44,111 research outputs found
On the Ages of Exoplanet Host Stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanet host
(EH) stars, observable from the southern hemisphere. We measured the
chromospheric activity index, Rhk. We compiled previously published values of
this index for the observed objects as well as the remaining EH stars in an
effort to better smooth temporal variations and derive a more representative
value of the average chromospheric activity for each object. We used the
average index to obtain ages for the group of EH stars. In addition we applied
other methods, such as: Isochrone, lithium abundance, metallicity and
transverse velocity dispersions, to compare with the chromospheric results. The
kinematic method is a less reliable age estimator because EH stars lie red-ward
of Parenago's discontinuity in the transverse velocity dispersion vs dereddened
B-V diagram. The chromospheric and isochrone techniques give median ages of 5.2
and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The median age of F and
G EH stars derived by the isochrone technique is 1--2 Gyr older than that of
identical spectral type nearby stars not known to be associated with planets.
However, the dispersion in both cases is large, about 2--4 Gyr. We searched for
correlations between the chromospheric and isochrone ages and Lir/L* (the
excess over the stellar luminosity) and the metallicity of the EH stars. No
clear tendency is found in the first case, whereas the metallicy dispersion
seems to slightly increase with age.Comment: 22 pages, 25 figures, A&A accepte
The formation of spiral arms and rings in barred galaxies
In this and in a previous paper (Romero-Gomez et al. 2006) we propose a
theory to explain the formation of both spirals and rings in barred galaxies
using a common dynamical framework. It is based on the orbital motion driven by
the unstable equilibrium points of the rotating bar potential. Thus, spirals,
rings and pseudo-rings are related to the invariant manifolds associated to the
periodic orbits around these equilibrium points. We examine the parameter space
of three barred galaxy models and discuss the formation of the different
morphological structures according to the properties of the bar model. We also
study the influence of the shape of the rotation curve in the outer parts, by
making families of models with rising, flat, or falling rotation curves in the
outer parts. The differences between spiral and ringed structures arise from
differences in the dynamical parameters of the host galaxies. The results
presented here will be discussed and compared with observations in a
forthcoming paper.Comment: 16 pages, 13 figures, accepted in A&A. High resolution version
available at http://www.oamp.fr/dynamique/pap/merce.htm
Synaptic plasticity and cognitive function are disrupted in the absence of Lrp4.
Lrp4, the muscle receptor for neuronal Agrin, is expressed in the hippocampus and areas involved in cognition. The function of Lrp4 in the brain, however, is unknown, as Lrp4-/- mice fail to form neuromuscular synapses and die at birth. Lrp4-/- mice, rescued for Lrp4 expression selectively in muscle, survive into adulthood and showed profound deficits in cognitive tasks that assess learning and memory. To learn whether synapses form and function aberrantly, we used electrophysiological and anatomical methods to study hippocampal CA3-CA1 synapses. In the absence of Lrp4, the organization of the hippocampus appeared normal, but the frequency of spontaneous release events and spine density on primary apical dendrites were reduced. CA3 input was unable to adequately depolarize CA1 neurons to induce long-term potentiation. Our studies demonstrate a role for Lrp4 in hippocampal function and suggest that patients with mutations in Lrp4 or auto-antibodies to Lrp4 should be evaluated for neurological deficits
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