56 research outputs found
A population of binaries in the Asymptotic Giant Branch of 47 Tucanae?
We have used a set of archived Hubble Space Telescope/ACS images to probe the
evolved populations of the globular cluster 47 Tucanae. We find an excess of
Asymptotic Giant Branch (AGB) stars in the cluster core. We interpret this
feature as the signature of an extra-population likely made by the progeny of
massive stars originated by the evolution of binary systems. Indeed the
comparison with theoretical tracks suggests that the AGB population of 47 Tuc
can be significantly contaminated by more massive stars currently experiencing
the first ascending Red Giant Branch.Comment: 14 pages, 4 figures, in press on ApJ Letter
Photometric determination of the mass accretion rates of pre-main sequence stars. VI. The case of LH 95 in the Large Magellanic Cloud
We report on the accretion properties of low-mass stars in the LH95
association within the Large Magellanic Cloud (LMC). Using non-contemporaneous
wide-band and narrow-band photometry obtained with the HST, we identify 245
low-mass pre-main sequence (PMS) candidates showing H excess emission
above the 4 level. We derive their physical parameters, i.e. effective
temperatures, luminosities, masses (), ages, accretion luminosities,
and mass accretion rates (). We identify two different
stellar populations: younger than ~8Myr with median ~5.4x10/yr (and ~0.15-1.8) and older than
~8Myr with median ~4.8x10/yr (and
~0.6-1.2). We find that the younger PMS candidates are
assembled in groups around Be stars, while older PMS candidates are uniformly
distributed within the region without evidence of clustering. We find that
in LH95 decreases with time more slowly than what is
observed in Galactic star-forming regions (SFRs). This agrees with the recent
interpretation according to which higher metallicity limits the accretion
process both in rate and duration due to higher radiation pressure. The relationship shows different behaviour at different ages,
becoming progressively steeper at older ages, indicating that the effects of
mass and age on cannot be treated independently. With the
aim to identify reliable correlations between mass, age, and , we used for our PMS candidates a multivariate linear regression fit
between these parameters. The comparison between our results with those
obtained in other SFRs of our Galaxy and the MCs confirms the importance of the
metallicity for the study of the evolution in clusters with
different environmental conditions.Comment: Accepted for publication in ApJ; 26 pages, 12 pages, 3 tables;
abstract shortened. Fixed a typo in the name of a co-autho
The dynamical state of the globular clusters Rup 106 and IC 4499
The dynamical evolution of globular clusters is theoretically described by a
series of well known events typical of N-body systems. Still, the
identification of observational signatures able to empirically describe the
stage of dynamical evolution of a stellar system of the density typical of a
globular cluster, represents a challenge. In this paper we study the dynamical
age of the globular clusters Rup 106 and IC 4499. To this aim, we study the
radial distribution of the Blue Straggler Stars via the A+ parameter and of the
slope of the Main Sequence Mass Function. Both tracers show that Rup 106 and IC
4499 are dynamically young clusters where dynamical friction has just started
to segregate massive stars towards the clusters' centre. In fact, we observe
that the Blue Straggler stars are more centrally concentrated in both clusters
than the reference population. On the same line, we find that in both cases the
slope of the mass function significantly decreases as a function of the
cluster-centric distances. This result provides additional support for the use
of the the radial distribution of the blue stragglers as a powerful
observationally convenient indicator of the cluster dynamical age.Comment: Accepted for publication on A&
Chemical abundance analysis of the old, rich open cluster Trumpler 20
Trumpler 20 is an open cluster located at low Galactic longitude, just beyond
the great Carina spiral arm, and whose metallicity and fundamental parameters
were very poorly known until now. As it is most likely a rare example of an
old, rich open cluster -- possibly a twin of NGC 7789 -- it is useful to
characterize it. To this end, we determine here the abundance of several
elements and their ratios in a sample of stars in the clump of Trumpler 20. The
primary goal is to measure Trumpler 20 metallicity, so far very poorly
constrained, and revise the cluster's fundamental parameters. We present
high-resolution spectroscopy of eight clump stars. Based on their radial
velocities, we identify six bona fide cluster members, and for five of them
(the sixth being a fast rotator) we perform a detailed abundance analysis.
We find that Trumpler 20 is slightly more metal-rich than the Sun, having
[Fe/H]=+0.090.10. The abundance ratios of alpha-elements are generally
solar. In line with recent studies of clusters as old as Trumpler 20, Ba is
overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr
and Ni) does not reveal anything anomalous. Based on these results, we
re-estimate the cluster age to be 1.5 Gyr. Its distance to the
Galactic centre turns out to be 7.3 kpc. With this distance and metallicity,
Trumpler 20 fits fairly well in the metallicity gradient for the galactic inner
disc. With this new study, the characterization of Trumpler~20 is now on much
more solid ground. Further studies should focus on the estimate of the binary
fraction and on its main sequence membership.Comment: 13 pages, 6 eps figures, in press in Astronomy and Astrophysic
A complete census of emitters in NGC 6397
We used a dataset of archival Hubble Space Telescope images obtained through
the F555W, F814W and F656N filters, to perform a complete search for objects
showing emission in the globular cluster NGC 6397. As photometric
diagnostic, we used the color excess in the
- color-color diagram. In the analysed field of view,
we identified 53 emitters. In particular, we confirmed the optical
counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars
and 11 active binaries) and identified 33 previously unknown sources, thus
significantly enlarging the population of known active binaries in this
cluster. We report the main characteristics for each class of objects.
Photometric estimates of the equivalent width of the emission line,
were derived from the -excess and, for the first time, compared
to the spectroscopic measurements obtained from the analysis of MUSE spectra.
The very good agreement between the spectroscopic and photometric measures
fully confirmed the reliability of the proposed approach to measure the
emission. The search demonstrated the efficiency of this novel
approach to pinpoint and measure -emitters, thus offering a powerful
tool to conduct complete census of objects whose formation and evolution can be
strongly affected by dynamical interactions in star clusters.Comment: Accepted for publication by ApJ; 14 pages, 8 Figures, 1 Tabl
Deep multi-telescope photometry of NGC 5466. II. The radial behaviour of the mass function slope
We use a combination of data acquired with the Advanced Camera for Survey
(ACS) on board the Hubble Space Telescope and the Large Binocular Camera
(LBC-blue) mounted on the Large Binocular Telescope, to sample the main
sequence stars of the globular cluster NGC~5466 in the mass range
. We derive the cluster's Luminosity Function in several
radial regions, from the center of the cluster out to the tidal radius. After
corrections for incompleteness and field-contamination, this has been compared
to theoretical Luminosity Functions, obtained by multiplying a simple power law
Mass Function in the form dN/dm by the derivative of the
mass-luminosity relationship of the best-fit isochrone. We find that
varies from -0.6 in the core region to -1.9 in the outer region. This fact
allows us to observationally prove that the stars in NGC 5466 have experienced
the effects of mass segregation. We compare the radial variation of
from the center out to 5 core radii (r) in NGC 5466 and the globular
cluster M10, finding that the gradient of in the first 5r is more
than a factor of 2 shallower in NGC 5466 than in M10, in line with the
differences in the clusters' relaxation timescales. NGC 5466 is dynamically
younger than M10, with two-body relaxation processes only recently starting to
shape the distribution of main sequence stars. This result fully agrees with
the conclusion obtained in our previous works on the radial distribution of
Blue Straggler Stars, further confirming that this can be used as an efficient
clock to measure the dynamical age of stellar systems.Comment: Accepted for publications on Ap
Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters
We measure how the slope of the stellar mass function (MF) changes
as a function of clustercentric distance in five Galactic globular clusters
and compare to predictions from direct -body star cluster
simulations. Theoretical studies predict that (which traces the
degree of mass segregation in a cluster) should steepen with time as a cluster
undergoes two-body relaxation and that the amount by which the global MF can
evolve from its initial state due to stellar escape is directly linked to
. We find that the amount of mass segregation in M10, NGC 6218, and
NGC 6981 is consistent with their dynamical ages, but only the global MF of M10
is consistent with its degree of mass segregation as well. NGC 5466 and NGC
6101 on the other hand appear to be less segregated than their dynamical ages
would indicate. Furthermore, despite the fact that the escape rate of stars in
non-segregated clusters is independent of stellar mass, both NGC 5466 and NGC
6101 have near-flat MFs. We discuss various mechanisms which could produce
non-segregated clusters with near-flat MFs, including higher mass-loss rates
and black hole retention, but argue that for some clusters (NGC 5466 and NGC
6101) explaining the present-day properties might require either a
non-universal IMF or a much more complex dynamical history.Comment: 12 pages, 9 figures, Accepted for publication in MNRA
- …