14,275 research outputs found

    Study of alkaline hydrothermal activation of belite cements by thermal analysis

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    The effect of alkaline hydrothermal activation of class-C fly ash belite cement was studied using thermal analysis (TG/DTG) by determining the increase in the combined water during a period of hydration of 180 days. The results were compared with those obtained for a belite cement hydrothermally activated in water. The two belite cements were fabricated via the hydrothermal-calcination route of class-C fly ash in 1 M NaOH solution (FABC-2-N) or demineralised water (FABC-2-W). From the results, the effect of the alkaline hydrothermal activation of belite cement (FABC-2-N) was clearly differentiated, mainly at early ages of hydration, for which the increase in the combined water was markedly higher than that of the belite cement that was hydrothermally activated in water. Important direct quantitative correlations were obtained among physicochemical parameters, such as the combined water, the BET surface area, the volume of nano-pores, and macro structural engineering properties such as the compressive mechanical strength

    Absence of hot gas within the Wolft-Rayet Bubble around WR16

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    We present the analysis of XMM-Newton archival observations towards the Wolf-Rayet (WR) bubble around WR16. Despite the closed bubble morphology of this WR nebula, the XMM-Newton observations show no evidence of diffuse emission in its interior as in the similar WR bubbles NGC6888 and S308. We use the present observations to estimate a 3-\sigma upper limit to the X-ray luminosity in the 0.3-1.5 keV energy band equal to 7.4x10^{32} erg s^{-1} for the diffuse emission from the WR nebula, assuming a distance of 2.37 kpc. The WR nebula around WR16 is the fourth observed by the current generation of X-ray satellites and the second not detected. We also examine FUSE spectra to search for nebular O VI absorption lines in the stellar continuum of WR16. The present far-UV data and the lack of measurements of the dynamics of the optical WR bubble do not allow us to confirm the existence of a conductive layer of gas at T~3x10^5 K between the cold nebular gas and the hot gas in its interior. The present observations result in an upper limit of n_e < 0.6 cm^-3 on the electron density of the X-ray emitting material within the nebula.Comment: 6 pages, 3 figures, 1 table; To appear in Astronomy and Astrophysic

    XMM-Newton Detection of Hot Gas in the Eskimo Nebula: Shocked Stellar Wind or Collimated Outflows?

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    The Eskimo Nebula (NGC 2392) is a double-shell planetary nebula (PN) known for the exceptionally large expansion velocity of its inner shell, ~90 km/s, and the existence of a fast bipolar outflow with a line-of-sight expansion velocity approaching 200 km/s. We have obtained XMM-Newton observations of the Eskimo and detected diffuse X-ray emission within its inner shell. The X-ray spectra suggest thin plasma emission with a temperature of ~2x10^6 K and an X-ray luminosity of L_X = (2.6+/-1.0)x10^31 (d/1150 pc)^2 ergs/s, where d is the distance in parsecs. The diffuse X-ray emission shows noticeably different spatial distributions between the 0.2-0.65 keV and 0.65-2.0 keV bands. High-resolution X-ray images of the Eskimo are needed to determine whether its diffuse X-ray emission originates from shocked fast wind or bipolar outflows.Comment: 4 pages, 2 figures, accepted in Astronomy and Astrophysics Letter

    WISE morphological study of Wolf-Rayet nebulae

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    We present a morphological study of nebulae around Wolf-Rayet (WR) stars using archival narrow-band optical and Wide-field Infrared Survey Explorer (WISE) infrared images. The comparison among WISE images in different bands and optical images proves to be a very efficient procedure to identify the nebular emission from WR nebulae, and to disentangle it from that of the ISM material along the line of sight. In particular, WR nebulae are clearly detected in the WISE W4 band at 22 őľ\mum. Analysis of available mid-IR Spitzer spectra shows that the emission in this band is dominated by thermal emission from dust spatially coincident with the thin nebular shell or most likely with the leading edge of the nebula. The WR nebulae in our sample present different morphologies that we classified into well defined WR bubbles (bubble B{\cal B}-type nebulae), clumpy and/or disrupted shells (clumpy/disrupted C{\cal C}-type nebulae), and material mixed with the diffuse medium (mixed M{\cal M}-type nebulae). The variety of morphologies presented by WR nebulae shows a loose correlation with the central star spectral type, implying that the nebular and stellar evolutions are not simple and may proceed according to different sequences and time-lapses. We report the discovery of an obscured shell around WR35 only detected in the infrared.Comment: 11 pages, 6 figures, plus 23 appendix figures; to appear in Astronomy and Astrophysic

    Hidden IR structures in NGC 40: signpost of an ancient born-again event

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    We present the analysis of infrared (IR) observations of the planetary nebula NGC 40 together with spectral analysis of its [WC]-type central star HD 826. Spitzer IRS observations were used to produce spectral maps centred at polycyclic aromatic hydrocarbons (PAH) bands and ionic transitions to compare their spatial distribution. The ionic lines show a clumpy distribution of material around the main cavity of NGC 40, with the emission from [Ar II] being the most extended, whilst the PAHs show a rather smooth spatial distribution. Analysis of ratio maps shows the presence of a toroidal structure mainly seen in PAH emission, but also detected in a Herschel PACS 70 mic image. We argue that the toroidal structure absorbs the UV flux from HD 826, preventing the nebula to exhibit lines of high-excitation levels as suggested by previous authors. We discuss the origin of this structure and the results from the spectral analysis of HD 826 under the scenario of a late thermal pulse.Comment: 10 pages, 10 figures; Accepted to MNRA
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