4,237 research outputs found
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 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 , 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 Ly, H 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, , clustered around the central galaxies may play a major role in generating
spatially extended Ly, continuum () and H
halos. We apply the analytic formalism developed in Mas-Ribas & Dijkstra (2016)
to model the halos around Lyman Alpha Emitters (LAEs) at , for several
different satellite clustering prescriptions. In general, our UV and Ly
surface brightness profiles match the observations well at 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 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 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
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