Normal type Ia supernovae from violent mergers of white dwarf binaries

One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 M and 1.1 M combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms in the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic color light curves of our merger, which produces about 0.62 M of Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia. © 2012 The American Astronomical Society. All rights reserved.

On the brightness distribution of type ia supernovae from violent white dwarf mergers

We investigate the brightness distribution expected for thermonuclear explosions that might result from the ignition of a detonation during the violent merger of white dwarf (WD) binaries. Violent WD mergers are a subclass of the canonical double degenerate scenario where two carbon-oxygen (CO) WDs merge when the larger WD fills its Roche lobe. Determining their brightness distribution is critical for evaluating whether such an explosion model could be responsible for a significant fraction of the observed population of Type Ia supernovae (SNe Ia). We argue that the brightness of an explosion realized via the violent merger model is mainly determined by the mass of Ni produced in the detonation of the primary COWD. To quantify this link, we use a set of sub-Chandrasekhar mass WD detonation models to derive a relationship between primary WD mass (m) and expected peak bolometric brightness (M). We use this m-M relationship to convert the masses of merging primary WDs from binary population models to a predicted distribution of explosion brightness. We also investigate the sensitivity of our results to assumptions about the conditions required to realize a detonation during violent mergers ofWDs. We find a striking similarity between the shape of our theoretical peak-magnitude distribution and that observed for SNe Ia: our model produces a M distribution that roughly covers the range and matches the shape of the one observed for SNe Ia. However, this agreement hinges on a particular phase of mass accretion during binary evolution: the primary WD gains ~0.15-0.35M? from a slightly evolved helium star companion. In our standard binary evolution model, such an accretion phase is predicted to occur for about 43 per cent of all binary systems that ultimately give rise to binary CO WD mergers. We also find that with high probability, violent WD mergers involving the most massive primaries (?1.3M?, which should produce bright SNe) have delay times ?500 Myr. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Three-dimensional delayed-detonation models with nucleosynthesis for type ia supernovae

We present results for a suite of 14 three-dimensional, high-resolution hydrodynamical simulations of delayed-detonation models of Type Ia supernova (SN Ia) explosions. This model suite comprises the first set of three-dimensional SN Ia simulations with detailed isotopic yield information. As such, it may serve as a data base for Chandrasekhar-mass delayed-detonation model nucleosynthetic yields and for deriving synthetic observables such as spectra and light curves. We employ aphysically motivated, stochastic model based on turbulent velocity fluctuations and fuel density to calculate in situ the deflagration-to-detonation transition probabilities. To obtain different strengths of the deflagration phase and thereby different degrees of pre-expansion, we have chosen a sequence of initial models with 1, 3, 5, 10, 20, 40, 100, 150, 200, 300 and 1600 (two different realizations) ignition kernels in a hydrostatic white dwarf with a central density of 2.9 × 10 g cm, as well as one high central density (5.5 × 10 g cm) and one low central density (1.0 × 10 g cm) rendition of the 100 ignition kernel configuration. For each simulation, we determined detailed nucleosynthetic yields by postprocessing10 tracer particles with a 384 nuclide reaction network. All delayed-detonation models result in explosions unbinding thewhite dwarf, producing a range of 56Ni masses from 0.32 to 1.11M. As a general trend, the models predict that the stableneutron-rich iron-group isotopes are not found at the lowest velocities, but rather at intermediate velocities (~3000×10 000 km s) in a shell surrounding a Ni-rich core. The models further predict relatively low-velocity oxygen and carbon, with typical minimum velocities around 4000 and 10 000 km s, respectively. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with mass 0.9M

Type Ia supernovae are thought to result from thermonuclear explosions of carbong'oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of 0.9M. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83M to 0.9M. © 2010 Macmillan Publishers Limited. All rights reserved.

Re-organisation of the cytoskeleton during developmental programmed cell death in Picea abies embryos

Cell and tissue patterning in plant embryo development is well documented. Moreover, it has recently been shown that successful embryogenesis is reliant on programmed cell death (PCD). The cytoskeleton governs cell morphogenesis. However, surprisingly little is known about the role of the cytoskeleton in plant embryogenesis and associated PCD. We have used the gymnosperm, Picea abies , somatic embryogenesis model system to address this question. Formation of the apical-basal embryonic pattern in P. abies proceeds through the establishment of three major cell types: the meristematic cells of the embryonal mass on one pole and the terminally differentiated suspensor cells on the other, separated by the embryonal tube cells. The organisation of microtubules and F-actin changes successively from the embryonal mass towards the distal end of the embryo suspensor. The microtubule arrays appear normal in the embryonal mass cells, but the microtubule network is partially disorganised in the embryonal tube cells and the microtubules disrupted in the suspensor cells. In the same embryos, the microtubule-associated protein, MAP-65, is bound only to organised microtubules. In contrast, in a developmentally arrested cell line, which is incapable of normal embryonic pattern formation, MAP-65 does not bind the cortical microtubules and we suggest that this is a criterion for proembryogenic masses (PEMs) to passage into early embryogeny. In embryos, the organisation of F-actin gradually changes from a fine network in the embryonal mass cells to thick cables in the suspensor cells in which the microtubule network is completely degraded. F-actin de-polymerisation drugs abolish normal embryonic pattern formation and associated PCD in the suspensor, strongly suggesting that the actin network is vital in this PCD pathway.

Super-luminous Type Ic Supernovae: Catching a Magnetar by the Tail

We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely, PTF10hgi, SN2011ke, PTF11rks, SN2011kf, and SN2012il. Photometric imaging of the transients at +50 to +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase for four of these SL-SNe. A high-resolution, optical, and near-infrared spectrum from xshooter provides detection of a broad He I ?10830 emission line in the spectrum (+50 days) of SN2012il, revealing that at least some SL-SNe Ic are not completely helium-free. At first sight, the tail luminosity decline rates that we measure are consistent with the radioactive decay of 56Co, and would require 1-4 M ? of 56Ni to produce the luminosity. These 56Ni masses cannot be made consistent with the short diffusion times at peak, and indeed are insufficient to power the peak luminosity. We instead favor energy deposition by newborn magnetars as the power source for these objects. A semi-analytical diffusion model with energy input from the spin-down of a magnetar reproduces the extensive light curve data well. The model predictions of ejecta velocities and temperatures which are required are in reasonable agreement with those determined from our observations. We derive magnetar energies of 0.4 <~ E(1051 erg) lsim 6.9 and ejecta masses of 2.3 <~ M ej(M ?) lsim 8.6. The sample of five SL-SNe Ic presented here, combined with SN 2010gx—the best sampled SL-SNe Ic so far—points toward an explosion driven by a magnetar as a viable explanation for all SL-SNe Ic.

Testing the efficacy of the glacial buzzsaw: insights from the Sredinny Mountains, Kamchatka

Peak altitudes, hypsometry, geology, and former equilibrium-line altitudes (ELAs) are analyzed across the Sredinny Mountains (Kamchatka). Overall, evidence is found to suggest that the glacial buzzsaw has operated to shape the topography of this mountain range, but the strength of this signature is not spatially uniform. In the southern sector of the mountains, we see evidence that an efficient glacial buzzsaw has acted to impose constraints upon topography, limiting peak altitudes, and concentrating land-surface area (hypsometric maxima) close to palaeo-ELAs. By contrast, in the northern sector of the mountains, a number of peaks rise high above the surrounding topography, and land-surface area is concentrated well below palaeo-ELAs. This deviation from a classic ‘buzzsaw signature’, in the northern sector of the mountains, is considered to reflect volcanic construction during the Quaternary, resulting in a series of high altitude peaks, combined with the action of dynamic glaciers, acting to skew basin topography toward low altitudes, well below palaeo-ELAs. These glaciers are considered to have been particularly dynamic because of their off-shore termination, their proximity to moisture-bearing air masses from the North Pacific, and because accumulation was supplemented by snow and ice avalanching from local high altitude peaks. Overall, the data suggest that the buzzsaw remains a valid mechanism to generally explain landscape evolution in mountain regions, but its signature is significantly weakened in mountain basins that experience both volcanic construction and climatic conditions favouring dynamic glaciation.

Lucky Imaging of transiting planet host stars with LuckyCam

We obtained high-resolution, high-contrast optical imaging in the Sloan Digital Sky Survey i′ band with the LuckyCam camera mounted on the 2.56 m Nordic Optical Telescope, to search for faint stellar companions to 16 stars harbouring transiting exoplanets. The Lucky imaging technique uses very short exposures to obtain near diffraction-limited images yielding sub-arcsecond sensitivity, allowing us to search for faint stellar companions within the seeing disc of the primary planet host. Here, we report the detection of two candidate stellar companions to the planet host TrES-1 at separations <6.5 arcsec and we confirm stellar companions to CoRoT-2, CoRoT-3, TrES-2, TrES-4 and HAT-P-7 already known in the literature. We do not confirm the candidate companions to HAT-P-8 found via Lucky imaging by Bergfors et al., however, most probably because HAT-P-8 was observed in poor seeing conditions. Our detection sensitivity limits allow us to place constraints on the spectral types and masses of the putative bound companions to the planet host stars in our sample. If bound, the stellar companions identified in this work would provide stringent observational constraints to models of planet formation and evolution. In addition, these companions could affect the derived physical properties of the exoplanets in these systems.

CI Aql: a Type Ia supernova progenitor?

If recurrent novae are progenitors of Type Ia supernovae, their white dwarfs must have masses close to the Chandrasekhar limit. The most reliable means of determining white dwarf masses in recurrent novae is dynamically, via radial-velocity and rotational-broadening measurements of the companion star. Such measurements require the system to be both eclipsing and to show absorption features from the secondary star. Prior to the work reported here, the only dynamical mass estimate of a recurrent nova was for U Sco, which has a white dwarf mass of 1.55 +/- 0.24 Msolar (Thoroughgood et al. 2001). We present new time-resolved, intermediate-resolution spectroscopy of the eclipsing recurrent nova CI Aquilae (CI Aql) during quiescence. We find the mass of the white dwarf to be 1.00 +/- 0.14 Msolar and the mass of the secondary star to be 2.32 +/- 0.19 Msolar. We estimate the radius of the secondary to be 2.07 +/- 0.06 Rsolar, implying that it is a slightly-evolved early A-type star. The high mass ratio of q = 2.35 +/- 0.24 and the high secondary-star mass implies that the mass transfer occurs on a thermal timescale. We suggest that CI Aql is rapidly evolving into a supersoft X-ray source, and ultimately may explode as a Type Ia supernova within 10 Myr.

Discovery of WASP-65b and WASP-75b: Two hot Jupiters without highly inflated radii

We report the discovery of two transiting hot Jupiters, WASP-65b (M_pl = 1.55 ± 0.16 M_J; R_pl = 1.11 ± 0.06 R_J), and WASP-75b (M_pl = 1.07 ± 0.05 M_J; R_pl = 1.27 ± 0.05 R_J). They orbit their host star every ∼2.311, and ∼2.484 days, respectively. The planet host WASP-65 is a G6 star (T_eff = 5600 K, [Fe/H] = −0.07 ± 0.07, age 8 Gyr); WASP-75 is an F9 star (T_eff = 6100 K, [Fe/H] = 0.07 ± 0.09, age ∼ 3 Gyr). WASP-65b is one of the densest known exoplanets in the mass range 0.1 and 2.0 MJ (ρpl = 1.13 ± 0.08 ρJ), a mass range where a large fraction of planets are found to be inflated with respect to theoretical planet models. WASP-65b is one of only a handful of planets with masses of ∼1.5 MJ, a mass regime surprisingly underrepresented among the currently known hot Jupiters. The radius of WASP-75b is slightly inflated (10%) as compared to theoretical planet models with no core, and has a density similar to that of Saturn (ρpl = 0.52 ± 0.06 ρJ). Key words. planetary systems – stars: individual:

WASP-54b, WASP-56b and WASP-57b: Three new sub-Jupiter mass planets from SuperWASP

We present three newly discovered sub-Jupiter mass planets from the SuperWASP survey: WASP-54b is a heavily bloated planet of mass 0.636$^{+0.025}_{-0.024}$ \mj and radius 1.653$^{+0.090}_{-0.083}$ \rj. It orbits a F9 star, evolving off the main sequence, every 3.69 days. Our MCMC fit of the system yields a slightly eccentric orbit ($e=0.067^{+0.033}_{-0.025}$) for WASP-54b. We investigated further the veracity of our detection of the eccentric orbit for WASP-54b, and we find that it could be real. However, given the brightness of WASP-54 V=10.42 magnitudes, we encourage observations of a secondary eclipse to draw robust conclusions on both the orbital eccentricity and the thermal structure of the planet. WASP-56b and WASP-57b have masses of 0.571$^{+0.034}_{-0.035}$ \mj and $0.672^{+0.049}_{-0.046}$ \mj, respectively; and radii of $1.092^{+0.035}_{-0.033}$ \rj for WASP-56b and $0.916^{+0.017}_{-0.014}$ \rj for WASP-57b. They orbit main sequence stars of spectral type G6 every 4.67 and 2.84 days, respectively. WASP-56b and WASP-57b show no radius anomaly and a high density possibly implying a large core of heavy elements; possibly as high as $\sim$50 M$_{\oplus}$ in the case of WASP-57b. However, the composition of the deep interior of exoplanets remain still undetermined. Thus, more exoplanet discoveries such as the ones presented in this paper, are needed to understand and constrain giant planets' physical properties.

The not-so-extreme white dwarf of the CV GD 552

GD 552 is a cataclysmic binary which was previously believed to be composed of an M-star and a white dwarf, the latter having an extreme mass of 1.4 solar masses. In a recent paper we showed that this is not compatible with new observational evidence and presented an alternative model in which the white dwarf has a typical mass and the companion is a brown dwarf, making the system a likely member of the elusive group of CVs which have already evolved through minimum orbital period. Here we present additional spectroscopical evidence supporting this conclusion by means of skew mapping.

The Type IIb Supernova 2011dh from a Supergiant Progenitor

A set of hydrodynamical models based on stellar evolutionary progenitors is used to study the nature of SN 2011dh. Our modeling suggests that a large progenitor star ---with R ~200 Rsun---, is needed to reproduce the early light curve of SN 2011dh. This is consistent with the suggestion that the yellow super-giant star detected at the location of the SN in deep pre-explosion images is the progenitor star. From the main peak of the bolometric light curve and expansion velocities we constrain the mass of the ejecta to be ~2 Msun, the explosion energy to be E= 6-10 x 10^50 erg, and the 56Ni mass to be approximately 0.06 Msun. The progenitor star was composed of a helium core of 3 to 4 Msun and a thin hydrogen-rich envelope of ~0.1 M_sun with a main sequence mass estimated to be in the range of 12--15 Msun. Our models rule out progenitors with helium-core masses larger than 8 Msun, which correspond to M_ZAMS > 25 Msun. This suggests that a single star evolutionary scenario for SN 2011dh is unlikely.

Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae that radiate more than 10 44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56 Ni are synthesized; this isotope decays to 56 Fe via 56 Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10 -6 times that of the core-collapse rate. © 2013 Macmillan Publishers Limited. All rights reserved.

CD2AP is associated with end-stage renal disease in patients with type 1 diabetes

CD2-associated protein (CD2AP) is essential for podocyte function. CD2AP mutations have been found in patients with focal segmental glomerulosclerosis, a disease histologically resembling diabetic nephropathy and often progressing to end-stage renal disease (ESRD). We hypothesised that variations in the CD2AP gene may contribute to susceptibility to glomerular injury in diabetes and investigated if single-nucleotide polymorphisms (SNPs) in CD2AP are associated with diabetic nephropathy in patients with type 1 diabetes. The discovery cohort consisted of 2,251 Finnish patients with type 1 diabetes. SNPs were selected from the HapMap database to cover the CD2AP gene. The associations between genotyped SNPs and diabetic nephropathy or ESRD were analysed with the chi-squared test and logistic regression. Three SNPs were selected for replication in cohorts from Denmark, Italy, the United Kingdom and Ireland. None of the 15 successfully genotyped SNPs were associated with diabetic nephropathy when compared to patients with normal albumin excretion rate. However, when genotype frequencies in patients with ESRD were compared with all other patients, two CD2AP SNPs, rs9369717 and rs9349417, were found to be associated with ESRD. The meta-analysis of the original and two additional European cohorts resulted in significant p values