45 research outputs found
Kinematic complexity around NGC 419: resolving the proper motion of the cluster, the Small Magellanic Cloud, and the Magellanic bridge
We present proper motions in the
direction of the star cluster NGC419 in the Small Magellanic Cloud. Because
of the high precision of our measurements, for the first time it is possible to
resolve the complex kinematics of the stellar populations located in the field,
even along the tangential direction. In fact, the proper motions we measured
allow us to separate cluster stars, which move on average with
() =
(, ) mas yr, from those of the Small
Magellanic Cloud and those belonging to a third kinematic feature that we
recognise as part of the Magellanic Bridge. Resolving such a kinematic
complexity enables the construction of decontaminated colour-magnitude
diagrams, as well as the measurement of the absolute proper motion of the three
separate components. Our study therefore sets the first steps towards the
possibility of dynamically investigating the Magellanic system by exploiting
the resolved kinematics of its stellar clusters.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Extended Main-Sequence Turnoff and Red Clump in intermediate-age star clusters: A study of NGC 419
With the goal of untangling the origin of extended main-sequence turnoffs
(eMSTOs) and extended red clumps (eRCs) in star clusters, in this work we
present the study of the intermediate-age cluster NGC 419, situated along the
Bridge of the Small Magellanic Cloud. To this aim, we analyzed multi-epoch,
high angular resolution observations acquired with the Hubble Space Telescope
for this dynamically young cluster, which enabled the determination of precise
proper motions and therefore the assessment of the cluster membership for each
individual star in the field of view. With this unprecedented information at
hand, we first studied the radial distribution of kinematically selected member
stars in different eMSTO subregions. The absence of segregation supports the
rotation scenario as the cause for the turnoff color extension and disfavors
the presence of a prolonged period of star formation in the cluster. A similar
analysis on the eRC of NGC 419 confirms the absence of segregation, providing
further evidence against an age spread, which is at odds with previous
investigations. Even so, the currently available evolutionary models including
stellar rotation fail at reproducing the two photometric features
simultaneously. We argue that either shortcomings in these models or a
different origin for the red clump feature, such as a nonstandard differential
mass loss along the red giant branch phase, are the only way to reconcile our
observational findings with theoretical expectations.Comment: 6 pages, 8 figures, Accepted for publication on A&
A PSF-based approach to Kepler/K2data – III. Search for exoplanets and variable stars within the open cluster M 67 (NGC 2682)
In the third paper of this series we continue the exploitation of Kepler/K2
data in dense stellar fields using our PSF-based method. This work is focused
on a ~720-arcmin^2 region centred on the Solar-metallicity and Solar-age open
cluster M 67. We extracted light curves for all detectable sources in the
Kepler channels 13 and 14, adopting our technique based on the usage of a
high-angular-resolution input catalogue and target-neighbour subtraction. We
detrended light curves for systematic errors, and searched for variables and
exoplanets using several tools. We found 451 variables, of which 299 are new
detection. Three planetary candidates were detected by our pipeline in this
field. Raw and detrended light curves, catalogues, and K2 stacked images used
in this work will be released to the community.Comment: 14 pages, 9 figures (1 at low resolution), 3 tables. Accepted for
publication in MNRAS on August 24, 2016. Electronic materials are available
at http://groups.dfa.unipd.it/ESPG/Kepler-K2.htm
Blue Stragglers as tracers of the dynamical state of two clusters in the Small Magellanic Cloud: NGC 339 and NGC 419
The level of central segregation of Blue Straggler stars proved to be an
excellent tracer of the dynamical evolution of old star clusters (the so-called
"dynamical clock"), both in the Milky Way and in the Large Magellanic Cloud.
The parameter, used to measure the Blue Stragglers degree of
segregation, has in fact been found to strongly correlate with the parent
cluster central relaxation time. Here we studied the Blue-Straggler population
of two young stellar systems in the Small Magellanic Cloud, namely NGC 339
(which is 6 Gyr old) and NGC 419 (with an age of only 1.5 Gyr), in order to
study their dynamical state. Thanks to multi-epoch, high angular resolution
Hubble Space Telescope observations available for both clusters, we took
advantage of the stellar proper motions measured in the regions of the two
systems and we selected a population of likely cluster members, removing the
strong contamination from Small Magellanic Cloud stars. This enabled us to
study, with unprecedented accuracy, the radial distribution of Blue Stragglers
in these two extragalactic clusters and to measure their dynamical age. As
expected for such young clusters, we found that both systems are poorly evolved
from the dynamical point of view, also fully confirming that the
parameter is a sensitive "clock hand" even in the dynamically-young regime.Comment: 12 pages,11 figures, Accepted for publication by Ap
Ages of the bulge globular clusters NGC 6522 and NGC 6626 (M28) from HST proper-motion-cleaned color–magnitude diagrams
Bulge globular clusters (GCs) with metallicities [Fe/H]−1.0 and blue horizontal branches are candidates to harbor the oldest populations in the Galaxy. Based on the analysis of HST proper-motion-cleaned color–magnitude diagrams in filters F435W and F625W, we determine physical parameters for the old bulge GCs NGC 6522 and NGC 6626 (M28), both with well-defined blue horizontal branches. We compare these results with similar data for the inner halo cluster NGC 6362. These clusters have similar metallicities (−1.3[Fe/H]−1.0) obtained from high-resolution spectroscopy. We derive ages, distance moduli, and reddening values by means of statistical comparisons between observed and synthetic fiducial lines employing likelihood statistics and the Markov chain Monte Carlo method. The synthetic fiducial lines were generated using α-enhanced BaSTI and Dartmouth stellar evolutionary models, adopting both canonical (Y∼0.25) and enhanced (Y∼0.30–0.33) helium abundances. RR Lyrae stars were employed to determine the HB magnitude level, providing an independent indicator to constrain the apparent distance modulus and the helium enhancement. The shape of the observed fiducial line could be compatible with some helium enhancement for NGC 6522 and NGC 6626, but the average magnitudes of RR Lyrae stars tend to rule out this hypothesis. Assuming canonical helium abundances, BaSTI and Dartmouth models indicate that all three clusters are coeval, with ages between ∼12.5 and 13.0 Gyr. The present study also reveals that NGC 6522 has at least two stellar populations, since its CMD shows a significantly wide subgiant branch compatible with 14%±2% and 86%±5% for first and second generations, respectively
Astrometry with the Wide-Field InfraRed Space Telescope
The Wide-Field InfraRed Space Telescope (WFIRST) will be capable of
delivering precise astrometry for faint sources over the enormous field of view
of its main camera, the Wide-Field Imager (WFI). This unprecedented combination
will be transformative for the many scientific questions that require precise
positions, distances, and velocities of stars. We describe the expectations for
the astrometric precision of the WFIRST WFI in different scenarios, illustrate
how a broad range of science cases will see significant advances with such
data, and identify aspects of WFIRST's design where small adjustments could
greatly improve its power as an astrometric instrument.Comment: version accepted to JATI
Astro2020 Science White Paper: The Local Relics of of Supermassive Black Hole Seeds
We have compelling evidence for stellar-mass black holes (BHs) of ~5-80 M_sun
that form through the death of massive stars. We also have compelling evidence
for so-called supermassive BHs (10^5-10^10 M_sun) that are predominantly found
in the centers of galaxies. We have very good reason to believe there must be
BHs with masses in the gap between these ranges: the first ~10^9 M_sun BHs are
observed only hundreds of millions of years after the Big Bang, and all
theoretically viable paths to making supermassive BHs require a stage of
"intermediate" mass. However, no BHs have yet been reliably detected in the
100-10}^5 M_sun mass range. Uncovering these intermediate-mass BHs of 10^3-10^5
M_sun is within reach in the coming decade. In this white paper we highlight
the crucial role that 30-m class telescopes will play in dynamically detecting
intermediate-mass black holes, should they exist.Comment: Science White Paper Submitted for the Astro2020 Decadal Survey on
Astronomy and Astrophysic
GaiaHub: A method for combining data from the Gaia and Hubble space telescopes to derive improved proper motions for faint stars
We present GaiaHub, a publicly available tool that combines
measurements with () archival images to
derive proper motions (PMs). It increases the scientific impact of both
observatories beyond their individual capabilities. provides PMs across
the whole sky, but the limited mirror size and time baseline restrict the best
PM performance to relatively bright stars. can measure accurate PMs for
much fainter stars over a small field, but this requires two epochs of
observation which are not always available. GaiaHub yields considerably
improved PM accuracy compared to -only measurements, especially for faint
sources , requiring only a single epoch of data observed
more than years ago (before 2012). This provides considerable
scientific value especially for dynamical studies of stellar systems or
structures in and beyond the Milky Way (MW) halo, for which the member stars
are generally faint. To illustrate the capabilities and demonstrate the
accuracy of GaiaHub, we apply it to samples of MW globular clusters (GCs) and
classical dwarf spheroidal (dSph) satellite galaxies. This allows us, e.g., to
measure the velocity dispersions in the plane of the sky for objects out to and
beyond kpc. We find, on average, mild radial velocity anisotropy in
GCs, consistent with existing results for more nearby samples. We observe a
correlation between the internal kinematics of the clusters and their
ellipticity, with more isotropic clusters being, on average, more round. Our
results also support previous findings that Draco and Sculptor dSph galaxies
appear to be radially anisotropic systems.Comment: 24 pages, 18 figures. Accepted for publication in Ap