148 research outputs found
The Distribution of Mass in the Orion Dwarf Galaxy
Dwarf galaxies are good candidates to investigate the nature of Dark Matter,
because their kinematics are dominated by this component down to small
galactocentric radii. We present here the results of detailed kinematic
analysis and mass modelling of the Orion dwarf galaxy, for which we derive a
high quality and high resolution rotation curve that contains negligible
non-circular motions and we correct it for the asymmetric drift. Moreover, we
leverage the proximity (D = 5.4 kpc) and convenient inclination (47{\deg}) to
produce reliable mass models of this system. We find that the Universal
Rotation Curve mass model (Freeman disk + Burkert halo + gas disk) fits the
observational data accurately. In contrast, the NFW halo + Freeman disk + gas
disk mass model is unable to reproduce the observed Rotation Curve, a common
outcome in dwarf galaxies. Finally, we attempt to fit the data with a MOdified
Newtonian Dynamics (MOND) prescription. With the present data and with the
present assumptions on distance, stellar mass, constant inclination and
reliability of the gaseous mass, the MOND "amplification" of the baryonic
component appears to be too small to mimic the required "dark component". The
Orion dwarf reveals a cored DM density distribution and a possible tension
between observations and the canonical MOND formalism.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Reinforcing the pulmonary artery autograft in the aortic position with a textile mesh: a histological evaluation
OBJECTIVES
The Ross procedure involves replacing a patientâs diseased aortic valve with their own pulmonary valve. The most common failure mode is dilatation of the autograft. Various strategies to reinforce the autograft have been proposed. Personalized external aortic root support has been shown to be effective in stabilizing the aortic root in Marfan patients. In this study, the use of a similar external mesh to support a pulmonary artery autograft was evaluated.
METHODS
The pulmonary artery was translocated as an interposition autograft in the descending thoracic aortas of 10 sheep. The autograft was reinforced with a polyethylene terephthalate mesh (nâ=â7) or left unreinforced (nâ=â3). After 6 months, a computed tomography scan was taken, and the descending aorta was excised and histologically examined using the haematoxylinâeosin and Elastica van Gieson stains.
RESULTS
The autograft/aortic diameter ratio was 1.59 in the unreinforced group but much less in the reinforced group (1.11) (Pâ<â0.05). A fibrotic sheet, variable in thickness and containing fibroblasts, neovessels and foreign body giant cells, was incorporated in the mesh. Histological examination of the reinforced autograft and the adjacent aorta revealed thinning of the vessel wall due to atrophy of the smooth muscle cells. Potential spaces between the vessel wall and the mesh were filled with oedema.
CONCLUSIONS
Reinforcing an interposition pulmonary autograft in the descending aorta with a macroporous mesh showed promising results in limiting autograft dilatation in this sheep model. Histological evaluation revealed atrophy of the smooth muscle cell and consequently thinning of the vessel wall within the mesh support
Biomechanical evaluation of a personalized external aortic root support applied in the Ross procedure
A commonly heard concern in the Ross procedure, where a diseased aortic valve is replaced by the patient's own pulmonary valve, is the possibility of pulmonary autograft dilatation. We performed a biomechanical investigation of the use of a personalized external aortic root support or exostent as a possibility for supporting the autograft.
In ten sheep a short length of pulmonary artery was interposed in the descending aorta, serving as a simplified version of the Ross procedure. In seven of these cases, the autograft was supported by an external mesh or so-called exostent. Three sheep served as control, of which one was excluded from the mechanical testing. The sheep were sacrificed six months after the procedure. Samples of the relevant tissues were obtained for subsequent mechanical testing: normal aorta, normal pulmonary artery, aorta with exostent, pulmonary artery with exostent, and pulmonary artery in aortic position for six months. After mechanical testing, the material parameters of the Gasser-Ogden-Holzapfel model were determined for the different tissue types.
Stress-strain curves of the different tissue types show significantly different mechanical behavior. At baseline, stress-strain curves of the pulmonary artery are lower than aortic stress-strain curves, but at the strain levels at which the collagen fibers are recruited, the pulmonary artery behaves stiffer than the aorta. After being in aortic position for six months, the pulmonary artery tends towards aorta-like behavior, indicating that growth and remodeling processes have taken place. When adding an exostent around the pulmonary autograft, the mechanical behavior of the composite artery (exostent + artery) differs from the artery alone, the non-linearity being more evident in the former
Local stellar kinematics from RAVE data: III. Radial and Vertical Metallicity Gradients based on Red Clump Stars
We investigate radial and vertical metallicity gradients for a sample of red
clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We
select a total of 6781 stars, using a selection of colour, surface gravity and
uncertainty in the derived space motion, and calculate for each star a
probabilistic (kinematic) population assignment to a thin or thick disc using
space motion and additionally another (dynamical) assignment using stellar
vertical orbital eccentricity. We derive almost equal metallicity gradients as
a function of Galactocentric distance for the high probability thin disc stars
and for stars with vertical orbital eccentricities consistent with being
dynamically young, e_v<=0.07, i.e. d[M/H]/dR_m = -0.041(0.003) and d[M/H]/dR_m
= -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from
the Galactic plane for the same populations are steeper, i.e. d[M/H]/dz_{max} =
-0.109(0.008) and d[M/H]/dz_{max} = -0.260(0.031) dex/kpc, respectively. R_m
and z_{max} are the arithmetic mean of the perigalactic and apogalactic
distances, and the maximum distance to the Galactic plane, respectively.
Samples including more thick disc red clump giant stars show systematically
shallower abundance gradients. These findings can be used to distinguish
between different formation scenarios of the thick and thin discs.Comment: 27 pages, including 15 figures and 4 tables, accepted for publication
in MNRA
Galaxy Bulges As Tests of CDM vs MOND in Strong Gravity
The tight correlation between galaxy bulges and their central black hole
masses likely emerges in a phase of rapid collapse and starburst at high
redshift, due to the balance of gravity on gas with the feedback force from
starbursts and the wind from the black hole; the average gravity on per unit
mass of gas is ~ 2 x 10^-10 m/sec^2 during the star burst phase. This level of
gravity could come from the real r^{-1} cusps of Cold Dark Matter (CDM) halos,
but the predicted gravity would have a large scatter due to dependence on
cosmological parameters and formation histories. Better agreement is found with
the gravity from the scalar field in some co-variant versions of MOND, which
can create the mirage of a Newtonian effective dark halo of density Pi r^{-1}
near the center, where the characteristic surface density Pi=130alpha^{-1} Msun
pc^{-2} and alpha is a fundamental constant of order unity fixed by the
Lagrangian of the co-variant theory if neglecting environmental effects.
We show with a toy analytical model and a hydrodynamical simulation that a
constant background gravity due to MOND/TeVeS scalar field implies a critical
pressure synchronizing starbursts and the formation of galaxy bulges and
ellipticals. A universal threshold for the formation of the brightest regions
of galaxies in a MONDian universe suggests that the central BHs, bulges and
ellipticals would respect tight correlations like the M_{bulge}-M_{BH}-sigma
relations. In general MOND tends to produce tight correlations in galaxy
properties because its effective halo has less freedom and scatter than CDM
halos.Comment: 30p, 6 figs, expanded. Accpeted for Ap
Interstellar absorptions and shocked clouds towards supernova remnant RX J0852.0-4622
We present results of survey of interstellar absorptions towards supernova
remnant (SNR) RX J0852.0-4622. The distribution of KI absorbers along the
distance of the background stars is indicative of a local region (d<600pc)
strongly depopulated by KI line-absorbing clouds. This fact is supported by the
behavior of the interstellar extinction. We find four high-velocity CaII
components with velocities of >100km/s towards three stars and identify them
with shocked clouds of Vela SNR. We reveal and measure acceleration of two
shocked clouds at the approaching and receding sides of Vela SNR along the same
sight line. The clouds acceleration, velocity, and CaII column density are used
to probe cloud parameters. The total hydrogen column density of both
accelerating clouds is found to be similar (~6*10^{17} cm) which
indicates that possibly there is a significant amount of small-size clouds in
the vicinity of Vela SNR.Comment: accepted in MNRA
XHIP-II: Clusters and associations
Context. In the absence of complete kinematic data it has not previously been
possible to furnish accurate lists of member stars for all moving groups. There
has been an unresolved dispute concerning the apparent inconsistency of the
Hipparcos parallax distance to the Pleiades.
Aims. To find improved candidate lists for clusters and associations
represented among Hipparcos stars, to establish distances, and to cast light on
the Pleiades distance anomaly.
Methods. We use a six dimensional fitting procedure to identify candidates,
and plot CMDs for 20 of the nearest groups. We calculate the mean parallax
distance for all groups.
Results. We identify lists of candidates and calculated parallax distances
for 42 clusters and 45 associations represented within the Hipparcos catalogue.
We find agreement between parallax distance and photometric distances for the
most important clusters. For single stars in the Pleiades we find mean parallax
distance 125.6 \pm 4.2 pc and photometric distance 132 \pm 3 pc calibrated to
nearby groups of similar in age and composition. This gives no reason to doubt
either the Hipparcos database or stellar evolutionary theory.Comment: Accepted for publication in Astronomy Letters, 10 pages, 2 fig
Dark Energy, scalar-curvature couplings and a critical acceleration scale
We study the effects of coupling a cosmologically rolling scalar field to
higher order curvature terms. We show that when the strong coupling scale of
the theory is on the 10^{-3}-10^{-1}eV range, the model passes all experimental
bounds on the existence of fifth forces even if the field has a mass of the
order of the Hubble scale in vacuum and non-suppressed couplings to SM fields.
The reason is that the coupling to certain curvature invariant acts as an
effective mass that grows in regions of large curvature. This prevents the
field from rolling down its potential near sources and makes its effects on
fifth-force search experiments performed in the laboratory to be observable
only at the sub-mm scale. We obtain the static spherically symmetric solutions
of the theory and show that a long-range force appears but it is turned on only
below a fixed Newtonian acceleration scale of the order of the Hubble constant.
We comment on the possibility of using this feature of the model to alleviate
the CDM small scale crisis and on its possible relation to MOND.Comment: 12 pages, 2 figure
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