ALBERT

Beschreibung: We report on interferometric observations with the CHARA Array of two classical Wolf–Rayet (WR) stars in suspected binary systems, namely WR 137 and WR 138. In both cases, we resolve the component stars to be separated by a few milliarcseconds. The data were collected in the H band, and provide a measure of the fractional flux for both stars in each system. We find that the WR star is the dominant H -band light source in both systems ( f WR,137 = 0.59 ± 0.04; f WR,138 = 0.67 ± 0.01), which is confirmed through both comparisons with estimated fundamental parameters for WR stars and O dwarfs, as well as through spectral modelling of each system. Our spectral modelling also provides fundamental parameters for the stars and winds in these systems. The results on WR 138 provide evidence that it is a binary system which may have gone through a previous mass-transfer episode to create the WR star. The separation and position of the stars in the WR 137 system together with previous results from the IOTA interferometer provides evidence that the binary is seen nearly edge-on. The possible edge-on orbit of WR 137 aligns well with the dust production site imaged by the Hubble Space Telescope during a previous periastron passage, showing that the dust production may be concentrated in the orbital plane.

Beschreibung: We present analyses of new optical spectra of three WC9 stars, WR 88, WR 92 and WR 103 to test the suggestion that they exemplify an evolutionary sequence amongst the WC9 stars. The spectrum of WR 88 shows conspicuous lines of N  iii and N  iv , leading to classification as a transitional WN8o/WC9 star. The three stars show a sequence of increasing O  ii and O  iii line strengths, confirming and extending earlier studies. The spectra were analysed using cmfgen models, finding greater abundances of oxygen and carbon in WR 103 than in WR 92 and, especially, in WR 88. Of the three stars, only WR 103 makes circumstellar dust. We suggest that oxygen itself does not enhance this process but that it is its higher carbon abundance that allows WR 103 to make dust.

Beschreibung: Large peridotite massifs are scattered along the 1500 km length of the Yarlung–Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low f O 2 (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO 2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high- T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high- P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr–Al spinel ± cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to 〉 13 GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375 Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low- P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low- f O 2 phases. A proposed P – T – t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c . 375 Ma, residence in the upper Transition Zone for 〉200 Myr, and rapid exhumation at c . 170–150 Ma or 130–120 Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in 〈 10 Myr. This rapid upwelling, which may explain some characteristics of the diamonds, appears to have brought some massifs to the surface in forearc or back-arc basins, where they provided a basement for oceanic crust. This model can reconcile many apparently contradictory petrological and geological datasets. It also defines an important, previously unrecognized geodynamic process that may have operated along other large suture zones such as the Urals.

Beschreibung: Visual photometry of 16 WC8–9 dust-making Wolf–Rayet (WR) stars during 2001–2009 was extracted from the All-Sky Automated Survey All Star Catalogue (ASAS-3) to search for eclipses attributable to extinction by dust formed in clumps in our line of sight. Data for a comparable number of dust-free WC6–9 stars were also examined to help characterize the data set. Frequent eclipses were observed from WR 104, and several from WR 106, extending the 1994–2001 studies by Kato et al., but not supporting their phasing the variations in WR 104 with its ‘pinwheel’ rotation period. Only four other stars showed eclipses, WR 50 (one of the dust-free stars), WR 69, WR 95 and WR 117, and there may have been an eclipse by WR 121, which had shown two eclipses in the past. No dust eclipses were shown by the ‘historic’ eclipsers WR 103 and WR 113. The atmospheric eclipses of the latter were observed but the suggestion by David-Uraz et al. that dust may be partly responsible for these is not supported. Despite its frequent eclipses, there is no evidence in the infrared images of WR 104 for dust made in its eclipses, demonstrating that any dust formed in this process is not a significant contributor to its circumstellar dust cloud and suggesting that the same applies to the other stars showing fewer eclipses.

Beschreibung: We present a detection in pre-explosion Hubble Space Telescope ( HST ) imaging of a point source consistent with being the progenitor star of the Type IIb supernova (SN IIb) 2016gkg. Post-explosion imaging from the Keck adaptive optics system was used to perform relative astrometry between the Keck and HST imaging. We identify a single point source in the HST images coincident with the SN position to 0.89. The HST photometry is consistent with the progenitor star being an A0 Ia star with T  = 9500 K and log ( L /L ) = 5.15. We find that the SN 2016gkg progenitor star appears more consistent with binary than single-star evolutionary models. In addition, early-time light-curve data from SN 2016gkg revealed a rapid rise in luminosity within ~0.4 d of non-detection limits, consistent with models of the cooling phase after shock break-out. We use these data to determine an explosion date of 2016 September 20.15 and progenitor-star radius of log ( R / R ) = 2.41, which agrees with photometry from the progenitor star. Our findings are also consistent with detections of other SNe IIb progenitor stars, although more luminous and bluer than most other examples.

Beschreibung: Infrared photometry of the probable triple WC4(+O?)+O8I: Wolf–Rayet system HD 36402 (= BAT99-38) in the Large Magellanic Cloud shows emission characteristic of heated dust. The dust emission is variable on a time-scale of years, with a period near 4.7 yr, possibly associated with orbital motion of the O8 supergiant and the inner P ~= 3.03-d WC4+O binary. The phase of maximum dust emission is close to that of the X-ray minimum, consistent with both processes being tied to colliding wind effects in an elliptical binary orbit. It is evident that Wolf–Rayet dust formation occurs also in metal-poor environments.

Beschreibung: $Kepler, K2$ long cadence data are used to study white light flares in a sample of 45 L dwarfs. We identified 11 flares on 9 L dwarfs with equivalent durations of (1.3–198) h and total (UV/optical/IR) energies of ≥0.9 × 1032 erg. Two superflares with energies of >1033 erg were detected on an L5 dwarf (VVV BD001): this is the coolest object so far on which flares have been identified. The larger superflare on this L5 dwarf has an energy of 4.6 × 1034 erg and an amplitude of >300 times the photospheric level: so far, this is the largest amplitude flare detected by the Kepler/K2 mission. The next coolest star on which we identified a flare was an L2 dwarf: 2MASS J08585891+1804463. Combining the energies of all the flares which we have identified on 9 L dwarfs with the total observation time which was dedicated by Kepler to all 45 L dwarfs, we construct a composite flare frequency distribution (FFD). The FFD slope is quite shallow (−0.51 ± 0.17), consistent with earlier results reported by Paudel et al. for one particular L0 dwarf, for which the FFD slope was found to be −0.34. Using the composite FFD, we predict that, in early- and mid-L dwarfs, a superflare of energy 1033 erg occurs every 2.4 yr and a superflare of energy 1034 erg occurs every 7.9 yr. Analysis of our L dwarf flares suggests that magnetic fields of ≥0.13–1.3 kG are present on the stellar surface: such fields could suppress Type II radio bursts.

Beschreibung: Infrared imaging of the colliding-wind binary Apep has revealed a spectacular dust plume with complicated internal dynamics that challenges standard colliding-wind binary physics. Such challenges can be potentially resolved if a rapidly rotating Wolf–Rayet star is located at the heart of the system, implicating Apep as a Galactic progenitor system to long-duration gamma-ray bursts. One of the difficulties in interpreting the dynamics of Apep is that the spectral composition of the stars in the system was unclear. Here, we present visual to near-infrared spectra that demonstrate that the central component of Apep is composed of two classical Wolf–Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes. We argue that such an assignment represents the strongest case of a classical Wolf–Rayet+Wolf–Rayet binary system in the Milky Way. The terminal line-of-sight wind velocities of the WC8 and WN4-6b stars are measured to be 2100 ± 200 and 3500 ± 100 km s−1, respectively. If the mass-loss rate of the two stars are typical for their spectral class, the momentum ratio of the colliding winds is expected to be ≈0.4. Since the expansion velocity of the dust plume is significantly smaller than either of the measured terminal velocities, we explore the suggestion that one of the Wolf–Rayet winds is anisotropic. We can recover a shock-compressed wind velocity consistent with the observed dust expansion velocity if the WC8 star produces a significantly slow equatorial wind with a velocity of ≈530 km s−1. Such slow wind speeds can be driven by near-critical rotation of a Wolf–Rayet star.