Stellar granulation as seen in disk-integrated intensity. I. Simplified theoretical modeling

The solar granulation is known for a long time to be a surface manifestation of convection. Thanks to the current space-borne missions CoRoT and Kepler, it is now possible to observe in disk-integrated intensity the signature of this phenomena in a growing number of stars. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aim at providing a simple theoretical modeling to reproduce these scaling relations and subsequently at inferring the physical properties of granulation properties across the HR diagram. We develop a simple 1D theoretical model that enable us to test any prescription concerning the time-correlation between granules. The input parameters of the model are extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model are calibrated with solar observations. Two different prescriptions for representing the eddy time-correlation in the Fourier space are compared: a Lorentzian and an exponential form. Finally, we compare our theoretical prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (ab-initio approach). Provided that the free parameters are appropriately adjusted, our theoretical model satisfactorily reproduces the shape and the amplitude of the observed solar granulation spectrum. The best agreement is obtained with an exponential form. Furthermore, our theoretical model results in granulation spectra that consistently agree with the these calculated on the basis of the ab-initio approach with two 3D RHD models. Comparison between theoretical granulation spectra calculated with the present model and high precision photometry measurements of stellar granulation is undertaken in a companion paper.Comment: 10 pages, 2 figures, accepted for publication in A&

Surface-effect corrections for solar-like oscillations using 3D hydrodynamical simulations

The space-borne missions have provided us with a wealth of high-quality observational data that allows for seismic inferences of stellar interiors. This requires the computation of precise and accurate theoretical frequencies, but imperfect modeling of the uppermost stellar layers introduces systematic errors. To overcome this problem, an empirical correction has been introduced by Kjeldsen et al. (2008, ApJ, 683, L175) and is now commonly used for seismic inferences. Nevertheless, we still lack a physical justification allowing for the quantification of the surface-effect corrections. We used a grid of these simulations computed with the CO$^5$BOLD code to model the outer layers of solar-like stars. Upper layers of the corresponding 1D standard models were then replaced by the layers obtained from the horizontally averaged 3D models. The frequency differences between these patched models and the 1D standard models were then calculated using the adiabatic approximation and allowed us to constrain the Kjeldsen et al. power law, as well as a Lorentzian formulation. We find that the surface effects on modal frequencies depend significantly on both the effective temperature and the surface gravity. We further provide the variation in the parameters related to the surface-effect corrections using their power law as well as a Lorentzian formulation. Scaling relations between these parameters and the elevation (related to the Mach number) is also provided. The Lorentzian formulation is shown to be more robust for the whole frequency spectrum, while the power law is not suitable for the frequency shifts in the frequency range above $\nu_{\rm max}$.Comment: 11 pages, 14 figures, 4 tables; accepted for publication in Astronomy & Astrophysic

Comparison of date-palm wastes and perlite as growth substrates on some tomato growing indexes

Today, various organic matters are used as growth media. Most of them are combined with various materials, such as a mixture of peat moss and pine bark, or a mixture of peat moss and pine bark with mineral materials like perlite or vermiculite. All over the world, stone wool and other materials like perlite, pumice, polyortan phome, zeolite, coco peat and sawdust are used as growth media in soilless culture. The objective of this study was to assess the effects of incubation time on the physicochemical properties of date palm wastes and their effects on growing indexes of tomato in comparison with perlite when used as culture substrates. The date palm wastes were first manually chopped into small pieces, after which they were kept in plastic bags. They were adjusted to 55% moisture content and thoroughly mixed once every week. Treatments were done for 4 incubation times (0, 1, 3 and 9 months). The physicochemical properties of materials were analyzed during 3 months. After 3 months of incubation, for comparison of this composting materials (date-palm) with other substrates, perlite and the composting materials were used as growth media. The research was conducted in a completely randomized design with 6 replications of tomato for 6 months. Treatments included perlite, date-palm1 (without incubation time) and date-palm2 (with 3 months incubation time). During plant growth irrigation rate, temperature, humidity and pest control for all treatments were similar. During plant growth, Papadopolus formula with fertigation method was used for the nutrient solution. The plants were kept for 6 months, then after this period, the physicochemical properties of these materials were analyzed again. So, the growing indexes of tomato, which included fruit yield, plant height, fruit  number, stem diameter, TSS and biomass, were measured. The results of incubation time period showed that changes of porosity, bulk density, pH, C/N ratio and CEC had significant differences (p<0.05). The C/N ratio reduced, while the bulk density and CEC increased by the increase in incubation time period. Comparison of tomato growing indexes in the different substrates showed that fruit yield, plant height and fruit number of tomato had no significant differences at 5% level, but stem diameter and biomass in date-palm1 was higher than other media and thus, had significant differences at 5% level.Key words: Date palm wastes, incubation time, aerating, perlite, fertigation, tomato

Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933

Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions. We aim to model the photometric granulation background of HD49933 as well as the Sun, and compare the predictions to the measurements obtained by the COROT and SOHO satellite missions. We construct hydrodynamical model atmospheres representing HD49933 and the Sun, and use a previously developed scaling technique to obtain the observable disk-integrated brightness fluctuations. We further performed exploratory magneto-hydrodynamical simulations to gauge the impact of small scale magnetic fields on the synthetic light-curves. We find that the granulation-related brightness fluctuations depend on metallicity. We obtain a satisfactory correspondence between prediction and observation for the Sun, validating our approach. For HD49933, we arrive at a significant over-estimation by a factor of two to three in total power. Locally generated magnetic fields are unlikely to be responsible, otherwise existing fields would need to be rather strong to sufficiently suppress the granulation signal. Presently suggested updates on the fundamental stellar parameters do not improve the correspondence; however, an ad-hoc increase of the HD49933 surface gravity by about 0.2dex would eliminate most of the discrepancy. We diagnose a puzzling discrepancy between the predicted and observed granulation background in HD49933, with only rather ad-hoc ideas for remedies at hand.Comment: 7 pages, 5 figures, accepted for publication in A&

Density assessment of muddy shore crabs in the eastern part of Bandar Abbas

Crabs of intertidal zone of a muddy shore in Bandar Abbas were studied using monthly examination of Goharshad and Terminal stations during August 2005 to July 2006. Sampling was done by Corer and 14 species from 6 families were identified. Crabs were from Ocypodidae, Pinnotheridae, Pilumnidae, Trapeziidae, Leucosiidae and Varunidae families. Results showed Ocypodidae, the most abundant and diverse with 1196 individuals/m^2, was more abundant but less diverse in Goharshad station compared to Terminal which could be attributed to the urban pollution in Terminal station. Pinnotheridae with 64 individuals/m 2 showed an adaptation to organic materials but was not resistant to being out of water. Pilumnidae and Trapeziidae were found in small numbers in the stattions. Genus Ebalia from Leucosiidae with 27.18 individuals/m^2 showed positive reaction to organic materials and was resistant to being out of water. Varunidae were present with a few specimens probably because of human disturbance or speed in burrowing and escaping from sampling. Because civic pollution enter Terminal station, The results of this research showed that crabs react to urban pollution and hence are suitable as environmental indicators

Stochastic excitation of acoustic modes in stars

For more than ten years, solar-like oscillations have been detected and frequencies measured for a growing number of stars with various characteristics (e.g. different evolutionary stages, effective temperatures, gravities, metal abundances ...). Excitation of such oscillations is attributed to turbulent convection and takes place in the uppermost part of the convective envelope. Since the pioneering work of Goldreich & Keely (1977), more sophisticated theoretical models of stochastic excitation were developed, which differ from each other both by the way turbulent convection is modeled and by the assumed sources of excitation. We review here these different models and their underlying approximations and assumptions. We emphasize how the computed mode excitation rates crucially depend on the way turbulent convection is described but also on the stratification and the metal abundance of the upper layers of the star. In turn we will show how the seismic measurements collected so far allow us to infer properties of turbulent convection in stars.Comment: Notes associated with a lecture given during the fall school organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39 pages ; 11 figure