1,723 research outputs found
Tomography of the red supergiant star {\mu} Cep
A tomographic method, aiming at probing velocity fields at depth in stellar
atmospheres, is applied to the red supergiant star {\mu} Cep and to snapshots
of 3D radiative-hydrodynamics simulation in order to constrain atmospheric
motions and relate them to photometric variability.Comment: 2 pages, 2 figures, accepted as Proceedings of IAU Symposium No. 343,
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Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER
Thanks to their large angular dimension and brightness, red giants and
supergiants are privileged targets for optical long-baseline interferometers.
Sixteen red giants and supergiants have been observed with the VLTI/AMBER
facility over a two-years period, at medium spectral resolution (R=1500) in the
K band. The limb-darkened angular diameters are derived from fits of stellar
atmospheric models on the visibility and the triple product data. The angular
diameters do not show any significant temporal variation, except for one
target: TX Psc, which shows a variation of 4% using visibility data. For the
eight targets previously measured by Long-Baseline Interferometry (LBI) in the
same spectral range, the difference between our diameters and the literature
values is less than 5%, except for TX Psc, which shows a difference of 11%. For
the 8 other targets, the present angular diameters are the first measured from
LBI. Angular diameters are then used to determine several fundamental stellar
parameters, and to locate these targets in the Hertzsprung-Russell Diagram
(HRD). Except for the enigmatic Tc-poor low-mass carbon star W Ori, the
location of Tc-rich stars in the HRD matches remarkably well the
thermally-pulsating AGB, as it is predicted by the stellar-evolution models.
For pulsating stars with periods available, we compute the pulsation constant
and locate the stars along the various sequences in the Period -- Luminosity
diagram. We confirm the increase in mass along the pulsation sequences, as
predicted by the theory, except for W Ori which, despite being less massive,
appears to have a longer period than T Cet along the first-overtone sequence.Comment: 15 pages, 9 figures, 6 table
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
Radiative hydrodynamics simulations of red supergiant stars: II. simulations of convection on Betelgeuse match interferometric observations
Context. The red supergiant (RSG) Betelgeuse is an irregular variable star.
Convection may play an important role in understanding this variability.
Interferometric observations can be interpreted using sophisticated simulations
of stellar convection. Aims. We compare the visibility curves and closure
phases obtained from our 3D simulation of RSG convection with CO5BOLD to
various interferometric observations of Betelgeuse from the optical to the H
band in order to characterize and measure the convection pattern on this star.
Methods. We use 3D radiative-hydrodynamics (RHD) simulation to compute
intensity maps in different filters and we thus derive interferometric
observables using the post-processing radiative transfer code OPTIM3D. The
synthetic visibility curves and closure phases are compared to observations.
Results. We provide a robust detection of the granulation pattern on the
surface of Betelgeuse in the optical and in the H band based on excellent fits
to the observed visibility points and closure phases. Moreover, we determine
that the Betelgeuse surface in the H band is covered by small to medium scale
(5-15 mas) convection-related surface structures and a large (30 mas)
convective cell. In this spectral region, H2O molecules are the main absorbers
and contribute to the small structures and to the position of the first null of
the visibility curve (i.e. the apparent stellar radius).Comment: 11 pages, Accepted for publication on A&
Use of the GATE Monte Carlo package for dosimetry applications
6 pages, 3 figures - submitted to NIM A, presented by D. VisvikisInternational audienceOne of the roles for MC simulation studies is in the area of dosimetry. A number of different codes dedicated to dosimetry applications are available and widely used today, such as MCNP, EGSnrc and PTRAN. However, such codes do not easily facilitate the description of complicated 3D sources or emission tomography systems and associated data flow, which may be useful in different dosimetry application domains. Such problems can be overcome by the use of specific MC codes such as GATE, which is based on Geant4 libraries, providing a scripting interface with a number of advantages for the simulation of SPECT and PET systems. Despite this potential, its major disadvantage is in terms of efficiency involving long execution times for applications such as dosimetry. The strong points and disadvantages of GATE in comparison to other dosimetry specific codes are discussed and illustrated in terms of accuracy, efficiency and flexibility. A number of features, such as the use of voxelised and moving sources, as well as developments such as advanced visualisation tools and the development of dose estimation maps allowing GATE to be used for dosimetry applications are presented. In addition, different examples from dosimetry applications with GATE are given. Finally, future directions with respect to the use of GATE for dosimetry applications are outlined
The EEE Project
The new experiment ``Extreme Energy Events'' (EEE) to detect extensive air
showers through muon detection is starting in Italy. The use of particle
detectors based on Multigap Resistive Plate Chambers (MRPC) will allow to
determine with a very high accuracy the direction of the axis of cosmic ray
showers initiated by primaries of ultra-high energy, together with a high
temporal resolution. The installation of many of such 'telescopes' in numerous
High Schools scattered all over the Italian territory will also allow to
investigate coincidences between multiple primaries producing distant showers.
Here we present the experimental apparatus and its tasks.Comment: 4 pages, 29th ICRC 2005, Pune, Indi
The close circumstellar environment of Betelgeuse - II. Diffraction-limited spectro-imaging from 7.76 to 19.50 microns with VLT/VISIR
Context: Mass-loss occurring in red supergiants (RSGs) is a major contributor
to the enrichment of the interstellar medium in dust and molecules. The
physical mechanism of this mass loss is however relatively poorly known.
Betelgeuse is the nearest RSG, and as such a prime object for high angular
resolution observations of its surface (by interferometry) and close
circumstellar environment. Aims: The goal of our program is to understand how
the material expelled from Betelgeuse is transported from its surface to the
interstellar medium, and how it evolves chemically in this process. Methods: We
obtained diffraction-limited images of Betelgeuse and a PSF calibrator
(Aldebaran) in six filters in the N band (7.76 to 12.81 mic) and two filters in
the Q band (17.65 and 19.50 mic), using the VLT/VISIR instrument. Results: Our
images show a bright, extended and complex circumstellar envelope at all
wavelengths. It is particularly prominent longwards of 9-10 mic, pointing at
the presence of O-rich dust, such as silicates or alumina. A partial circular
shell is observed between 0.5 and 1.0" from the star, and could correspond to
the inner radius of the dust envelope. Several knots and filamentary structures
are identified in the nebula, one of the knots, located at a distance of 0.9"
west of the star, being particularly bright and compact. Conclusions: The
circumstellar envelope around Betelgeuse extends at least up to several tens of
stellar radii. Its relatively high degree of clumpiness indicates an
inhomogeneous spatial distribution of the material lost by the star. Its
extension corresponds to an important intermediate scale, where most of the
dust is probably formed, between the hot and compact gaseous envelope observed
previously in the near infrared and the interstellar medium.Comment: 10 page
Heavy Meson Production in Proton-Nucleus Reactions with Empirical Spectral Functions
We study the production of and mesons in reactions on the basis of empirical spectral functions. The high
momentum, high removal energy part of the spectral function is found to be
negligible in all cases close to the absolute threshold. Furthermore, the
two-step process () dominates the cross section at threshold energies in line with
earlier calculations based on the folding model.Comment: 18 pages, LaTeX, plus 14 postscript figures, submitted to Z. Phys.
Fundamental properties of the Population II fiducial stars HD 122563 and Gmb 1830 from CHARA interferometric observations
We have determined the angular diameters of two metal-poor stars, HD 122563
and Gmb 1830, using CHARA and Palomar Testbed Interferometer observations. For
the giant star HD 122563, we derive an angular diameter theta_3D = 0.940 +-
0.011 milliarcseconds (mas) using limb-darkening from 3D convection simulations
and for the dwarf star Gmb 1830 (HD 103095) we obtain a 1D limb-darkened
angular diameter theta_1D = 0.679 +- 0.007 mas. Coupling the angular diameters
with photometry yields effective temperatures with precisions better than 55 K
(Teff = 4598 +- 41 K and 4818 +- 54 K --- for the giant and the dwarf star,
respectively). Including their distances results in very well-determined
luminosities and radii (L = 230 +- 6 L_sun, R = 23.9 +- 1.9 R_sun and L = 0.213
+- 0.002 L_sun, R = 0.664 +- 0.015 R_sun, respectively). We used the CESAM2k
stellar structure and evolution code in order to produce models that fit the
observational data. We found values of the mixing-length parameter alpha (which
describes 1D convection) that depend on the mass of the star. The masses were
determined from the models with precisions of <3% and with the well-measured
radii excellent constraints on the surface gravity are obtained (log g = 1.60
+- 0.04, 4.59 +- 0.02, respectively). The very small errors on both log g and
Teff provide stringent constraints for spectroscopic analyses given the
sensitivity of abundances to both of these values. The precise determination of
Teff for the two stars brings into question the photometric scales for
metal-poor stars.Comment: accepted A&A, 8 dbl-column pages, incl. 7 tables and 4 figure
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