2,820 research outputs found
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
Climate change impact assessment as function of model inaccuracy
International audienceNumerical simulation models are frequently applied to assess the impact of climate change on hydrology and agriculture. A common hypothesis is that unavoidable model errors are reflected in the reference situation as well as in the climate change situation so that by comparing reference to scenario model errors will level out. For a polder in The Netherlands an innovative procedure has been introduced, referred to as the Model-Scenario-Ratio (MSR), to express model inaccuracy on climate change impact assessment. MSR values close to 1, indicating that impact assessment is mainly a function of the scenario itself rather than of the quality of the model, were found for most indicators evaluated. More extreme climate change scenarios and indicators based on threshold values showed lower MSR values, indicating that model accuracy is an important component of the climate change impact assessment. It was concluded that the MSR approach can be applied easily and will lead to more robust impact assessment analyses
Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud
We present ESO/VLT spectra in the 2.9--4.1 micron range for a large sample of
infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of
MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out
of 30 targets, 28 are positively identified as carbon stars, significantly
adding to the known population of optically invisible carbon stars in the LMC.
We also present spectra for six IR-bright stars in or near three clusters in
the LMC, identifying four of them as carbon stars and two as oxygen-rich
supergiants. We analyse the molecular bands of C2H2 at 3.1 and 3.8 micron, HCN
at 3.57 micron, and sharp absorption features in the 3.70--3.78 micron region
that we attribute to C2H2. There is evidence for a generally high abundance of
C2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios
at the low metallicity in the LMC. The low initial metallicity is also likely
to have resulted in less abundant HCN and CS. The sample of IR carbon stars
exhibits a range in C2H2:HCN abundance ratio. We do not find strong
correlations between the properties of the molecular atmosphere and
circumstellar dust envelope, but the observed differences in the strengths and
shapes of the absorption bands can be explained by differences in excitation
temperature. High mass-loss rates and strong pulsation would then be seen to be
associated with a large scale height of the molecular atmosphere.Comment: Accepted for publication in Astronomy and Astrophysics. 20 pages.
Figure 11 is degraded for posting on astro-p
Quantifying the impact of model inaccuracy in climate change impact assessment studies using an agro-hydrological model
International audienceNumerical simulation models are frequently applied to assess the impact of climate change on hydrology and agriculture. A common hypothesis is that unavoidable model errors are reflected in the reference situation as well as in the climate change situation so that by comparing reference to scenario model errors will level out. For a polder in The Netherlands an innovative procedure has been introduced, referred to as the Model-Scenario-Ratio (MSR), to express model inaccuracy on climate change impact assessment studies based on simulation models comparing a reference situation to a climate change situation. The SWAP (Soil Water Atmosphere Plant) model was used for the case study and the reference situation was compared to two climate change scenarios. MSR values close to 1, indicating that impact assessment is mainly a function of the scenario itself rather than of the quality of the model, were found for most indicators evaluated. A climate change scenario with enhanced drought conditions and indicators based on threshold values showed lower MSR values, indicating that model accuracy is an important component of the climate change impact assessment. It was concluded that the MSR approach can be applied easily and will lead to more robust impact assessment analyses
Plasmons in strongly correlated systems: spectral weight transfer and renormalized dispersion
We study the charge-density dynamics within the two-dimensional extended
Hubbard model in the presence of long-range Coulomb interaction across the
metal-insulator transition point. To take into account strong correlations we
start from self-consistent extended dynamical mean-field theory and include
non-local dynamical vertex corrections through a ladder approximation to the
polarization operator. This is necessary to fulfill charge conservation and to
describe plasmons in the correlated state. The calculated plasmon spectra are
qualitatively different from those in the random-phase approximation: they
exhibit a spectral density transfer and a renormalized dispersion with enhanced
deviation from the canonical -behavior. Both features are reminiscent
of interaction induced changes found in single-electron spectra of strongly
correlated systems.Comment: 5 pages, 5 figures + appendix (3 pages, 1 figure
Socioeconomic status and colon cancer incidence: a prospective cohort study.
The association between socioeconomic status and colon cancer was investigated in a prospective cohort study that started in 1986 in The Netherlands among 120,852 men and women aged 55-69 years. At baseline, data on socioeconomic status, alcohol consumption and other dietary and non-dietary covariates were collected by means of a self-administered questionnaire. For data analysis a case-cohort approach was used, in which the person-years at risk were estimated using a randomly selected subcohort (1688 men and 1812 women). After 3.3 years of follow-up, 312 incident colon cancer cases were detected: 157 men and 155 women. After adjustment for age, we found a positive association between colon cancer risk and highest level of education (trend P = 0.13) and social standing (trend P = 0.008) for men. Also, male, upper white-collar workers had a higher colon cancer risk than blue-collar workers (RR = 1.42, 95% CI 0.95-2.11). Only the significant association between social standing and colon cancer risk persisted after additional adjustment for other risk factors for colon cancer (trend P = 0.005), but the higher risk was only found in the highest social standing category (RR highest/lowest social standing = 2.60, 95% CI 1.31-5.14). In women, there were no clear associations between the socioeconomic status indicators and colon cancer
Self-consistent Dual Boson approach to single-particle and collective excitations in correlated systems
We propose an efficient dual boson scheme, which extends the DMFT paradigm to
collective excitations in correlated systems. The theory is fully
self-consistent both on the one- and on the two-particle level, thus describing
the formation of collective modes as well as the renormalization of electronic
and bosonic spectra on equal footing. The method employs an effective impurity
model comprising both fermionic and bosonic hybridization functions. Only
single- and two-electron Green's functions of the reference problem enter the
theory, due to the optimal choice of the self-consistency condition for the
effective bosonic bath. We show that the theory is naturally described by a
dual Luttinger-Ward functional and obeys the relevant conservation laws.Comment: 17 pages, 12 figure
Chemical evolution of star clusters
I discuss the chemical evolution of star clusters, with emphasis on old
globular clusters, in relation to their formation histories. Globular clusters
clearly formed in a complex fashion, under markedly different conditions from
any younger clusters presently known. Those special conditions must be linked
to the early formation epoch of the Galaxy and must not have occurred since.
While a link to the formation of globular clusters in dwarf galaxies has been
suggested, present-day dwarf galaxies are not representative of the
gravitational potential wells within which the globular clusters formed.
Instead, a formation deep within the proto-Galaxy or within dark-matter
minihaloes might be favoured. Not all globular clusters may have formed and
evolved similarly. In particular, we may need to distinguish Galactic halo from
Galactic bulge clusters.Comment: 27 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 6 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Conservation in two-particle self-consistent extensions of dynamical-mean-field-theory
Extensions of dynamical-mean-field-theory (DMFT) make use of quantum impurity
models as non-perturbative and exactly solvable reference systems which are
essential to treat the strong electronic correlations. Through the introduction
of retarded interactions on the impurity, these approximations can be made
two-particle self-consistent. This is of interest for the Hubbard model,
because it allows to suppress the antiferromagnetic phase transition in
two-dimensions in accordance with the Mermin-Wagner theorem, and to include the
effects of bosonic fluctuations. For a physically sound description of the
latter, the approximation should be conserving. In this paper we show that the
mutual requirements of two-particle self-consistency and conservation lead to
fundamental problems. For an approximation that is two-particle self-consistent
in the charge- and longitudinal spin channel, the double occupancy of the
lattice and the impurity are no longer consistent when computed from
single-particle properties. For the case of self-consistency in the charge- and
longitudinal as well as transversal spin channels, these requirements are even
mutually exclusive so that no conserving approximation can exist. We illustrate
these findings for a two-particle self-consistent and conserving DMFT
approximation.Comment: 17 pages, 9 figure
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