79 research outputs found
A Mechanism for the Oxygen and Iron Bimodal Radial Distribution Formation in the Disc of our Galaxy
Recently it has been proposed that there are two types of SN Ia progenitors
-- short-lived and long-lived. On the basis of this idea, we develope a theory
of a unified mechanism for the formation of the bimodal radial distribution of
iron and oxygen in the Galactic disc. The underlying cause for the formation of
the fine structure of the radial abundance pattern is the influence of spiral
arms, specifically, the combined effect of the corotation resonance and
turbulent diffusion. From our modelling we conclude that to explain the bimodal
radial distributions simultaneously for oxygen and iron and to obtain
approximately equal total iron output from different types of supernovae, the
mean ejected iron mass per supernova event should be the same as quoted in
literature if maximum mass of stars, that eject heavy elements, is . For the upper mass limit of the production of iron
by a supernova II explosion should be increased by about 1.5 times.Comment: 7 pages, 6 figures, MNRAS submitte
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc
The Bulge is the least understood major stellar population of the Milky Way.
Most of what we know about the formation and evolution of the Bulge comes from
bright giant stars. The underlying assumption that giants represent all the
stars, and accurately trace the chemical evolution of a stellar population, is
under debate. In particular, recent observations of a few microlensed dwarf
stars give a very different picture of the evolution of the Bulge from that
given by the giant stars. [ABRIDGED] We perform a detailed elemental abundance
analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra
that were obtained while the stars were optically magnified during
gravitational microlensing events. [ABRIDGED] We present detailed elemental
abundances and stellar ages for six new dwarf stars in the Galactic bulge.
Combining these with previous events, here re-analysed with the same methods,
we study a homogeneous sample of 15 stars, which constitute the largest sample
to date of microlensed dwarf stars in the Galactic bulge. We find that the
stars span the full range of metallicities from [Fe/H]=-0.72 to +0.54, and an
average metallicity of =-0.08+/-0.47, close to the average metallicity
based on giant stars in the Bulge. Furthermore, the stars follow well-defined
abundance trends, that for [Fe/H]<0 are very similar to those of the local
Galactic thick disc. This suggests that the Bulge and the thick disc have had,
at least partially, comparable chemical histories. At sub-solar metallicities
we find the Bulge dwarf stars to have consistently old ages, while at
super-solar metallicities we find a wide range of ages. Using the new age and
abundance results from the microlensed dwarf stars we investigate possible
formation scenarios for the Bulge.Comment: New version accepted for publication in Astronomy and Astrophysic
Chemical gradients in the Milky Way from the RAVE data
Aims. We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besancon model. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane.
Methods. We analysed three different samples selected from three independent datasets: a sample of 19 962 dwarf stars selected from the RAVE database, a sample of 10 616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besancon model. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (R-g) and the maximum distances from the Galactic plane reached by the stars along their orbits (Z(max)). We measured the chemical gradients as functions of R-g at different Z(max).
Results. We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Z(max) < 0.4 kpc and 4.5 < R-g(kpc) < 9.5, the iron gradient for the RAVE sample is d[Fe/H]/dR(g) = -0.065 dex kpc(-1), whereas for the GCS sample it is d[Fe/H]/dR(g) = -0.043 dex kpc(-1) with internal errors of +/-0.002 and +/-0.004 dex kpc(-1), respectively. The gradients of the RAVE and GCS samples become flatter at larger Z(max). Conversely, the mock sample has a positive iron gradient of d[Fe/H]/dR(g) = +0.053 +/- 0.003 dex kpc(-1) at Z(max) < 0.4 kpc and remains positive at any Z(max). These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besancon model. In addition, the low metallicity and asymmetric drift of the thick disc causes a shift of the stars towards lower R-g and metallicity which, together with the thin-disc stars with a higher metallicity and R-g, generates a fictitious positive gradient of the full sample. The flatter gradient at larger Z(max) found in the RAVE and the GCS samples may therefore be due to the superposition of thin-and thick-disc stars, which mimicks a flatter or positive gradient. This does not exclude the possibility that the thick disc has no chemical gradient. The discrepancies between the observational samples and the mock sample can be reduced by i) decreasing the density; ii) decreasing the vertical velocity; and iii) increasing the metallicity of the thick disc in the Besancon model
The globular cluster system of NGC 1399 V. dynamics of the cluster system out to 80 kpc
(Abridged) We use the largest set of globular cluster velocities obtained so
far of any elliptical galaxy to revise and extend the previous investigations
of the dynamics of NGC 1399, the central dominant galaxy of the nearby Fornax
cluster of galaxies. Our sample now comprises velocities for almost 700 GCs
with projected galactocentric radii between 6 and 100 kpc. In addition, we use
velocities published by Bergond et al. (2007). We study the kinematics of the
metal-poor and metal-rich subpopulations and perform spherical Jeans modelling.
The most important results are: The metal-rich (red) GCs resemble the stellar
field population of NGC 1399 in the region of overlap. Both subpopulations are
kinematically distinct and do not show a smooth transition. It is not possible
to find a common dark halo which reproduces simultaneously the properties of
both subpopulations. Some velocities of blue GCs are only to be explained by
orbits with very large apogalactic distances, thus indicating a contamination
with GCs which belong to the entire Fornax cluster rather than to NGC 1399.
Stripped GCs from nearby elliptical galaxies, particularly NGC 1404, may also
contaminate the metal-poor sample. We argue in favour of a scenario in which
the majority of the blue cluster population has been accreted during the
assembly of the Fornax cluster. The red cluster population shares the dynamical
history of the galaxy itself. Therefore we recommend to use a dark halo based
on the red GCs alone. The dark halo which fits best is marginally less massive
than the halo quoted by Richtler et al. (2004). The comparison with X-ray
analyses is satisfactory in the inner regions, but without showing evidence for
a transition from a galaxy to a cluster halo, as suggested by X-ray work.Comment: 31 pages, accepted for publication in A&A. Online material will be
available from the CD
Hepatitis B Virus Alters the Antioxidant System in Transgenic Mice and Sensitizes Hepatocytes to Fas Signaling
Hepatitis B virus (HBV) is a major etiological factor of hepatocellular carcinoma (HCC). However, the precise pathogenetic mechanisms linking HBV infection and HCC remain uncertain. It has been reported that decreased antioxidant enzyme activities are associated with severe liver injury and hepatocarcinogenesis in mouse models. It is unclear if HBV can interfere with the activities of antioxidant enzymes. We established a HBV transgenic mouse line, which spontaneously developed HCC at 2 years of age. We studied the activities of the antioxidant enzymes in the liver of the HBV transgenic mice. Our results showed that the antioxidant enzymes glutathione peroxidase and superoxide dismutase 2 were down-regulated in HBV transgenic mice and correlated with JNK activation. HBV enhanced the Fas-mediated activation of caspase 6, caspase 8 and JNK without enhancing the activation of caspase 3 and hepatocellular apoptosis. As a proper redox balance is important for maintaining cellular homeostasis, these effects of HBV on the host antioxidant system and Fas-signaling may play an important role in HBV-induced hepatocarcinogenesis
The cytoprotective drug amifostine modifies both expression and activity of the pro-angiogenic factor VEGF-A
Peer reviewedPublisher PD
Diagnosis, monitoring and prevention of exposure-related non-communicable diseases in the living and working environment: DiMoPEx-project is designed to determine the impacts of environmental exposure on human health
Preconceito social na Internet: a reprodução de preconceitos e desigualdades sociais a partir da análise de sites de redes sociais
Ellagic acid, a phenolic compound, exerts anti-angiogenesis effects via VEGFR-2 signaling pathway in breast cancer
- …