496 research outputs found
On the Fractal Distribution of HII Regions in Disk Galaxies
In this work we quantify the degree to which star-forming events are clumped.
We apply a precise and accurate technique to calculate the correlation
dimension Dc of the distribution of HII regions in a sample of disk galaxies.
Our reliable results are distributed in the range 1.5<Dc<2.0. We get
significant variations in the fractal dimension among galaxies, contrary to a
universal picture sometimes claimed in literature. The faintest galaxies tend
to distribute their HII regions in more clustered (less uniform) patterns.
Moreover, the fractal dimension for the brightest HII regions within the same
galaxy seems to be smaller than for the faintest ones suggesting some kind of
evolutionary effect.Comment: 6 pages including 4 figures. To appear in Highlights of Spanish
Astrophysics V: Proceedings of the VIII Scientific Meeting of the Spanish
Astronomical Society held in Santander, July 7-11, 200
Stellar Open Clusters' Membership Probabilities: an N-Dimensional Geometrical Approach
We present a new geometrical method aimed at determining the members of open
clusters. The methodology estimates, in an N-dimensional space, the membership
probabilities by means of the distances between every star and the cluster
central overdensity. It can handle different sets of variables, which have to
satisfy the simple condition of being more densely distributed for the cluster
members than for the field stars (as positions, proper motions, radial
velocities and/or parallaxes are). Unlike other existing techniques, this fact
makes the method more flexible and so can be easily applied to different
datasets. To quantify how the method identifies the clus- ter members, we
design series of realistic simulations recreating sky regions in both position
and proper motion subspaces populated by clusters and field stars. The re-
sults, using different simulated datasets (N = 1, 2 and 4 variables), show that
the method properly recovers a very high fraction of simulated cluster members,
with a low number of misclassified stars. To compare the goodness of our
methodology, we also run other existing algorithms on the same simulated data.
The results show that our method has a similar or even better performance than
the other techniques. We study the robustness of the new methodology from
different subsamplings of the ini- tial sample, showing a progressive
deterioration of the capability of our method as the fraction of missing
objects increases. Finally, we apply all the methodologies to the real cluster
NGC 2682, indicating that our methodology is again in good agreement with
preceding studies.Comment: 15 pages, 9 figures, 6 tables, accepted for publication in MNRA
A catalogue of open cluster radii determined from Gaia proper motions
In this work we improve a previously published method to calculate in a
reliable way the radius of an open cluster. The method is based on the
behaviour of stars in the proper motion space as the sampling changes in the
position space. Here we describe the new version of the method and show its
performance and robustness. Additionally, we apply it to a large number of open
clusters using data from Gaia DR2 to generate a catalogue of 401 clusters with
reliable radius estimations. The range of obtained apparent radii goes from
Rc=1.4+-0.1 arcmin (for the cluster FSR 1651) to Rc=25.5+-3.5 arcmin (for
NGC~2437). Cluster linear sizes follow very closely a lognormal distribution
with a mean characteristic radius of Rc=3.7 pc, and its high radius tail can be
fitted by a power law as N \propto Rc^(-3.11+-0.35). Additionally, we find that
number of members, cluster radius and age follow the relationship Nc \propto
Rc^(1.2+-0.1) Tc^(-1.9+-0.4) where the younger and more extensive the cluster,
the more members it presents. The proposed method is not sensitive to low
density or irregular spatial distributions of stars and, therefore, is a good
alternative or complementary procedure to calculate open cluster radii not
having previous information on star memberships.Comment: 13 pages including 9 figures and 2 tables (main table will be
available online). Accepted for publication in MNRA
NGC 2548: clumpy spatial and kinematic structure in an intermediate-age galactic cluster
NGC 2548 is a 400-500 Myr old open cluster with evidence of spatial
substructures likely caused by its interaction with the Galactic disk. In this
work we use precise astrometric data from the Carte du Ciel - San Fernando
(CdC-SF) catalogue to study the clumpy structure in this cluster. We confirm
the fragmented structure of NGC 2548 but, additionally, the relatively high
precision of our kinematic data lead us to the first detection of substructures
in the proper motion space of a stellar cluster. There are three spatially
separated cores each of which has its own counterpart in the proper motion
distribution. The two main cores lie nearly parallel to the Galactic plane
whereas the third one is significantly fainter than the others and it moves
toward the Galactic plane separating from the rest of the cluster. We derive
core positions and proper motions, as well as the stars belonging to each core.Comment: 9 pages, 8 figures, 4 tables. Accepted for publication in MNRA
A method for determining the radius of an open cluster from stellar proper motions
We propose a method for calculating the radius of an open cluster in an
objective way from an astrometric catalogue containing, at least, positions and
proper motions. It uses the minimum spanning tree (hereinafter MST) in the
proper motion space to discriminate cluster stars from field stars and it
quantifies the strength of the cluster-field separation by means of a
statistical parameter defined for the first time in this paper. This is done
for a range of different sampling radii from where the cluster radius is
obtained as the size at which the best cluster-field separation is achieved.
The novelty of this strategy is that the cluster radius is obtained
independently of how its stars are spatially distributed. We test the
reliability and robustness of the method with both simulated and real data from
a well-studied open cluster (NGC 188), and apply it to UCAC4 data for five
other open clusters with different catalogued radius values. NGC 188, NGC 1647,
NGC 6603 and Ruprecht 155 yielded unambiguous radius values of 15.2+/-1.8,
29.4+/-3.4, 4.2+/-1.7 and 7.0+/-0.3 arcmin, respectively. ASCC 19 and Collinder
471 showed more than one possible solution but it is not possible to know
whether this is due to the involved uncertainties or to the presence of complex
patterns in their proper motion distributions, something that could be inherent
to the physical object or due to the way in which the catalogue was sampled.Comment: 12 pages including 14 figures and 1 table. Accepted for publication
in MNRA
Analysis of the stellar population in the central area of the HII region Sh 2-284
There is a lack of state-of-the-art information on very young open clusters,
with implications for determining the structure of the Galaxy. Our main
objective is to study the timing and location of the star formation processes
which yielded the generation of the giant HII region Sh 2-284. The analysis is
based on UBVRcIc CCD measurements and JHKs photometry in the central part of
the HII region, where the cluster Dolidze 25 is located.The determination of
cluster distance, reddening and age is carried out through comparison with
ZAMS, post-MS and PMS isochrones. Reference lines for metallicity Z=0.004 are
used, in agreement with spectroscopic metallicity determination published for
several cluster members. The results are: E(B-V)=0.78+-0.02, M=12.8+-0.2,
LogAge(yr)=6.51+-0.07. A PMS member sequence is proposed, coeval within the
errors with the post-MS cluster age (LogAge(yr)=6.7+-0.2). The mass function
for this PMS population in the mass range above 1.3-3.5 Msun is well fitted by
a Salpeter mass function.The presence of a different star generation in the
cluster with a distinctly older age, around 40 Myr, is suggested. The NIR
results indicate a large number of sources with H-Ks excess, practically
distinct from the optical PMS candidate members.
The distance determined for the cluster is distinctly lower than previously
published values. This result originates in the consistent use of low
metallicity models for ZAMS fitting, applying published metallicity values for
the cluster.Comment: gzipped tar file: 1 aa.cls, 1 tex file, 1 bbl file, 13 figures.
Accepted in Astronomy & Astrophysics (9.11.2009
Looking for phase-space structures in star-forming regions: An MST-based methodology
We present a method for analysing the phase space of star-forming regions. In particular we are searching for clumpy structures in the 3D sub-space formed by two position coordinates and radial velocity. The aim of the method is the detection of kinematic segregated radial velocity groups, that is, radial velocity intervals whose associated stars are spatially concentrated. To this end we define a kinematic segregation index, Λ(RV), based on the Minimum Spanning Tree graph algorithm, which is estimated for a set of radial velocity intervals in the region. When Λ(RV) is significantly greater than 1 we consider that this bin represents a grouping in the phase space. We split a star-forming region into radial velocity bins and calculate the kinematic segregation index for each bin, and then we obtain the spectrum of kinematic groupings, which enables a quick visualization of the kinematic behaviour of the region under study. We carried out numerical models of different configurations in the sub-space of the phase space formed by the coordinates and the that various case studies illustrate. The analysis of the test cases demonstrates the potential of the new methodology for detecting different kind of groupings in phase space.We acknowledge support from the Spanish Ministry for Economy and Competitiveness and FEDER funds through grant AYA2013-40611-P.Peer Reviewe
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