Hubble Space Telescope Observations of UV Oscillations in WZ Sagittae During the Decline from Outburst

We present a time series analysis of Hubble Space Telescope observations of WZ Sge obtained in 2001 September, October, November and December as WZ Sge declined from its 2001 July superoutburst. Previous analysis of these data showed the temperature of the white dwarf decreased from ~29,000 K to ~18,000 K. In this study we binned the spectra over wavelength to yield ultraviolet light curves at each epoch that were then analyzed for the presence of the well-known 27.87 s and 28.96 s oscillations. We detect the 29 s periodicity at all four epochs, but the 28 s periodicity is absent. The origin of these oscillations has been debated since their discovery in the 1970s and competing hypotheses are based on either white dwarf non-radial g-mode pulsations or magnetically-channelled accretion onto a rotating white dwarf. By analogy with the ZZ Ceti stars, we argue that the non-radial g-mode pulsation model demands a strong dependence of pulse period on the white dwarf's temperature. However, these observations show the 29 s oscillation is independent of the white dwarf's temperature. Thus we reject the white dwarf non-radial g-mode pulsation hypothesis as the sole origin of the oscillations. It remains unclear if magnetically-funnelled accretion onto a rapidly rotating white dwarf (or belt on the white dwarf) is responsible for producing the oscillations. We also report the detection of a QPO with period ~18 s in the September light curve. The amplitudes of the 29 s oscillation and the QPO vary erratically on short timescales and are not correlated with the mean system brightness nor with each other.Comment: 20 pages, 3 figures, 1 table; accepted for publication in Ap

Post-irradiation chemical processing of DNA damage generates double-strand breaks in cells already engaged in repair

In cells exposed to ionizing radiation (IR), double-strand breaks (DSBs) form within clustered-damage sites from lesions disrupting the DNA sugar–phosphate backbone. It is commonly assumed that these DSBs form promptly and are immediately detected and processed by the cellular DNA damage response (DDR) apparatus. This assumption is questioned by the observation that after irradiation of naked DNA, a fraction of DSBs forms minutes to hours after exposure as a result of temperature dependent, chemical processing of labile sugar lesions. Excess DSBs also form when IR-exposed cells are processed at 50°C, but have been hitherto considered method-related artifact. Thus, it remains unknown whether DSBs actually develop in cells after IR exposure from chemically labile damage. Here, we show that irradiation of ‘naked’ or chromatin-organized mammalian DNA produces lesions, which evolve to DSBs and add to those promptly induced, after 8–24 h in vitro incubation at 37°C or 50°C. The conversion is more efficient in chromatin-associated DNA, completed within 1 h in cells and delayed in a reducing environment. We conclude that IR generates sugar lesions within clustered-damage sites contributing to DSB formation only after chemical processing, which occurs efficiently at 37°C. This subset of delayed DSBs may challenge DDR, may affect the perceived repair kinetics and requires further characterization

GW Lib: a Unique Laboratory for White Dwarf Pulsations

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in mixed H/He/Z atmospheres, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Lib stands out by having a well-established observational record of three independent pulsation modes that were wiped out during its 2007 outburst. We have obtained new HST ultraviolet observations in May 2013 that show an unexpected behaviour: besides some activity near the ̃280 s period that has been observed in the past, the white dwarf underwent a large-amplitude brightening. We demonstrate that the brightening is related to an increase of the photospheric temperature, argue against an accretion episode as explanation, and discuss this event in the context of non-radial pulsations on a rapidly rotating star

The space density and X-ray luminosity function of non-magnetic cataclysmic variables

We combine two complete, X-ray flux-limited surveys, the ROSAT Bright Survey (RBS) and the ROSAT North Ecliptic Pole (NEP) survey, to measure the space density (\rho) and X-ray luminosity function (\Phi) of non-magnetic CVs. The combined survey has a flux limit of F_X \ga 1.1 \times 10^{-12} erg cm^{-2}s^{-1} over most of its solid angle of just over 2\pi, but is as deep as \simeq 10^{-14} erg cm^{-2}s^{-1} over a small area. The CV sample that we construct from these two surveys contains 20 non-magnetic systems. We carefully include all sources of statistical error in calculating \rho and \Phi by using Monte Carlo simulations; the most important uncertainty proves to be the often large errors in distances estimates. If we assume that the 20 CVs in the combined RBS and NEP survey sample are representative of the intrinsic population, the space density of non-magnetic CVs is 4^{+6}_{-2} \times 10^{-6} pc^{-3}. We discuss the difficulty in measuring \Phi in some detail---in order to account for biases in the measurement, we have to adopt a functional form for \Phi. Assuming that the X-ray luminosity function of non-magnetic CVs is a truncated power law, we constrain the power law index to -0.80 \pm 0.05. It seems likely that the two surveys have failed to detect a large, faint population of short-period CVs, and that the true space density may well be a factor of 2 or 3 larger than what we have measured; this is possible, even if we only allow for undetected CVs to have X-ray luminosities in the narrow range 28.7< log(L_X/erg\,s^{-1})<29.7. However, \rho as high as 2 \times 10^{-4} pc^{-3} would require that the majority of CVs has X-ray luminosities below L_X = 4 \times 10^{28} erg s^{-1} in the 0.5--2.0 keV band.Comment: MNRAS, accepted. 14 pages, 8 figure

Les Années 1900 à la Trinité-sur-Mer et à Saint-Philibert / Jean-Christian Godon

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Molecular cloning and developmental expression of AtGR1, a new growth-related Arabidopsis gene strongly induced by ionizing radiation

International audienceScreening for mRNAs that accumulate after DNA damage induced by ionizing radiation, we have isolated a 2.0-kb cDNA coding for a new Arabidopsis PEST-box protein named AtGR1 (A. thaliana gamma response 1) with an expression profile similar to that observed for several plant cell cycle-related proteins. Using an anti-AtGR1 antibody, we have shown that the AtGR1 protein is expressed at basal levels in mitotically dividing cells (meristematic tissues and organ primordia) and at a strongly enhanced level in endoreduplicating cells (stipules, trichomes). Using transgenic Arabidopsis plants that express the GUS reporter gene under the control of the AtGR1 promoter, we have demonstrated that the observed AtGR1 protein distribution is due to the promoter activity. Our results suggest that basal AtGR1 levels are associated with progression through mitosis, whereas elevated intracellular levels of AtGR1 seem to induce changes between the S and M phases of the cell cycle that trigger somatic cells to enter the endoreduplication cycle. Ionizing radiation-induced rapid and dose-dependent accumulation of AtGR1 mRNA in cell cultures and plant tissues leads to tissue-specific accumulation of AtGR1 protein, best observed in ovules, which never undergo an endoreduplication cycle. It therefore appears that the radiation-induced transient AtGR1 accumulation reflects DNA damage-dependent transient cell cycle arrest before mitosis, which is necessary to accomplish DNA repair prior to chromosome segregation and cytokinesis

Preparation and Deposition of Plant Roots for AFM Nanomechanical Measurements

International audienceIn general, cell growth is a mechanical process that balances internal and externalstresses allowing or limiting expansion. Plant cells are often compared to“hydraulicmachines”due to the similar concept of balanced counterforces between the primarywall stresses and the turgor pressure. Knowledge of the mechanics of plant root cellsis essential to understand how plant wall works and, therefore, how plants grow. Onemethod for qualitative and quantitative analysis of mechanical properties is atomicforce microscopy (AFM) that measures mechanical properties of living cells or tissuesunder conditions close to relevant physiological environments (Arnould et al., 2017,Kozlova et al., 2019, Milani et al., 2011, 2014, Peaucelle et al., 2012, 2011, Torode et al.,2018, Yakubov et al., 2016, Zdunek and Kurenda, 2013, Zhao et al., 2005). In our recentstudy, AFM has been used to investigate properties of root epidermal cells (Balzergueet al., 2017), which are the cells that form the outermost layer of the root. These epi-dermal cells were studied in situ on living seedlings and were therefore still influ-enced by the inherent multicellular properties of the root, which is an improvementover studies conducted on isolated living cells.This chapter explains and illustrates the different steps required to prepare anddeposit the seedling, and in particular the root tip, of the plant on a glass slideready for nanomechanical measurements by AF

Short- and long-term dynamics of the toxic potential and genotypic structure in benthic populations of Microcystis.

International audienceMicrocystis colonies are known to overwinter on the surface of the sediment of freshwater ecosystems. However, little is known about the genotypic and toxicological dynamics of Microcystis populations during this benthic life stage. In this study, we report a two-year-long survey of benthic populations of Microcystis, which had spent from a few days to more than six years in the sediment. In order to avoid any interaction with the planktonic proliferations, we chose two deeply buried benthic populations, which could be easily dated. Quantitative PCR on mcyB gene and protein phosphatase inhibition assays were performed to measure their toxic potential, and their genotypic structure was assessed by Capillary Electrophoresis-Single Strand Conformation Polymorphism (CE-SSCP), based on 16S-23S Intergenic Transcribed Spacer (ITS). The microcystin content of the cells seemed to change sharply during the first few months of benthic survival, whereas this content was low and decreased steadily after several years of benthic life. No genetic selection was observed in either the proportion of potentially toxic clones or the ITS sequences for any of the populations considered. From these results, the benthic life stage of Microcystis appears to preserve the structure and the composition of the population over a far larger time scale than classical overwintering period. Finally, some genotypes were common in both of the benthic populations, even though they originated from planktonic blooms that had developed five years apart, suggesting a major overlap of planktonic proliferations in successive years

Nano-structural stiffness measure for soft biomaterials of heterogeneous elasticity

International audienceMeasuring the structural stiffness aims to reveal the impact of nanostructured components or various physiological circumstances on the elastic response of material to an external indentation. With a pyramidal tip at a nano-scale, we employed the atomic force microscopy (AFM) to indent the surfaces of two compositions of polyacrylamide gels with different softness and seedling roots of Arabidopsis thaliana. We found that the stiffness curve derived from the measured force exhibits a heterogeneous character in elasticity. According to the tendency of stiffness curve, we decomposed the responding force into depth-impact (FC), Hookean (FH) and tip-shape (FS) components, called trimechanic, and represent their strengths by the respective spring constants (kC, kH, kS) of three parallel-connected spring (3PCS) analogs to differentiate restoring nanomechansims of indented materials. The effective Young's modulus Ê and the total stiffness kT (= kH + kS) globally unambiguously distinguish the softness between the two gel categories. Data fluctuations were observed in the elasticity parameters of individual samples, reflecting nanostructural variations in the gel matrix. Similar tendencies were found in the results from growing plant roots, though the data fluctuations are expectedly much more dramatic. The zone-wise representation of stiffness by the trimechanic-3PCS framework demonstrates a stiffness measure that reflects beneath nanostructures encountered by deepened depth. It provides a new paradigm for analyzing restoring nanomechanics of soft biomaterials in response to indenting forces