455 research outputs found
Modelling resonances and orbital chaos in disk galaxies. Application to a Milky Way spiral model
Context: Resonances in the stellar orbital motion under perturbations from
spiral arms structure play an important role in the evolution of the disks of
spiral galaxies. The epicyclic approximation allows the determination of the
corresponding resonant radii on the equatorial plane (for nearly circular
orbits), but is not suitable in general.
Aims: We expand the study of resonant orbits by analysing stellar motions
perturbed by spiral arms with Gaussian-shaped profiles without any restriction
on the stellar orbital configurations, and we expand the concept of Lindblad
(epicyclic) resonances for orbits with large radial excursions.
Methods: We define a representative plane of initial conditions, which covers
the whole phase space of the system. Dynamical maps on representative planes
are constructed numerically, in order to characterize the phase-space structure
and identify the precise location of resonances. The study is complemented by
the construction of dynamical power spectra, which provide the identification
of fundamental oscillatory patterns in the stellar motion.
Results: Our approach allows a precise description of the resonance chains in
the whole phase space, giving a broader view of the dynamics of the system when
compared to the classical epicyclic approach, even for objects in retrograde
motion. The analysis of the solar neighbourhood shows that, depending on the
current azimuthal phase of the Sun with respect to the spiral arms, a star with
solar kinematic parameters may evolve either inside the stable co-rotation
resonance or in a chaotic zone.
Conclusions: Our approach contributes to quantifying the domains of resonant
orbits and the degree of chaos in the whole Galactic phase-space structure. It
may serve as a starting point to apply these techniques to the investigation of
clumps in the distribution of stars in the Galaxy, such as kinematic moving
groups.Comment: 17 pages, 15 figures. Matches accepted version in A&
Bimodal chemical evolution of the Galactic disk and the Barium abundance of Cepheids
In order to understand the Barium abundance distribution in the Galactic disk
based on Cepheids, one must first be aware of important effects of the
corotation resonance, situated a little beyond the solar orbit. The thin disk
of the Galaxy is divided in two regions that are separated by a barrier
situated at that radius. Since the gas cannot get across that barrier, the
chemical evolution is independent on the two sides of it. The barrier is caused
by the opposite directions of flows of gas, on the two sides, in addition to a
Cassini-like ring void of HI (caused itself by the flows). A step in the
metallicity gradient developed at corotation, due to the difference in the
average star formation rate on the two sides, and to this lack of communication
between them. In connection with this, a proof that the spiral arms of our
Galaxy are long-lived (a few billion years) is the existence of this step. When
one studies the abundance gradients by means of stars which span a range of
ages, like the Cepheids, one has to take into account that stars, contrary to
the gas, have the possibility of crossing the corotation barrier. A few stars
born on the high metallicity side are seen on the low metallicity one, and
vice-versa. In the present work we re-discuss the data on Barium abundance in
Cepheids as a function of Galactic radius, taking into account the scenario
described above. The [Ba/H] ratio, plotted as a function of Galactic radius,
apparently presents a distribution with two branches in the external region
(beyond corotation). One can re-interpret the data and attribute the upper
branch to the stars that were born on the high metallicity side. The lower
branch, analyzed separately, indicates that the stars born beyond corotation
have a rising Barium metallicity as a function of Galactic radius.Comment: 6 pages, 7 figures, Proceedings of IAU Symposium 29
Combined dynamical effects of the bar and spiral arms in a Galaxy model. Application to the solar neighbourhood
Observational data indicate that the Milky Way is a barred spiral galaxy.
Computation facilities and availability of data from Galactic surveys stimulate
the appearance of models of the Galactic structure. More efforts to build
dynamical models containing both spiral arms and the central bar/bulge are
needed.
We expand the study of the stellar dynamics in the Galaxy by adding the
bar/bulge component to a model with spiral arms introduced in our previous
paper. The model is tested by applying it to the solar neighborhood, where
observational data are more precise.
We model analytically the potential of the Galaxy to derive the force field
in its equatorial plane. The model comprises an axisymmetric disc derived from
the observable rotation curve, four arms with Gaussian-shaped groove profiles,
and a classical elongated/oblate ellipsoidal bar/bulge structure. The
parameters describing the bar/bulge are constrained by observations and the
stellar dynamics, and their possible limits are determined.
A basic model results in a bar of 2.9 kpc in length, with a mass of the order
of a few 10. The size and orientation of the bar are also restricted
by the position of masers with VLBI distances. The bar's rotation speed is
constrained to km s kpc taking into account
the allowed mass range.
We conclude that our basic model is compatible with observations and with the
dynamical constraints. The model explains simultaneously the bulk of the main
moving groups, associated here with the spiral corotation resonance, and the
Hercules stream, associated with several inner high-order spiral resonances; in
particular, with the 8/1 resonance. From the dynamical constraints on the bar's
angular speed, it is unlikely that the bar's OLR lies near the solar circle;
moreover, its proximity would compromise the stability of the Local Arm
structure.Comment: 17 pages, 16 figures. Accepted in A&
Evaluation of Potential Protective Factors Against Metabolic Syndrome in Bottlenose Dolphins: Feeding and Activity Patterns of Dolphins in Sarasota Bay, Florida
Free-ranging bottlenose dolphins (Tursiops truncatus) living in Sarasota Bay, Florida appear to have a lower risk of developing insulin resistance and metabolic syndrome compared to a group of dolphins managed under human care. Similar to humans, differences in diet and activity cycles between these groups may explain why Sarasota dolphins have lower insulin, glucose, and lipids. To identify potential protective factors against metabolic syndrome, existing and new data were incorporated to describe feeding and activity patterns of the Sarasota Bay wild dolphin community. Sarasota dolphins eat a wide variety of live fish and spend 10–20% of daylight hours foraging and feeding. Feeding occurs throughout the day, with the dolphins eating small proportions of their total daily intake in brief bouts. The natural pattern of wild dolphins is to feed as necessary and possible at any time of the day or night. Wild dolphins rarely eat dead fish or consume large amounts of prey in concentrated time periods. Wild dolphins are active throughout the day and night; they may engage in bouts of each key activity category at any time during daytime. Dive patterns of radio-tagged dolphins varied only slightly with time of day. Travel rates may be slightly lower at night, suggesting a diurnal rhythm, albeit not one involving complete, extended rest. In comparison, the managed dolphins are older; often fed a smaller variety of frozen-thawed fish types; fed fish species not in their natural diet; feedings and engaged activities are often during the day; and they are fed larger but fewer meals. In summary, potential protective factors against metabolic syndrome in dolphins may include young age, activity, and small meals fed throughout the day and night, and specific fish nutrients. These protective factors against insulin resistance and type 2 diabetes are similar to those reported in humans. Further studies may benefit humans and dolphins
- …