47 research outputs found
Soft L_e-L_mu-L_tau flavour symmetry breaking and sterile neutrino keV Dark Matter
We discuss how a flavour symmetry that is softly broken
leads to keV sterile neutrinos, which are a prime candidate for Warm Dark
Matter. This is to our knowledge the first model where flavour symmetries are
applied simultaneously to active and sterile neutrinos explaining at the same
time active neutrino properties and this peculiar Dark Matter scenario. The
essential point is that different scales of the symmetry breaking and the
symmetry preserving entries in the mass matrix lead to one right-handed
neutrino which is nearly massless compared to the other two. Furthermore, we
naturally predict vanishing and maximal , while the
correct value of must come from the mixing of the charged
leptons. We can furthermore predict an exact mass spectrum for the light
neutrinos, which will be testable in the very near future.Comment: 14 page
The Faint Cepheids of the Small Magellanic Cloud: an evolutionary selection effect?
Two problems about the faintest Small Magellanic Cloud (SMC) Cepheids are
addressed. On one hand evolutionary tracks fail to cross the Cepheid
Instability Strip for the highest magnitudes (i.e. I-mag~17) where Cepheids are
observed; Mass-Luminosity relations (ML) obtained from evolutionary tracks
disagree with Mass-Luminosity relations derived from observations. We find that
the above failures concern models built with standard input physics as well as
with non-standard ones. The present work suggests that towards highest
magnitudes, Cepheids stars undergo a selection effect caused by evolution: only
the most metal poor stars cross the Instability Strip during the ``blue loop''
phase and are therefore the only ones which can be observed at low luminosity.
This solution enables us to reproduce the shape of the lower part of the
Instability Strip and improves the agreement between observed and theoretical
ML-relations. Some issues are discussed, among them Beat Cepheids results argue
strongly in favor of our hypothesis.Comment: 13 pages, 8 figure
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure
Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment
The OLYMPUS collaboration reports on a precision measurement of the
positron-proton to electron-proton elastic cross section ratio, ,
a direct measure of the contribution of hard two-photon exchange to the elastic
cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams
were directed through a hydrogen gas target internal to the DORIS storage ring
at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and
time-of-flight scintillators detected elastically scattered leptons in
coincidence with recoiling protons over a scattering angle range of to . The relative luminosity between the two beam species
was monitored using tracking telescopes of interleaved GEM and MWPC detectors
at , as well as symmetric M{\o}ller/Bhabha calorimeters at
. A total integrated luminosity of 4.5~fb was collected. In
the extraction of , radiative effects were taken into account
using a Monte Carlo generator to simulate the convolutions of internal
bremsstrahlung with experiment-specific conditions such as detector acceptance
and reconstruction efficiency. The resulting values of , presented
here for a wide range of virtual photon polarization ,
are smaller than some hadronic two-photon exchange calculations predict, but
are in reasonable agreement with a subtracted dispersion model and a
phenomenological fit to the form factor data.Comment: 5 pages, 3 figures, 2 table
Convective Core Mixing: a Metallicity Dependence?
The main scope of this paper is to investigate the possible existence of a
metallicity dependence of the overshooting from main sequence stars turbulent
cores. We focus on objects with masses in the range ~2.5 Msol - ~25 Msol.
Basically, evolutionary time scale ratios are compared with star numbers ratios
on the main sequence. Star populations are synthesized using grids of
evolutionary tracks computed with various overshooting amounts. Observational
material is provided by the large and homogeneous photometric database of OGLE
2 project for the Magellanic clouds. Attention is paid to the study of
uncertainties: distance modulus, intergalactic and interstellar reddening, IMF
slope and average binarity rate. Rotation and chemical composition gradient are
also considered. The result for the overshooting distance is l_over(SMC)= 0.40
+0.12-0.06 Hp (Z=0.004) and l_over(LMC)= 0.10+0.17-0.10 Hp (Z=0.008) suggesting
a possible dependence of the extent of the mixed central regions with
metallicity within the considered mass range. Unfortunately it is not yet
possible to fully disentangle effects of mass and chemical composition.Comment: 13 pages, 8 figures, accepted in Astronomy & Astrophysic
Seismic constraints on open clusters
We derive knowledge on the global and structural parameters of low-mass stars
using asteroseismology and taking advantage of the stellar collective behavior
within open clusters. We build stellar models and compute the seismic signal
expected from main sequence objects in the 0.8-1.6 Msun range. We first
evaluate apparent magnitudes and oscillations-induced luminosity fluctuations
expected in the Hyades, the Pleiades and the alpha Persei clusters. The closest
cluster presents a feasible challenge to observational asteroseismology in the
present and near future. We combine seismological and classical computations to
address three questions: what can be inferred about 1) mass, 2) composition and
3) extension of outer convection zones of solar analogs in the Hyades. The
first issue relies on the strong sensitivity of the large separation to mass.
Then large separations and second differences are used to respectively
constrain metal and helium fractions in the Hyades.When plotted for several
masses, the relation of effective temperature vs large separation is found to
be strongly dependent on the metal content. Besides this the second difference
main modulation is related to the second ionization of helium.The second
difference modulations are also partly due to the discontinuity in stellar
stratification at the convective envelope / radiative core transition. They
permit direct insight in the stellar structure. We compute acoustic radii of
the convective bases for different values of the mixing length theoryparameter
alpha_MLT in convection modelling, i.e. different convective efficiency in the
superadiabatic layers. For a given effectivetemperature we show that the
acoustic radius changes with convection efficiency
Updated Nucleosynthesis Constraints on Unstable Relic Particles
We revisit the upper limits on the abundance of unstable massive relic
particles provided by the success of Big-Bang Nucleosynthesis calculations. We
use the cosmic microwave background data to constrain the baryon-to-photon
ratio, and incorporate an extensively updated compilation of cross sections
into a new calculation of the network of reactions induced by electromagnetic
showers that create and destroy the light elements deuterium, he3, he4, li6 and
li7. We derive analytic approximations that complement and check the full
numerical calculations. Considerations of the abundances of he4 and li6 exclude
exceptional regions of parameter space that would otherwise have been permitted
by deuterium alone. We illustrate our results by applying them to massive
gravitinos. If they weigh ~100 GeV, their primordial abundance should have been
below about 10^{-13} of the total entropy. This would imply an upper limit on
the reheating temperature of a few times 10^7 GeV, which could be a potential
difficulty for some models of inflation. We discuss possible ways of evading
this problem.Comment: 40 pages LaTeX, 18 eps figure
Low and intermediate mass star yields: The evolution of carbon abundances
We present a set of low and intermediate mass star yields based on a modeling
of the TP--AGB phase which affects the production of nitrogen and carbon. These
yields are evaluated by using them in a Galaxy Chemical Evolution model, with
which we analyze the evolution of carbon abundances. By comparing the results
with those obtained with other yield sets, and with a large amount of
observational data, we conclude that the model using these yields combined with
those from Woosley & Weaver (1995) for massive stars properly reproduce all the
data. The model reproduces well the increase of C/O with increasing O/H
abundances. Since these massive star yields do not include winds, it implies
that these stellar winds might have a smoother dependence on metallicity than
usually assumed and that a significant quantity of carbon proceeds from LIM
stars.Comment: 21 pages, 11 figures. To be published in Astronomy and Astrophysic