Overshooting by convective settling

We study a process of slow mixing in stars with convective envelopes, which is driven by the settling of cool downward plumes below the base of the convection zone. If a small fraction (of order $10^{-7}$) of the material cooled at the surface retains a significant entropy deficit while descending in plumes, it can reach the depth where lithium burning takes place. The model calculates the thermal response and mixing below the convection zone due to the settling process, assuming that the plumes arrive at the base of the convection zone with a broad range of entropy contrasts. We obtain a good fit to the observed lithium depletion in the Sun by assuming that the settling mass flux is distributed with respect to the entropy contrast as a power law with a slope around -2. We find convective settling to have a negligible influence on the stratification below the convection zone, although mixing induced by it could modify the gradient of helium concentration.Comment: 8 pages, 4 figures, accepted for publication in A&

Overshooting by differential heating

On the long nuclear time scale of stellar main-sequence evolution, even weak mixing processes can become relevant for redistributing chemical species in a star. We investigate a process of "differential heating," which occurs when a temperature fluctuation propagates by radiative diffusion from the boundary of a convection zone into the adjacent radiative zone. The resulting perturbation of the hydrostatic equilibrium causes a flow that extends some distance from the convection zone. We study a simplified differential-heating problem with a static temperature fluctuation imposed on a solid boundary. The astrophysically relevant limit of a high Reynolds number and a low P\'eclet number (high thermal diffusivity) turns out to be interestingly non-intuitive. We derive a set of scaling relations for the stationary differential heating flow. A numerical method adapted to a high dynamic range in flow amplitude needed to detect weak flows is presented. Our two-dimensional simulations show that the flow reaches a stationary state and confirm the analytic scaling relations. These imply that the flow speed drops abruptly to a negligible value at a finite height above the source of heating. We approximate the mixing rate due to the differential heating flow in a star by a height-dependent diffusion coefficient and show that this mixing extends about $4\%$ of the pressure scale height above the convective core of a $10\,M_\odot$ zero-age main sequence star.Comment: 16 pages, 9 figures, accepted for publication in A&

Convective settling in main sequence stars: Li and Be depletion

The process of convective settling is based on the assumption that a small fraction of the low-entropy downflows sink from the photosphere down to the bottom of the star's envelope convection zone retaining a substantial entropy contrast. We have previously shown that this process could explain the slow Li depletion observed in the Sun. We construct a parametric model of convective settling to investigate the dependence of Li and Be depletion on stellar mass and age. Our model is generally in good agreement with the Li abundances measured in open clusters and solar twins, although it seems to underestimate the Li depletion in the first ~1 Gyr. The model is also compatible with the Be abundances measured in a sample of field stars.Comment: 8 pages, 9 figures, accepted for publication in A&

Two new nematode species of the subfamily Brittonematinae (Dorylaimida: Actinolaimidae)

Abstract. Two new species of actinolaimoid nematodes of the subfamily Brittonematinae are described and illustrated.Actinca marisae sp. n. from Brazil is characterized by the long (on average 2.92 mm) and slender body, 30–32 distinct longitudinalridges on cuticle, narrow head, slender odontostyle, onchial tips facing each other, cylindrus occupying somewhat lessthan one-half of pharynx, broad vulval lips, and by medum long tail. Afractinca eburnea sp. n. from Côte d’Ivoire can bedistinguished by a relatively long body (on average 1.88 mm), thin cuticle provided with 14 longitudinal ridges, cap-like offsetlabial ring, very slender odontostyle, long prerectum, vulva sunk in body contour, and by the elongate-conoid female tail. Mainmorphological structures of Actinca and Afractinca species are summarized. Some comments on further brittonematine speciesare added

Another species of the genus Aporcelinus Andrássy, 2009 (Nematoda: Dorylaimida)

A further new species of the recently established genus Aporcelinus Andrássy, 2009 is presented. Aporcelinus decimus sp. n. from Ecuador can be differentiated from all known species by the shape of the vulval lips and the tail. The former are sclerotized lying parallel with the body axis, the latter is relatively long, rapidly tapered at its middle, and more or less bent dorsally