From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco

The 10th recorded outburst of the recurrent eclipsing nova USco was observed simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9 after outburst. Two full passages of the companion in front of the nova ejecta were observed, witnessing the reformation of the accretion disk. On day 22.9, we observed smooth eclipses in UV and optical but deep dips in the X-ray light curve which disappeared by day 34.9, then yielding clean eclipses in all bands. X-ray dips can be caused by clumpy absorbing material that intersects the line of sight while moving along highly elliptical trajectories. Cold material from the companion could explain the absence of dips in UV and optical light. The disappearance of X-ray dips before day 34.9 implies significant progress in the formation of the disk. The X-ray spectra contain photospheric continuum emission plus strong emission lines, but no clear absorption lines. Both continuum and emission lines in the X-ray spectra indicate a temperature increase from day 22.9 to day 34.9. We find clear evidence in the spectra and light curves for Thompson scattering of the photospheric emission from the white dwarf. Photospheric absorption lines can be smeared out during scattering in a plasma of fast electrons. We also find spectral signatures of resonant line scattering that lead to the observation of the strong emission lines. Their dominance could be a general phenomenon in high-inclination systems such as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure

The Super Lithium-Rich Red Giant Rapid Rotator G0928+73.2600: A Case for Planet Accretion?

We present the discovery of a super lithium-rich K giant star, G0928+73.2600. This red giant (T_eff = 4885 K and log g = 2.65) is a fast rotator with a projected rotational velocity of 8.4 km/s and an unusually high lithium abundance of A(Li) = 3.30 dex. Although the lack of a measured parallax precludes knowing the exact evolutionary phase, an isochrone-derived estimate of its luminosity places the star on the Hertzsprung-Russell diagram in a location that is not consistent with either the red bump on the first ascent of the red giant branch or with the second ascent on the asymptotic giant branch, the two evolutionary stages where lithium-rich giant stars tend to cluster. Thus, even among the already unusual group of lithium-rich giant stars, G0928+73.2600 is peculiar. Using 12C/13C as a tracer for mixing---more mixing leads to lower 12C/13C---we find 12C/13C = 28, which is near the expected value for standard first dredge-up mixing. We can therefore conclude that "extra" deep mixing has not occurred. Regardless of the ambiguity of the evolutionary stage, the extremely large lithium abundance and the rotational velocity of this star are unusual, and we speculate that G0928+73.2600 has been enriched in both lithium and angular momentum from a sub-stellar companion.Comment: 5 pages, 4 figures, Accepted by ApJ Letter

Susceptibility of salt marshes to nutrient enrichment and predator removal

Salt marsh ecosystems have been considered not susceptible to nitrogen overloading because early studies suggested that salt marshes adsorbed excess nutrients in plant growth. However, the possible effect of nutrient loading on species composition, and the combined effects of nutrients and altered species composition on structure and function, was largely ignored. Failure to understand interactions between nutrient loading and species composition may lead to severe underestimates of the impacts of stresses. We altered whole salt marsh ecosystems (similar to 60 000 m(2)/treatment) by addition of nutrients in flooding waters and by reduction of a key predatory fish, the mummichog. We added nutrients (N and P; 15-fold increase over ambient conditions) directly to the flooding tide to mimic the way anthropogenic nutrients are delivered to marsh ecosystems. Despite the high concentrations (70 mmol N/L) achieved in the water column, our annual N loadings (15-60 g N.m(-2).yr(-1)) were an order of magnitude less than most plot-level fertilization experiments, yet we detected responses at several trophic levels. Preliminary calculations suggest that 30-40% of the added N was removed by the marsh during each tidal cycle. Creek bank Spartina alterniflora and high marsh S. patens production increased, but not stunted high marsh S. alterniflora. Microbial production increased in the fertilized creek bank S. alterniflora habitat where benthic microalgae also increased. We found top-down control of benthic microalgae by killifish, but only under nutrient addition and in the opposite direction (increase) than that predicted by a fish-invertebrate-microalgae trophic cascade. Surprisingly, infauna declined in abundance during the first season of fertilization and with fish removal. Our results demonstrate ecological effects of both nutrient addition and mummichog reduction at the whole-system level, including evidence for synergistic interactions