109 research outputs found
Star count density profiles and structural parameters of 26 Galactic globular clusters
We used a proper combination of high-resolution HST observations and
wide-field ground based data to derive the radial star density profile of 26
Galactic globular clusters from resolved star counts (which can be all freely
downloaded on-line). With respect to surface brightness (SB) profiles (which
can be biased by the presence of sparse, bright stars), star counts are
considered to be the most robust and reliable tool to derive cluster structural
parameters. For each system a detailed comparison with both King and Wilson
models has been performed and the most relevant best-fit parameters have been
obtained. This is the largest homogeneous catalog collected so far of star
count profiles and structural parameters derived therefrom. The analysis of the
data of our catalog has shown that: (1) the presence of the central cusps
previously detected in the SB profiles of NGC 1851, M13 and M62 is not
confirmed; (2) the majority of clusters in our sample are fitted equally well
by the King and the Wilson models; (3) we confirm the known relationship
between cluster size (as measured by the effective radius) and galactocentric
distances; (4) the ratio between the core and the effective radii shows a
bimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a
secondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak
turns out to be in agreement with what expected from simulations of cluster
dynamical evolution and the ratio between these two radii well correlates with
an empirical dynamical age indicator recently defined from the observed shape
of blue straggler star radial distribution, thus suggesting that no exotic
mechanisms of energy generation are needed in the cores of the analyzed
clusters.Comment: Accepted for publication in The Astrophysical Journal; 19 pages
(emulateapj style), 15 figures, 2 table
Merging black holes in young star clusters
Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitationalwave detections.Here, we investigate the demography ofmerging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed 4
7 103 N-body simulations of SCs with metallicity Z = 0.002, initial binary fraction 0.4, and fractal initial conditions, to mimic the clumpiness of star-forming regions. Our simulations include a novel population-synthesis approach based on the code MOBSE. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 per cent of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to ~120M 99 form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only ~70 M 99. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally ~2 per cent of merging BBHs in young SCs have mass in the pair-instability mass gap (~60-120 M 99). This represents a unique fingerprint of merging BBHs in SCs
3D Morphology of Open Clusters in the Solar Neighborhood with Gaia EDR3: its Relation to Cluster Dynamics
We analyze the 3D morphology and kinematics of 13 open clusters (OCs) located
within 500 pc of the Sun, using Gaia EDR3 and kinematic data from literature.
Members of OCs are identified using the unsupervised machine learning method
StarGO, using 5D parameters (X, Y, Z, ). The
OC sample covers an age range of 25Myr--2.65Gyr. We correct the asymmetric
distance distribution due to the parallax error using Bayesian inversion. The
uncertainty in the corrected distance for a cluster at 500~pc is 3.0--6.3~pc,
depending on the intrinsic spatial distribution of its members. We determine
the 3D morphology of the OCs in our sample and fit the spatial distribution of
stars within the tidal radius in each cluster with an ellipsoid model. The
shapes of the OCs are well-described with oblate spheroids (NGC2547, NGC2516,
NGC2451A, NGC2451B, NGC2232), prolate spheroids (IC2602, IC4665, NGC2422,
Blanco1, Coma Berenices), or triaxial ellipsoids (IC2391, NGC6633, NGC6774).
The semi-major axis of the fitted ellipsoid is parallel to the Galactic plane
for most clusters. Elongated filament-like substructures are detected in three
young clusters (NGC2232, NGC2547, NGC2451B), while tidal-tail-like
substructures (tidal tails) are found in older clusters (NGC2516, NGC6633,
NGC6774, Blanco1, Coma Berenices). Most clusters may be super-virial and
expanding. -body models of rapid gas expulsion with an SFE of
are consistent with clusters more massive than , while clusters
less massive than 250 tend to agree with adiabatic gas expulsion
models. Only six OCs (NGC2422, NGC6633, and NGC6774, NGC2232, Blanco1, Coma
Berenices) show clear signs of mass segregation.Comment: 35 pages, 17 figures, accepted by Ap
Halo millisecond pulsars ejected by intermediate mass black holes in globular clusters
Intermediate mass black holes (IMBHs) are among the most elusive objects in
contemporary astrophysics. Both theoretical and observational evidence of their
existence is subject of debate. Conversely, both theory and observations
confirm the presence of a large population of millisecond pulsars (MSPs) with
low mass companions residing in globular cluster (GC) centers. If IMBHs are
common in GC centers as well, then dynamical interactions will inevitably break
up many of these binaries, causing the ejection of several fast MSPs in the
Galactic halo. Such population of fast halo MSPs, hard to produce with
'standard' MSP generation mechanisms, would provide a strong, albeit indirect,
evidence of the presence of a substantial population of IMBHs in GCs. In this
paper we study in detail the dynamical formation and evolution of such fast
MSPs population, highlighting the relevant observational properties and
assessing detection prospects with forthcoming radio surveys.Comment: 13 pages, 7 figures, 2 tables, accepted for publication in MNRA
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters
Empirical evidence for both stellar mass black holes M_bh<10^2 M_sun) and
supermassive black holes (SMBHs, M_bh>10^5 M_sun) is well established.
Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is
correlated with the bulge mass, and even more strongly with the central stellar
velocity dispersion sigma_c, the `M-sigma' relation. On the other hand,
evidence for "intermediate-mass" black holes (IMBHs, with masses in the range
1^2 - 10^5 M_sun) is relatively sparse, with only a few mass measurements
reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We
explore the question of whether globular clusters extend the M-sigma
relationship for galaxies to lower black hole masses and find that available
data for globular clusters are consistent with the extrapolation of this
relationship. We use this extrapolated M-sigma relationship to predict the
putative black hole masses of those globular clusters where existence of
central IMBH was proposed. We discuss how globular clusters can be used as a
constraint on theories making specific predictions for the low-mass end of the
M-sigma relation.Comment: 14 pages, 3 figures, accepted for publication in Astrophysics and
Space Science; fixed typos and a quote in Sec.
Constructing Impactful Machine Learning Research for Astronomy: Best Practices for Researchers and Reviewers
Machine learning has rapidly become a tool of choice for the astronomical
community. It is being applied across a wide range of wavelengths and problems,
from the classification of transients to neural network emulators of
cosmological simulations, and is shifting paradigms about how we generate and
report scientific results. At the same time, this class of method comes with
its own set of best practices, challenges, and drawbacks, which, at present,
are often reported on incompletely in the astrophysical literature. With this
paper, we aim to provide a primer to the astronomical community, including
authors, reviewers, and editors, on how to implement machine learning models
and report their results in a way that ensures the accuracy of the results,
reproducibility of the findings, and usefulness of the method.Comment: 14 pages, 3 figures; submitted to the Bulletin of the American
Astronomical Societ
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
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