Nouvelles observations sur la glaciation des montagnes du nord-ouest du Portugal

[Resumé] Les observations réalisées récemment par les auteurs confirment en les nuançant~ les conclusions antérieures de G. Coudé-Gaussen et ouvrent de nouvelles perspectives d'étude. Le haut plateau isolé de la Serra do Larouco (1527 m) et les Cotos de Fonte Fria qui attei gnent 1456 m Vers l'extrémité orientale du Geré n'ont révélé aucune trace d'englacement en territoire portugais mais seulement d'imposants pavages superficiels accompagnés de quelques bourrelets ou de niches ce qui implique une importante nivation mais sans formation de glace. Une route en construction a montré l'existence dans la profonde vallée de Gavieira (Serra da Peneda) de deux gros bourre lets de moraine frontale entaillés jusqu'a 580 et 505 m par le Rio Grande. C'est peut-etre l'altitude la plus basse atteinte par la glace dans le Nord-Ouest de la Péninsule Iberique. Sur le versant nord-ouest de la montagne la °langue glaciaire isolée de Corga dos Cortelhos ne descendait qu'a environ 750 m. Cette dissymétrie démontre l 'importance de la suralimentation neigeuse par les vents d'Ouest qui balayaient les croupes sommitales (14l5 m) dominant un large plateau établi Vers 1200 m. Plus al 'Ouest~ la Serra de Arga (823 m) n'a pas été englacée~ en dépit d'un modelé tres favorable. Ces faits confirment~ en accentuant légerement les schémas antérieurs~ la rapide montée d'Ouest en Est de la limite régionale des neiges persistantes; située Vers 1100 m dans l'Ouest de la Serra da Peneda~ Vers 1200 m dans l 'Ouest du Geres, elle atteignait 1400 m a l'Est du massif et environ 1500 m au Larouco. Les indices d'une possible glaciation antérieurede plus large extension et la distinction de plusieurs phases au cours de la seule glaciation actuellement démontrée exigeront de nouvelles recherches sur le terrain~ a propos desquelles queZques réfZexions méthodologiques sont présentées.[Resumen] Nuevas observaciones sobre la glaciación de las montañas del Noroeste de Portugal. Las observaciones recientemente hechas por los autores confirman y matizan las conclusiones anteriores de G. Coudé-Gaussen e abrem nuevas persvectivas de estudio. El altiplano aislado de la Serra do Larouco (1527 m) y los Cotos de Fonte Fria que alcanzan 1456 m cerca de la extremidad oriental del Geres no muestran ningún rasgo de cobertura de hielo en territorio portugués, mas solamente imponentes lanchas acompañadas de algunos caballones y nichos lo que indica una importante nivación pero sin formación de hielo. La carretera en construción en el hondo valle de Gavieira (Serra da Peneda) enseña dos grandes arcos de morrena terminal cortados hasta 580 m y 505 m por el Rio Grande. Es posiblemente la altitud la más baja alcanzada por el hielo en todo el Noroeste de la Península Ibérica. En la vertiente NW de la montaña la lengua glaciaria aislada de Corga dos Cortelhos no baja a más de cerca 750 m. Esta disimetría muestra la importancia de la sobrealimentación en nieve por los vientos del Oeste desde las cumbres (1416 m)~ que dominan un ancho altiplano situado a cerca de 1200 m. Más al Oeste la Serra de Arga (823 m) pese a un relieve muy favorable no estuvo cubierta por el hie lo. Estos hechos confirman acentuando ligeramente los esquemas'anteriores la rápida subida de Oeste para Este del limite regional de las nieves perpetuas; situado cerca de 1100 m al Oeste de la Serra da Peneda cerca de 1200 m al Oeste del Geres alcanza 1400 m al Este del mismo macizo y cerca de 1500 m en Larouco. Los indicios de una posible glaciación anterior más extensa y distinción de varias fases durante la única glaciación hasta hora reconocida exigen nuevas investigaciones de campo a propósito de las quales se plantean algunas reflexiones metodológicas

Estimating the phase in ground-based interferometry: performance comparison between single-mode and multimode schemes

In this paper we compare the performance of multi and single-mode interferometry for the estimation of the phase of the complex visibility. We provide a theoretical description of the interferometric signal which enables to derive the phase error in presence of detector, photon and atmospheric noises, for both multi and single-mode cases. We show that, despite the loss of flux occurring when injecting the light in the single-mode component (i.e. single-mode fibers, integrated optics), the spatial filtering properties of such single-mode devices often enable higher performance than multimode concepts. In the high flux regime speckle noise dominated, single-mode interferometry is always more efficient, and its performance is significantly better when the correction provided by adaptive optics becomes poor, by a factor of 2 and more when the Strehl ratio is lower than 10%. In low light level cases (detector noise regime), multimode interferometry reaches better performance, yet the gain never exceeds 20%, which corresponds to the percentage of photon loss due to the injection in the guides. Besides, we demonstrate that single-mode interferometry is also more robust to the turbulence in both cases of fringe tracking and phase referencing, at the exception of narrow field of views (<1 arcsec).Comment: 9 pages (+ 11 online material appendices) -- 8 Figures. Accepted in A&

Photometric stability analysis of the Exoplanet Characterisation Observatory

Photometric stability is a key requirement for time-resolved spectroscopic observations of transiting extrasolar planets. In the context of the Exoplanet Characterisation Observatory (EChO) mission design, we here present and investigate means of translating spacecraft pointing instabilities as well as temperature fluctuation of its optical chain into an overall error budget of the exoplanetary spectrum to be retrieved. Given the instrument specifications as of date, we investigate the magnitudes of these photometric instabilities in the context of simulated observations of the exoplanet HD189733b secondary eclipse.Comment: submitted to MNRA

Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band

Our objective is to prove that integrated optics (IO) is not only a good concept for astronomical interferometry but also a working technique with high performance. We used the commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner which now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 0.005 for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a new astronomical prodecure for calibrating diameters of simple stars by simultaneously fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors of the order of previous ones but without the need of an already known calibrator. These results are an important step of integrated optics and paves the road to incoming imaging interferometer projects

A near-infrared interferometric survey of debris disk stars. I. Probing the hot dust content around epsilon Eridani and tau Ceti with CHARA/FLUOR

We probed the first 3AU around tau Ceti and epsilon Eridani with the CHARA array (Mt Wilson, USA) in order to gauge the 2micron excess flux emanating from possible hot dust grains in the debris disks and to also resolve the stellar photospheres. High precision visibility amplitude measurements were performed with the FLUOR single mode fiber instrument and telescope pairs on baselines ranging from 22 to 241m of projected length. The short baseline observations allow us to disentangle the contribution of an extended structure from the photospheric emission, while the long baselines constrain the stellar diameter. We have detected a resolved emission around tau Cet, corresponding to a spatially integrated, fractional excess flux of 0.98 +/- 0.21 x 10^{-2} with respect to the photospheric flux in the K'-band. Around eps Eri, our measurements can exclude a fractional excess of greater than 0.6x10^{-2} (3sigma). We interpret the photometric excess around tau Cet as a possible signature of hot grains in the inner debris disk and demonstrate that a faint, physical or background, companion can be safely excluded. In addition, we measured both stellar angular diameters with an unprecedented accuracy: Theta_LD(tau Cet)= 2.015 +/- 0.011 mas and Theta_LD(eps Eri)=2.126 +/- 0.014 mas.Comment: 8 pages, 5 figures, to appear in Astronomy and Astrophysic

Asteroseismology and interferometry of the red giant star epsilon Oph

The GIII red giant star epsilon Oph has been found to exhibit several modes of oscillation by the MOST mission. We interpret the observed frequencies of oscillation in terms of theoretical radial p-mode frequencies of stellar models. Evolutionary models of this star, in both shell H-burning and core He-burning phases of evolution, are constructed using as constraints a combination of measurements from classical ground-based observations (for luminosity, temperature, and chemical composition) and seismic observations from MOST. Radial frequencies of models in either evolutionary phase can reproduce the observed frequency spectrum of epsilon Oph almost equally well. The best-fit models indicate a mass in the range of 1.85 +/- 0.05 Msun with radius of 10.55 +/- 0.15 Rsun. We also obtain an independent estimate of the radius of epsilon Oph using high accuracy interferometric observations in the infrared K' band, using the CHARA/FLUOR instrument. The measured limb darkened disk angular diameter of epsilon Oph is 2.961 +/- 0.007 mas. Together with the Hipparcos parallax, this translates into a photospheric radius of 10.39 +/- 0.07 Rsun. The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic

EChOSim: The Exoplanet Characterisation Observatory software simulator

EChOSim is the end-to-end time-domain simulator of the Exoplanet Characterisation Observatory (EChO) space mission. EChOSim has been developed to assess the capability EChO has to detect and characterize the atmospheres of transiting exoplanets, and through this revolutionize the knowledge we have of the Milky Way and of our place in the Galaxy. Here we discuss the details of the EChOSim implementation and describe the models used to represent the instrument and to simulate the detection. Software simulators have assumed a central role in the design of new instrumentation and in assessing the level of systematics affecting the measurements of existing experiments. Thanks to its high modularity, EChOSim can simulate basic aspects of several existing and proposed spectrometers for exoplanet transits, including instruments on the Hubble Space Telescope and Spitzer, or ground-based and balloon borne experiments. A discussion of different uses of EChOSim is given, including examples of simulations performed to assess the EChO mission

Circumstellar material in the Vega inner system revealed by CHARA/FLUOR

Only a handful of debris disks have been imaged up to now. Due to the need for high dynamic range and high angular resolution, very little is known about the inner planetary region, where small amounts of warm dust are expected to be found. We investigate the close neighbourhood of Vega with the help of infrared stellar interferometry and estimate the integrated K-band flux originating from the central 8 AU of the debris disk. We performed precise visibility measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to separately resolve the emissions from the extended debris disk (short baselines) and from the stellar photosphere (long baselines). After revising Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant deficit in squared visibility (1.88+/-0.34%) is detected at short baselines with respect to the best-fit uniform disk stellar model. This deficit can be either attributed to the presence of a low-mass stellar companion around Vega, or as the signature of the thermal and scattered emissions from the debris disk. We show that the presence of a close companion is highly unlikely, as well as other possible perturbations (stellar morphology, calibration), and deduce that we have most probably detected the presence of dust in the close neighbourhood of Vega. The resulting flux ratio between the stellar photosphere and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view (~7.8 AU). Finally, we complement our K-band study with archival photometric and interferometric data in order to evaluate the main physical properties of the inner dust disk. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.Comment: A&A, accepted -- Press release available at http://www2.cnrs.fr/presse/communique/848.ht

The angular diameter and distance of the Cepheid Zeta Geminorum

Cepheids are the primary distance indicators for extragalactic astronomy and therefore are of very high astrophysical interest. Unfortunately, they are rare stars, situated very far from Earth.Though they are supergiants, their typical angular diameter is only a few milliarcseconds, making them very challenging targets even for long-baseline interferometers. We report observations that were obtained in the K prime band (2-2.3 microns), on the Cepheid Zeta Geminorum with the FLUOR beam combiner, installed at the IOTA interferometer. The mean uniform disk angular diameter was measured to be 1.64 +0.14 -0.16 mas. Pulsational variations are not detected at a significant statistical level, but future observations with longer baselines should allow a much better estimation of their amplitude. The distance to Zeta Gem is evaluated using Baade-Wesselink diameter determinations, giving a distance of 502 +/- 88 pc.Comment: 8 pages, 3 figure

Study of molecular layers in the atmosphere of the supergiant star mu Cep by interferometry in the K band

Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant mu Cep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter 14.11+/-0.60 mas is surrounded by a molecular layer of diameter 18.56+/-0.26 mas, with an optical thickness varying from nearly zero at 2.15 microns to >1 at 2.39 microns. Although mu Cep and alpha Ori have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants.Comment: 9 pages, 2 figures, to appear in A&