3,541 research outputs found
On the assessment of the nature of open star clusters and the determination of their basic parameters with limited data
Our knowledge of stellar evolution and of the structure and chemical
evolution of the Galactic disk largely builds on the study of open star
clusters. Because of their crucial role in these relevant topics, large
homogeneous catalogues of open cluster parameters are highly desirable.
Although efforts have been made to develop automatic tools to analyse large
numbers of clusters, the results obtained so far vary from study to study, and
sometimes are very contradictory when compared to dedicated studies of
individual clusters. In this work we highlight the common causes of these
discrepancies for some open clusters, and show that at present dedicated
studies yield a much better assessment of the nature of star clusters, even in
the absence of ideal data-sets. We make use of deep, wide-field, multi-colour
photometry to discuss the nature of six strategically selected open star
clusters: Trumpler~22, Lynga~6, Hogg~19, Hogg~21, Pismis~10 and Pismis~14. We
have precisely derived their basic parameters by means of a combination of star
counts and photometric diagrams. Trumpler~22 and Lynga~6 are included in our
study because they are widely known, and thus provided a check of our data and
methodology. The remaining four clusters are very poorly known, and their
available parameters have been obtained using automatic tools only. Our results
are in some cases in severe disagreement with those from automatic surveys.Comment: 22 pages, 16 figures, in pres
UBVI CCD photometry and star counts in 9 inner disk Galactic star clusters
We present and discuss new CCD-based photometric material in the UBVI
pass-bands for nine Galactic star clusters located inside the solar ring, for
which no CCD data are currently available. They are: IC 2714, NGC 4052,
ESO131SC09, NGC 5284, NGC 5316, NGC 5715, VdB-Hagen ~164, NGC 6268, and Czernik
38. We first perform star counts by combining our optical photometry wi th
2MASS, and derive cluster sizes and radial density profiles. The fundamental
parameters - age, reddening and distance- are then inferred from the analysis
of the star distribution in color-color and color-magnitude diagrams of only
the spatially selected likely members. Our analysis shows that ESO131SC09, NGC
5284, and VdB-Hagen 164 are most probably not clusters, but random enhancements
of a few bright stars along the line of sight, with prop erties much similar to
open cluster remnants. The remaining clusters are physical groups, and are all
younger than about 1 Gyr . We use the newly derived set of parameters, in
particular distance and reddening, to investigate their position in the Galaxy
in the context of the spiral structure of the Milky Way. We find that the
youngest clusters (IC 2714, NGC 5316, and NGC 6268) are located close to or
inside the Carina-Sagittarius arm, and are therefore {\i t bona fide} spiral
structure tracers. On the other hand, the oldest (Czernik 38, NGC 4052, and NGC
5715) are floating in the inter-arm space between the Carina-Sagittarius and
the more distant Scutum-Crux arm. Interestingly enough, the oldest clusters of
this sample - Czernik 38 and NGC 5715- are among the few known open clusters to
be older or as old as the Hyades in the inner Galactic disk, where star
clusters are not expected to survive for a long time, because of the strong
tidal field and the higher probability of close encounters .Comment: 17 pages, 11 eps figures, in press in MNRA
Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk
We perform a detailed photometric study of the stellar populations in a
Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We
present the first U,B,V,I photometry of the old open cluster Auner1 and
determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the
background of the cluster, at more than 9 kpc, we detect a young population
most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect
the signature of an older population and identify its Turn Off and Red Giant
Branch. This population is found to have a mean age of 7 Gyrs and a mean
metallicity of Z = 0.006 . We reconstruct the geometry of the stellar
distribution and argue that this older population - often associated to the
Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk
component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in
the Astronomical Journa
The complex stellar populations in the lines of sight to open clusters in the third Galactic quadrant
Multi-color photometry of the stellar populations in five fields in the third
Galactic quadrant centred on the clusters NGC 2215, NGC 2354, Haffner 22,
Ruprecht 11, and ESO489SC01 is interpreted in terms of a warped and flared
Galactic disk, without resort to an external entity such as the popular
Monoceros or Canis Major overdensities. Except for NGC 2215, the clusters are
poorly or unstudied previously. The data generate basic parameters for each
cluster, including the distribution of stars along the line of sight. We use
star counts and photometric analysis, without recourse to Galactic-model-based
predictions or interpretations, and confirms earlier results for NGC 2215 and
NGC 2354. ESO489SC01 is not a real cluster, while Haffner~22 is an overlooked
cluster aged about 2.5 Gyr. Conclusions for Ruprecht~11 are preliminary,
evidence for a cluster being marginal. Fields surrounding the clusters show
signatures of young and intermediate-age stellar populations. The young
population background to NGC~2354 and Ruprecht~11 lies 8-9 kpc from the Sun and
1 kpc below the formal Galactic plane, tracing a portion of the
Norma-Cygnus arm, challenging Galactic models that adopt a sharp cut-off of the
disk 12-14 kpc from the Galactic center. The old population is metal poor with
an age of 2-3 Gyr, resembling star clusters like Tombaugh 2 or NGC 2158. It has
a large color spread and is difficult to locate precisely. Young and old
populations follow a pattern that depends critically on the vertical location
of the thin and/or thick disk, and whether or not a particular line of sight
intersects one, both, or none.Comment: 16 pages, 9 eps figures, in press in MNRA
No evidence for a dark matter disk within 4 kpc from the Galactic plane
We estimated the dynamical surface mass density (Sigma) at the solar
Galactocentric distance between 2 and 4 kpc from the Galactic plane, as
inferred from the observed kinematics of the thick disk. We find Sigma(z=2
kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range
considered by our investigation. We compared our results with the expectations
for the visible mass, adopting the most recent estimates in the literature for
contributions of the Galactic stellar disk and interstellar medium, and
proposed models of the dark matter distribution. Our results match the
expectation for the visible mass alone, never differing from it by more than
0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark
component. We assume that the dark halo could be undetectable with our method,
but the dark disk, recently proposed as a natural expectation of the LambdaCDM
models, should be detected. Given the good agreement with the visible mass
alone, models including a dark disk are less likely, but within errors its
existence cannot be excluded. In any case, these results put constraints on its
properties: thinner models (scale height lower than 4 kpc) reconcile better
with our results and, for any scale height, the lower-density models are
preferred. We believe that successfully predicting the stellar thick disk
properties and a dark disk in agreement with our observations could be a
challenging theoretical task.Comment: Accepted for publication in ApJ Letter
Star clusters in the Carina complex: UBVRI photometry of NGC 3114, Collinder 228 and vdB-Hagen 99
We report on UBVRI photometry of two 5' x 5' fields in the region of the young open cluster NGC 3324. One of our fields covers the core region, while the other is closer to the tidal radius of the cluster. Our study provides the first CCD photometry of NGC 3324. We find that the cluster is very young and probably contains several pre Main Sequence (MS) stars. 25 members are identified on the basis of their position in the (U-B) vs. (B-V) diagram. We investigate the relation of the red super-giant HD 92207 with NGC 3324, suggesting that it probably does not belong to the cluster. Our second field is close to Loden 165, a possible cluster of stars that has never been studied so far. We show that this object is a probable open cluster, much older than NGC 3324 and much closer to the Su
BVRIJHK photometry and proper motion analysis of NGC 6253 and the surrounding field
Context. We present a photometric and astrometric catalog of 187963 stars
located in the field around the old super-metal-rich Galactic open cluster NGC
6253. The total field-of-view covered by the catalog is 34' x 33'. In this
field, we provide CCD BVRI photometry. For a smaller region close to the
cluster's center, we also provide near-infrared JHK photometry. Aims. We
analyze the properties of NGC 6253 by using our new photometric data and
astrometric membership. Methods. In June 2004, we targeted the cluster during a
10 day multi-site campaign, which involved the MPG/ESO 2.2m telescope with its
wide-field imager and the Anglo-Australian 3.9m telescope, equipped with the
IRIS2 near-infrared imager. Archival CCD images of NGC 6253 were used to derive
relative proper motions and to calculate the cluster membership probabilities.
Results. We have refined the cluster's fundamental parameters, deriving
(V_0-M_v)=11.15, E(B - V)=0.15, E(V - I)=0.25, E(V - J)=0.50, and E(V -
H)=0.55. The color excess ratios obtained using both the optical and near
infrared colors indicate a normal reddening law in the direction of NGC 6253.
The age of NGC 6253 at 3.5 Gyr, determined from our best-fitting isochrone
appears to be slightly older than the previous estimates. Finally, we estimated
the binary fraction among the cluster members to be \sim20%-30% and identified
11 blue straggler candidates.Comment: 16 pages, 13 figures, 11 tables. Accepted for publication in A&A.
Catalog avaiable via CD
In operando XAS investigation of reduction and oxidation processes in cobalt and iron mixed spinels during the chemical loop reforming of ethanol
FeCo2O4 and CoFe2O4 nanoparticles have been studied as oxygen carriers for the Chemical Loop Reforming (CLR) of ethanol. By using in operando X-ray absorption spectroscopy we have followed in real time the chemical and structural changes that take place on the materials as a function of temperature and reactive atmosphere (i.e. ethanol/water streams). During the first step of CLR for both oxides the most active chemical species are the cations in the tetrahedral sites, irrespective of their chemical nature. Quite rapidly the spinel structure is transformed into a mix of wustite-type oxide and metal alloys, but the formation of a metal phase is easier in the case of cobalt, while iron shows a marked preference to form wustite type oxide. Despite the good reducibility of FeCo2O4 imparted by the high amount of cobalt, its performance in the production of hydrogen is quite poor due to an inefficient oxidation by water steam, which is able to oxidize only the outer shell of the nanoparticles. In contrast, CoFe2O4 due to the residual presence of a reducible wustite phase shows a higher hydrogen yield. Moreover, by combining the structural information provided by X-ray absorption spectroscopy with the analysis of the byproducts of ethanol decomposition we could infer that FeCo2O4 is more selective than CoFe2O4 for the selective dehydrogenation of ethanol to acetaldehyde because of the higher amount of Fe(III) ions in tetrahedral sites
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