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

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 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 $10^{5}$ up to $3\cdot 10^{6}$ 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 ($>10\%$) fraction of stars unless a disk lifetime larger ($t_{disk}\sim20~Myr$) 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 $r_{h}/r_{J}>0.15$ 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 $N$-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 $\approx 30\%-60\%$ 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 $\approx 6$ within the stellar half-light radius ($\approx0.3$ kpc) and $>50$ within the maximum radius of the analysed dataset ($\approx1.5^\circ\approx2$ 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

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

Modeling the chemical evolution of Omega Centauri using three-dimensional hydrodynamical simulations

We present a hydrodynamical and chemical model for the globular cluster Omega Cen, under the assumption that it is the remnant of an ancient dwarf spheroidal galaxy (dSph), the bulk of which was disrupted and accreted by our Galaxy ~10 Gyr ago. We highlight the very different roles played by Type II and Type Ia supernovae (SNe) in the chemical enrichment of the inner regions of the putative parent dSph. While the SNe II pollute the interstellar medium rather uniformly, the SNe Ia ejecta may remain confined inside dense pockets of gas as long as succesive SNe II explosions spread them out. Stars forming in such pockets have lower alpha-to-iron ratios than the stars forming elsewhere. Owing to the inhomogeneous pollution by SNe Ia, the metal distribution of the stars in the central region differs substantially from that of the main population of the dwarf galaxy, and resembles that observed in Omega Cen. This inhomogeneous mixing is also responsible for a radial segregation of iron-rich stars with depleted [alpha/Fe] ratios, as observed in some dSphs. Assuming a star formation history of ~1.5 Gyr, our model succeeds in reproducing both the iron and calcium distributions observed in Omega Cen and the main features observed in the empirical alpha/Fe versus Fe/H plane. Finally, our model reproduces the overall spread of the color-magnitude diagram, but fails in reproducing the morphology of the SGB-a and the double morphology of the main sequence. However, the inhomogeneous pollution reduces (but does not eliminate) the need for a significantly enhanced helium abundance to explain the anomalous position of the blue main sequence. Further models taking into account the dynamical interaction of the parent dwarf galaxy with the Milky Way and the effect of AGB pollution will be required.Comment: 15 pages, 13 figures. MNRAS accepte