332 research outputs found
Cosmic rays measurements around the knee of the primary spectrum
In this contribution I will summarize and discuss some recent results about the study of the knee of the cosmic rays energy spectrum, indicating that this spectral feature is originated by astrophysical processes. I will then discuss the current experimental efforts that are giving further insights. Latest all particle spectrum measurements have shown that, between 1016eV and 1018eV, the spectrum cannot be described by a single slope power law: an hardening around 1016eV and a steepeningaround 1017eV have been observed. This last feature has been attributed, by the KASCADE-Grande experiment, to the heavy primary component, confirming that the energy of the elemental spectra change of slope increase with the mass of the primary particle
The Stagger-grid: A grid of 3D stellar atmosphere models - IV. Limb darkening coefficients
We compute the emergent stellar spectra from the UV to far infrared for
different viewing angles using realistic 3D model atmospheres for a large range
in stellar parameters to predict the stellar limb darkening. We have computed
full 3D LTE synthetic spectra based on 3D radiative hydrodynamic atmosphere
models from the Stagger-grid. From the resulting intensities at different
wavelength, we derived coefficients for the standard limb darkening laws
considering a number of often-used photometric filters. Furthermore, we
calculated theoretical transit light curves, in order to quantify the
differences between predictions by the widely used 1D model atmosphere and our
3D models. The 3D models are often found to predict steeper limb darkening
compared to the 1D models, mainly due to the temperature stratifications and
temperature gradients being different in the 3D models compared to those
predicted with 1D models based on the mixing length theory description of
convective energy transport. The resulting differences in the transit light
curves are rather small; however, these can be significant for high-precision
observations of extrasolar transits, and are able to lower the residuals from
the fits with 1D limb darkening profiles. We advocate the use of the new limb
darkening coefficients provided for the standard four-parameter non-linear
power law, which can fit the limb darkening more accurately than other choices.Comment: Accepted for publication in A&A, 10 pages, 9 figures, 1 tabl
The convection of close red supergiant stars observed with near-infrared interferometry
Our team has obtained observations of the photosphere of the two closest red
supergiant stars Betelgeuse ( Ori) and Antares ( Sco) using
near infrared interferometry. We have been monitoring the photosphere of
Betelgeuse with the VLTI/PIONIER instrument for three years. On Antares, we
obtained an unprecedented sampling of the visibility function. These data allow
us to probe the convective photosphere of massive evolved stars.Comment: 5 pages, 3 figures. Published in the proceedings of the Physics Of
Evolved Stars conference, dedicated to the memory of Olivier Chesneau (Nice,
France, 2015
Planet transit and stellar granulation detection with interferometry
Aims. We used realistic three-dimensional (3D) radiative hydrodynamical (RHD)
simulations from the Stagger-grid and synthetic images computed with the
radiative transfer code Optim3D to provide interferometric observables to
extract the signature of stellar granulation and transiting planets. Methods.
We computed intensity maps from RHD simulations for twelve interferometric
instruments covering wavelengths ranging from optical to infrared. The stellar
surface asymmetries in the brightness distribution mostly affect closure
phases. We compared the closure phases of the system star with a transiting
planet and the star alone and considered the impact of magnetic spots
constructing a hypothetical starspots image. Results. All the simulations show
departure from the axisymmetric case at all wavelengths. We presented two
possible targets (Beta Com and Procyon) and found that departures up to 16 deg
can be detected on the 3rd lobe and higher. In particular, MIRC is the most
appropriate instrument because it combines good UV coverage and long baselines.
Moreover, we explored the impact of convection on interferometric planet
signature for three prototypes of planets. It is possible to disentangle the
signature of the planet at particular wavelengths (either in the infrared or in
the optical) by comparing the closure phases of the star at difference phases
of the planetary transit. Conclusions. The detection and characterisation of
planets must be based on a comprehensive knowledge of the host star; this
includes the detailed study of the stellar surface convection with
interferometric techniques. In this context, RHD simulations are crucial to
reach this aim. We emphasize that interferometric observations should be pushed
at high spatial frequencies by accumulating observations on closure phases at
short and long baselines.Comment: accepted in Astronomy and Astrophysics, 13 pages. Some figures have
reduced resolution to decrease the size of the output file. Please contact
[email protected] to have the high resolution version of the pape
<3D> NLTE line formation in the atmospheres of red supergiants
Red supergiants with their enormous brightness at J-band are ideal probes of
cosmic chemical composition. It is therefore crucial to have realistic models
of radiative transfer in their atmospheres, which will permit determination of
abundances accurate to 0.15 dex, the precision attainable with future telescope
facilities in galaxies as distant as tens of Mpc. Here, we study the effects of
non-local thermodynamic equilibrium (NLTE) on the formation of iron, titanium,
and silicon lines, which dominate J-band spectra of red supergiants. It is
shown that the NLTE radiative transfer models enable accurate derivation of
metallicity and effective temperature in the J-band. We also discuss
consequences for RSG spectrum synthesis in different spectral windows,
including the heavily TiO-blanketed optical region, and atmospheric structure.
We then touch upon challenges of NLTE integration with new generation of 3D
hydrodynamical RSG models and present the first calculations of NLTE spectra
with the mean 3D model of Betelgeuse.Comment: 7 pages, proceedings of the Betelgeuse Workshop, Paris, 201
- …