A disk inside the bipolar planetary nebula M2-9

Bipolarity in proto-planetary and planetary nebulae is associated with events occurring in or around their cores. Past infrared observations have revealed the presence of dusty structures around the cores, many in the form of disks. Characterising those dusty disks provides invaluable constraints on the physical processes that govern the final mass expulsion of intermediate-mass stars. We focus this study on the famous M2-9 bipolar nebula, where the moving lighthouse beam pattern indicates the presence of a wide binary. The compact and dense dusty core in the center of the nebula can be studied by means of optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are interpreted using a dust radiative transfer Monte Carlo code. A disk-like structure is detected perpendicular to the lobes and a good fit is found with a stratified disk model composed of amorphous silicates. The disk is compact, 25$\times$35 mas at 8$\rm \mu m$, and 37$\times$46 mas at 13$\rm \mu m$. For the adopted distance of 1.2 kpc, the inner rim of the disk is $\sim$15 AU. The mass represents a few percent of the mass found in the lobes. The compactness of the disk puts strong constraints on the binary content of the system, given an estimated orbital period 90-120yr. We derive masses of the binary components between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an evolved star. We present different scenarios on the geometric structure of the disk accounting for the interactions of the binary system, which includes an accretion disk as well.Comment: 9 figures, A&A accepte

Live imaging of targeted cell ablation in Xenopus:a new model to study demyelination and repair

Live imaging studies of the processes of demyelination and remyelination have so far been technically limited in mammals. We have thus generated a Xenopus laevis transgenic line allowing live imaging and conditional ablation of myelinating oligodendrocytes throughout the central nervous system (CNS). In these transgenic pMBP-eGFP-NTR tadpoles the myelin basic protein (MBP) regulatory sequences, specific to mature oligodendrocytes, are used to drive expression of an eGFP (enhanced green fluorescent protein) reporter fused to the E. coli nitroreductase (NTR) selection enzyme. This enzyme converts the innocuous pro-drug metronidazole (MTZ) to a cytotoxin. Using two-photon imaging in vivo, we show that pMBP-eGFP-NTR tadpoles display a graded oligodendrocyte ablation in response to MTZ, which depends on the exposure time to MTZ. MTZ-induced cell death was restricted to oligodendrocytes, without detectable axonal damage. After cessation of MTZ treatment, remyelination proceeded spontaneously, but was strongly accelerated by retinoic acid. Altogether, these features establish the Xenopus pMBP-eGFP-NTR line as a novel in vivo model for the study of demyelination/remyelination processes and for large-scale screens of therapeutic agents promoting myelin repair

Radio, X-ray, and gamma-ray Emission Models of the Colliding Winds Binary WR 140

We use hydrodynamical models of the wind-collision region (WCR) in the archetype colliding-wind system WR140 to determine the spatial and spectral distribution of the radio, X-ray and gamma-ray emission from shock accelerated electrons. Our calculations are for orbital phase 0.837 when the observed radio emission is close to maximum. Using the observed thermal X-ray emission together with the radio emission to constrain the mass-loss rates, we find that the O-star mass-loss rate is consistent with recent reductions for O4-5 supergiants. We demonstrate that radio VLBI observations of the WCR fail to constrain the opening angle. The observed low frequency turnover at ~3 GHz in the radio emission is due to free-free absorption, since models based on the Razin effect have an unacceptably large fraction of energy in non-thermal particles. The index of the non-thermal electron energy distribution is flatter than the canonical value for diffusive shock acceleration, namely p<2. Several mechanisms are discussed that could lead to such an index. Tighter constraints on p and the nature of the shocks in WR140 will be obtained from future observations at MeV and GeV energies, for which we generally predict lower fluxes than previous work. Since the high stellar photon fluxes prevent the acceleration of electrons beyond gamma > 1e5-1e6, TeV emission from CWB systems will provide unambiguous evidence of pion-decay emission from accelerated ions. We finish by commenting on the emission and physics of the multiple wind collisions in dense stellar clusters, paying particular attention to the Galactic Centre (abridged).Comment: Accepted by MNRAS, 29 pages, 23 figures; substantial changes made following referee's repor

Stem Cells and Regeneration in the Xenopus Retina

International audienceThe ability to regenerate damaged cells in the retina varies tremendously among species, being restricted for most of them to specific developmental stages. Regarding vertebrates, only the newt was thought to exhibit full regenerative capacity upon retinectomy in the adulthood. The recent discovery that the anuran amphibian Xenopus can regenerate its retina after metamorphosis opened new avenues to investigate the cellular and molecular mechanisms involved in this process. In this review, we provide an historical overview of regeneration studies in Xenopus. Particular emphasis is given to the cellular sources contributing to retinal replacement, the involvement of tissue interactions and the importance of the injury paradigm. We also describe recent progress and promises in the field brought by the development of 3D tissue culture methods and transgenic Xenopus models