4,025 research outputs found
Star formation and chemical evolution in SPH simulations: a statistical approach
In Smoothed Particles Hydrodynamics (SPH) codes with a large number of
particles, star formation as well as gas and metal restitution from dying stars
can be treated statistically. This approach allows to include detailed chemical
evolution and gas re-ejection with minor computational effort. Here we report
on a new statistical algorithm for star formation and chemical evolution,
especially conceived for SPH simulations with large numbers of particles, and
for parallel SPH codes.
For the sake of illustration, we present also two astrophysical simulations
obtained with this algorithm, implemented into the Tree-SPH code by Lia &
Carraro (2000). In the first one, we follow the formation of an individual
disc-like galaxy, predict the final structure and metallicity evolution, and
test resolution effects. In the second one we simulate the formation and
evolution of a cluster of galaxies, to demonstrate the capabilities of the
algorithm in investigating the chemo-dynamical evolution of galaxies and of the
intergalactic medium in a cosmological context.Comment: 17 pages, 20 figures, accepted for publication on MNRA
THE EFFECT OF INTERSTATE BANKING ON FARM LENDER MARKET SHARES IN NEW YORK STATE
Commercial bank loans to New York farmers are significantly overestimated in the reported USDA statistics due to out-of-state lending and reporting of some agribusiness loans as agricultural loans by New York State banks. Correcting for this distortion lowers the 1978-84 average New York agricultural credit market share held by banks from 36 to 24 percent. As deregulation allows more interstate banking activity, the overestimate of agricultural loan volume in states with money center banks and the corresponding underestimate of loan levels and market shares in nonmoney center states could cause increased distortion of state level farm debt statistics.Agricultural Finance,
The cost of checkable deposits in the United States
Checking accounts ; Bank deposits ; Banks and banking - Costs
UB CCD photometry of the old, metal rich, open clusters NGC 6791, NGC 6819 and NGC 7142
We report on a UV-oriented imaging survey in the fields of the old,
metal-rich open clusters, NGC 6791, NGC 6819 and NGC 7142. These three clusters
represent both very near and ideal stellar aggregates to match the distinctive
properties of the evolved stellar populations, as in elliptical galaxies and
bulges of spirals. The CMD of the three clusters is analyzed in detail, with
special emphasis to the hot stellar component. We report, in this regard, one
new extreme horizontal-branch star candidate in NGC 6791. For NGC 6819 and
7142, the stellar luminosity function points to a looser radial distribution of
faint lower Main Sequence stars, either as a consequence of cluster dynamical
interaction with the Galaxy or as an effect of an increasing fraction of binary
stars toward the cluster core, as actually observed in NGC 6791 too.Comment: 12 pages with 15 Figures & 5 Tables. To appear in the Astronomical
Journa
A new look at Sco OB1 association with Gaia DR2
We present and discuss photometric optical data in the area of the OB
association Sco OB1 covering about 1 squared degree. UBVI photometry is
employed in tandem with Gaia DR2 data to investigate the 3 dimensional
structure and the star formation history of the region. By combining parallaxes
and proper motions we identify 7 physical groups located between the young open
cluster NGC 6231 and the bright nebula IC4628. The most prominent group
coincides with the sparse open cluster Trumpler 24. We confirm the presence of
the intermediate age star cluster VdB-Hagen 202, which is unexpected in this
environment, and provide for the first time estimates of its fundamental
parameters. After assessing individual groups membership, we derive mean proper
motion components, distances, and ages. The seven groups belong to two
different families. To the younger family (family I) belong several pre-Main
Sequence stars as well. These are evenly spread across the field, and also in
front of VdB-Hagen 202. VdB-Hagen 202 and two smaller, slightly detached,
groups of similar properties form family II, which do not belong to the
association, but are caught in the act of passing through it. As for the
younger population, this forms an arc-like structure from the bright nebula IC
4628 down to NGC 6231, as previously found. Moreover, the pre-Main Sequence
stars density seems to increase from NGC 6231 northward to Trumpler 24
Spectroscopy and BVI photometry of the young open cluster NGC 6604
BVI photometry (from South Africa Astron. Obs.), Echelle high resolution
spectroscopy and AFOSC integral field spectroscopy (from Asiago, Italy) of the
young open cluster NGC 6604 are presented. Age, distance, reddening,
membership, radial and rotational velocities are derived and discussed. An age
of 5 million years, a distance of 1.7 kpc and a reddening E(B-V)=1.02 are
found. The cluster radial velocity is in agreement with the Hron (1987) model
for the Galaxy disk rotation. Pre-ZAMS objects are not present down to M_V =
+1.5 mag.Comment: accepted in Astron.Astrophys.Suppl. Figure 2 is degraded in
resolutio
Evidence of tidal distortions and mass loss from the old open cluster NGC 6791
We present the first evidence of clear signatures of tidal distortions in the
density distribution of the fascinating open cluster NGC 6791. We used deep and
wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2x2
square degrees area around the cluster. The two-dimensional density map
obtained with the optimal matched filter technique shows a clear elongation and
an irregular distribution starting from ~300" from the cluster center. At
larger distances, two tails extending in opposite directions beyond the tidal
radius are also visible. These features are aligned to both the absolute proper
motion and to the Galactic center directions. Moreover, other overdensities
appear to be stretched in a direction perpendicular to the Galactic plane.
Accordingly to the behaviour observed in the density map, we find that both the
surface brightness and the star count density profiles reveal a departure from
a King model starting from ~600" from the center. These observational evidence
suggest that NGC 6791 is currently experiencing mass loss likely due to
gravitational shocking and interactions with the tidal field. We use this
evidence to argue that NGC 6791 should have lost a significant fraction of its
original mass. A larger initial mass would in fact explain why the cluster
survived so long. Using available recipes based on analytic studies and N-body
simulations, we derived the expected mass loss due to stellar evolution and
tidal interactions and estimated the initial cluster mass to be M_ini=(1.5-4) x
10^5 M_sun.Comment: Accepted for publication in the MNRAS (9 pages, 8 Figures
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
Extended star formation history of the star cluster NGC 2154 in the Large Magellanic Cloud
The colour-magnitude diagram (CMD) of the intermediate-age Large Magellanic Cloud star cluster NGC 2154 and its adjacent field has been analysed using Padova stellar models to determine the cluster´s fundamental parameters and its star formation history. Deep BR CCD photometry, together with synthetic CMDs and integrated luminosity functions, has allowed us to infer that the cluster experienced an extended star formation period of about 1.2 Gyr, which began approximately 2.3 Gyr ago and ended 1.1 Gyr ago. However, the physical reality of such a prolonged period of star formation is questionable, and could be the result of inadequacies in the stellar evolutionary tracks themselves. A substantial fraction of binaries (70 per cent) seems to exist in NGC 2154.Fil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Carraro, Giovanni. Università di Padova; ItaliaFil: Costa, E.. Universidad de Chile; ChileFil: Mendez, R. A.. Universidad de Chile; ChileFil: Girardi, L.. Osservatorio Astronomico di Padova; Itali
Ca II TRIPLET SPECTROSCOPY OF SMALL MAGELLANIC CLOUD RED GIANTS. IV. ABUNDANCES FOR A LARGE SAMPLE OF FIELD STARS AND COMPARISON WITH THE CLUSTER SAMPLE
This paper represents a major step forward in the systematic and homogeneous study of Small Magellanic Cloud (SMC) star clusters and field stars carried out by applying the calcium triplet technique. We present in this work the radial velocity and metallicity of approximately 400 red giant stars in 15 SMC fields, with typical errors of about 7 km s-1 and 0.16 dex, respectively. We added to this information our previously determined metallicity values for 29 clusters and approximately 350 field stars using the identical techniques. Using this enlarged sample, we analyze the metallicity distribution and gradient in this galaxy. We also compare the chemical properties of the clusters and of their surrounding fields. We find a number of surprising results. While the clusters, taken as a whole, show no strong evidence for a metallicity gradient (MG), the field stars exhibit a clear negative gradient in the inner region of the SMC, consistent with the recent results of Dobbie et al. For distances to the center of the galaxy less than 4\ub0, field stars show a considerably smaller metallicity dispersion than that of the clusters. However, in the external SMC regions, clusters and field stars exhibit similar metallicity dispersions. Moreover, in the inner region of the SMC, clusters appear to be concentrated in two groups: one more metal-poor and another more metal-rich than field stars. Individually considered, neither cluster group presents an MG. Most surprisingly, the MG for both stellar populations (clusters and field stars) appears to reverse sign in the outer regions of the SMC. The difference between the cluster metallicity and the mean metallicity of the surrounding field stars turns out to be a strong function of the cluster metallicity. These results could be indicating different chemical evolution histories for these two SMC stellar populations. They could also indicate variations in the chemical behavior of the SMC in its internal and external regions
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