77 research outputs found
Wide-field multi-color photometry of the Galactic globular cluster NGC 1261
(Abriged)This work studies in more detail the stellar population, including
its photometric properties and characteristics, in the rarely studied southern
Galactic globular cluster NGC 1261. We focus on the brighter sequences of the
cluster's color-magnitude diagram (CMD). Like in our previous works, we rely
upon photometry in several passbands to achieve more reliable results and
conclusions. We carried out and analyzed new multi-color photometry of NGC 1261
in UBVI reaching below the turnoff point in all passbands in a fairly extended
cluster field, about 14'x14'. We found several signs of the inhomogeneity
("multiplicity") in the stellar population. The most prominent of them are: (1)
the dependence of the radial distribution of sub-giant branch (SGB) stars in
the cluster on their U magnitude, with brighter stars less centrally
concentrated at the 99.9 \% level than their fainter counterparts; (2) the
dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD
on their radial distance from the cluster center, with the portion of stars
bluer in the (U-B) color increasing towards the cluster outskirts.
Additionally, the radial variation of the RGB luminosity function in the bump
region is suspected. We assume that both the SGB stars brighter in the U and
the RGB stars bluer in the (U-B) color are probably associated with blue
horizontal branch stars, because of a similarity in their radial distribution
in the cluster. We estimated the metalicity of NGC 1261 from the slope of the
RGB in U-based CMDs and the location of the RGB bump on the branch. These
metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41
+/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates.
They are more centrally concentrated than the lower red giants of comparable
brightness at the 97.9 \% level.Comment: 8 pages, 9 figures, accepted for publication in A&
The Infrared Ca II triplet as metallicity indicator
From observations of almost 500 RGB stars in 29 Galactic open and globular
clusters, we have investigated the behaviour of the infrared Ca II triplet
(8498, 8542 and 8662 \AA) in the age range 13Age/Gyr0.25 and the
metallicity range [Fe/H] +0.47. These are the widest ranges of
ages and metallicities in which the behaviour of the Ca II triplet lines has
been investigated in a homogeneous way. We report the first empirical study of
the variation of the CaII triplet lines strength, for given metallicities, with
respect to luminosity. We find that the sequence defined by each cluster in the
Luminosity-Ca plane is not exactly linear. However, when only stars in
a small magnitude interval are observed, the sequences can be considered as
linear. We have studied the the Ca II triplet lines on three metallicities
scales. While a linear correlation between the reduced equivalent width (
or ) versus metallicity is found in the \citet{cg97} and \citet{ki03}
scales, a second order term needs to be added when the \citet{zw84} scale is
adopted. We investigate the role of age from the wide range of ages covered by
our sample. We find that age has a weak influence on the final relationship.
Finally, the relationship derived here is used to estimate the metallicities of
three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110.
For the latter, the metallicity derived here is the first spectroscopic
estimate available.Comment: 52 pages, 16 figures, accepted for publication in Astronomical
Journa
VLT FORS-1 observations of NGC 6397: Evidence for mass segregation
We present (V,V-I) VLT-FORS1 observations of the Galactic Globular Cluster
NGC 6397. We derive accurate color--magnitude diagrams and luminosity functions
(LFs) of the cluster Main Sequence (MS) for two fields extending from a region
near the centre of the cluster out to ~ 10 arcmin. The photometry of these
fields produces a narrow MS extending down to V ~ 27, much deeper than any
previous ground based study on this system and comparable to previous HST
photometry. The V, V-I CMD also shows a deep white dwarf cooling sequence
locus, contaminated by many field stars and spurious objects. We concentrate
the present work on the analysis of the MSLFs derived for two annuli at
different radial distance from the center of the cluster. Evidence of a
clear-cut correlation between the slope of the observed LFs before reaching the
turn-over, and the radial position of the observed fields inside the cluster
area is found. We find that the LFs become flatter with decreasing radius (x ~
0.15 for 1'< R1 < 5.5'; x ~ 0.24 for 5.5'< R2 <9.8'; core radius, rc = 0.05'),
a trend that is consistent with the interpretation of NGC 6397 as a dynamically
relaxed system. This trend is also evident in the mass function.Comment: 11 pages; 15 figures; accepted for publication on Astronomy and
Astrophysic
VI Photometry of Globular Clusters NGC6293 and NGC6541: The Formation of the Metal-Poor Inner Halo Globular Clusters
We present VI photometry of the metal-poor inner halo globular clusters
NGC6293 and NGC6541 using the planetary camera of the WFPC2 on board HST. Our
color-magnitude diagrams of the clusters show well-defined BHB populations,
consistent with their low metallicities and old ages. NGC6293 appears to have
blue straggler stars in the cluster's central region. We discuss the
interstellar reddening and the distance modulus of NGC6293 and NGC6541 and
obtain E(B-V) = 0.40 and (m-M)_0 = 14.61 for NGC6293 and E(B-V) = 0.14 and
(m-M)_0 = 14.19 for NGC6541. Our results confirm that NGC6293 and NGC6541 are
clearly located in the Galaxy's central regions (R_{GC} < 3 kpc). We also
discuss the differential reddening across NGC6293. The interstellar reddening
value of NGC6293 appears to vary by Delta E(B-V) ~ 0.02 -- 0.04 mag within our
small field of view.
The most notable result of our study is that the inner halo clusters NGC6293
and NGC6541 essentially have the same ages as M92, confirming the previous
result from the HST NIC3 observations of NGC6287.Comment: AJ, accepte
A standard stellar library for evolutionary synthesis. III. Metallicity calibration
We extend the colour calibration of the widely used BaSeL standard stellar
library (Lejeune, Cuisinier, & Buser 1997, 1998) to non-solar metallicities,
down to [Fe/H] ~ -2.0 dex. Surprisingly, we find that at the present epoch it
is virtually impossible to establish a unique calibration of UBVRIJHKL colours
in terms of stellar metallicity [Fe/H] which is consistent simultaneously with
both colour-temperature relations and colour-absolute magnitude diagrams (CMDs)
based on observed globular cluster photometry data and on published, currently
popular standard stellar evolutionary tracks and isochrones. The problem
appears to be related to the long-standing incompleteness in our understanding
of convection in late-type stellar evolution, but is also due to a serious lack
of relevant observational calibration data that would help resolve, or at least
further significant progress towards resolving this issue. In view of the most
important applications of the BaSeL library, we here propose two different
metallicity calibration versions: (1) the "WLBC 99" library, which consistently
matches empirical colour-temperature relations and which, therefore, should
make an ideal tool for the study of individual stars; and (2), the "PADOVA
2000" library, which provides isochrones from the Padova 2000 grid (Girardi et
al., 2000) that successfully reproduce Galactic globular-cluster
colour-absolute magnitude diagrams and which thus should prove particularly
useful for studies of collective phenomena in stellar populations in clusters
and galaxies.Comment: 15 pages, LaTeX, documentclass aa, accepted for A and
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752
We present the results of a photometric survey for variable stars in the
field of the nearby globular cluster NGC 6752. The cluster was monitored in
1996 and 1997 for a total of 54 hours with 3 different CCD cameras mounted on
the 1.0-m Swope telescope. Eleven new variables were identified: 3 SX Phe
stars, 7 contact binaries and 1 candidate detached eclipsing binary. All 3 SX
Phe variables are likely members of the cluster while only 1 out of the 7
contact binaries is a potential cluster member. As a by-product of our survey
we obtained UBV photometry for a large sample of stars in the cluster field.
Two stars with U-B \approx -1.0 and V=19.3 and V=20.6 were identified. They lie
along the extended horizontal branch of the cluster, and are likely to be faint
sdB stars from NGC 6752.Comment: 18 pages, LaTex, 9 figures (Fig. 1 not available), accepted for
publication in the Astronomical Journa
Homogeneous Photometry V: The Globular Cluster NGC 4147
New BVRI broad-band photometry and astrometry are presented for the globular
cluster NGC 4147, based upon measurements derived from 524 ground-based CCD
images mostly either donated by colleagues or retrieved from public archives.
We have also reanalysed five exposures of the cluster obtained with WFPC2 on
the Hubble Space Telescope in the F439W and F555W (B and V) filters. We present
calibrated color-magnitude and color-color diagrams. Analysis of the
color-magnitude diagram reveals morphogical properties generally consistent
with published metal-abundance estimates for the cluster, and an age typical of
other Galactic globular clusters of similar metallicity. We have also
redetermined the periods and mean magnitudes for the RR Lyrae variables,
including a new c-type variable reported here for the first time. Our data do
not show clear evidence for photometric variability in candidate V18, recently
reported by Arellano Ferro et al. (2004). These observations also support the
non-variable status of candidates V5, V9, and V15. The union of our light-curve
data with those of Newburn (1957), Mannino (1957) and Arellano Ferro et al.
(op. cit.) permits the derivation of significantly improved periods. The mean
periods and the Bailey period-amplitude diagrams support the classification of
the cluster as Oosterhoff I despite its predominantly blue horizontal branch.
The number ratio of c- to ab-type RR Lyrae stars, on the other hand, is
unusually high for an Oosterhoff I cluster. The calibrated results have been
made available through the first author's web site.Comment: 30 pages, 30 figures. PASP, in press. Uses 11/2004 version of
emulateapj (included). For the full-resolution preprint, the reader is
encouraged to download the ps file (123 pages, manuscript format) available
at http://cadcwww.hia.nrc.ca/stetson/NGC4147/ms.ps.gz, or the pdf file (30
pages, preprint format) available at
http://www.astro.puc.cl/~mcatelan/N4147/full-res.pd
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