3,513 research outputs found
Uncovering Multiple Populations with Washington Photometry: I. The Globular Cluster NGC 1851
The analysis of multiple populations (MPs) in globular clusters (GCs) has
become a forefront area of research in astronomy. Multiple red giant branches
(RGBs), subgiant branches (SGBs), and even main sequences (MSs) have now been
observed photometrically in many GCs. UV photometry has been crucial in
discovering and analyzing these MPs, but the Johnson U and the Stromgren and
Sloan u filters that have generally been used are relatively inefficient and
very sensitive to reddening and atmospheric extinction. In contrast, the
Washington C filter is much broader and redder than these competing UV filters.
Here we investigate the use of the Washington system to uncover MPs using only
a 1-meter telescope. Our analysis of the well-studied GC NGC 1851 finds that
the C filter is both very efficient and effective at detecting its previously
discovered MPs in the RGB and SGB. Remarkably, we have also detected an
intrinsically broad MS best characterized by two distinct but heavily
overlapping populations that cannot be explained by binaries, field stars, or
photometric errors. The MS distribution is in very good agreement with that
seen on the RGB, with ~30% of the stars belonging to the second population.
There is also evidence for two sequences in the red horizontal branch, but this
appears to be unrelated to the MPs in this cluster. Neither of these latter
phenomena have been observed previously in this cluster. The redder MS stars
are also more centrally concentrated than the blue MS. This is the first time
MPs in a MS have been discovered from the ground, and using only a 1-meter
telescope. The Washington system thus proves to be a very powerful tool for
investigating MPs, and holds particular promise for extragalactic objects where
photons are limited.Comment: 25 pages, 10 figure
On the existence of young embedded clusters at high Galactic latitude
Careful analyses of photometric and star count data available for the nine
putative young clusters identified by Camargo et al. (2015, 2016) at high
Galactic latitudes reveal that none of the groups contain early-type stars, and
most are not significant density enhancements above field level. 2MASS colours
for stars in the groups match those of unreddened late-type dwarfs and giants,
as expected for contamination by (mostly) thin disk objects. A simulation of
one such field using only typical high latitude foreground stars yields a
colour-magnitude diagram that is very similar to those constructed by Camargo
et al. (2015, 2016) as evidence for their young groups as well as the means of
deriving their reddenings and distances. Although some of the fields are
coincident with clusters of galaxies, one must conclude that there is no
evidence that the putative clusters are extremely young stellar groups.Comment: Accepted for publication (MNRAS
Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66
We report on high resolution spectroscopy of four giant stars in the Galactic
old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck
telescope. We find that and for
Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise
the fundamental parameters of the clusters, and then discuss them in the
context of the Galactic disk radial abundance gradient. We found that both
clusters nicely obey the most updated estimate of the slope of the gradient
from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical
Journa
Old open clusters in the outer Galactic disk
The outer parts of the Milky Way disk are believed to be one of the main
arenas where the accretion of external material in the form of dwarf galaxies
and subsequent formation of streams is taking place. The Monoceros stream and
the Canis Major and Argo over-densities are notorious examples. VLT high
resolution spectra have been acquired for five distant open clusters. We derive
accurate radial velocities to distinguish field interlopers and cluster
members. For the latter we perform a detailed abundance analysis and derive the
iron abundance [Fe/H] and the abundance ratios of several elements.
Our analysis confirms previous indications that the radial abundance gradient
in the outer Galactic disk does not follow the expectations extrapolated from
the solar vicinity, but exhibits a shallower slope. By combining the
metallicity of the five program clusters with eight more clusters for which
high resolution spectroscopy is available, we find that the mean metallicity in
the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H]
, with only marginal indications for a radial variation. In
addition, all the program clusters exhibit solar scaled or slightly enhanced
elements, similar to open clusters in the solar vicinity and thin disk
stars. We investigate whether this outer disk cluster sample might belong to an
extra-galactic population, like the Monoceros ring. However, close scrutiny of
their properties - location, kinematics and chemistry - does not convincingly
favor this hypothesis. On the contrary, they appear more likely genuine
Galactic disk clusters. We finally stress the importance to obtain proper
motion measurements for these clusters to constrain their orbits.Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit i
Model-Independent Diagnostics of Highly Reddened Milky Way Star Clusters: Age Calibration
The next generation near- and mid-infrared Galactic surveys will yield a
large number of new highly obscured star clusters. Detailed characterization of
these new objects with spectroscopy is time-consuming. Diagnostic tools that
will be able to characterize clusters based only on the available photometry
will be needed to study large samples of the newly found objects. The
brightness difference between the red clump and the main-sequence turn-off
point have been used as a model-independent age calibrator for clusters with
ages from a few 10 to 10 yr in the optical. Here we apply for the
first time the method in the near-infrared. We calibrated this difference in
-band, which is likely to be available for obscured clusters, and we apply
it to a number of test clusters with photometry comparable to the one that will
be yielded by the current or near-future surveys. The new calibration yields
reliable ages over the range of ages for which the red clump is present in
clusters. The slope of the relation is smoother than that of the corresponding
-band relation, reducing the uncertainty in the age determinations with
respect to the optical ones.Comment: 5 pages, 5 eps figure, accepted for publication in Astronomy and
Astrophysic
The intermediate-age open clusters Ruprecht 61, Czernik 32, NGC 2225 and NGC 2262
We present the first CCD photometry to of 4 fields centered on
the region of the southern Galactic star clusters Ruprecht~61, Czernik~32, NGC
2225 and NGC 2262 and of 4 displaced control fields. These clusters were never
studied before, and we provide for the first time estimates of their
fundamental parameters, namely radial extent, age, distance and reddening. We
find that the four clusters are all of intermediate age (around 1 Gyr), close
to the Sun and possess lower than solar metal abundance.Comment: 10 pages, 14 figures, in press in MNRA
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
Properties of the Young Milky Way Globular Cluster Whiting 1 from Near-Infrared Photometry
Whiting 1 is a member of the fast-growing group of young globular clusters in
the Milky Way halo. Preliminary estimates of its fundamental parameters have
been provided using optical photometry and low resolution spectroscopy. In an
attempt to strengthen our knowledge of Whiting 1, in this study we employ a
complementary approach. Isochrone fitting method was applied on the
Near-Infrared Color-Magnitude Diagram and yields an age t=5.70.3 Gyr,
metallicity =0.0060.001 ([Fe/H]=0.50.1) and distance modulus
=17.480.10. Our results confirm that Whiting 1 is a young and
moderately metal-rich globular cluster. It is one of the youngest from the Sgr
dSph. We fitted an Elson, Fall and Freeman (EFF) profile to the near-infrared
number counts, and measured cluster core radius
=9.13.9. Two probable eclipsing
variables in the cluster were found from multi-epoch band photometry.
Finally, an unknown galaxy cluster was identified on our vs.
color-magnitude diagram. It has a redshift z1, and it is located at about
1 from the center of Whiting 1 at , ,
contaminating the cluster photometry.Comment: Accepted for publication in MNRAS, 6 pages, 8 figure
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