Post-flare UV light curves explained with thermal instability of loop plasma

In the present work we study the C8 flare occurred on September 26, 2000 at 19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the impulsive phase to well beyond the disappearance in the X-rays. The emission first decayed progressively through equilibrium states until the plasma reached 2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium conditions), are emitted at the same time and all evolve in a similar way. Here we show that the simultaneous emission of lines with such a different formation temperature is due to thermal instability occurring in the flaring plasma as soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative start time of the light curves of each line in the correct order with a simple (and standard) model of a single flaring loop. The agreement with the observed light curves is greatly improved, and a slower evolution of the line emission is predicted, if we assume that the model loop consists of an ensemble of subloops or strands heated at slightly different times. Our analysis can be useful for flare observations with SDO/EVE.Comment: 24 pages, 7 figures, accepted for publicatio

Search for Rare and Forbidden Dilepton Decays of the D+, Ds, and D0 Charmed Mesons

We report the results of a search for flavor-changing neutral current, lepton-flavor violating, and lepton-number violating decays of D+, Ds, and D0 mesons (and their antiparticles) into modes containing muons and electrons. Using data from Fermilab charm hadroproduction experiment E791, we examine the pi,l,l and K,l,l decay modes of D+ and Ds and the l+l- decay modes of D0. No evidence for any of these decays is found. Therefore, we present branching-fraction upper limits at 90% confidence level for the 24 decay modes examined. Eight of these modes have no previously reported limits, and fourteen are reported with significant improvements over previously published results.Comment: 12 pages, 3 figures, LaTeX, elsart.cls, epsf.sty, amsmath.sty Submitted to Physics Letters

Modelling the chromosphere and transition region of Epsilon Eri (K2 V)

Measurements of ultraviolet line fluxes from Space Telescope Imaging Spectrograph and Far-Ultraviolet Spectroscopic Explorer spectra of the K2-dwarf Epsilon Eri are reported. These are used to develop new emission measure distributions and semi-empirical atmospheric models for the chromosphere and lower transition region of the star. These models are the most detailed constructed to date for a main-sequence star other than the Sun. New ionisation balance calculations, which account for the effect of finite density on dielectronic recombination rates, are presented for carbon, nitrogen, oxygen and silicon. The results of these calculations are significantly different from the standard Arnaud & Rothenflug ion balance, particularly for alkali-like ions. The new atmospheric models are used to place constraints on possible First Ionisation Potential (FIP) related abundance variations in the lower atmosphere and to discuss limitations of single-component models for the interpretation of certain optically thick line fluxes.Comment: 22 pages, 9 figure

Search for Rare and Forbidden Charm Meson Decays D0--> V l+ l- and h h l l

We report the results of a search for flavor-changing neutral current, lepton-flavor, and lepton-number violating decays of the 3 and 4-body decay modes of D0 (and its antiparticle) containing muons and electrons. Using data from Fermilab charm hadroproduction experiment E791, we examine modes with two leptons and either a Rho(0), Kstar, or Phi vector meson or a non-resonant Pi Pi, K Pi, or K K pair of pseudoscalar mesons. No evidence for any of these decays is found. Therefore, we present branching-fraction upper limits at 90% confidence level for the 27 decay modes examined (18 new).Comment: 5 pages, 1 Figure, Submitted to PR

Interstitial Spaces: Microinteraction Settings and the Genesis of New Practices Between Institutional Fields

I develop a model linking specific microinteraction dynamics between members of different institutional fields and the genesis of new practices. The model centers on the concept of interstitial spaces—that is, small-scale settings where individuals from different fields interact occasionally and informally around common activities to which they devote limited time (e.g., hobbyist clubs, hangouts, workshops, meet-ups). I argue that the features of interstitial spaces (e.g., their institutional diversity and their occasional and informal nature) facilitate the individuals interacting in these settings to temporarily break free from existing institutions and experiment collectively with new activities and ideas. However, these very same features hinder the constitution of such new activities and ideas into new practices. I identify two microlevel conditions that enable the new activities and ideas developed in interstitial spaces to be constituted into new practices: the emergence of successful interaction rituals, and the presence of catalysts sustaining others' interactions and assisting the construction of shared meanings

A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars

The Curiosity rover discovered fine-grained sedimentary rocks, inferred to represent an ancient lake, preserve evidence of an environment that would have been suited to support a Martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. C, H, O, S, N, and P were measured directly as key biogenic elements, and by inference N and P are assumed to have been available. The environment likely had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars