Numerical study of Bingham flow in macrosopic two dimensional heterogenous porous media

The flow of non-Newtonian fluids is ubiquitous in many applications in the geological and industrial context. We focus here on yield stress fluids (YSF), i.e. a material that requires minimal stress to flow. We study numerically the flow of yield stress fluids in 2D porous media on a macroscopic scale in the presence of local heterogeneities. As with the microscopic problem, heterogeneities are of crucial importance because some regions will flow more easily than others. As a result, the flow is characterized by preferential flow paths with fractal features. These fractal properties are characterized by different scale exponents that will be determined and analyzed. One of the salient features of these results is that these exponents seem to be independent of the amplitude of heterogeneities for a log-normal distribution. In addition, these exponents appear to differ from those at the microscopic level, illustrating the fact that, although similar, the two scales are governed by different sets of equations

3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits

We examine the stability and observational consequences of mixing induced by 3He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have finger-like structures with a length to diameter ratio l/d > 10. We identify some potentially serious obstacles for thermohaline convection as the predominant mixing mechanism for giants. We show that rotation-induced horizontal turbulent diffusion may suppress the 3He-driven thermohaline convection. Another potentially serious problem for it is to explain observational evidence of enhanced extra mixing. The 3He exhaustion in stars approaching the red giant branch (RGB) tip should make the 3He mixing inefficient on the asymptotic giant branch (AGB). In spite of this, there are observational data indicating the presence of extra mixing in low-mass AGB stars similar to that operating on the RGB. Overmixing may also occur in carbon-enhanced metal-poor stars.Comment: 25 pages, 6 figures, modified version, accepted by Ap

WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap

We report discovery of significant depletion of Li on the surfaces of F dwarf stars in the 150-Myr-old open cluster M35, analagous to a feature in the 700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have caught the gap in the act of forming: using high resolution, high S/N, WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted) stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen from 6000K to 6700K or hotter; b) shows a significant dispersion in Li abundance at all T_eff, even with stars of the same T_eff; and c) contains undepleted stars (as well as depleted ones) in the (narrow) classical Hyades gap region, which itself shows no undepleted stars. All of these M35 Li depletion properties support rotationally-induced slow mixing as the primary physical mechanism that forms the gap, and argues against other proposed mechanisms, particularly diffusion and steady main sequence mass loss. When viewed in the context of the M35 Li depletion properties, the Hyades Li gap may well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter

Persistent time intervals between features in solar flare hard X-ray emission

Several solar hard X-ray events (greater than 100 keV) were observed simultaneously with identical instruments on the Venera 11, 12, 13, 14, and Prognoz spacecraft. High time resolution (= 2 ms) data were stored in memory when a trigger occurred. The observations of modulation are presented with a period of 1.6 s for the event on December 3, 1978. Evidence is also presented for fast time fluctuations from an event on November 6, 1979, observed from Venera 12 and another on September 6, 1981, observed from the Solar Maximum Mission. Power spectrum analysis, epoch folding, and Monte Carlo simulation were used to evaluate the statistical significance of persistent time delays between features. The results are discussed in light of the MHD model proposed by Zaitsev and Stepanov

The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT Lib

A number of blue stars which appear to be similar to Population I B-stars in the star forming regions of the galactic disk are found more than 1 kpc from the galactic plane. Uncertainties about the true distances and masses of these high latitude B-stars has fueled a debate as to their origin and evolutionary status. The eclipsing binary IT Lib is composed of two B-stars, is approximately one kiloparsec above the galactic plane, and is moving back toward the plane. Observations of the light and velocity curves presented here lead to the conclusion that the B-stars in this system are massive young main-sequence stars. While there are several possible explanations, it appears most plausible that the IT Lib system formed in the disk about 30 million years ago and was ejected on a trajectory taking it to its present position.Comment: 26 pages, 3 figures, accepted for publication in the PASP (January 2003

The Geneva stellar evolution code

This paper presents the Geneva stellar evolution code with special emphasis on the modeling of solar-type stars. The basic input physics used in the Geneva code as well as the modeling of atomic diffusion is first discussed. The physical description of rotation is then presented. Finally, the modeling of magnetic instabilities and transport of angular momentum by internal gravity waves is briefly summarize

Solar-like oscillations in a massive star

Seismology of stars provides insight into the physical mechanisms taking place in their interior, with modes of oscillation probing different layers. Low-amplitude acoustic oscillations excited by turbulent convection were detected four decades ago in the Sun and more recently in low-mass main-sequence stars. Using data gathered by the Convection Rotation and Planetary Transits mission, we report here on the detection of solar-like oscillations in a massive star, V1449 Aql, which is a known large-amplitude (b Cephei) pulsator.Comment: Published in Sience, 19 June 2009, vol. 324, p. 154

Light elements in massive single and binary stars

We highlight the role of the light elements (Li, Be, B) in the evolution of massive single and binary stars, which is largely restricted to a diagnostic value, and foremost so for the element boron. However, we show that the boron surface abundance in massive early type stars contains key information about their foregoing evolution which is not obtainable otherwise. In particular, it allows to constrain internal mixing processes and potential previous mass transfer event for binary stars (even if the companion has disappeared). It may also help solving the mystery of the slowly rotating nitrogen-rich massive main sequence stars.Comment: 10 pages, 8 figures, to appear in proc. IAU-Symp. 268. C. Charbonnel et al., eds

Main-Sequence and sub-giant stars in the Globular Cluster NGC6397: The complex evolution of the lithium abundance

Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence (MS) stars of a Globular Cluster. At the same time we observed Li in a sample of Sub-Giant (SG) stars of the same B-V colour. Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis. We derived the effective temperatures, from H$\alpha$ fitting, and NLTE Li abundances of the stars in our the sample, using 3D and 1D models. We find that SG stars have a mean Li abundance higher by 0.1dex than MS stars, using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status. The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.Comment: Proceedings of the contributed talk presented at the IAU Symposium 26

Congenital pulmonary adenomatoid malformation: Indication and management of fetal shunting in our center

Objective: Congenital pulmonary adenomatoid malformations (CPAM) are increasingly diagnosed in recent years thanks to the expert hands and the technology of ultrasonography used in antenatal diagnosis. The purpose was to examine a series of cases referred to our fetalmaternal tertiary center for lung disease and to present the natural course, treatment and outcome of antenatally diagnosed cystic lung disease. Material and Methods: We proposed a retrospective study (October 2003 to September 2016) in a single center. Prenatal and postnatal data of fetus, with suspected CPAM, where analyzed. Results: We studied 34 cases. One child was excluded post-natally because at birth diagnosis of esophageal duplication was made. Mean gestational age at the diagnosis was 22 weeks (range 21 to 28). Thoracoamniotic shunting was performed in 9 cases. In 8 cases, it was an intra-cystic drain and in 1 case, a pleuroamniotic drain. Mean gestational age for drainage was 25 weeks (range 22 to 28). Mean gestational age at delivery was 38 weeks. All patients underwent postnatal radiological assessment. Anatomical surgical resection was performed in all cases: immediately at birth in 7 symptomatic cases, in the first year of life for 1 case that became symptomatic and 1 patient died immediately at birth due to severe respiratory distress. Conclusion: Antenatally diagnosed CPAM have an excellent prognosis. We propose invasive prenatal thoracoamniotic shunting in rare cases with unequivocal fetal compromise (i.e., polydramnios and hydrops)