328 research outputs found
The RR Lyrae Period - K Luminosity relation for Globular Clusters: an observational approach
The Period - metallicity - K band luminosity (PLK) relation for RR Lyrae
stars in 15 Galactic globular clusters and in the LMC globular cluster
Reticulum has been derived. It is based on accurate near infrared (K)
photometry combined with 2MASS and other literature data. The PLK relation has
been calibrated and compared with the previous empirical and theoretical
determinations in literature. The zero point of the absolute calibration has
been obtained from the K magnitude of RR Lyr whose distance modulus has been
measured via trigonometric parallax with HST. Using this relation we obtain a
distance modulus to the LMC of (m-M)_0 = 18.54 \pm 0.15 mag, in good agreement
with recent determinations based on the analysis of Cepheid variable stars.Comment: 6 pages, 4 figures, accepted for publication by MNRA
Monte Carlo simulations of multiple populations in globular clusters: constraints on the cooling flow vs. accretion scenario using million bodies simulations
I simulate the evolution of a stellar system hosting two stellar populations
whose initial set up is defined according to the two main scenarios proposed
for the origin of multiple populations in Galactic globular clusters: (i)
formation of a second generation from a cooling flow of pristine+polluted gas
and (ii) accretion of polluted gas onto the proto-stellar disks of a fraction
of low-mass stars. For this purpose, Monte Carlo simulations containing from
up to particles have been run including the effect of
stellar evolution, binary interactions, external tidal field and a detailed
modelling of the proto-stellar disk structure. The early accretion of gas onto
proto-stellar disks is unable to produce discrete populations and to alter the
chemical composition of a significant () fraction of stars unless a disk
lifetime larger () than that predicted by models is
assumed. Moreover, in this scenario the mixing timescale of the two populations
is too short to reproduce the observed segregation of the chemically enriched
population. On the other hand, simulations run within the cooling flow scenario
can evolve after a Hubble time into stellar systems with a first-to-second
population mass ratio similar to that observed in globular clusters, provided
that an initial filling-factor is adopted. However, in the
weak tidal field regime a radial segregation of the second population stronger
than what observed in Milky Way globular clusters at large Galactocentric
distances is predicted. This discrepancy disappears in simulations following
eccentric orbits in a realistic axisymmetric potential.Comment: 19 pages, 15 figures, accepted for publication by MNRA
Infrared photometry of Young Massive Clusters in the starburst galaxy NGC 4214
We present the results of an infrared photometric survey performed with
NICS@TNG in the nearby starburst galaxy NGC 4214. We derived accurate
integrated JK magnitudes of 10 young massive clusters and compared them with
the already available Hubble Space Telescope ultraviolet colors. These clusters
are located in the combined ultraviolet-infrared colors planes on well defined
sequences, whose shapes allow a precise determination of their age. By means of
the comparison with suitable stellar evolution models we estimated ages,
metallicities, reddening and masses of these clusters. All the analyzed
clusters appear to be younger than log(t/yr)<8.4, moderately metal-rich and
slightly less massive than present-day Galactic globular clusters. The derived
ages for clusters belonging to the secondary HII star forming complex are
significantly larger than those previously estimated in the literature. We also
discuss the possibility of using the ultraviolet-infrared color-color diagram
to select candidate young massive clusters hosting multiple stellar
populations.Comment: 12 pages, 9 figures, accepted for publication by MNRA
Globular clusters in modified Newtonian dynamics: velocity-dispersion profiles from self-consistent models
We test the modified Newtonian dynamics (MOND) theory with the
velocity-dispersion profiles of Galactic globular clusters populating the
outermost region of the Milky Way halo, where the Galactic acceleration is
lower than the characteristic MOND acceleration a_0. For this purpose, we
constructed self-consistent, spherical models of stellar systems in MOND, which
are the analogues of the Newtonian King models. The models are spatially
limited, reproduce well the surface-brightness profiles of globular clusters,
and have velocity-dispersion profiles that differ remarkably in shape from the
corresponding Newtonian models. We present dynamical models of six globular
clusters, which can be used to efficiently test MOND with the available
observing facilities. A comparison with recent spectroscopic data obtained for
NGC2419 suggests that the kinematics of this cluster might be hard to explain
in MOND.Comment: 13 pages, 9 figures, accepted for publication by MNRA
New Metallicities of RR Lyrae Stars in omega Centauri: Evidence for a Non He-Enhanced Metal-Intermediate Population
We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega
Cen obtained with FLAMES. The well-known metallicity spread is visible among
the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~
-1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~
-1.7), in good agreement with the corresponding zero-age horizontal branch
models with cosmological helium abundance Y = 0.246. This result conflicts with
the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to
the anomalous blue main-sequence stars, which share a similar metallicity but
whose properties are currently explained by assuming for them a large helium
enhancement. Therefore, in this scenario, the coexistence within the cluster of
two different populations with similar metallicities ([Fe/H] ~ -1.2) and
different helium abundances has to be considered.Comment: 9 pages, 4 figures, accepted for publication by ApJ
Spectroscopy of Red Giants in the globular cluster Terzan 8: kinematics and evidence for the surrounding Sagittarius stream
We present the results of a spectroscopic survey of Red Giants in the
globular cluster Terzan 8 with the aim of studying its kinematics. We derived
accurate radial velocities for 82 stars located in the innermost 7 arcmin from
the cluster center identifying 48 bona fide cluster members. The kinematics of
the cluster have been compared with a set of dynamical models accounting for
the effect of mass segregation and a variable fraction of binaries. The derived
velocity dispersion appears to be larger than that predicted for
mass-segregated stellar systems without binaries, indicating that either the
cluster is dynamically young or it contains a large fraction of binaries
(>30%). We detected 7 stars with a radial velocity compatible with the cluster
systemic velocity but with chemical patterns which stray from those of both the
cluster and the Galactic field. These stars are likely members of the
Sagittarius stream surrounding this stellar system.Comment: 10 pages, 8 figures, accepted for publication by MNRA
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