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S. Dehaes † M. A. T. Groenewegen L. Decin S. Hony G. Raskin J. A. D. L. Blommaert 《Monthly notices of the Royal Astronomical Society》2007,377(2):931-944
It is generally acknowledged that the mass-loss of asymptotic giant branch (AGB) stars undergoes variations on different time-scales. We constructed models for the dust envelopes for a sample of AGB stars to assess whether mass-loss variations influence the spectral energy distribution. To constrain the variability, extra observations at millimetre wavelengths (1.2 mm) were acquired. From the analysis of the dust models, two indications for the presence of mass-loss variations can be found, being (1) a dust temperature at the inner boundary of the dust envelope that is far below the dust condensation temperature and (2) an altered density distribution with respect to ρ( r ) ∝ r −2 resulting from a constant mass-loss rate. For five out of the 18 studied sources a two-component model of the envelope is required, consisting of an inner region with a constant mass-loss rate and an outer region with a less steep density distribution. For one source an outer region with a steeper density distribution was found. Moreover, in a search for time variability in our data set at 1.2 mm, we found that WX Psc shows a large relative time variation of 34 per cent which might partially be caused by variable molecular line emission. 相似文献
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Eric Lagadec Albert A. Zijlstra G. C. Sloan Peter R. Wood Mikako Matsuura Jeronimo Bernard-Salas J. A. D. L. Blommaert M.-R. L. Cioni M. W. Feast M. A. T. Groenewegen Sacha Hony J. W. Menzies J. Th. van Loon P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2009,396(1):598-608
We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C2 H2 and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range 1.0–3.3 × 10−8 M⊙ yr−1 . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample. 相似文献
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Allan Sacha Brun 《Solar physics》2004,220(2):333-345
We have performed 3-D numerical simulations of compressible convection under the influence of rotation and magnetic fields
in spherical shells. They aim at understanding the subtle coupling between convection, rotation and magnetic fields in the
solar convection zone. We show that as the magnetic Reynolds number is increased in the simulations, the magnetic energy saturates
via nonlinear dynamo action, to a value smaller but comparable to the kinetic energy contained in the shell, leading to increasingly
strong Maxwell stresses that tend to weaken the differential rotation driven by the convection. These simulations also indicate
that the mean toroidal and poloidal magnetic fields are small compared to their fluctuating counterparts, most of the magnetic
energy being contained in the non-axisymmetric fields. The intermittent nature of the magnetic fields generated by such a
turbulent convective dynamo confirms that in the Sun the large-scale ordered dynamo responsible for the 22-year cycle of activity
can hardly be located in the solar convective envelope. 相似文献
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M. A. T. Groenewegen P. R. Wood G. C. Sloan J. A. D. L. Blommaert M.-R. L. Cioni M. W. Feast S. Hony † M. Matsuura J. W. Menzies E. A. Olivier E. Vanhollebeke J. Th. van Loon P. A. Whitelock A. A. Zijlstra H. J. Habing E. Lagadec 《Monthly notices of the Royal Astronomical Society》2007,376(1):313-337
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Nicholas J. Nelson Benjamin P. Brown A. Sacha Brun Mark S. Miesch Juri Toomre 《Solar physics》2014,289(2):441-458
Our global 3D simulations of convection and dynamo action in a Sun-like star reveal that persistent wreaths of strong magnetism can be built within the bulk of the convention zone. Here we examine the characteristics of buoyant magnetic structures that are self-consistently created by dynamo action and turbulent convective motions in a simulation with solar stratification but rotating at three times the current solar rate. These buoyant loops originate within sections of the magnetic wreaths in which turbulent flows amplify the fields to much higher values than is possible through laminar processes. These amplified portions can rise through the convective layer by a combination of magnetic buoyancy and advection by convective giant cells, forming buoyant loops. We measure statistical trends in the polarity, twist, and tilt of these loops. Loops are shown to preferentially arise in longitudinal patches somewhat reminiscent of active longitudes in the Sun, although broader in extent. We show that the strength of the axisymmetric toroidal field is not a good predictor of the production rate for buoyant loops or the amount of magnetic flux in the loops that are produced. 相似文献
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Albert A. Zijlstra Mikako Matsuura Peter R. Wood G. C. Sloan Eric Lagadec Jacco Th. van Loon M. A. T. Groenewegen M. W. Feast J. W. Menzies P. A. Whitelock J. A. D. L. Blommaert M.-R. L. Cioni H. J. Habing S. Hony C. Loup L. B. F. M. Waters 《Monthly notices of the Royal Astronomical Society》2006,370(4):1961-1978
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Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud 总被引：1，自引：0，他引：1
M. Matsuura P. R. Wood G. C. Sloan A. A. Zijlstra J. Th. van Loon M. A. T. Groenewegen J. A. D. L. Blommaert M.-R. L. Cioni M. W. Feast H. J. Habing S. Hony E. Lagadec C. Loup J. W. Menzies L. B. F. M. Waters P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2006,371(1):415-420
We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope ( SST ) over the 5–38 μm range. All 26 low-resolution spectra show acetylene (C2 H2 ) bands at 7 and 14 μm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 μm C2 H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 μm C2 H2 band by circumstellar dust emission. This 14-μm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2 H2 column density, derived from the 13.7 μm band, shows a gas mass-loss rate in the range 3 × 10−6 to 5 × 10−5 M⊙ yr−1 . This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 μm C2 H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2 H2 abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity. 相似文献
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As the world’s largest fossil fuels exporter, Russia is one of the key countries for addressing global climate change. However, it has never demonstrated any significant ambitions to reduce greenhouse gas (GHG) emissions. This paper applies ideational research methodology to identify the structural differences in economic, political, and social normative contexts between industrialized fossil fuel importing economies and Russia that lead to the fundamental gap in motivations driving decarbonization efforts. Russia is unlikely to replicate the approach to the green transition and climate policy instruments of energy-importing countries. In order to launch decarbonization in Russia, interested stakeholders need to frame climate policies in Russia differently. Specifically, the framing must address the priority of diversification as a means to adapting the national economy to a new green landscape, the combination of diverse channels for decarbonization, the promotion of energy-efficiency, closer attention to climate-related forest projects, and linkage of climate change with other environmental problems. Moreover, considering Russia’s emissions as a part of the global economic system and shifting from a simplistic national focus on GHG emissions reduction would help coordinate policies through dialogue between exporters and importers of fossil fuel energy-intensive goods, which is essential for the global movement towards a net-zero future.
相似文献10.
Marcelo Bevilacqua Remor Silvio César Sampaio Sacha de Rijk Marcio Antônio Vilas Boas Jackeline Tatiane Gotardo Everton Tiago Pinto Fábio Augusto Schardong 《国际泥沙研究》2018,33(4):406-414
The objective of this study was to evaluate the concentration and distribution of heavy metals in the sediments of Paulo Gorski Lake, as well as the metals’ bioavailability and potential ecological risk, and to define the anthropogenic and natural heavy metal contributions to the lake. The chemical elements calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), nickel (Ni), lead (Pb), and zinc (Zn) were quantified by flame atomic absorption spectrophotometry with two extraction methods to quantify the bioavailable and non-bioavailable fractions. The data were evaluated using multivariate statistics and sediment quality indices. All sediment collection points (S1, S2, S3, S4 and S5) are different in terms of the concentration of heavy metals, except for S4 and S5, which were statistically equal. The bioavailable fraction of the elements in the sediment follows the sequence Pb>Cu>Mn>Zn>Ni>Cr>phosphorus (P) for all points. The elements Co, Cr, Pb, and Zn showed moderate to considerable contamination at all points. Only points S3 and S5 had moderate ecological risk. Urbanization has been affecting Paulo Gorski Lake via the input of chemical elements, especially Co and Pb. The points most affected by heavy metal contamination are S3 and S5 when the sedimentological sensitivity factor is considered. The lake has high hydrodynamics, causing some of the contaminants that enter the system to leave it, leading to potential negative impacts downstream. 相似文献