658 research outputs found
The stellar Initial Mass Function of the solar neighbourhood revealed by Gaia
I use a sample of more than 120,000 stars in the solar neighbourhood with
parallaxes, magnitudes and colours estimated with unprecedented accuracy by the
second data release of the Gaia mission to derive the initial mass function of
the Galactic disc. A full-forward technique is used to take into account for
the population of unresolved binaries, the metallicity distribution, the star
formation history and their variation across the Galactic disk as well as all
the observational effects. The shape of the initial mass function is well
represented by a segmented power-law with two breaks at characteristic masses.
It has a maximum at M~0.15 Ms with significant flattening (possibly a
depletion) at lower masses and a slope of alpha=-1.34 +/- 0.07 in the range
0.25<M/Ms<1. Above 1 Ms the IMF shows an abrupt decline with a slope ranging
from alpha=-2.68 +/- 0.09 to alpha=-2.41 +/- 0.11 depending on the adopted
resolution of the star formation history.Comment: 20 pages, 12 figures, accepted for publication by MNRA
The Global Mass Functions of 35 Galactic globular clusters: II. Clues on the Initial Mass Function and Black Hole Retention Fraction
In this paper we compare the mass function slopes of Galactic globular
clusters recently determined by Sollima & Baumgardt (2017) with a set of
dedicated N-body simulations of star clusters containing between 65,000 to
200,000 stars. We study clusters starting with a range of initial mass
functions (IMFs), black hole retention fractions and orbital parameters in the
parent galaxy. We find that the present-day mass functions of globular clusters
agree well with those expected for star clusters starting with Kroupa or
Chabrier IMFs, and are incompatible with clusters starting with single
power-law mass functions for the low-mass stars. The amount of mass segregation
seen in the globular clusters studied by Sollima & Baumgardt (2017) can be
fully explained by two-body relaxation driven mass segregation from initially
unsegregated star clusters. Based on the present-day global mass functions, we
expect that a typical globular cluster in our sample has lost about 75% of its
mass since formation, while the most evolved clusters have already lost more
than 90% of their initial mass and should dissolve within the next 1 to 2 Gyr.
Most clusters studied by Sollima & Baumgardt also show a large difference
between their central and global MF slopes, implying that the majority of
Galactic globular clusters is either near or already past core collapse. The
strong mass segregation seen in most clusters also implies that only a small
fraction of all black holes formed in globular clusters still reside in them.Comment: 8 pages, 6 figures, MNRAS, 472, 74
Treatment of realistic tidal field in Monte Carlo simulations of star clusters
We present a new implementation of the Monte Carlo method to simulate the
evolution of star clusters. The major improvement with respect to the
previously developed codes is the treatment of the external tidal field taking
into account for both the loss of stars from the cluster boundary and the
disk/bulge shocks. We provide recipes to handle with eccentric orbits in
complex galactic potentials. The first calculations for stellar systems
containing 21000 and 42000 equal-mass particles show good agreement with direct
N-body simulations in terms of the evolution of both the enclosed mass and the
Lagrangian radii provided that the mass-loss rate does not exceed a critical
value.Comment: 17 pages, 13 figures, accepted for publication by MNRA
The effect of tides on the Fornax dwarf spheroidal galaxy
Estimates of the mass distribution and dark-matter (DM) content of dwarf
spheroidal galaxies (dSphs) are usually derived under the assumption that the
effect of the tidal field of the host galaxy is negligible over the radial
extent probed by kinematic data-sets. We assess the implications of this
assumption in the specific case of the Fornax dSph by means of N-body
simulations of a satellite orbiting around the Milky Way. We consider
observationally-motivated orbits and we tailor the initial distributions of the
satellite's stars and DM to match, at the end of the simulations, the observed
structure and kinematics of Fornax. In all our simulations the present-day
observable properties of Fornax are not significantly influenced by tidal
effects. The DM component is altered by the interaction with the Galactic field
(up to 20% of the DM mass within 1.6 kpc is lost), but the structure and
kinematics of the stellar component are only mildly affected even in the more
eccentric orbit (more than 99% of the stellar particles remain bound to the
dwarf). In the simulations that successfully reproduce Fornax's observables,
the dark-to-luminous mass ratio within 1.6 kpc is in the range 5-6, and up to
16-18 if measured within 3 kpc.Comment: 19 pages, 16 figures. Accepted for publication in MNRA
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
The effect of tides on the Sculptor dwarf spheroidal galaxy
Dwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter
dominated objects in the Universe. Their dynamical masses are commonly derived
using the kinematics of stars under the assumption of equilibrium. However,
these objects are satellites of massive galaxies (e.g.\ the Milky Way) and thus
can be influenced by their tidal fields. We investigate the implication of the
assumption of equilibrium focusing on the Sculptor dSph by means of ad-hoc
-body simulations tuned to reproduce the observed properties of Sculptor
following the evolution along some observationally motivated orbits in the
Milky Way gravitational field. For this purpose, we used state-of-the-art
spectroscopic and photometric samples of Sculptor's stars. We found that the
stellar component of the simulated object is not directly influenced by the
tidal field, while the mass of the more diffuse DM halo is
stripped. We conclude that, considering the most recent estimate of the
Sculptor proper motion, the system is not affected by the tides and the stellar
kinematics represents a robust tracer of the internal dynamics. In the
simulations that match the observed properties of Sculptor, the present-day
dark-to-luminous mass ratio is within the stellar half-light radius
( kpc) and within the maximum radius of the analysed dataset
( kpc).Comment: 19 pages, 10 figures, accepted for publication in MNRAS. V3: updated
after editor comments See our playlist for simulation videos:
https://av.tib.eu/series/633/supplemental+videos+of+the+paper+the+effect+of+tides+on+the+sculptor+dwarf+spheroidal+galax
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
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