A low-energy core-collapse supernova without a hydrogen envelope

The final fate of massive stars depends on many factors. Theory suggests that some with initial masses greater than 25 to 30 solar masses end up as Wolf-Rayet stars, which are deficient in hydrogen in their outer layers because of mass loss through strong stellar winds. The most massive of these stars have cores which may form a black hole and theory predicts that the resulting explosion of some of them produces ejecta of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel. An alternative origin for low-energy supernovae is the collapse of the oxygen-neon core of a star of 7-9 solar masses. No weak, hydrogen-deficient, core-collapse supernovae have hitherto been seen. Here we report that SN 2008ha is a faint hydrogen-poor supernova. We propose that other similar events have been observed but have been misclassified as peculiar thermonuclear supernovae (sometimes labelled SN 2002cx-like events). This discovery could link these faint supernovae to some long-duration gamma-ray bursts, because extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce such long gamma-ray bursts, the afterglows of which do not show evidence of associated supernovae.

A deeper search for the progenitor of the Type Ic supernova 2002ap

Images of the site of the Type Ic supernova (SN) 2002ap taken before explosion were analysed previously by Smartt et al. We have uncovered new unpublished, archival pre-explosion images from the Canada-France-Hawaii Telescope (CFHT) that are vastly superior in depth and image quality. In this paper we present a further search for the progenitor star of this unusual Type Ic SN. Aligning high-resolution Hubble Space Telescope observations of the SN itself with the archival CFHT images allowed us to pinpoint the location of the progenitor site on the groundbased observations. We find that a source visible in the B- and R-band pre-explosion images close to the position of the SN is (1) not coincident with the SN position within the uncertainties of our relative astrometry and (2) is still visible similar to 4.7-yr post-explosion in late-time observations taken with the William Herschel Telescope. We therefore conclude that it is not the progenitor of SN 2002ap. We derived absolute limiting magnitudes for the progenitor of M-B >= -4.2 +/- 0.5 and M-R >= -5.1 +/- 0.5. These are the deepest limits yet placed on a Type Ic SN progenitor. We rule out all massive stars with initial masses greater than 7-8 M-circle dot (the lower mass limit for stars to undergo core collapse) that have not evolved to become Wolf-Rayet stars. This is consistent with the prediction that Type Ic SNe should result from the explosions of Wolf-Rayet stars. Comparing our luminosity limits with stellar models of single stars at appropriate metallicity (Z = 0.008) and with standard mass-loss rates, we find no model that produces a Wolf-Rayet star of low enough mass and luminosity to be classed as a viable progenitor. Models with twice the standard mass-loss rates provide possible single star progenitors but all are initially more massive than 30-40 M-circle dot. We conclude that any single star progenitor must have experienced at least twice the standard mass-loss rates, been initially more massive than 30-40 M-circle dot and exploded as a Wolf-Rayet star of final mass 10-12 M-circle dot. Alternatively a progenitor star of lower initial mass may have evolved in an interacting binary system. Mazzali et al. propose such a binary scenario for the progenitor of SN 2002ap in which a star of initial mass 15-20 M-circle dot is stripped by its binary companion, becoming a 5 M-circle dot Wolf-Rayet star prior to explosion. We constrain any possible binary companion to a main-sequence star of

The nebular spectra of SN 2012aw and constraints on stellar nucleosynthesis from oxygen emission lines

We present nebular-phase optical and near-infrared spectroscopy of the Type IIP supernova SN 2012aw combined with non-local thermodynamic equilibrium radiative transfer calculations applied to ejecta from stellar evolution/explosion models. Our spectral synthesis models generally show good agreement with the ejecta from a M_ZAMS = 15 M_⊙progenitor star. The emission lines of oxygen, sodium, and magnesium are all consistent with the nucleosynthesis in a progenitor in the 14-18 M_⊙ range.We also demonstrate how the evolution of the oxygen cooling lines of [O I] λ5577, [O I] λ6300, and [O I] λ6364 can be used to constrain the mass of oxygen in the non-molecularly cooled ashes to < 1 M_⊙, independent of the mixing in the ejecta. This constraint implies that any progenitor model of initial mass greater than 20 M_⊙ would be difficult to reconcile with the observed line strengths. A stellar progenitor of around M_ZAMS = 15 M_⊙ can consistently explain the directly measured luminosity of the progenitor star, the observed nebular spectra, and the inferred pre-supernova mass-loss rate.We conclude that there is still no convincing example of a Type IIP supernova showing the nucleosynthesis products expected from an M_ZAMS > 20 M_⊙ progenitor. © 2014 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

EARLY RADIO AND X-RAY OBSERVATIONS OF THE YOUNGEST NEARBY TYPE Ia SUPERNOVA PTF 11kly (SN 2011fe)

On 2011 August 24 (UT) the Palomar Transient Factory (PTF) discovered PTF11kly (SN 2011fe), the youngest and most nearby Type Ia supernova (SN Ia) in decades. We followed this event up in the radio (centimeter and millimeter bands) and X-ray bands, starting about a day after the estimated explosion time. We present our analysis of the radio and X-ray observations, yielding the tightest constraints yet placed on the pre-explosion mass-loss rate from the progenitor system of this supernova. We find a robust limit of from sensitive X-ray non-detections, as well as a similar limit from radio data, which depends, however, on assumptions about microphysical parameters. We discuss our results in the context of single-degenerate models for SNe Ia and find that our observations modestly disfavor symbiotic progenitor models involving a red giant donor, but cannot constrain systems accreting from main-sequence or sub-giant stars, including the popular supersoft channel. In view of the proximity of PTF11kly and the sensitivity of our prompt observations, we would have to wait for a long time (a decade or longer) in order to more meaningfully probe the circumstellar matter of SNe Ia.

The high-energy environment in the super-Earth system CoRoT-7

High-energy irradiation of exoplanets has been identified to be a key influence on the stability of these planets' atmospheres. So far, irradiation-driven mass-loss has been observed only in two Hot Jupiters, and the observational data remain even more sparse in the super-Earth regime. We present an investigation of the high-energy emission in the CoRoT-7 system, which hosts the first known transiting super-Earth. To characterize the high-energy XUV radiation field into which the rocky planets CoRoT-7b and CoRoT-7c are immersed, we analyzed a 25 ks XMM-Newton observation of the host star. Our analysis yields the first clear (3.5σ) X-ray detection of CoRoT-7. We determine a coronal temperature of ≈ 3 MK and an X-ray luminosity of 3 × 1028 erg s-1. The level of XUV irradiation on CoRoT-7b amounts to ≈37 000 erg cm-2 s-1. Current theories for planetary evaporation can only provide an order-of-magnitude estimate for the planetary mass loss; assuming that CoRoT-7b has formed as a rocky planet, we estimate that CoRoT-7b evaporates at a rate of about 1.3 × 1011 g s-1 and has lost ≈4-10 earth masses in total.

X-Ray Emission from the Super-Earth Host GJ 1214

Stellar activity can produce large amounts of high-energy radiation, which is absorbed by the planetary atmosphere leading to irradiation-driven mass loss. We present the detection and an investigation of high-energy emission in a transiting super-Earth host system, GJ 1214, based on XMM-Newton observations. We derive an X-ray luminosity of LX = 7.4 × 1025 erg s-1 and a corresponding activity level of log (LX /L bol) ~ -5.3. Further, we determine a coronal temperature of about ~3.5 MK, which is typical for coronal emission of moderately active low-mass stars. We estimate that GJ 1214 b evaporates at a rate of 1.3× 1010 g s-1 and has lost a total of ≈2-5.6 M ⊕.

X-ray driven evaporation of exoplanet atmospheres: discovery of a uniquely suited test system

The evaporation of exoplanetary atmospheres is thought to be driven by high-energy irradiation. However, the actual mass loss rates are not well constrained. Co-I Kipping has recently discovered that the star KOI-314, an M1V dwarf at 65 pc distance, is orbited by two earth-sized planets, the inner one of them rocky and the outer one gaseous (P_orb = 14d and 23d). Other recent works have shown an abundance of small rocky planets in very close orbits around their host stars, suggesting that the stellar high-energy irradiation evaporates away gaseous envelopes. KOI-314 is the first nearby system in which earth-sized planets of both types are detected, allowing us to constrain the efficiency of planetary evaporation if the stellar X-ray irradiation is measured. We therefore propose a 10 ks Chandra ACIS-S pointing to determine the stellar X-ray luminosity and hardness ratio. The accuracy of the orbital solution decreases quickly due to Transit-Timing Variations, which is why we ask for DDT.

Photoionization of Co+ and electron-impact excitation of Co2 + using the Dirac R-matrix method

Modelling of massive stars and supernovae (SNe) plays a crucial role in understanding galaxies. From this modelling we can derive fundamental constraints on stellar evolution, mass-loss processes, mixing, and the products of nucleosynthesis. Proper account must be taken of all important processes that populate and depopulate the levels (collisional excitation, de-excitation, ionization, recombination, photoionization, bound–bound processes). For the analysis of Type Ia SNe and core collapse SNe (Types Ib, Ic and II) Fe group elements are particularly important. Unfortunately little data is currently available and most noticeably absent are the photoionization cross-sections for the Fe-peaks which have high abundances in SNe. Important interactions for both photoionization and electron-impact excitation are calculated using the relativistic Dirac atomic R-matrix codes (DARC) for low-ionization stages of Cobalt. All results are calculated up to photon energies of 45 eV and electron energies up to 20 eV. The wavefunction representation of Co III has been generated using GRASP0 by including the dominant 3d⁷, 3d⁶[4s, 4p], 3p⁴3d⁹ and 3p⁶3d⁹ configurations, resulting in 292 fine structure levels. Electron-impact collision strengths and Maxwellian averaged effective collision strengths across a wide range of astrophysically relevant temperatures are computed for Co III. In addition, statistically weighted level-resolved ground and metastable photoionization cross-sections are presented for Co II and compared directly with existing work.

Degradation of poly-L-lactide. Part 1: in vitro and in vivo physiological temperature degradation

Poly-L-Lactide is a bioresorbable polymer which degrades through hydrolysis of its ester linkage influenced by initial molecular weight and degree of crystallinity. Polymers belonging to the aliphatic polyester family currently represent the most attractive group of polymers that meet the medical and physical demands for safe clinical applications. Compression moulded PLLA pellets were produced as rods, sterilized and degraded both in vitro and in vivo (sub-dermal implantation model). The material molecular weight, crystallinity, mechanical strength and thermal properties were evaluated. In both in vitro and in vivo environments, degradation proceeded at the same rate and followed the general sequence of aliphatic polyester degradation, ruling out enzymes accelerating the degradation rate in vivo. By 44 weeks duration of implantation the PLLA rods were still biocompatible, before any mass loss was observed.

Degradation of poly-L-lactide. Part2 : increased temperature accelerated degradation

Poly-L-lactide (PLLA) is one of the most significant members of a group of polymers regarded as bioresorbable. The degradation of PLLA proceeds through hydrolysis of the ester linkages in the polymer's backbone; however, the time for the complete resorption of orthopaedic devices manufactured from PLLA is known to be in excess of five years in a normal physiological environment. To evaluate the degradation of PLLA in an accelerated time period, PLLA pellets were processed by compression moulding into tensile test specimens, prior to being sterilized by ethylene oxide gas (EtO) and degraded in a phosphate-buffered solution (PBS) at both 50°C and 70°C. On retrieval, at predetermined time intervals, procedures were used to evaluate the material's molecular weight, crystallinity, mechanical strength, and thermal properties. The results from this study suggest that at both 50°C and 70°C, degradation proceeds by a very similar mechanism to that observed at 37°C in vitro and in vivo. The degradation models developed also confirmed the dependence of mass loss, melting temperature, and glass transition temperature (Tg) on the polymer's molecular weight throughout degradation. Although increased temperature appears to be a suitable method for accelerating the degradation of PLLA, relative to its physiological degradation rate, concerns still remain over the validity of testing above the polymer's Tg and the significance of autocatalysis at increased temperatures.

On the nature of the circumstellar medium of the remarkable Type Ia/IIn supernova SN 2002ic

We present results from the first high-resolution, high signal-to-noise ratio spectrum of SN 2002ic. The resolved Ha line has a P Cygni-type profile, clearly demonstrating the presence of a dense, slow-moving (~100 km s-1) outflow. We have additionally found a huge near-infrared excess, hitherto unseen in Type Ia supernovae. We argue that this is due to an infrared light-echo arising from the pre-existing dusty circumstellar medium. We deduce a circumstellar medium mass probably exceeding 0.3 Msolar produced by a mass-loss rate greater than several times 10-4 Msolar yr-1. For the progenitor, we favour a single-degenerate system where the companion is a post-asymptotic giant branch star. As a by-product of our optical data, we are able to provide a firm identification of the host galaxy of SN 2002ic.

Luminous blue variables as the progenitors of supernovae with quasi-periodic radio modulations

The interaction between supernova ejecta and circumstellar matter, arising from previous episodes of mass loss, provides us with a means of constraining the progenitors of supernovae. Radio observations of a number of supernovae show quasi-periodic deviations from a strict power-law decline at late times. Although several possibilities have been put forward to explain these modulations, no single explanation has proven to be entirely satisfactory. Here we suggest that Luminous blue variables undergoing S-Doradus type variations give rise to enhanced phases of mass loss that are imprinted on the immediate environment of the exploding star as a series of density enhancements. The variations in mass loss arise from changes in the ionization balance of Fe, the dominant ion that drives the wind. With this idea, we find that both the recurrence timescale of the variability and the amplitude of the modulations are in line with the observations. Our scenario thus provides a natural, single-star explanation for the observed behaviour that is, in fact, expected on theoretical grounds.

Assessment of Self-Compacting Concrete Immersed in Acidic Solutions

Ettringite and thaumasite can be found among the deterioration products of cementitious materials exposed to sulfate and hydrochloric attack. The results of a test program to investigate the acid resistance of self-compacting concrete (SCC) and conventional concrete (CC), immersed up to 18 weeks at 20°C in sulfuric and hydrochloric acid solutions, are described. The SCC was prepared with 47% carboniferous limestone powder, as a replacement for cement, and an ordinary portland cement. The CC was prepared with portland cement only. The water-binder ratios of the SCC and CC were 0.36 and 0.46, respectively. The parameter investigated was the time, in weeks, taken to cause 10% mass loss of fully immersed concrete specimens in a 1% solution of sulfuric acid and the same amount of loss in a 1% solution of hydrochloric acid. The investigation indicated that the SCC performed better than the CC in sulfuric solution but was slightly more vulnerable to hydrochloric acid attack compared to CC. The mode of attack between the two solutions was different.

Effect of Mixture Composition on Washout Resistance of Highly Flowable Underwater Concrete

The underwater casting of relatively thin lifts of concrete in water requires the proportioning of highly flowable concrete that can resist water dilution and segregation and spread readily into place. An investigation was carried out to determine the effects of antiwashout admixture concentration, water-cementitious materials ratio, and binder composition on the washout resistance of highly flowable concrete. Two main types of antiwashout admixtures were used: 1) a powdered welan gum at concentrations of 0.07 and 0.15% (by mass of binder); and 2) a liquid-based cellulosic admixture with dosages up to 1.65 L/100 kg of binder. The water-cementitious materials ratios were set at 0.41 and 0.47, corresponding to high-quality underwater concrete. Four binder compositions were used: a standard Canadian Type 10 cement, the same cement with 10% silica fume replacement, the cement with 50% granulated blast-furnace slag replacement, and a ternary cement containing 6% silica fume and 20% Class F fly ash. The concentrations of anti-washout admixture have direct impact on washout resistance. For a given washout loss, greater slump flow consistency can be achieved with the increases in anti-washout admixture concentration and decreases in water-binder ratio. The washout mass loss can be reduced, for a given consistency

Effect of Mixture Composition on Relative Strength of Highly Flowable Underwater Concrete

Concrete used for underwater repair is often proportioned to spread readily into place and self-consolidate, and to develop high resistance to segregation and water dilution. An investigation was carried out to determine the effect of the dosage of antiwashout admixture, water-cementitious materials ratio (w/cm), and binder composition on the relative residual strength of highly flowable underwater concrete. Two types of antiwashout admixtures were used: a powdered welan gum at 0.07 and 0.15% by mass of binder, and a liquid-based cellulosic admixture employed at a high dosage of 1 to 1.65 L/100 kg of cementitious materials. The w/cms were set at 0.41 and 0.47 to secure adequate performance of underwater concrete for construction and repair. Four binder compositions were used: a Canadian Type 10 cement; a cement with 10% silica fume replacement; a cement with 50% replacement of granulated blast-furnace slag; and a ternary binder containing 6% silica fume and 20% Class F fly ash. Test results indicate that for a given washout mass loss and slump flow consistency, greater relative residual strength can be secured when the dosage of antiwashout admixture is increased, the w/cm is reduced, and a binary binder with 10% silica fume substitution or the ternary binder are employed. Such mixtures can develop relative residual compressive strengths of 85 and 80%, compared to mixtures cast in air, when the value of washout loss is limited to 4 and 6% for mixtures with slump flow values of 450 and 550 mm, respectively.