63 research outputs found
Gas accretion on spiral galaxies: bar formation and renewal
The effects of gas accretion on spiral disk dynamics and stability are
studied through N-body simulations, including star formation and gas/stars mass
exchange. The detailed processes of bar formation, bar destruction and bar
re-formation are followed, while in the same time the disk to bulge ratio is
varying. The accreted gas might be first prevented to flow inwards to the
center by the bar gravity torques, which maintains it to the outer Lindblad
resonance. While the first bar is weakening, the accreted gas replenishes the
disk, increasing the disk-to-bulge ratio, and the disk self-gravity. A second
bar is then unstable, with a higher pattern speed, due both to the increased
mass, and shorter bar length. Three or four bar episodes have been followed
over a Hubble time. Their strength is decreasing with time, while their pattern
speed is increasing. Detailed balance of the angular momentum transfer and
evolution can account for these processes. The gas recycled through star
formation, and rejected through stellar mass loss plays also a role in the disk
dynamics. Implications on the spiral galaxy dynamics and evolution along the
Hubble sequence, and as a function of redshift are discussed.Comment: 21 pages, 21 figures. Accepted for publication in A&A. Figs. 1, 2, 5,
10, 14, 15, 19 are available as jpeg file
Dust Production and Mass Loss in the Galactic Globular Cluster NGC 362
We investigate dust production and stellar mass loss in the Galactic globular
cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud
(SMC), NGC 362 was imaged with the IRAC and MIPS cameras onboard the Spitzer
Space Telescope as part of the Surveying the Agents of Galaxy Evolution
(SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the
tip of the Red Giant Branch that exhibit infrared excesses indicative of
circumstellar dust and find that dust is not present in measurable quantities
in stars below the tip of the Red Giant Branch. We modeled the spectral energy
distribution (SED) of the stars with the strongest IR excess and find a total
cluster dust mass-loss rate of 3.0(+2.0/-1.2) x 10^-9 solar masses per year,
corresponding to a gas mass-loss rate of 8.6(+5.6/-3.4) x 10^-6 solar masses
per year, assuming [Fe/H] = -1.16. This mass loss is in addition to any
dust-less mass loss that is certainly occurring within the cluster. The two
most extreme stars, variables V2 and V16, contribute up to 45% of the total
cluster dust-traced mass loss. The SEDs of the more moderate stars indicate the
presence of silicate dust, as expected for low-mass, low-metallicity stars.
Surprisingly, the SED shapes of the stars with the strongest mass-loss rates
appear to require the presence of amorphous carbon dust, possibly in
combination with silicate dust, despite their oxygen-rich nature. These results
corroborate our previous findings in omega Centauri.Comment: 13 pages, 11 figures. Accepted to Ap
The short term debt vs. long term debt puzzle: a model for the optimal mix
This paper argues that the existing finance literature is inadequate with respect to its coverage of capital structure of small and medium sized enterprises (SMEs). In particular it is argued that the cost of equity (being both conceptually ill defined and empirically non quantifiable) is not applicable to the capital structure decisions for a large proportion of SMEs and the optimal capital structure depends only on the mix of short and long term debt. The paper then presents a model, developed by practitioners for optimising the debt mix and demonstrates its practical application using an Italian firm's debt structure as a case study
Secular evolution versus hierarchical merging: galaxy evolution along the Hubble sequence, in the field and rich environments
In the current galaxy formation scenarios, two physical phenomena are invoked
to build disk galaxies: hierarchical mergers and more quiescent external gas
accretion, coming from intergalactic filaments. Although both are thought to
play a role, their relative importance is not known precisely. Here we consider
the constraints on these scenarios brought by the observation-deduced star
formation history on the one hand, and observed dynamics of galaxies on the
other hand: the high frequency of bars and spirals, the high frequency of
perturbations such as lopsidedness, warps, or polar rings.
All these observations are not easily reproduced in simulations without
important gas accretion. N-body simulations taking into account the mass
exchange between stars and gas through star formation and feedback, can
reproduce the data, only if galaxies double their mass in about 10 Gyr through
gas accretion. Warped and polar ring systems are good tracers of this
accretion, which occurs from cold gas which has not been virialised in the
system's potential. The relative importance of these phenomena are compared
between the field and rich clusters. The respective role of mergers and gas
accretion vary considerably with environment.Comment: 18 pages, 8 figures, review paper to "Penetrating Bars through Masks
of Cosmic Dust: the Hubble Tuning Fork Strikes a New Note", Pilanesberg, ed.
D. Block et al., Kluwe
Spatial variations of dust abundances across the Large Magellanic Cloud
Using the data obtained with the Spitzer Space telescope as part of the
Surveying the Agents of a Galaxy's Evolution (SAGE) legacy survey, we have
studied the variations of the dust composition and abundance across the Large
Magellanic Cloud (LMC). Such variations are expected, as the explosive events
which have lead to the formation of the many HI shells observed should have
affected the dust properties. Using a model and comparing with a reference
spectral energy distribution from our Galaxy, we deduce the relative abundance
variations of small dust grains across the LMC. We examined the infrared color
ratios as well as the relative abundances of very small grains (VSGs) and
polycyclic aromatic hydrocarbons (PAHs) relative to the big grain (BG)
abundance. Results show that each dust component could have different origins
or evolution in the interstellar medium (ISM). The VSG abundance traces the
star formation activity and could result from shattering of larger grains,
whereas the PAH abundance increases around molecular clouds as well as in the
stellar bar, where they could have been injected into the ISM during mass loss
from old stars.Comment: Received 2009 February 13; Accepted 2009 April 2
Effects of betaine on lipopolysaccharide-induced memory impairment in mice and the involvement of GABA transporter 2
<p>Abstract</p> <p>Background</p> <p>Betaine (glycine betaine or trimethylglycine) plays important roles as an osmolyte and a methyl donor in animals. While betaine is reported to suppress expression of proinflammatory molecules and reduce oxidative stress in aged rat kidney, the effects of betaine on the central nervous system are not well known. In this study, we investigated the effects of betaine on lipopolysaccharide (LPS)-induced memory impairment and on mRNA expression levels of proinflammatory molecules, glial markers, and GABA transporter 2 (GAT2), a betaine/GABA transporter.</p> <p>Methods</p> <p>Mice were continuously treated with betaine for 13 days starting 1 day before they were injected with LPS, or received subacute or acute administration of betaine shortly before or after LPS injection. Then, their memory function was evaluated using Y-maze and novel object recognition tests 7 and 10-12 days after LPS injection (30 μg/mouse, i.c.v.), respectively. In addition, mRNA expression levels in hippocampus were measured by real-time RT-PCR at different time points.</p> <p>Results</p> <p>Repeated administration of betaine (0.163 mmol/kg, s.c.) prevented LPS-induced memory impairment. GAT2 mRNA levels were significantly increased in hippocampus 24 hr after LPS injection, and administration of betaine blocked this increase. However, betaine did not affect LPS-induced increases in levels of mRNA related to inflammatory responses. Both subacute administration (1 hr before, and 1 and 24 hr after LPS injection) and acute administration (1 hr after LPS injection) of betaine also prevented LPS-induced memory impairment in the Y-maze test.</p> <p>Conclusions</p> <p>These data suggest that betaine has protective effects against LPS-induced memory impairment and that prevention of LPS-induced changes in GAT2 mRNA expression is crucial to this ameliorating effect.</p
Effects of leucine supplemented diet on intestinal absorption in tumor bearing pregnant rats
BACKGROUND: It is known that amino acid oxidation is increased in tumor-bearing rat muscles and that leucine is an important ketogenic amino acid that provides energy to the skeletal muscle. METHODS: To evaluate the effects of a leucine supplemented diet on the intestinal absorption alterations produced by Walker 256, growing pregnant rats were distributed into six groups. Three pregnant groups received a normal protein diet (18% protein): pregnant (N), tumor-bearing (WN), pair-fed rats (Np). Three other pregnant groups were fed a diet supplemented with 3% leucine (15% protein plus 3% leucine): leucine (L), tumor-bearing (WL) and pair-fed with leucine (Lp). Non pregnant rats (C), which received a normal protein diet, were used as a control group. After 20 days, the animals were submitted to intestinal perfusion to measure leucine, methionine and glucose absorption. RESULTS: Tumor-bearing pregnant rats showed impairment in food intake, body weight gain and muscle protein content, which were less accentuated in WL than in WN rats. These metabolic changes led to reduction in both fetal and tumor development. Leucine absorption slightly increased in WN group. In spite of having a significant decrease in leucine and methionine absorption compared to L, the WL group has shown a higher absorption rate of methionine than WN group, probably due to the ingestion of the leucine supplemented diet inducing this amino acid uptake. Glucose absorption was reduced in both tumor-bearing groups. CONCLUSIONS: Leucine supplementation during pregnancy in tumor-bearing rats promoted high leucine absorption, increasing the availability of the amino acid for neoplasic cells and, mainly, for fetus and host utilization. This may have contributed to the better preservation of body weight gain, food intake and muscle protein observed in the supplemented rats in relation to the non-supplemented ones
Human Microglia Transplanted in Rat Focal Ischemia Brain Induce Neuroprotection and Behavioral Improvement
BACKGROUND AND PURPOSE: Microglia are resident immunocompetent and phagocytic cells of central nervous system (CNS), which produce various cytokines and growth factors in response to injury and thereby regulate disease pathology. The purpose of this study is to investigate the effects of microglial transplantation on focal cerebral ischemia model in rat. METHODS: Transient middle cerebral artery occlusion (MCAO) in rats was induced by the intraluminal filament technique. HMO6 cells, human microglial cell line, were transplanted intravenously at 48 hours after MCAO. Functional tests were performed and the infarct volume was measured at 7 and 14 days after MCAO. Migration and cell survival of transplanted microglial cells and host glial reaction in the brain were studied by immunohistochemistry. Gene expression of neurotrophic factors, cytokines and chemokines in transplanted cells and host rat glial cells was determined by laser capture microdissection (LCM) and quantitative real time-PCR. RESULTS: HMO6 human microglial cells transplantation group demonstrated significant functional recovery compared with control group. At 7 and 14 days after MCAO, infarct volume was significantly reduced in the HMO group. In the HMO6 group, number of apoptotic cells was time-dependently reduced in the infarct core and penumbra. In addition, number of host rat microglia/macrophages and reactive astrocytes was significantly decreased at 7 and 14 days after MCAO in the penumbra. Gene expression of various neurotrophic factors (GDNF, BDNF, VEGF and BMP7) and anti-inflammatory cytokines (IL4 and IL5) was up-regulated in transplanted HMO6 cells of brain tissue compared with those in culture. The expression of GDNF and VEGF in astrocytes in penumbra was significantly up-regulated in the HMO6 group. CONCLUSIONS: Our results indicate that transplantation of HMO6 human microglial cells reduces ischemic deficits and apoptotic events in stroke animals. The results were mediated by modulation of gliosis and neuroinflammation, and neuroprotection provided by neurotrophic factors of endogenous and transplanted cells-origin
The population of barred galaxies in the local universe I. Detection and characterisation of bars
(Abridge) Bars are very common in the centre of the disc galaxies, and they
drive the evolution of their structure. A volume-limited sample of 2106 disc
galaxies extracted from the Sloan Digital Sky Survey Data Release 5 was studied
to derive the bar fraction, length, and strength as a function of the
morphology, size, local galaxy density, light concentration, and colour of the
host galaxy. The bars were detected using the ellipse fitting method and
Fourier analysis method. The ellipse fitting method was found to be more
efficient in detecting bars in spiral galaxies. The fraction of barred galaxies
turned out to be 45%. A bar was found in 29% of the lenticular galaxies, in 55%
and 54% of the early- and late-type spirals, respectively. The bar length
(normalised by the galaxy size) of late-type spirals is shorter than in
early-type or lenticular ones. A correlation between the bar length and galaxy
size was found with longer bars hosted by larger galaxies. The bars of the
lenticular galaxies are weaker than those in spirals. Moreover, the unimodal
distribution of the bar strength found for all the galaxy types argues against
a quick transition between the barred and unbarred statues. There is no
difference between the local galaxy density of barred and unbarred galaxies.
Besides, neither the length nor strength of the bars are correlated with the
local density of the galaxy neighbourhoods. In contrast, a statistical
significant difference between the central light concentration and colour of
barred and unbarred galaxies was found. Bars are mostly located in less
concentrated and bluer galaxies. These results indicate that the properties of
bars are strongly related to those of their host galaxies, but do not depend on
the local environment.Comment: 15 pages, 13 figures. Accepted for publication in A&
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