Toward a consistent use of overshooting parametrizations in 1D stellar evolution codes

Several parametrizations for overshooting in 1D stellar evolution calculations coexist in the literature. These parametrizations are used somewhat arbitrarily in stellar evolution codes, based on what works best for a given problem, or even for historical reasons related to the development of each code. We bring attention to the fact that these different parametrizations correspond to different physical regimes of overshooting, depending whether the effects of radiation are dominant, marginal, or negligible. Our analysis is based on previously published theoretical results, as well as multidimensional hydrodynamical simulations of stellar convection where the interaction between the convective region and a stably-stratified region is observed. Although the underlying hydrodynamical processes are the same, the outcome of the overshooting process is profoundly affected by radiative effects. Using a simple picture of the scales involved in the overshooting process, we show how three regimes are obtained, depending on the importance of radiative effects. These three regimes correspond to the different behaviors observed in hydrodynamical simulations so far, and to the three types of parametrizations used in 1D codes. We suggest that the existing parametrizations for overshooting should coexist in 1D stellar evolution codes, and should be applied consistently at convective boundaries depending on the local physical conditions.Comment: 5 pages, 2 figures, to appear in A&A as a regular paper. Last version: language editing + typos in Eq. (6) & (9) correcte

Formalized linear algebra over Elementary Divisor Rings in Coq

This paper presents a Coq formalization of linear algebra over elementary divisor rings, that is, rings where every matrix is equivalent to a matrix in Smith normal form. The main results are the formalization that these rings support essential operations of linear algebra, the classification theorem of finitely presented modules over such rings and the uniqueness of the Smith normal form up to multiplication by units. We present formally verified algorithms computing this normal form on a variety of coefficient structures including Euclidean domains and constructive principal ideal domains. We also study different ways to extend B\'ezout domains in order to be able to compute the Smith normal form of matrices. The extensions we consider are: adequacy (i.e. the existence of a gdco operation), Krull dimension $\leq 1$ and well-founded strict divisibility

On the relevance of bubbles and potential flows for stellar convection

Recently Pasetto et al. have proposed a new method to derive a convection theory appropriate for the implementation in stellar evolution codes. Their approach is based on the simple physical picture of spherical bubbles moving within a potential flow in dynamically unstable regions, and a detailed computation of the bubble dynamics. Based on this approach the authors derive a new theory of convection which is claimed to be parameter free, non-local and time-dependent. This is a very strong claim, as such a theory is the holy grail of stellar physics. Unfortunately we have identified several distinct problems in the derivation which ultimately render their theory inapplicable to any physical regime. In addition we show that the framework of spherical bubbles in potential flows is unable to capture the essence of stellar convection, even when equations are derived correctly.Comment: 14 pages, 3 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society. (Comments and criticism are welcomed

Recent and Ancient Signature of Balancing Selection around the S-Locus in Arabidopsis halleri and A. lyrata

Balancing selection can maintain different alleles over long evolutionary times. Beyond this direct effect on the molecular targets of selection, balancing selection is also expected to increase neutral polymorphism in linked genome regions, in inverse proportion to their genetic map distances from the selected sites. The genes controlling plant self-incompatibility are subject to one of the strongest forms of balancing selection, and they show clear signatures of balancing selection. The genome region containing those genes (the S-locus) is generally described as nonrecombining, and the physical size of the region with low recombination has recently been established in a few species. However, the size of the region showing the indirect footprints of selection due to linkage to the S-locus is only roughly known. Here, we improved estimates of this region by surveying synonymous polymorphism and estimating recombination rates at 12 flanking region loci at known physical distances from the S-locus region boundary, in two closely related self-incompatible plants Arabidopsis halleri and A. lyrata. In addition to studying more loci than previous studies and using known physical distances, we simulated an explicit demographic scenario for the divergence between the two species, to evaluate the extent of the genomic region whose diversity departs significantly from neutral expectations. At the closest flanking loci, we detected signatures of both recent and ancient indirect effects of selection on the S-locus flanking genes, finding ancestral polymorphisms shared by both species, as well as an excess of derived mutations private to either species. However, these effects are detected only in a physically small region, suggesting that recombination in the flanking regions is sufficient to quickly break up linkage disequilibrium with the S-locus. Our approach may be useful for distinguishing cases of ancient versus recently evolved balancing selection in other systems

Microstructure evolution of gold thin films under spherical indentation for micro switches contact applications

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this work to a sphere / plane contact, represent a major reliability issue. A first step towards failure mechanism understanding is the investigation of the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high resolution Electron Back Scatter Diffraction (EBSD) is used to investigate contact area. Grain rotation against {111} fiber texture of 1 μm thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading by a spherical diamond indenter with 50 μm tip radius. A heterogeneity in grain rotation is observed corresponding to a more important plastic deformation in the middle of the indent than at the edge. A 30° Grain rotation is observed for a half million mechanical cycles under 300 μN load by a spherical gold tip (20 μm radius) due to cyclic work hardening. The same test in hot switching mode induces a grain growth in the contact area. Therefore thermal effects occurring during hot switching are underline

On the topicality and research impact of special issues

The publication of special issues constitute an important yet underinvestigated phenomenon of scholarly communication. In an attempt to draw attention to the proliferation of special issues, Priem (2006) suggested that their commissioning has an underestimated opportunity cost, given the relative scarcity of publication space: by distorting the “marketplace for ideas” through the commanding of preselected topical distributions, special issues undermine the total research output by “squeezing out” high-quality but topically unrelated articles. The present paper attempts to test this hypothesis by providing a topicality and research impact analysis of conference-based, monographic, and regular issues published between 2010 and 2015 inclusive and indexed in Clarivate Analytics’ Web of Science. The results show that the titles and abstracts of articles copublished are topically closer to each other than those copublished in regular issues, which suggests that their relative importance might influence the total topical distribution. However, disciplinary and overall comparison of relative citations for both special and regular issues shows that intraissue averages and variances in the former case are respectively higher and lower than in the regular issue context, which undermines not only the abovementioned hypothesis, but also the belief that editors often “fill up” special issues by accepting substandard papers