820 research outputs found
The Determination Of Reddening From Intrinsic VR Colors Of RR Lyrae Stars
New R-band observations of 21 local field RR Lyrae variable stars are used to
explore the reliability of minimum light (V-R) colors as a tool for measuring
interstellar reddening. For each star, R-band intensity mean magnitudes and
light amplitudes are presented. Corresponding V-band light curves from the
literature are supplemented with the new photometry, and (V-R) colors at
minimum light are determined for a subset of these stars as well as for other
stars in the literature. Two different definitions of minimum light color are
examined, one which uses a Fourier decomposition to the V and R light curves to
find (V-R) at minimum V-band light, (V-R)_{min}^F, and the other which uses the
average color between the phase interval 0.5-0.8, (V-R)_{min}^{\phi(0.5-0.8)}.
From 31 stars with a wide range of metallicities and pulsation periods, the
mean dereddened RR Lyrae color at minimum light is (V-R)_{min,0}^F = 0.28 pm
0.02 mag and (V-R)_{min,0}^{\phi(0.5-0.8)} = 0.27 pm 0.02 mag. As was found by
Guldenschuh et al. (2005) using (V-I) colors, any dependence of the star's
minimum light color on metallicity or pulsation amplitude is too weak to be
formally detected. We find that the intrinsic (V-R) of Galactic bulge RR Lyrae
stars are similar to those found by their local counterparts and hence that
Bulge RR0 Lyrae stars do not have anomalous colors as compared to the local RR
Lyrae stars.Comment: accepted by A
Spectroscopic confirmation of the low-latitude object FSR 1716 as an old globular cluster
Star clusters are invaluable tracers of the Galactic components and the
discovery and characterization of low-mass stellar systems can be used to
appraise their prevailing disruption mechanisms and time scales. However, owing
to the significant foreground contamination, high extinction, and still
uncharted interfaces of the underlying Milky Way components, objects at low
Galactic latitudes are notoriously difficult to characterize. Here, we present
the first spectroscopic campaign to identify the chemodynamical properties of
the low-latitude star cluster FSR 1716. While its photometric age and distance
are far from settled, the presence of RR Lyrae variables indicates a rather old
cluster variety. Using medium-resolution (R10600) calcium triplet (CaT)
spectroscopy obtained with the wide-field multi-fibre AAOmega instrument, we
identified six member candidates with a mean velocity of km s and
a velocity dispersion of 2.50.9 km s. The latter value implies a
dynamic mass of 1.310 M, typical of a low-mass
globular cluster. Combined with our derived CaT metallicity of
dex, this object is finally confirmed as an old, metal-poor globular cluster.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Detailed chemical abundance analysis of the thick disk star cluster Gaia 1
Star clusters, particularly those objects in the disk-bulge-halo interface
are as of yet poorly charted, albeit carrying important information about the
formation and the structure of the Milky Way. Here, we present a detailed
chemical abundance study of the recently discovered object Gaia 1. Photometry
has previously suggested it as an intermediate-age, moderately metal-rich
system, although the exact values for its age and metallicity remained
ambiguous in the literature. We measured detailed chemical abundances of 14
elements in four red giant members, from high-resolution (R=25000) spectra that
firmly establish Gaia 1 as an object associated with the thick disk. The
resulting mean Fe abundance is 0.03(stat.)0.10(sys.) dex, which
is more metal-poor than indicated by previous spectroscopy from the literature,
but it is fully in line with values from isochrone fitting. We find that Gaia 1
is moderately enhanced in the -elements, which allowed us to
consolidate its membership with the thick disk via chemical tagging. The
cluster's Fe-peak and neutron-capture elements are similar to those found
across the metal-rich disks, where the latter indicate some level of
-process activity. No significant spread in iron nor in other heavy elements
was detected, whereas we find evidence of light-element variations in Na, Mg,
and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations,
typically seen in old globular clusters, are not seen in our data. This
confirms that Gaia 1 is rather a massive and luminous open cluster than a
low-mass globular cluster. Finally, orbital computations of the target stars
bolster our chemical findings of Gaia 1's present-day membership with the thick
disk, even though it remains unclear, which mechanisms put it in that place.Comment: 11 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics. Some figure sizes reduce
Spectroscopic study of the elusive globular cluster ESO452-SC11 and its surroundings
Globular clusters (GCs) are amongst the oldest objects in the Galaxy and play
a pivotal role in deciphering its early history. We present the first
spectroscopic study of the GC ESO452-SC11 using the AAOmega spectrograph at
medium resolution. Given the sparsity of this object and high degree of
foreground contamination due to its location toward the bulge, few details are
known for this cluster: there is no consensus of its age, metallicity, or its
association with the disk or bulge. We identify 5 members based on radial
velocity, metallicity, and position within the GC. Using spectral synthesis,
accurate abundances of Fe and several -, Fe-peak, neutron-capture
elements (Si,Ca,Ti,Cr,Co,Ni,Sr,Eu) were measured. Two of the 5 cluster
candidates are likely non-members, as they have deviant Fe abundances and
[/Fe] ratios. The mean radial velocity is 192 km s with a
low dispersion of 2.83.4 km s, in line with its low mass. The mean
Fe-abundance from spectral fitting is , with a spread driven by
observational errors. The -elements of the GC candidates are marginally
lower than expected for the bulge at similar metallicities. As spectra of
hundreds of stars were collected in a 2 degree field around ESO452-SC11,
detailed abundances in the surrounding field were measured. Most non-members
have higher [/Fe] ratios, typical of the nearby bulge population. Stars
with measured Fe-peak abundances show a large scatter around Solar values,
though with large uncertainties. Our study provides the first systematic
measurement of Sr in a Galactic bulge GC. The Eu and Sr abundances of the GC
candidates are consistent with a disk or bulge association. Our calculations
place ESO452 on an elliptical orbit in the central 3 kpc of the bulge. We find
no evidence of extratidal stars in our data. (Abridged)Comment: 10 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Distance to the Sagittarius Dwarf Galaxy using MACHO Project RR Lyrae stars
We derive the distance to the northern extension of the Sagittarius (Sgr)
dwarf spheroidal galaxy from 203 Sgr RR0 Lyrae stars found in the MACHO
database. Their distances are determined differentially with respect to 288
Galactic Bulge RR0 Lyrae stars also found in the MACHO data. We find a distance
modulus difference of 2.41 mags at = 5 and = -8 and
that the extension of the Sgr galaxy towards the galactic plane is inclined
toward us. Assuming = 8 kpc, this implies the distance to these
stars is = 16.97 0.07 mags, which corresponds to D = 24.8
0.8 kpc. Although this estimate is smaller than previous determinations for
this galaxy and agrees with previous suggestions that Sgr's body is truly
closer to us, this estimate is larger than studies at comparable galactic
latitudes.Comment: accepted in A
An Oosterhoff Analysis of the Galactic Bulge Field RR Lyrae stars: Implications On Their Absolute Magnitudes
We present an analysis of the period--amplitude plane for RR0 Lyrae stars
(fundamental mode pulsators) with "normal" light curves in the bulge using the
MACHO bulge fields. Although bulge globular clusters have RR Lyraes that divide
into two reasonable distinct groups according to the average period of the RR0
Lyraes (Oosterhoff 1939), there is no evidence of a gap between Oosterhoff I
and II stars in the bulge field star sample. The majority of the bulge RR0
Lyrae field star population have a difference in period compared to the
Oosterhoff I cluster M3 (Delta log P) that is shifted by about 0.02 days with
regard to the Milky Way Oosterhoff I population, and the sample includes stars
with Delta log P > 0.06 days, a characteristic hardly seen in Milky Way
globular clusters. The metal-rich RR0 Lyrae stars in the Galactic bulge sample
have Delta log P values on the other side of the spectrum as those in the
metal-rich globular clusters NGC 6388 and NGC 6441. We find that the
-amplitude for a given period is a function of Delta log P, and not of metal
abundance, similar to the result found by Clement & Shelton (1999) for RR Lyrae
stars in Milky Way globular clusters. A comparative study of the bulge field
stars with similar metallicities but different Oosterhoff types is carried out.
Bulge field RR0 Lyrae variables with Delta log P values similar to Oosterhoff
II clusters are about 0.2 mag brighter than RR0 Lyrae variables with Delta log
P similar to Oosterhoff I clusters. Reliance upon a single absolute
magnitude-[Fe/H] relation may be inappropriate when considering populations
with different Delta log P.Comment: accepted to AJ, 9 figure
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