| 1. |
- Berger, E., et al.
(författare)
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An Intermediate Luminosity Transient in NGC 300 : The Eruption of a Dust-Enshrouded Massive Star
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699, s. 1850-1865
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Tidskriftsartikel (refereegranskat)abstract
- We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discovered in the nearby galaxy NGC 300. We find that the transient (NGC 300 OT2008-1) has a peak absolute magnitude of M bol ≈ -11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85 OT2006-1 and SN 2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (~200-1000 km s-1) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect asymmetric Ca II H&K absorption with a broad red wing extending to ~103 km s-1, indicative of gas inflow at high velocity (possibly the wind of a massive binary companion). The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points to a dust-enshrouded star with a luminosity of about 6 × 104 L sun, indicative of a ~10-20 M sun progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation.
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| 2. |
- Ludwig, Hans-Günter, et al.
(författare)
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3D Hydrodynamical Modelling of the Solar Chromosphere
- 2003
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Ingår i: International Astronomical Union Symposium. - 158381163X ; 219, s. 40-40
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Konferensbidrag (refereegranskat)abstract
- We report on 3D radiation-hydrodynamics modelling of the non-magneticsolar atmosphere with emphasis on the problem of the acoustic heating ofthe chromosphere. Our models include the generation of acoustic waves inthe uppermost layers of the solar convection zone their propagationthrough the photospheric layers and their ultimate transformation intoshock fronts which are dissipated in the chromospheric layers. In thechromosphere we find a bimodal temperature distribution consisting of acool non-shocked and hot shocked gas component. The hot gas is locatedin small-scale filamentary structures associated with propagating shockfronts which evolve rapidly in time. The detailed spatial and temporalinformation provided by the models can be exploited to predictobservational diagnostics hitherto unavailable. We present syntheticimages of features in the millimetre and sub-millimetre radio continuumpotentially observable by the Atacama Large Millimetre Array (ALMA).
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| 3. |
- Frandsen, K. E. H., et al.
(författare)
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The molecular basis of polysaccharide cleavage by lytic polysaccharide monooxygenases
- 2016
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Ingår i: Nature Chemical Biology. - : Springer Science and Business Media LLC. - 1552-4450 .- 1552-4469. ; 12:4, s. 298-
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Tidskriftsartikel (refereegranskat)abstract
- Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that oxidatively break down recalcitrant polysaccharides such as cellulose and chitin. Since their discovery, LPMOs have become integral factors in the industrial utilization of biomass, especially in the sustainable generation of cellulosic bioethanol. We report here a structural determination of an LPMO-oligosaccharide complex, yielding detailed insights into the mechanism of action of these enzymes. Using a combination of structure and electron paramagnetic resonance spectroscopy, we reveal the means by which LPMOs interact with saccharide substrates. We further uncover electronic and structural features of the enzyme active site, showing how LPMOs orchestrate the reaction of oxygen with polysaccharide chains.
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| 4. |
- Matrozis, E., et al.
(författare)
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Galactic chemical evolution of sulphur Sulphur abundances from the [SI] lambda 1082 nm line in giants
- 2013
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Galactic chemical evolution of sulphur is still under debate. At low metallicities some studies find no correlation between [S/Fe] and [Fe/H], which is typical for a-elements, while others find [S/Fe] increasing towards lower metallicities, and still others find a combination of the two. Each scenario has different implications for the Galactic chemical evolution of sulphur. Aims. The aim of this study is to contribute to the discussion on the Galactic chemical evolution of sulphur by deriving sulphur abundances from non-local thermodynamic equilibrium (LTE) insensitive spectral diagnostics in disk and halo stars with homogeneously determined stellar parameters. Methods. We derived effective temperatures from photometric colours, surface gravities from stellar isochrones and Bayesian estimation, and metallicities and sulphur abundances from spectrum synthesis. We derived sulphur abundances from the [SI] lambda 1082 nm line in 39 mostly cool and metal-poor giants using 1D LTE MARCS model atmospheres to model our high-resolution near-infrared spectra obtained with the VLT, NOT, and Gemini South telescopes. Results. We derive homogeneous stellar parameters for 29 of the 39 stars. Our results argue for a chemical evolution of sulphur that is typical for a-elements, contrary to some previous studies that have found high sulphur abundances ([S/Fe] >= 0.6) for stars with -2.5 < [Fe/H] < -1. Our abundances are systematically higher by about 0.1 dex than those of other studies that arrived at similar conclusions using other sulphur diagnostics. Conclusions. We find the [SI] line to be a valuable diagnostic of sulphur abundances in cool giants down to [Fe/H] similar or equal to -2.3. We argue that a homogeneous determination of stellar parameters is necessary, since the derived abundances are sensitive to them. Our results ([S/Fe]) agree reasonably well with predictions of contemporary models of Galactic chemical evolution. In these models sulphur is predominantly created in massive stars by oxygen burning and is ejected into the interstellar medium during Type II supernovae explosions. Systematic differences with previous studies most likely fall within modelling uncertainties.
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| 5. |
- Wilson, L. F. L., et al.
(författare)
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The structure of EXTL3 helps to explain the different roles of bi-domain exostosins in heparan sulfate synthesis
- 2022
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:3314
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Tidskriftsartikel (refereegranskat)abstract
- Heparan sulfate is a highly modified O-linked glycan that performs diverse physiological roles in animal tissues. Though quickly modified, it is initially synthesised as a polysaccharide of alternating β-d-glucuronosyl and N-acetyl-α-d-glucosaminyl residues by exostosins. These enzymes generally possess two glycosyltransferase domains (GT47 and GT64)—each thought to add one type of monosaccharide unit to the backbone. Although previous structures of murine exostosin-like 2 (EXTL2) provide insight into the GT64 domain, the rest of the bi-domain architecture is yet to be characterised; hence, how the two domains co-operate is unknown. Here, we report the structure of human exostosin-like 3 (EXTL3) in apo and UDP-bound forms. We explain the ineffectiveness of EXTL3’s GT47 domain to transfer β-d-glucuronosyl units, and we observe that, in general, the bi-domain architecture would preclude a processive mechanism of backbone extension. We therefore propose that heparan sulfate backbone polymerisation occurs by a simple dissociative mechanism.
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