Subaru and Swift observations of V652 Herculis: resolving the photospheric pulsation

High-resolution spectroscopy with the Subaru High Dispersion Spectrograph, and Swift ultraviolet photometry are presented for the pulsating extreme helium star V652 Her. Swift provides the best relative ultraviolet photometry obtained to date, but shows no direct evidence for a shock at ultraviolet or X-ray wavelengths. Subaru has provided high spectral and high temporal resolution spectroscopy over six pulsation cycles (and eight radius minima). These data have enabled a line-by-line analysis of the entire pulsation cycle and provided a description of the pulsating photosphere as a function of optical depth. They show that the photosphere is compressed radially by a factor of at least 2 at minimum radius, that the phase of radius minimum is a function of optical depth and the pulse speed through the photosphere is between 141 and 239 km s−1 (depending how measured) and at least 10 times the local sound speed. The strong acceleration at minimum radius is demonstrated in individual line profiles; those formed deepest in the photosphere show a jump discontinuity of over 70 kms−1 on a time-scale of 150 s. The pulse speed and line profile jumps imply a shock is present at minimum radius. These empirical results provide input for hydrodynamical modelling of the pulsation and hydrodynamical plus radiative transfer modelling of the dynamical spectra

Accelerated Stress Testing of Hydrocarbon-Based Encapsulants for Medium-Concentration CPV Applications

Concentrating photovoltaic (CPV) systems have great potential to reduce photovoltaic (PV) electricity costs because of the relatively low cost of optical components as compared to PV cells. A transparent polymeric material is used to optically couple the PV cell to optical components and is thus exposed to the concentrated light source at elevated temperatures. In this work polymeric encapsulant materials are positioned close to a Xenon arc lamp to expose them to ultraviolet radiation (UV) that is about 42 times as intense as sunlight. Furthermore, different glass types are used as filters to modify the spectral distribution of light in the UV range. A strong sensitivity of non-silicone-based encapsulants to light below ~350 nm is demonstrated. Of all the materials examined in this study, the polydimethyl silicone samples performed the best. The next best material was an ionomer which maintained optical transmission but became photo-oxidized where exposed to the atmosphere

The Solar Neighborhood. XIX. Discovery and Characterization of 33 New Nearby White Dwarf Systems

We present spectra for 33 previously unclassified white dwarf systems brighter than V = 17 primarily in the southern hemisphere. Of these new systems, 26 are DA, 4 are DC, 2 are DZ, and 1 is DQ. We suspect three of these systems are unresolved double degenerates. We obtained VRI photometry for these 33 objects as well as for 23 known white dwarf systems without trigonometric parallaxes, also primarily in the southern hemisphere. For the 56 objects, we converted the photometry values to fluxes and fit them to a spectral energy distribution using the spectroscopy to determine which model to use (i.e. pure hydrogen, pure helium, or metal-rich helium), resulting in estimates of effective temperature and distance. Eight of the new and 12 known systems are estimated to be within the NStars and Catalogue of Nearby Stars (CNS) horizons of 25 pc, constituting a potential 18% increase in the nearby white dwarf sample. Trigonometric parallax determinations are underway via CTIOPI for these 20 systems. One of the DCs is cool so that it displays absorption in the near infrared. Using the distance determined via trigonometric parallax, we are able to constrain the model-dependent physical parameters and find that this object is most likely a mixed H/He atmosphere white dwarf similar to other cool white dwarfs identified in recent years with significant absorption in the infrared due to collision-induced absorptions by molecular hydrogen.Comment: 33 pages, 10 figures, accepted for publication in the Astronomical Journa

The human CTF4-orthologue AND-1 interacts with DNA polymerase α/primase via its unique C-terminal HMG box

A dynamic multi-protein assembly known as the replisome is responsible for DNA synthesis in eukaryotic cells. In yeast, the hub protein Ctf4 bridges DNA helicase and DNA polymerase and recruits factors with roles in metabolic processes coupled to DNA replication. An important question in DNA replication is the extent to which the molecular architecture of the replisome is conserved between yeast and higher eukaryotes. Here we describe the biochemical basis for the interaction of the human CTF4-orthologue AND-1 with Pol α/primase, the replicative polymerase that initiates DNA synthesis. AND-1 has maintained the trimeric structure of yeast Ctf4, driven by its conserved SepB domain. However, the primary interaction of AND-1 with Pol α/primase is mediated by its C-terminal HMG box, unique to mammalian AND-1, which binds the B subunit, at the same site targeted by the SV40 T antigen for viral replication. In addition, we report a novel DNA-binding activity in AND-1, which might promote the correct positioning of Pol α/primase on the lagging-strand template at the replication fork. Our findings provide a biochemical basis for the specific interaction between two critical components of the human replisome, and indicate that important principles of replisome architecture have changed significantly in evolution.This work was supported by a Wellcome Trust investigator award to L.P. (104641/Z/14/Z), a Cambridge Gates PhD scholarship to A.C.S., a PhD fellowship of the Boehringer-Ingelheim Fonds and awards from the Janggen-Pöhn-Stiftung and the Swiss National Science Foundation to S.H

The rapidly pulsating sdO star, SDSS J160043.6+074802.9

A spectroscopic analysis of SDSS J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields Teff = 70 000 +/- 5000 K and log g = 5.25 +/- 0.30, together with a most likely type of K3V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. (2008) and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. It therefore seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having log g > 5.3 to be more likely to be unstable and capable of driving pulsation in the observed frequency range.Comment: 14 pages, 12 figures, accepted for publication in MNRAS, 2009 September

The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey

Spectrophotometric observations at high signal-to-noise ratio were obtained of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey. Fits of observed Balmer lines to synthetic spectra calculated from pure-hydrogen model atmospheres were used to obtain robust values of Teff, log g, masses, radii, and cooling ages. The luminosity function of the sample, weighted by 1/Vmax, was obtained and compared with other determinations. The mass distribution of the white dwarfs is derived, after important corrections for the radii of the white dwarfs in this magnitude-limited survey and for the cooling time scales. The formation rate of DA white dwarfs from the PG is estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from a theoretical study of the white dwarf formation rate for single stars indicates that >80% of the high mass component requires a different origin, presumably mergers of lower mass double degenerate stars. In order to estimate the recent formation rate of all white dwarfs in the local Galactic disk, corrections for incompleteness of the PG, addition of the DB-DO white dwarfs, and allowance for stars hidden by luminous binary companions had to be applied to enhance the rate. An overall formation rate of white dwarfs recently in the local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two recent studies of samples of nearby Galactic planetary nebulae lead to estimates around twice as high. Difficulties in reconciling these determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen

Mass limits for the progenitor star of supernova 2001du and other type II-P supernovae

The supernova SN2001du in the galaxy NGC1365 (19+/-2Mpc), is a core-collapse event of type II-P. Images of this galaxy, have been taken with HST approximately 6.6 years before discovery and include the supernova position on the WFPC2 field of view. We have observed the supernova with the WFPC2 to allow accurate differential astrometry of SN2001du on the pre-explosion frames. There is a marginal detection (3-sigma) of a source close to the supernova position on the prediscovery V-band frame, however it is not precisely coincident and we do not believe it to be a robust detection of a point source. We conclude that there is no stellar progenitor at the supernova position and derive sensitivity limits of the prediscovery images which provide an upper mass limit for the progenitor star. We estimate that the progenitor had a mass of less than 15M_sol. We revisit two other nearby SNe II-P which have high quality pre-explosion images, and refine the upper mass limits for the progenitor stars. Finally we compile all the direct information available for the progenitors of eight nearby core-collapse supernovae and compare their mass estimates. These are compared with the latest stellar evolutionary models of pre-supernova evolution which have attempted to relate metallicity and mass to the supernovae type. Reasonable agreement is found for the lower mass events (generally the II-P), but some discrepancies appear at higher masses. (abridged).Comment: Minor changes, accepted for publication in MNRAS, full resolution version on http://www.ast.cam.ac.uk/~sjs/papers/sn2001du_smartt.p

Periodic variations in the O-C diagrams of five pulsation frequencies of the DB white dwarf EC 20058-5234

Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058−5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions.Web of Scienc

Fortnightly Fluctuations in the O-C Diagram of CS 1246

Dominated by a single, large-amplitude pulsation mode, the rapidly-pulsating hot subdwarf B star CS 1246 is a prime candidate for a long-term O-C diagram study. We collected nearly 400 hours of photometry with the PROMPT telescopes over a time span of 14 months to begin looking for secular variations in the pulse timings. Interestingly, the O-C diagram is dominated by a strong sinusoidal pattern with a period of 14.1 days and an amplitude of 10.7 light-seconds. Underneath this sine wave is a secular trend implying a decrease in the 371.7-s pulsational period of Pdot = -1.9 x 10^-11, which we attribute to the evolution of the star through the H-R diagram. The sinusoidal variation could be produced by the presence of a low-mass companion, with m sin i ~ 0.12 Msun, orbiting the subdwarf B star at a distance of 20 Rsun. An analysis of the combined light curve reveals the presence of a low-amplitude first harmonic to the main pulsation mode.Comment: Accepted for publication in MNRAS. 11 pages, 8 figures, 5 table