9,617 research outputs found
A new library of theoretical stellar spectra with scaled-solar and alpha-enhanced mixtures
Theoretical stellar libraries have been increasingly used to overcome
limitations of empirical libraries, e.g. by exploring atmospheric parameter
spaces not well represented in the latter. This work presents a new theoretical
library which covers 3000 Teff 25000\,K, -0.5 log g
5.5, and 12 chemical mixtures covering 0.0017 Z 0.049 at both
scaled-solar and -enhanced compositions. This library complements
previous ones by providing: (i) homogeneous computations of opacity
distribution functions, models atmospheres, statistical surface fluxes and high
resolution spectra; (ii) high resolution spectra with continua slopes corrected
by the effect of predicted lines, and; (iii) two families of -enhanced
mixtures for each scaled-solar iron abundance, to allow studies of the
-enhancement both at `fixed iron' and `fixed Z' cases. Comparisons to
observed spectra were performed and confirm that the synthetic spectra
reproduce well the observations, although there are wavelength regions which
should be still improved. The atmospheric parameter scale of the model library
was compared to one derived from a widely used empirical library, and no
systematic difference between the scales was found. This is particularly
reassuring for methods which use synthetic spectra for deriving atmospheric
parameters of stars in spectroscopic surveys
A grid of Synthetic Spectra for Hot DA White Dwarfs and Its Application in Stellar Population Synthesis
In this work we present a grid of LTE and non-LTE synthetic spectra of hot DA
white dwarfs (WDs). In addition to its usefulness for the determination of
fundamental stellar parameters of isolated WDs and in binaries, this grid will
be of interest for the construction of theoretical libraries for stellar
studies from integrated light. The spectral grid covers both a wide temperature
and gravity range, with 17,000 K <= T_eff <= 100,000 K and 7.0 <= log(g) <=
9.5. The stellar models are built for pure hydrogen and the spectra cover a
wavelength range from 900 A to 2.5 microns. Additionally, we derive synthetic
HST/ACS, HST/WFC3, Bessel UBVRI and SDSS magnitudes. The grid was also used to
model integrated spectral energy distributions of simple stellar populations
and our modeling suggests that DAs might be detectable in ultraviolet bands for
populations older than ~8 Gyr.Comment: to be published in The Astrophysical Journal Supplement Serie
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
Full spectral fitting of Milky Way and M31 globular clusters: ages and metallicities
Context: The formation and evolution of disk galaxies are long standing
questions in Astronomy. Understanding the properties of globular cluster
systems can lead to important insights on the evolution of its host galaxy.
Aims: We aim to obtain the stellar population parameters - age and metallicity
- of a sample of M31 and Galactic globular clusters. Studying their globular
cluster systems is an important step towards understanding their formation and
evolution in a complete way. Methods: Our analysis employs a modern
pixel-to-pixel spectral fitting technique to fit observed integrated spectra to
updated stellar population models. By comparing observations to models we
obtain the ages and metallicities of their stellar populations. We apply this
technique to a sample of 38 globular clusters in M31 and to 41 Galactic
globular clusters, used as a control sample. Results: Our sample of M31
globular clusters spans ages from 150 Myr to the age of the Universe.
Metallicities [Fe/H] range from -2.2 dex to the solar value. The
age-metallicity relation obtained can be described as having two components: an
old population with a flat age-[Fe/H] relation, possibly associated with the
halo and/or bulge, and a second one with a roughly linear relation between age
and metallicity, higher metallicities corresponding to younger ages, possibly
associated with the M31 disk. While we recover the very well known Galactic GC
metallicity bimodality, our own analysis of M31's metallicity distribution
function (MDF) suggests that both GC systems cover basically the same [Fe/H]
range yet M31's MDF is not clearly bimodal. These results suggest that both
galaxies experienced different star formation and accretion histories.Comment: A&A, in pres
Gene identification for the cblD defect of vitamin B12 metabolism
Background Vitamin B12 (cobalamin) is an essential cofactor in several metabolic pathways. Intracellular conversion of cobalamin to its two coenzymes, adenosylcobalamin in mitochondria and methylcobalamin in the cytoplasm, is necessary for the homeostasis of methylmalonic acid and homocysteine. Nine defects of intracellular cobalamin metabolism have been defined by means of somatic complementation analysis. One of these defects, the cblD defect, can cause isolated methylmalonic aciduria, isolated homocystinuria, or both. Affected persons present with multisystem clinical abnormalities, including developmental, hematologic, neurologic, and metabolic findings. The gene responsible for the cblD defect has not been identified.
Methods We studied seven patients with the cblD defect, and skin fibroblasts from each were investigated in cell culture. Microcell-mediated chromosome transfer and refined genetic mapping were used to localize the responsible gene. This gene was transfected into cblD fibroblasts to test for the rescue of adenosylcobalamin and methylcobalamin synthesis.
Results The cblD gene was localized to human chromosome 2q23.2, and a candidate gene, designated MMADHC (methylmalonic aciduria, cblD type, and homocystinuria), was identified in this region. Transfection of wild-type MMADHC rescued the cellular phenotype, and the functional importance of mutant alleles was shown by means of transfection with mutant constructs. The predicted MMADHC protein has sequence homology with a bacterial ATP-binding cassette transporter and contains a putative cobalamin binding motif and a putative mitochondrial targeting sequence.
Conclusions Mutations in a gene we designated MMADHC are responsible for the cblD defect in vitamin B12 metabolism. Various mutations are associated with each of the three biochemical phenotypes of the disorder
Nuclear magnetic resonance measurements reveal the origin of the Debye process in monohydroxy alcohols
Monohydroxy alcohols show a structural relaxation and at longer time scales a
Debye-type dielectric peak. From spin-lattice relaxation experiments using
different nuclear probes an intermediate, slower-than-structural dynamics is
identified for n-butanol. Based on these findings and on diffusion
measurements, a model of self-restructuring, transient chains is proposed. The
model is demonstrated to explain consistently the so far puzzling observations
made for this class of hydrogen-bonded glass forming liquids.Comment: 4 pages, 4 figure
Effect of competitive interactions between ectomycorrhizal and saprotrophic fungi on castanea sativa performance
In Northeast of Portugal the macrofungal community associated to chestnut tree (Castanea sativa Mill.) is rich and diversified. Among fungal species, the ectomycorrhizal Pisolithus tinctorius and the saprotroph Hypholoma fasciculare are common in this habitat. The aim of the present work was to assess the effect of the interaction between both fungi on growth, nutritional status and physiology of C. sativa seedlings. In pot experiments, C. sativa seedlings were inoculated with P. tinctorius and H. fasciculare individually or in combination. Inoculation with P. tinctorius stimulated the plant growth and resulted in increased foliar-N, -P, and photosynthetic pigment contents. These effects were suppressed when H. fasciculare was simultaneously applied with P. tinctorius. This result could be related to the inhibition of ectomycorrhizal fungus root colonization as a result of antagonism or to the competition for nutrient sources. If chestnut seedlings have been previously inoculated with P. tinctorius, the subsequent inoculation of H. fasciculare 30 days later did not affect root colonization and mycorrhization benefits were observed. This work confirms an antagonistic interaction between ectomycorrhizal and saprotrophic fungi with consequences on the ectomycorrhizal host physiology. Although P. tinctorius is effective in promoting growth of host trees by establishing mycorrhizae, in the presence of other fungi it may not always be able to interact with host roots due to an inability to compete with certain fungi.Authors are grateful to Fundação para a Ciência e Tecnologia (FCT) for financial support (Project PTDC/AGR-AAM/099556/2008)
- …