Photospheric activity, rotation and magnetic interaction in LHS 6343 A

Context. The Kepler mission has recently discovered a brown dwarf companion transiting one member of the M4V+M5V visual binary system LHS 6343 AB with an orbital period of 12.71 days. Aims. The particular interest of this transiting system lies in the synchronicity between the transits of the brown dwarf C component and the main modulation observed in the light curve, which is assumed to be caused by rotating starspots on the A component. We model the activity of this star by deriving maps of the active regions that allow us to study stellar rotation and the possible interaction with the brown dwarf companion. Methods. An average transit profile was derived, and the photometric perturbations due to spots occulted during transits are removed to derive more precise transit parameters. We applied a maximum entropy spot model to fit the out-of-transit optical modulation as observed by Kepler during an uninterrupted interval of 500 days. It assumes that stellar active regions consist of cool spots and bright faculae whose visibility is modulated by stellar rotation. Results. Thanks to the extended photometric time series, we refine the determination of the transit parameters and find evidence of spots that are occulted by the brown dwarf during its transits. The modelling of the out-of-transit light curve of LHS 6343 A reveals several starspots rotating with a slightly longer period than the orbital period of the brown dwarf, i.e., 13.13 +- 0.02 days. No signature attributable to differential rotation is observed. We find evidence of a persistent active longitude on the M dwarf preceding the sub- companion point by 100 deg and lasting for at least 500 days. This can be relevant for understanding how magnetic interaction works in low-mass binary and star-planet systems.Comment: 14 pages, 16 figure

Habitable planets around the star Gl 581?

Radial velocity surveys are now able to detect terrestrial planets at habitable distance from M-type stars. Recently, two planets with minimum masses below 10 Earth masses were reported in a triple system around the M-type star Gliese 581. Using results from atmospheric models and constraints from the evolution of Venus and Mars, we assess the habitability of planets Gl 581c and Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ) boundaries determination. We provide simplified formulae to estimate the HZ limits that may be used to evaluate the astrobiological potential of terrestrial exoplanets that will hopefully be discovered in the near future. Planets Gl 581c and 'd' are near, but outside, what can be considered as the conservative HZ. Planet 'c' receives 30% more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. Its habitability cannot however be positively ruled out by theoretical models due to uncertainties affecting cloud properties. Irradiation conditions of planet 'd' are comparable with those of early Mars. Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better candidate for the first exoplanet known to be potentially habitable. A mixture of various greenhouse gases could also maintain habitable conditions on this planet.Comment: Astronomy and Astrophysics (2007) accepted for publicatio

Eclipsing binaries suitable for distance determination in the Andromeda galaxy

The Local Group galaxies constitute a fundamental step in the definition of cosmic distance scale. Therefore, obtaining accurate distance determinations to the galaxies in the Local Group, and notably to the Andromeda Galaxy (M31), is essential to determining the age and evolution of the Universe. With this ultimate goal in mind, we started a project to use eclipsing binaries as distance indicators to M31. Eclipsing binaries have been proved to yield direct and precise distances that are essentially assumption free. To do so, high-quality photometric and spectroscopic data are needed. As a first step in the project, broad band photometry (in Johnson B and V) has been obtained in a region (34'x34') at the North-Eastern quadrant of the galaxy over 5 years. The data, containing more than 250 observations per filter, have been reduced by means of the so-called difference image analysis technique and the DAOPHOT program. A catalog with 236238 objects with photometry in both B and V passbands has been obtained. The catalog is the deepest (V<25.5 mag) obtained so far in the studied region and contains 3964 identified variable stars, with 437 eclipsing binaries and 416 Cepheids. The most suitable eclipsing binary candidates for distance determination have been selected according to their brightness and from the modelling of the obtained light curves. The resulting sample includes 24 targets with photometric errors around 0.01 mag. Detailed analysis (including spectroscopy) of some 5-10 of these eclipsing systems should result in a distance determination to M31 with a relative uncertainty of 2-3% and essentially free from systematic errors, thus representing the most accurate and reliable determination to date.Comment: 12 pages, 9 figures; accepted for publication in A&A; see electronic tables and full resolution images at http://www.am.ub.es/~fvilarde/download/A+A

Fine structure of the age-chromospheric activity relation in solar-type stars I: The Ca II infrared triplet: Absolute flux calibration

Strong spectral lines are useful indicators of stellar chromospheric activity. They are physically linked to the convection efficiency, differential rotation, and angular momentum evolution and are a potential indicator of age. However, for ages > 2 Gyr, the age-activity relationship remains poorly constrained thus hampering its full application. The Ca II infrared triplet (IRT lines) has been poorly studied compared to classical chromospheric indicators. We report in this paper absolute chromospheric fluxes in the three Ca II IRT lines, based on a new calibration tied to up-to-date model atmospheres. We obtain the Ca II IRT absolute fluxes for 113 FGK stars from high signal-to-noise ratio and high-resolution spectra covering an extensive domain of chromospheric activity levels. We perform an absolute continuum flux calibration for the Ca II IRT lines anchored in atmospheric models calculated as an explicit function of effective temperatures, metallicity, and gravities avoiding the degeneracy present in photometric continuum calibrations based solely on color indices. The internal uncertainties achieved for continuum absolute flux calculations are 2\% of the solar chromospheric flux, one order of magnitude lower than photometric calibrations. We gauge the impact of observational errors on the final chromospheric fluxes due to the absolute continuum flux calibration and find that $T_{\rm eff}$ uncertainties are properly mitigated by the photospheric correction leaving [Fe/H] as the dominating factor in the chromospheric flux uncertainty. Across the FGK spectral types, the Ca II IRT lines are sensitive to chromospheric activity. The reduced internal uncertainties reported here enable us to build a new chromospheric absolute flux scale and explore the age-activity relation from the active regime down to very low activity levels and a wide range of $T_{\rm eff}$, mass, [Fe/H], and age.Comment: 12 pages, 12 figures, 6 tables, Accepted for publication on A&A. Abstract edited to comply with arXiv standards regarding the number of character

Small-scale Intensity Mapping: Extended Lyα\alpha, Hα\alpha and Continuum emission as a Probe of Halo Star Formation in High-redshift Galaxies

Lyman alpha halos are observed ubiquitously around star-forming galaxies at high redshift, but their origin is still a matter of debate. We demonstrate that the emission from faint unresolved satellite sources, $M_{\rm UV} \gtrsim -17$, clustered around the central galaxies may play a major role in generating spatially extended Ly$\alpha$, continuum (${\rm UV + VIS}$) and H$\alpha$ halos. We apply the analytic formalism developed in Mas-Ribas & Dijkstra (2016) to model the halos around Lyman Alpha Emitters (LAEs) at $z=3.1$, for several different satellite clustering prescriptions. In general, our UV and Ly$\alpha$ surface brightness profiles match the observations well at $20\lesssim r \lesssim 40$ physical kpc from the centers of LAEs. We discuss how our profiles depend on various model assumptions and how these can be tested and constrained with future H$\alpha$ observations by the James Webb Space Telescope (JWST). Our analysis shows how spatially extended halos constrain (i) the presence of otherwise undetectable satellite sources, (ii) the integrated, volumetric production rates of Ly$\alpha$ and LyC photons, and (iii) their population-averaged escape fractions. These quantities are all directly relevant for understanding galaxy formation and evolution and, for high enough redshifts, cosmic reionization.Comment: 13 pages, 6 figures, edited to match accepted ApJ version. Results unaffected. New descriptive flow-chart figure (Fig.6

Doppler-beaming in the Kepler light curve of LHS 6343 A

Context. Kepler observations revealed a brown dwarf eclipsing the M-type star LHS 6343 A with a period of 12.71 days. In addition, an out-of-eclipse light modulation with the same period and a relative semi-amplitude of 2 x 10^-4 was observed showing an almost constant phase lag to the eclipses produced by the brown dwarf. In a previous work, we concluded that this was due to the light modulation induced by photospheric active regions in LHS 6343 A. Aims. In the present work, we prove that most of the out-of-eclipse light modulation is caused by the Doppler-beaming induced by the orbital motion of the primary star. Methods. We introduce a model of the Doppler-beaming for an eccentric orbit and also considered the ellipsoidal effect. The data were fitted using a Bayesian approach implemented through a Monte Carlo Markov chain method. Model residuals were analysed by searching for periodicities using a Lomb-Scargle periodogram. Results. For the first seven quarters of Kepler observations and the orbit previously derived from the radial velocity measurements, we show that the light modulation of the system outside eclipses is dominated by the Doppler-beaming effect. A period search performed on the residuals shows a significant periodicity of 42.5 +- 3.2 days with a false-alarm probability of 5 x 10^-4, probably associated with the rotational modulation of the primary component.Comment: 6 pages, 7 figure

The morphodynamic responses of artificial embayed beaches to storm events

The morphological changes caused by storm events in two Barcelona beaches were recorded using video monitoring techniques during the period 2001–2006. Changes in shoreline position and configuration and submerged bar position and shape were analyzed during the 25 major storm events that occurred during the study period. Beach responses to storms were grouped into three categories: shoreline advance or retreat (including rotation), sandbar migration and/or configuration change (linear or crescentic shape) and formation of megacusps. This work provides examples of the differential adaptation of both beaches to the same storm and of some unexpected morphological responses of both beaches. The response of the beach to storm events is not straightforward because wave conditions are not the only relevant parameter to be considered. In particular, in such embayed beaches it is crucial to take into account their specific morphodynamic configuration prior to the storm