Atmospheric dynamics of red supergiant stars and applications to Interferometry

We have written a 3D radiative transfer code that computes emerging spectra and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of RSG stars carried out with CO5BOLD (Freytag et al. 2002) observables expected for red supergiant stars (RSG) especially for interferometric observations, with emphasis on small scale structures. We show that the convection-related surface structures are detectable in the H band with today's interferometers and that the diameter measurement should not be too dependent on the adopted model. The simulations are a great improvement over parametric models for the interpretation of interferometric observations.Comment: 6 pages, SF2A 2007 conferenc

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

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

Antifungal resistance regarding Malassezia pachydermatis: Where are we now?

Malassezia pachydermatis is a yeast inhabiting the skin and ear canals in healthy dogs. In the presence of various predisposing conditions it can cause otitis and dermatitis, which are treated with multiple antifungal agents, mainly azole derivatives. This manuscript aims to review the available evidence regarding the occurrence of resistance phenomena in this organism. Various findings support the capacity of M. pachydermatis for developing resistance. These include some reports of treatment failure in dogs, the reduced antifungal activity found against yeast isolates sampled from dogs with exposure to antifungal drugs and strains exposed to antifungal agents in vitro, and the description of resistance mechanisms. At the same time, the data reviewed may suggest that the development of resistance is a rare eventuality in canine practice. For example, only three publications describe confirmed cases of treatment failure due to antifungal resistance, and most claims of resistance made by past studies are based on interpretive breakpoints that lack sound support from the clinical perspective. However, it is possible that resistant cases are underreported in literature, perhaps due to the difficulty of obtaining a laboratory confirmation given that a standard procedure for susceptibility testing of M. pachydermatis is still unavailable. These considerations highlight the need for maintaining surveillance for the possible emergence of clinically relevant resistance, hopefully through a shared strategy put in place by the scientific community

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&

Baryonic contributions to the dilepton spectrum of nucleon-nucleon collisions

We study the production of dileptons in relativistic nucleon-nucleon collisions. Additionally to the traditional dilepton production channels (vector meson decays, meson and Delta(1232) Dalitz decays) we included in our model as new dilepton sources the Dalitz decay of higher unflavored baryon resonances with spin<=5/2 and mass<=2.25 GeV/c^2. The contributions of these new channels are estimated using experimental information about the Ngamma decays of the resonances and have large uncertainties. The obtained dilepton spectra are compared to the experimental data by the DLS collaboration. Predictions for the HADES detector (SIS, GSI) are also discussed. In spite of the large uncertainties of the higher resonance Dalitz decay contributions we are able to draw the conclusion that these contributions are negligible compared to the other dilepton sources and do not influence the detectability of the phi and omega vector meson peaks.Comment: 9 pages, 4 figures, version accepted for publication in Phys. Rev.

A Covariant OBE Model for η\eta Production in NN Collisions

A relativistic covariant one boson exchange model, previously applied to describe elastic nucleon-nucleon scattering, is extended to study $\eta$ production in NN collisions. The transition amplitude for the elementary BN->$\eta$N process with B being the meson exchanged (B=$\pi$, $|sigma$,$\eta$, $\rho$, $\omega$ and $\delta$) are taken to be the sum of four terms corresponding to s and u-channels with a nucleon or a nucleon isobar N*(1535MeV) in the intermediate states. Taking the relative phases of the various exchange amplitudes to be +1, the model reproduces the cross sections for the $NN\to X\eta$ reactions in a consistent manner. In the limit where all $\eta$'s are produced via N^* excitations, interference terms between the overall contributions from the exchange of pseudoscalart and scalar mesons with that of vector mesons cancel out. Consequently, much of the ambiguities in the model predictions due to unknown relative phases of different vector pseudoscalar exchanges are strongly reduced.Comment: 40 pages, 15 figure

Comparison of the Near-Threshold Production of eta- and K-Mesons in Proton-Proton Collisions

The pp -> pp eta and pp -> pLambda K^+ reactions near threshold are dominated by the first and second S_11 resonance respectively. It is shown that a one-pion-exchange model exciting these isobars reproduces well the ratio of the production cross sections. The consequences for this and other channels are discussed.Comment: 10 pages, LaTeX2e, 1 eps-figur

On The pp->pp\eta (eta') Reactions Near Threshold

The production rate for \eta' in pp ->pp \eta' at rest is calculated in a covariant one boson exchange model, previously applied to study pi^0 and \eta production in NN collisions. The transition amplitudes for the elementary BN -> eta' N processes with B being the meson exchanged (B = pi, sigma, eta, rho, omega and delta) are taken to be the sum of s and u channels with a nucleon in the intermediate states, and a delta meson pole in a t-channel. The couplings of the eta' to hadrons are a factor 0.437 weaker than the respective eta-hadron couplings, as suggested by a quark model and a singlet-octetmixing angle theta = -23 degree. The model reproduces near threshold cross sections for the quasielastic processes pi- p ->n eta (eta') and pp -> pp eta (eta') reactions.Comment: 15 pages, 6 figure

Heavy Meson Production in Proton-Nucleus Reactions with Empirical Spectral Functions

We study the production of $K^+, \rho, \omega$ and $\phi$ mesons in $p + ^{12}C$ 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 ($pN \rightarrow \pi N N; \pi N \rightarrow N + K^+, \rho, \omega, \phi$) 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.