H1413+117. The polarization of the microlensed image D is significantly different, both in the continuum and in the broad lines, from the polarization of image A, which is essentially unaffected by microlensing. The observations suggest that the continuum is scattered off two regions, spatially separated, and producing roughly perpendicular polarizations. These results are compatible with a model in which the microlensed polarized continuum comes from a compact region located in the equatorial plane close to the accretion disk and the non-microlensed continuum from an extended region located along the polar axis. Based on observations made with ESO Telescopes at the Paranal Observatory (Chile). ESO program ID: 386.B-0337. [less ▲]

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See detailTRAPPIST photometry and imaging monitoring of comet C/2013 R1 (Lovejoy): Implications for the origin of daughter species
Opitom, Cyrielle ULg; Jehin, Emmanuel ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (2015), 584

We report the results of the narrow-band photometry and imaging monitoring of comet C/2013 R1 (Lovejoy) with the robotic telescope TRAPPIST (La Silla observatory). We gathered around 400 images over 8 ... [more ▼]

We report the results of the narrow-band photometry and imaging monitoring of comet C/2013 R1 (Lovejoy) with the robotic telescope TRAPPIST (La Silla observatory). We gathered around 400 images over 8 months pre- and post-perihelion between September 12, 2013 and July 6, 2014. We followed the evolution of the OH, NH, CN, C[SUB]3[/SUB], and C[SUB]2[/SUB] production rates computed with the Haser model, as well as the evolution of the dust production. All five gas species display an asymmetry about perihelion, since the rate of brightening is steeper than the rate of fading. The study of the coma morphology reveals gas and dust jets that indicate one or several active zone(s) on the nucleus. The dust, C[SUB]2[/SUB], and C[SUB]3[/SUB] morphologies present some similarities, while the CN morphology is different. OH and NH are enhanced in the tail direction. The study of the evolution of the comet activity shows that the OH, NH, and C[SUB]2[/SUB] production rate evolution with the heliocentric distance is correlated to the dust evolution. The CN and, to a lesser extent, the C[SUB]3[/SUB] do not display such a correlation with the dust. This evidence and the comparison with parent species production rates indicate that C[SUB]2[/SUB] and C[SUB]3[/SUB], on one hand, and OH and NH, on the other, could be - at least partially - released from organic - rich grains and icy grains. On the contrary, all evidences point to HCN being the main parent of CN in this comet. [less ▲]

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See detailPost-coronagraphic tip-tilt sensing for vortex phase masks: the QACITS technique
Huby, Elsa ULg; Baudoz, Pierre; Mawet, Dimitri et al

in Astronomy and Astrophysics (2015), 584

Small inner working angle coronagraphs, like the vortex phase mask, are essential to exploit the full potential of ground-based telescopes in the context of exoplanet detection and characterization ... [more ▼]

Small inner working angle coronagraphs, like the vortex phase mask, are essential to exploit the full potential of ground-based telescopes in the context of exoplanet detection and characterization. However, the drawback of this attractive feature is a high sensitivity to pointing errors, which degrades the performance of the coronagraph. We propose a tip-tilt retrieval technique based on the analysis of the final coronagraphic image, hereafter called Quadrant Analysis of Coronagraphic Images for Tip-tilt Sensing (QACITS). Under the assumption of small phase aberrations, we show that the behaviour of the vortex phase mask can be simply described from the entrance pupil to the Lyot stop plane by Zernike polynomials. This convenient formalism is used to establish the theoretical basis of the QACITS technique. Simulations have been performed to demonstrate the validity and limits of the technique, including the case of a centrally obstructed pupil. The QACITS technique principle is further validated by experimental results in the case of an unobstructed circular aperture. The typical configuration of the Keck telescope (24% central obstruction) has been simulated with additional high order aberrations. In these conditions, our simulations show that the QACITS technique is still adapted to centrally obstructed pupils and performs tip-tilt retrieval with a precision of $5 \times 10^{-2}$ {\lambda}/D when wavefront errors amount to {\lambda}/14 rms and $10^{-2}$ {\lambda}/D for {\lambda}/70 rms errors (with {\lambda} the wavelength and D the pupil diameter). The implementation of the QACITS technique is based on the analysis of the scientific image and does not require any modification of the original setup. Current facilities equipped with a vortex phase mask can thus directly benefit from this technique to improve the contrast performance close to the axis. [less ▲]

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See detailLyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph
Ruane, Garreth J.; Huby, Elsa ULg; Absil, Olivier ULg et al

in Astronomy and Astrophysics (2015), 583

The vortex coronagraph is an optical instrument that precisely removes on-axis starlight allowing for high contrast imaging at small angular separation from the star, thereby providing a crucial ... [more ▼]

The vortex coronagraph is an optical instrument that precisely removes on-axis starlight allowing for high contrast imaging at small angular separation from the star, thereby providing a crucial capability for direct detection and characterization of exoplanets and circumstellar disks. Telescopes with aperture obstructions, such as secondary mirrors and spider support structures, require advanced coronagraph designs to provide adequate starlight suppression. We introduce a phase-only Lyot-plane optic to the vortex coronagraph that offers improved contrast performance on telescopes with complicated apertures. Potential solutions for the European Extremely Large Telescope (E-ELT) are described and compared. Adding a Lyot-plane phase mask relocates residual starlight away from a region of the image plane thereby reducing stellar noise and improving sensitivity to off-axis companions. The phase mask is calculated using an iterative phase retrieval algorithm. Numerically, we achieve a contrast on the order of $10^{-6}$ for a companion with angular displacement as small as $4~\lambda/D$ with an E-ELT type aperture. Even in the presence of aberrations, improved performance is expected compared to either a conventional vortex coronagraph or optimized pupil plane phase element alone. [less ▲]

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See detailDirect exoplanet detection and characterization using the ANDROMEDA method: Performance on VLT/NaCo data
Cantalloube, F.; Mouillet, D.; Mugnier, L. M. et al

in Astronomy and Astrophysics (2015), 582

Context. The direct detection of exoplanets with high-contrast imaging requires advanced data processing methods to disentangle potential planetary signals from bright quasi-static speckles. Among them ... [more ▼]

Context. The direct detection of exoplanets with high-contrast imaging requires advanced data processing methods to disentangle potential planetary signals from bright quasi-static speckles. Among them, angular differential imaging (ADI) permits potential planetary signals with a known rotation rate to be separated from instrumental speckles that are either statics or slowly variable. The method presented in this paper, called ANDROMEDA for ANgular Differential OptiMal Exoplanet Detection Algorithm is based on a maximum likelihood approach to ADI and is used to estimate the position and the flux of any point source present in the field of view. Aims. In order to optimize and experimentally validate this previously proposed method, we applied ANDROMEDA to real VLT/NaCo data. In addition to its pure detection capability, we investigated the possibility of defining simple and efficient criteria for automatic point source extraction able to support the processing of large surveys. Methods. To assess the performance of the method, we applied ANDROMEDA on VLT/NaCo data of TYC-8979-1683-1 which is surrounded by numerous bright stars and on which we added synthetic planets of known position and flux in the field. In order to accommodate the real data properties, it was necessary to develop additional pre-processing and post-processing steps to the initially proposed algorithm. We then investigated its skill in the challenging case of a well-known target, $\beta$ Pictoris, whose companion is close to the detection limit and we compared our results to those obtained by another method based on principal component analysis (PCA). Results. Application on VLT/NaCo data demonstrates the ability of ANDROMEDA to automatically detect and characterize point sources present in the image field. We end up with a robust method bringing consistent results with a sensitivity similar to the recently published algorithms, with only two parameters to be fine tuned. Moreover, the companion flux estimates are not biased by the algorithm parameters and do not require a posteriori corrections. Conclusions. ANDROMEDA is an attractive alternative to current standard image processing methods that can be readily applied to on-sky data. [less ▲]

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See detailEvidence for two spatially separated UV continuum emitting regions in the Cloverleaf broad absorption line quasar
Sluse, Dominique ULg; Hutsemekers, Damien ULg; Anguita, T. et al

in Astronomy and Astrophysics (2015), 582

Testing the standard Shakura-Sunyaev model of accretion is a challenging task because the central region of quasars where accretion takes place is unresolved with telescopes. The analysis of microlensing ... [more ▼]

Testing the standard Shakura-Sunyaev model of accretion is a challenging task because the central region of quasars where accretion takes place is unresolved with telescopes. The analysis of microlensing in gravitationally lensed quasars is one of the few techniques that can test this model, yielding to the measurement of the size and of temperature profile of the accretion disc. We present spectroscopic observations of the gravitationally lensed broad absorption line quasar H1413+117, which reveal partial microlensing of the continuum emission that appears to originate from two separated regions: a microlensed region, corresponding the compact accretion disc; and a non-microlensed region, more extended and contributing to at least 30% of the total UV-continuum flux. Because this extended continuum is occulted by the broad absorption line clouds, it is not associated with the host galaxy, but rather with light scattered in the neighbourhood of the central engine. We measure the amplitude of microlensing of the compact continuum over the rest-frame wavelength range 1000-7000 Å. Following a Bayesian scheme, we confront our measurements to microlensing simulations of an accretion disc with a temperature varying as T ∝ R[SUP]-1/ν[/SUP]. We find a most likely source half-light radius of R[SUB]1/2[/SUB] = 0.61 × 10[SUP]16[/SUP]cm (i.e., 0.002 pc) at 0.18 μm, and a most-likely index of ν = 0.4. The standard disc (ν = 4/3) model is not ruled out by our data, and is found within the 95% confidence interval associated with our measurements. We demonstrate that, for H1413+117, the existence of an extended continuum in addition to the disc emission only has a small impact on the inferred disc parameters, and is unlikely to solve the tension between the microlensing source size and standard disc sizes, as previously reported in the literature. Based on observations made with ESO Telescopes at the Paranal Observatory (Chile). ESO program ID: 386.B-0337.Appendices A and B are available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201526832/olm">http://www.aanda.org</A> [less ▲]

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See detailHigh spectral resolution monitoring of Nova V339 Delphini with TIGRE
De Gennaro Aquino, I.; Schröder, K.-P.; Mittag, M. et al

in Astronomy and Astrophysics (2015), 581

We investigate the early development of the classical nova V339 Del (Nova Delphini 2013) through high-resolution optical spectroscopy. To study the structure of the ejecta, we focus on the evolution of ... [more ▼]

We investigate the early development of the classical nova V339 Del (Nova Delphini 2013) through high-resolution optical spectroscopy. To study the structure of the ejecta, we focus on the evolution of the absorption and emission features and the changes within the line profiles. We obtained spectra with the robotic 1.2 m telescope TIGRE equipped with the HEROS spectrograph (R = 20 000, wavelength coverage from 3800 to 8800 Å). Our data set covers the outburst from 3 until 121 days after discovery. We provide a qualitative analysis of the spectra, describing the line profiles evolution and providing a rich list of identified lines. During the optically thick phase, we detected several blue-shifted absorption features from s-processed elements, whose origin is unclear. The presence of strong lines from C/O and the absence of Neon features confirm that the nature of the central white dwarf is a CO type. The later "nebular" phase spectra show evidence of the non-spherical, inhomogeneous structure of the ejecta. The detailed evolution of the line profiles and appearance of high ionization species (e.g. N III, O III, He II, [Fe VII]) are direct consequences of the re-ionization of the ejecta during the peak of the soft X-ray emission. [less ▲]

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See detailB fields in OB stars (BOB). Detection of a strong magnetic field in the O9.7 V star HD 54879
Castro, N.; Fossati, L.; Hubrig, S. et al

in Astronomy and Astrophysics (2015), 581

The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability ... [more ▼]

The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS 2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD 54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code fastwind results in an effective temperature and a surface gravity of 33 000 ± 1000 K and 4.0 ± 0.1 dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible within the errors. We investigate line-profile variability in HD 54879 by complementing our spectra with spectroscopic data from other recent OB-star surveys. The photospheric lines remain constant in shape between 2009 and 2014, although Hα shows a variable emission. The Hα emission is too strong for a standard O9.7 V and is probably linked to the magnetic field and the presence of circumstellar material. Its normal chemical composition and the absence of photospheric line profile variations make HD 54879 the most strongly magnetic, non-variable single O-star detected to date. Based on observations made with ESO telescopes at the La Silla and Paranal observatories under programme ID 191.D-0255(C, F).Appendix A is available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201425354/olm">http://www.aanda.org</A> [less ▲]

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See detailSimultaneous X-ray and optical spectroscopy of the Oef supergiant λ Cephei
Rauw, Grégor ULg; Hervé, A.; Nazé, Yaël ULg et al

in Astronomy and Astrophysics (2015), 580

Probing the structures of stellar winds is of prime importance for the understanding of massive stars. Based on their optical spectral morphology and variability, it has been suggested that the stars in ... [more ▼]

Probing the structures of stellar winds is of prime importance for the understanding of massive stars. Based on their optical spectral morphology and variability, it has been suggested that the stars in the Oef class feature large-scale structures in their wind. High-resolution X-ray spectroscopy and time-series of X-ray observations of presumably single O-type stars can help us understand the physics of their stellar winds. We have collected XMM-Newton observations and coordinated optical spectroscopy of the O6 Ief star λ Cep to study its X-ray and optical variability and to analyse its high-resolution X-ray spectrum. We investigate the line profile variability of the He ii λ 4686 and Hα emission lines in our time series of optical spectra, including a search for periodicities. We further discuss the variability of the broadband X-ray flux and analyse the high-resolution spectrum of λ Cep using line-by-line fits as well as a code designed to fit the full high-resolution X-ray spectrum consistently. During our observing campaign, the He ii λ 4686 line varies on a timescale of ~18 h. On the contrary, the Hα line profile displays a modulation on a timescale of 4.1 days which is likely the rotation period of the star. The X-ray flux varies on timescales of days and could in fact be modulated by the same 4.1-day period as Hα, although both variations are shifted in phase. The high-resolution X-ray spectrum reveals broad and skewed emission lines as expected for the X-ray emission from a distribution of wind-embedded shocks. Most of the X-ray emission arises within less than 2 R∗ above the photosphere. The properties of the X-ray emission of λ Cep generally agree with the expectations of the wind-embedded shock model. There is mounting evidence for the existence of large-scale structures that modulate the Hα line and about 10% of the X-ray emission of λ Cep. [less ▲]

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See detailHigh-resolution spectra of comet C/2013 R1 (Lovejoy)
Rousselot, P.; Decock, A.; Korsun, P. P. et al

in Astronomy and Astrophysics (2015), 580

Context. High-resolution spectra of comets permit deriving the physical properties of the coma. In the optical range, relative production rates can be computed, and information about isotopic ratios and ... [more ▼]

Context. High-resolution spectra of comets permit deriving the physical properties of the coma. In the optical range, relative production rates can be computed, and information about isotopic ratios and the origin of oxygen atoms can be obtained. <BR /> Aims: The main objective of the work presented here was to obtain information about the chemical composition of comet C/2013 R1 (Lovejoy), a bright and long-period comet that passed perihelion (0.81 au) on 22 December 2013. <BR /> Methods: We used the HARPS-North echelle spectrograph at the 3.5 m telescope TNG to obtain high-resolution spectra of comet C/2013 R1 (Lovejoy) in the optical range immediately after its perihelion passage during four consecutive nights in the period December 23 to 26, 2013. <BR /> Results: Our results demonstrate the ability of HARPS-North to efficiently obtain cometary spectra. Very faint emission lines, such as those of [SUP]15[/SUP]NH[SUB]2[/SUB], have been detected, leading to a rough estimate of the [SUP]14[/SUP]N/[SUP]15[/SUP]N ratio in NH[SUB]2[/SUB]. The [SUP]12[/SUP]C/[SUP]13[/SUP]C ratio was measured in the C[SUB]2[/SUB] lines and is equal to 80 ± 30. The oxygen lines were studied as well (green to red line intensity ratios and widths), confirming that H[SUB]2[/SUB]O is the main parent molecule that photodissociates to produce oxygen atoms. This suggests that this comet has a high CO[SUB]2[/SUB] abundance. Relative production rates for C[SUB]2[/SUB] and NH[SUB]2[/SUB] were computed, but we found no significant deviation from a typical NH[SUB]2[/SUB]/C[SUB]2[/SUB] ratio. Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. [less ▲]

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See detailModels of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints
Lagarde, N.; Miglio, A.; Eggenberger, P. et al

in Astronomy and Astrophysics (2015), 580

Context. The availability of asteroseismic constraints for a large sample of red giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of ... [more ▼]

Context. The availability of asteroseismic constraints for a large sample of red giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of stellar populations. <BR /> Aims: We use a detailed spectroscopic study of 19 CoRoT red giant stars to compare theoretical stellar evolution models to observations of the open cluster NGC 6633 and field stars. <BR /> Methods: In order to explore the effects of rotation-induced mixing and thermohaline instability, we compare surface abundances of carbon isotopic ratio and lithium with stellar evolution predictions. These chemicals are sensitive to extra-mixing on the red giant branch. <BR /> Results: We estimate mass, radius, and distance for each star using the seismic constraints. We note that the Hipparcos and seismic distances are different. However, the uncertainties are such that this may not be significant. Although the seismic distances for the cluster members are self consistent they are somewhat larger than the Hipparcos distance. This is an issue that should be considered elsewhere. Models including thermohaline instability and rotation-induced mixing, together with the seismically determined masses can explain the chemical properties of red giant targets. However, with this sample of stars we cannot perform stringent tests of the current stellar models. Tighter constraints on the physics of the models would require the measurement of the core and surface rotation rates, and of the period spacing of gravity-dominated mixed modes. A larger number of stars with longer times series, as provided by Kepler or expected with Plato, would help ensemble asteroseismology. [less ▲]

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See detailThe LEECH Exoplanet Imaging Survey. Further constraints on the planet architecture of the HR 8799 system (Corrigendum)
Maire, A.-L.; Skemer, A. J.; Hinz, P. M. et al

in Astronomy and Astrophysics (2015), 579

The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system ... [more ▼]

The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia. [less ▲]

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See detailPulsations of rapidly rotating stars. II. Realistic modelling for intermediate-mass stars
Ouazzani, R.-M.; Roxburgh, I. W.; Dupret, Marc-Antoine ULg

in Astronomy and Astrophysics (2015), 579

Context. Very high precision seismic space missions such as CoRoT and Kepler provide the means for testing the modelling of transport processes in stellar interiors. For some stars, such as δ Scuti, γ ... [more ▼]

Context. Very high precision seismic space missions such as CoRoT and Kepler provide the means for testing the modelling of transport processes in stellar interiors. For some stars, such as δ Scuti, γ Doradus, and Be stars, the observed pulsation spectra are modified by rotation to such an extent that it prevents any fruitful interpretation. <BR /> Aims: Our aim is to characterise acoustic pulsation spectra of realistic stellar models in order to be able to interpret asteroseismic data from such stars. <BR /> Methods: The 2D oscillation code ACOR, which treats rotation in a non-perturbative manner, is used to study pulsation spectra of highly distorted evolved models of stars. Two-dimensional models of stars are obtained by a self-consistent method that distorts spherically averaged stellar models a posteriori, at any stage of evolution, and for any type of rotation law. <BR /> Results: Four types of modes are calculated in a very dense frequency spectrum, among which are island modes. The regularity of the island modes spectrum is confirmed and yields a new set of quantum numbers, with which an échelle diagram can be built. Mixed gravito-acoustic modes are calculated in rapidly rotating models for the first time. [less ▲]

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See detailAngular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes
Belkacem, K.; Marques, J. P.; Goupil, M. J. et al

in Astronomy and Astrophysics (2015), 579

The detection of mixed modes in subgiants and red giants by the CoRoT and Kepler space-borne missions allows us to investigate the internal structure of evolved low-mass stars, from the end of the main ... [more ▼]

The detection of mixed modes in subgiants and red giants by the CoRoT and Kepler space-borne missions allows us to investigate the internal structure of evolved low-mass stars, from the end of the main sequence to the central helium-burning phase. In particular, the measurement of the mean core rotation rate as a function of the evolution places stringent constraints on the physical mechanisms responsible for the angular momentum redistribution in stars. It showed that the current stellar evolution codes including the modelling of rotation fail to reproduce the observations. An additional physical process that efficiently extracts angular momentum from the core is thus necessary. Our aim is to assess the ability of mixed modes to do this. To this end, we developed a formalism that provides a modelling of the wave fluxes in both the mean angular momentum and the mean energy equations in a companion paper. In this article, mode amplitudes are modelled based on recent asteroseismic observations, and a quantitative estimate of the angular momentum transfer is obtained. This is performed for a benchmark model of 1.3 M[SUB]⊙[/SUB] at three evolutionary stages, representative of the evolved pulsating stars observed by CoRoT and Kepler. We show that mixed modes extract angular momentum from the innermost regions of subgiants and red giants. However, this transport of angular momentum from the core is unlikely to counterbalance the effect of the core contraction in subgiants and early red giants. In contrast, for more evolved red giants, mixed modes are found efficient enough to balance and exceed the effect of the core contraction, in particular in the hydrogen-burning shell. Our results thus indicate that mixed modes are a promising candidate to explain the observed spin-down of the core of evolved red giants, but that an other mechanism is to be invoked for subgiants and early red giants. [less ▲]

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See detailAngular momentum redistribution by mixed modes in evolved low-mass stars. I. Theoretical formalism
Belkacem, K.; Marques, J. P.; Goupil, M. J. et al

in Astronomy and Astrophysics (2015), 579

Seismic observations by the space-borne mission Kepler have shown that the core of red giant stars slows down while evolving, requiring an efficient physical mechanism to extract angular momentum from the ... [more ▼]

Seismic observations by the space-borne mission Kepler have shown that the core of red giant stars slows down while evolving, requiring an efficient physical mechanism to extract angular momentum from the inner layers. Current stellar evolution codes fail to reproduce the observed rotation rates by several orders of magnitude and instead predict a drastic spin-up of red giant cores. New efficient mechanisms of angular momentum transport are thus required. In this framework, our aim is to investigate the possibility that mixed modes extract angular momentum from the inner radiative regions of evolved low-mass stars. To this end, we consider the transformed Eulerian mean (TEM) formalism, which allows us to consider the combined effect of both the wave momentum flux in the mean angular momentum equation and the wave heat flux in the mean entropy equation as well as their interplay with the meridional circulation. In radiative layers of evolved low-mass stars, the quasi-adiabatic approximation, the limit of slow rotation, and the asymptotic regime can be applied for mixed modes and enable us to establish a prescription for the wave fluxes in the mean equations. The formalism is finally applied to a 1.3 M[SUB]⊙[/SUB] benchmark model, representative of observed CoRoT and Kepler oscillating evolved stars. We show that the influence of the wave heat flux on the mean angular momentum is not negligible and that the overall effect of mixed modes is to extract angular momentum from the innermost region of the star. A quantitative and accurate estimate requires realistic values of mode amplitudes. This is provided in a companion paper. Appendix A is available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201526042/olm">http://www.aanda.org</A> [less ▲]

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See detailA radio-map of the colliding winds in the very massive binary system HD93129A
Benaglia, Paula; Marcote, Benito; Moldon, Javier et al

in Astronomy and Astrophysics (2015), 579

Radio observations are an effective tool to discover particle acceleration regions in colliding-wind binaries, through detection of synchrotron radiation; these regions are natural laboratories for the ... [more ▼]

Radio observations are an effective tool to discover particle acceleration regions in colliding-wind binaries, through detection of synchrotron radiation; these regions are natural laboratories for the study of relativistic particles.Wind-collision region (WCR) models can reproduce the radio continuum spectra of massive binaries that contain both thermal and non-thermal radio emission; however, key constraints for models come from high-resolution imaging. Only five WCRs have been resolved to date at radio frequencies at milliarcsec (mas) angular scales. The source HD93129A, prototype of the very few known O2 I stars, is a romising target for study: recently, a second massive, early-type star about 50 mas away was discovered, and a non-thermal radio source detected in the region. Preliminary long-baseline array data suggest that a significant fraction of the radio emission from the system comes from a putative WCR. We sought evidence that HD93129A is a massive binary system with colliding stellar winds that produce non-thermal radiation, through spatially resolved images of the radio emitting regions. Methods.We completed observations with the Australian Long Baseline Array (LBA) to resolve the system at mas angular resolutions and reduced archival Australia Telescope Compact Array (ATCA) data to derive the total radio emission. We also compiled optical astrometric data of the system in a homogeneous way. We reduced historical Hubble Space Telescope data and obtained absolute and relative astrometry with milliarcsec accuracy. The astrometric analysis leads us to conclude that the two stars in HD93129A form a gravitationally bound system. The LBA data reveal an extended arc-shaped non-thermal source between the two stars, indicative of a WCR. The wind momentum-rate ratio of the two stellar winds is estimated. The ATCA data show a point source with a change in flux level between 2003-4 and 2008-9, that is modeled with a non-thermal power-law spectrum with spectral indices of - 1:03 +/- 0:09 and -1:21 +/- 0:03 respectively. The mass-loss rates derived from the deduced thermal radio emission and from the characteristics of the WCR are consistent with estimates derived by other authors. [less ▲]

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See detailAsymmetric features in the protoplanetary disk MWC 758
Benisty, M.; Juhasz, A.; Boccaletti, A. et al

in Astronomy and Astrophysics (2015)

Context. The study of dynamical processes in protoplanetary disks is essential to understand planet formation. In this context, transition disks are prime targets because they are at an advanced stage of ... [more ▼]

Context. The study of dynamical processes in protoplanetary disks is essential to understand planet formation. In this context, transition disks are prime targets because they are at an advanced stage of disk clearing and may harbor direct signatures of disk evolution. Aims: We aim to derive new constraints on the structure of the transition disk MWC 758, to detect non-axisymmetric features and understand their origin. Methods: We obtained infrared polarized intensity observations of the protoplanetary disk MWC 758 with VLT/SPHERE at 1.04 μm to resolve scattered light at a smaller inner working angle (0.093'') and a higher angular resolution (0.027'') than previously achieved. Results: We observe polarized scattered light within 0.53'' (148 au) down to the inner working angle (26 au) and detect distinct non-axisymmetric features but no fully depleted cavity. The two small-scale spiral features that were previously detected with HiCIAO are resolved more clearly, and new features are identified, including two that are located at previously inaccessible radii close to the star. We present a model based on the spiral density wave theory with two planetary companions in circular orbits. The best model requires a high disk aspect ratio (H/r ~ 0.20 at the planet locations) to account for the large pitch angles which implies a very warm disk. Conclusions: Our observations reveal the complex morphology of the disk MWC 758. To understand the origin of the detected features, the combination of high-resolution observations in the submillimeter with ALMA and detailed modeling is needed. [less ▲]

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See detailB fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439
Hubrig, S.; Schöller, M.; Fossati, L. et al

in Astronomy and Astrophysics (2015), 578

<BR /> Aims: Massive B-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. Only two such stars, called σ ... [more ▼]

<BR /> Aims: Massive B-type stars with strong magnetic fields and fast rotation are very rare and pose a mystery for theories of star formation and magnetic field evolution. Only two such stars, called σ Ori E analogues, were known until recently. A team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD 23478 and HD 345439. The magnetic fields in these newly discovered σ Ori E analogues have not been investigated so far. <BR /> Methods: In the framework of our ESO Large Programme and one normal ESO programme, we carried out low-resolution FORS 2 spectropolarimetric observations of HD 23478 and HD 345439. <BR /> Results: In the measurements of hydrogen lines, we discover a rather strong longitudinal magnetic field of up to 1.5 kG in HD 23478 and up to 1.3 kG using the entire spectrum. The analysis of HD 345439 using four subsequent spectropolarimetric subexposures does not reveal a magnetic field at a significance level of 3σ. On the other hand, individual subexposures indicate that HD 345439 may host a strong magnetic field that rapidly varies over 88 min. The fast rotation of HD 345439 is also indicated by the behaviour of several metallic and He i lines in the low-resolution FORS 2 spectra that show profile variations already on this short time-scale. Based on observations obtained in the framework of the ESO Prgs. 191.D-0255(E) and 094.D-0355(B).Appendix A is available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201526262/olm">http://www.aanda.org</A> [less ▲]

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