Energy Distribution of Micro-events in the Quiet Solar Corona

Recent imaging observations of EUV line emissions have shown evidence for frequent flare-like events in a majority of the pixels in quiet regions of the solar corona. The changes in coronal emission measure indicate impulsive heating of new material to coronal temperatures. These heating or evaporation events are candidate signatures of "nanoflares" or "microflares" proposed to interpret the high temperature and the very existence of the corona. The energy distribution of these micro-events reported in the literature differ widely, and so do the estimates of their total energy input into the corona. Here we analyze the assumptions of the different methods, compare them by using the same data set and discuss their results. We also estimate the different forms of energy input and output, keeping in mind that the observed brightenings are most likely secondary phenomena. A rough estimate of the energy input observed by EIT on the SoHO satellite is of the order of 10% of the total radiative output in the same region. It is considerably smaller for the two reported TRACE observations. The discrepancy can be explained partially by different thresholds for flare detection. There is agreement on the slope and the absolute value of the distribution if the same method were used and a numerical error corrected. The extrapolation of the power law to unobserved energies that are many orders of magnitude smaller remains questionable. Nevertheless, these micro-events and unresolved smaller events are currently the best source of information on the heating process of the corona

Infrared Spectroscopy of Molecular Supernova Remnants

We present Infrared Space Observatory spectroscopy of sites in the supernova remnants W28, W44, and 3C391, where blast waves are impacting molecular clouds. Atomic fine-structure lines were detected from C, N, O, Si, P, and Fe. The S(3) and S(9) lines of H2 were detected for all three remnants. The observations require both shocks into gas with moderate (~ 100 /cm3) and high (~10,000 /cm3) pre-shock densities, with the moderate density shocks producing the ionic lines and the high density shock producing the molecular lines. No single shock model can account for all of the observed lines, even at the order of magnitude level. We find that the principal coolants of radiative supernova shocks in moderate-density gas are the far-infrared continuum from dust grains surviving the shock, followed by collisionally-excited [O I] 63.2 and [Si II] 34.8 micron lines. The principal coolant of the high-density shocks is collisionally-excited H2 rotational and ro-vibrational line emission. We systematically examine the ground-state fine structure of all cosmically abundant elements, to explain the presence or lack of all atomic fine lines in our spectra in terms of the atomic structure, interstellar abundances, and a moderate-density, partially-ionized plasma. The [P II] line at 60.6 microns is the first known astronomical detection. There is one bright unidentified line in our spectra, at 74.26 microns. The presence of bright [Si II] and [Fe II] lines requires partial destruction of the dust. The required gas-phase abundance of Fe suggests 15-30% of the Fe-bearing grains were destroyed. The infrared continuum brightness requires ~1 Msun of dust survives the shock, suggesting about 1/3 of the dust mass was destroyed, in agreement with the depletion estimate and with theoretical models for dust destruction.Comment: 40 pages; 10 figures; accepted by ApJ July 11, 200

The Heavy Element Enrichment of Lyman alpha Clouds in the Virgo Supercluster

Using high S/N STIS echelle spectra (FWHM=7 km/s) of 3C 273, we constrain the metallicities of two Lya clouds in the vicinity of the Virgo cluster. We detect C II, Si II, and Si III absorption lines in the Lya absorber at z = 0.00530. Previous observations with FUSE have revealed Ly beta - Ly theta lines at this redshift, thereby accurately constraining N(H I). We model the ionization of the gas and derive [C/H] = -1.2^{+0.3}_{-0.2}, [Si/C] = 0.2+/-0.1, and log n_{H} = -2.8+/-0.3. The model implies a small absorber thickness, ~70 pc, and thermal pressure p/k ~ 40 cm^{-3} K. It is most likely that the absorber is pressure confined by an external medium because gravitational confinement would require a very high ratio of dark matter to baryonic matter. Based on Milky Way sight lines in which carbon and silicon abundances have been reliably measured in the same interstellar cloud (including new measurements presented herein), we argue that the overabundance of Si relative to C is not due to dust depletion. Instead, this probably indicates that the gas has been predominately enriched by Type II supernovae. Such enrichment is most plausibly provided by an unbound galactic wind, given the absence of galaxies within a projected distance of 100 kpc and the presence of galaxies capable of driving a wind at larger distances. We also constrain the metallicity and physical conditions of the Virgo absorber at z = 0.00337 based on detections of O VI and H I and an upper limit on C IV. If this absorber is collisionally ionized, the O VI/C IV limit requires T > 10^{5.3} K. For either collisional ionization or photoionization, we find that [O/H] > -2.0 at z = 0.00337.Comment: Final Ap.J. versio

Composition Dependence of the Structure and Electronic Properties of Liquid Ga-Se Alloys Studied by Ab Initio Molecular Dynamics Simulation

Ab initio molecular dynamics simulation is used to study the structure and electronic properties of the liquid Ga-Se system at the three compositions Ga$_2$Se, GaSe and Ga$_2$Se$_3$, and of the GaSe and Ga$_2$Se$_3$ crystals. The calculated equilibrium structure of GaSe crystal agrees well with available experimental data. The neutron-weighted liquid structure factors calculated from the simulations are in reasonable agreement with recent neutron diffraction measurements. Simulation results for the partial radial distribution functions show that the liquid structure is closely related to that of the crystals. A close similarity between solid and liquid is also found for the electronic density of states and charge density. The calculated electronic conductivity decreases strongly with increasing Se content, in accord with experimental measurements.Comment: REVTeX, 8 pages and 12 uuencoded PostScript figures, submitted to Phys. Rev. B. corresponding author: [email protected]

The HST Quasar Absorption Line Key Project XII. The Unusual Absorption Line System in the Spectrum of PG 2302+029 ---Ejected or Intervening?

We report the discovery of a high-ionization broad absorption line system at a redshift of z_abs=0.695 in the spectrum of the z_em=1.052 radio-quiet quasar PG 2302+039. Broad absorption with FWHM from 3,000 to 5,000 km/s is detected from C IV, N V, and O VI in HST Faint Object Spectrograph spectra of the quasar. A narrow line system (FWHM approximately 250 km/s) at z_abs=0.7016 is resolved from the broad blend and includes absorption by Ly-alpha and the C IV, N V, and O VI doublets. No absorption by low-ionization metal species (e.g. Si II and Mg II) is detected in the HST or ground-based spectra for either the broad or the narrow system. The centroids of the broad system lines are displaced by approximately 56,000 km/s to the blue of the quasar's broad emission lines. The reddest extent of the broad line absorption is more than 50,000 km/s from the quasar. The properties of this system are unprecedented whether it is an intervening or an ejected system.Comment: 9 pages, LaTeX file, two postscript figures. Accepted for publication in The Astrophysical Journal Letters. Changes include removing TeX macros from abstrac

Spin-ice physics in cadmium cyanide

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1–5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.ISSN:2041-172

Exclusion of a major role for the PTEN tumour-suppressor gene in breast carcinomas

PTEN is a novel tumour-suppressor gene located on chromosomal band 10q23.3. This region displays frequent loss of heterozygosity (LOH) in a variety of human neoplasms including breast carcinomas. The detection of PTEN mutations in Cowden disease and in breast carcinoma cell lines suggests that PTEN may be involved in mammary carcinogenesis. We here report a mutational analysis of tumour specimens from 103 primary breast carcinomas and constitutive DNA from 25 breast cancer families. The entire coding region of PTEN was screened by single-strand conformation polymorphism (SSCP) analysis and direct sequencing using intron-based primers. No germline mutations could be identified in the breast cancer families and only one sporadic carcinoma carried a PTEN mutation at one allele. In addition, all sporadic tumours were analysed for homozygous deletions by differential polymerase chain reaction (PCR) and for allelic loss using the microsatellite markers D10S215, D10S564 and D10S573. No homozygous deletions were detected and only 10 out of 94 informative tumours showed allelic loss in the PTEN region. These results suggest that PTEN does not play a major role in breast cancer formation. 1999 Cancer Research Campaig

Evaluation of polygenic risk scores for breast and ovarian cancer risk prediction in BRCA1 and BRCA2 mutation carriers

Background: Genome-wide association studies (GWAS) have identified 94 common single-nucleotide polymorphisms (SNPs) associated with breast cancer (BC) risk and 18 associated with ovarian cancer (OC) risk. Several of these are also associated with risk of BC or OC for women who carry a pathogenic mutation in the high-risk BC and OC genes BRCA1 or BRCA2. The combined effects of these variants on BC or OC risk for BRCA1 and BRCA2 mutation carriers have not yet been assessed while their clinical management could benefit from improved personalized risk estimates. Methods: We constructed polygenic risk scores (PRS) using BC and OC susceptibility SNPs identified through population-based GWAS: for BC (overall, estrogen receptor [ER]-positive, and ER-negative) and for OC. Using data from 15 252 female BRCA1 and 8211 BRCA2 carriers, the association of each PRS with BC or OC risk was evaluated using a weighted cohort approach, with time to diagnosis as the outcome and estimation of the hazard ratios (HRs) per standard deviation increase in the PRS. Results: The PRS for ER-negative BC displayed the strongest association with BC risk in BRCA1 carriers (HR = 1.27, 95% confidence interval [CI] = 1.23 to 1.31, P = 8.2 x 10(53)). In BRCA2 carriers, the strongest association with BC risk was seen for the overall BC PRS (HR = 1.22, 95% CI = 1.17 to 1.28, P = 7.2 x 10(-20)). The OC PRS was strongly associated with OC risk for both BRCA1 and BRCA2 carriers. These translate to differences in absolute risks (more than 10% in each case) between the top and bottom deciles of the PRS distribution; for example, the OC risk was 6% by age 80 years for BRCA2 carriers at the 10th percentile of the OC PRS compared with 19% risk for those at the 90th percentile of PRS. Conclusions: BC and OC PRS are predictive of cancer risk in BRCA1 and BRCA2 carriers. Incorporation of the PRS into risk prediction models has promise to better inform decisions on cancer risk management