136 research outputs found
Star cluster evolution in barred disc galaxies. I. Planar periodic orbits
The dynamical evolution of stellar clusters is driven to a large extent by
their environment. Several studies so far have considered the effect of tidal
fields and their variations, such as, e.g., from giant molecular clouds,
galactic discs, or spiral arms. In this paper we will concentrate on a tidal
field whose effects on star clusters have not yet been studied, namely that of
bars. We present a set of direct N-body simulations of star clusters moving in
an analytic potential representing a barred galaxy. We compare the evolution of
the clusters moving both on different planar periodic orbits in the barred
potential and on circular orbits in a potential obtained by axisymmetrising its
mass distribution. We show that both the shape of the underlying orbit and its
stability have strong impact on the cluster evolution as well as the morphology
and orientation of the tidal tails and the sub-structures therein. We find that
the dissolution time-scale of the cluster in our simulations is mainly
determined by the tidal forcing along the orbit and, for a given tidal forcing,
only very little by the exact shape of the gravitational potential in which the
cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS.
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The regeneration of stellar bars by tidal interactions. Numerical simulations of fly-by encounters
We study the regeneration of stellar bars triggered by a tidal interaction,
using numerical simulations of either purely stellar or stellar+gas disc
galaxies. We find that interactions which are sufficiently strong to regenerate
the bar in the purely stellar models do not lead to a regeneration in the
dissipative models, owing to the induced gas inflow in those models. In models
in which the bar can be regenerated, we find a tight correlation between the
strength and the pattern speed of the induced bar. This relation can be
explained by a significant radial redistribution of angular momentum in the
disc due to the interaction, similar to the processes and correlations found
for isolated barred spirals. We furthermore show that the regenerated bars show
the same dynamical properties as their isolated counterparts.Comment: 18 pages, 26 figures, accepted for publication in MNRA
Formation and Evolution of Black Holes in Galactic Nuclei and Star Clusters
We study the formation, growth, and co-evolution of single and multiple supermassive black holes (SMBHs) and compact objects like neutron stars, white dwarfs, and stellar mass black holes in galactic nuclei and star clusters, focusing on the role of stellar dynamics. In this paper we focus on one exemplary topic out of a wider range of work done, the study of orbital parameters of binary black holes in galactic nuclei (binding energy, eccentricity, relativistic coalescence) as a function of initial parameters. In some cases the classical evolution of black hole binaries in dense stellar systems drives them to surprisingly high eccentricities, which is very exciting for the emission of gravitational waves and relativistic orbit shrinkage. Such results are interesting to the emerging field of gravitational wave astronomy, in relation to a number of ground and space based instruments designed to measure gravitational waves from astrophysical sources (VIRGO, Geo600, LIGO, LISA). Our models self-consistently cover the entire range from Newtonian dynamics to the relativistic coalescence of SMBH binaries
Stellar kinematics in double-barred galaxies: the sigma-hollows
We present SAURON integral-field stellar velocity and velocity dispersion
maps for four double-barred early-type galaxies: NGC2859, NGC3941, NGC4725 and
NGC5850. The presence of the inner bar does not produce major changes in the
line-of-sight velocity, but it appears to have an important effect in the
stellar velocity dispersion maps: we find two sigma-hollows of amplitudes
between 10 and 40 km/s on either side of the center, at the ends of the inner
bars. We have performed numerical simulations to explain these features. Ruling
out other possibilities, we conclude that the sigma-hollows are an effect of
the contrast between two kinematically different components: the high velocity
dispersion of the bulge and the more ordered motion (low velocity dispersion)
of the inner bar.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter
Changes in Waist Circumference and the Incidence of Diabetes in Middle-Aged Men and Women
Waist circumference (WC) is positively associated with diabetes, but the association with changes in WC (DWC) is less clear. We investigated the association between DWC and the subsequent risk of diabetes in middle-aged men and women, and evaluated the influence from concurrent changes in body mass index (DBMI).Data on 15,577 men and 20,066 women from the Danish Diet, Cancer and Health study were analyzed. Anthropometry was assessed in 1993–97 and 1999–02. Information on diabetes was obtained from The Danish National Diabetes Register. Hazard ratios (HR) were calculated from Cox' proportional hazard models with individuals considered at risk from 1999–02 until December 31 2006. During 5.4 years of follow-up, 1,027 and 876 new cases of diabetes occurred among men and women, respectively. WC was positively associated with diabetes in both sexes also with adjustment for covariates and BMI. DWC was positively associated with diabetes in women, but not in men (HR per 5 cm change = 1.09 (1.04∶1.15) in women, and 1.00 (0.94, 1.07) in men with adjustment for covariates, baseline WC, BMI and DBMI). Associations with DWC were not notably different in sub-groups stratified according to baseline WC or DBMI, or when individuals with diseases or diabetes occurring within the first years of follow-up were excluded.While this study confirmed that WC is positively associated with the risk of diabetes in middle-aged men and women, it surprisingly showed that changes in WC were not associated with the subsequent risk of diabetes in men, and only weakly positively associated with the risk of diabetes in women. Accordingly, these findings suggest that a reduction in WC may be a weak or insufficient or target for prevention of diabetes in middle-aged men and women
Changes in Waist Circumference and Mortality in Middle-Aged Men and Women
BACKGROUND: Waist circumference (WC) adjusted for body mass index (BMI) is positively associated with mortality, but the association with changes in WC is less clear. We investigated the association between changes in WC and mortality in middle-aged men and women, and evaluated the influence from concurrent changes in BMI. METHODOLOGY/PRINCIPAL FINDINGS: Data on 26,625 healthy men and women from the Danish Diet, Cancer and Health study was analyzed. WC and BMI were assessed in 1993-97 and in 1999-02. Information on mortality was obtained by linkage to the Danish central Person Register. Hazard ratios (HR) were estimated with Cox regression models. During 6.7 years of follow-up, 568 and 361 deaths occurred among men and women, respectively. Changes in WC were positively associated with mortality (HR per 5 cm for the sexes combined  = 1.09 (1.02 : 1.16) with adjustment for covariates, baseline WC, BMI and changes in BMI), whereas changes in BMI were inversely associated with mortality (HR per kg/m2 for the sexes combined  = 0.91 (0.86, 0.97) with adjustment for covariates, baseline WC, BMI and changes in WC). Associations between changes in WC and mortality were not notably different in sub-groups stratified according to changes in BMI, baseline WC or when smokers or deaths occurring within the first years of follow-up were excluded. CONCLUSIONS/SIGNIFICANCE: Changes in WC were positively associated with mortality in healthy middle-aged men and women throughout the range of concurrent changes in BMI. These findings suggest a need for development of prevention and treatment strategies targeted against redistribution of fat mass towards the abdominal region
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