59 research outputs found
Whiting 1: the youngest globular cluster associated with the Sgr dSph
Recently, Carraro (2005) drew attention to the remarkable star cluster
Whiting 1 by showing that it lies about 40 kpc from the Sun and is therefore
unquestionably a member of the Galactic halo (b=-60.6 deg.). Its CMD indicated
that Whiting 1 is very young (5 Gyrs) for a globular cluster. It is very likely
that Whiting 1 originated in a dwarf galaxy that has since been disrupted by
the Milky Way. Deep CCD photometry in the BVI pass-bands obtained with the VLT
is used to improve the quality of the CMD and to determine the cluster's
luminosity function and surface density profile. High-resolution spectrograms
obtained with Magellan are used to measure the cluster's radial velocity and to
place limits on its possible metallicity. The measurements of distance and
radial velocity are used to test the cluster's membership in the stellar
streams from the Sgr dSph. From our CMD of Whiting 1, we derive new estimates
for the cluster's age (6.5 Gyrs), metallicity ([Fe/H]=-0.65), and distance
(29.4 kpc). From echelle spectrograms of three stars, we obtain -130.6 km/s for
the cluster's radial velocity and show from measurements of two infra-red CaII
lines that the [Fe/H] of the cluster probably lies in the range -1.1 to -0.4.
We demonstrate that the position of Whiting 1 on the sky, its distance from the
Sun, and its radial velocity are identical to within the errors of both the
theoretical predictions of the trailing stream of stars from the Sgr dSph
galaxy and the previous observations of the M giant stars that delineate the
streams. With the addition of Whiting 1, there is now strong evidence that 6
globular clusters formed within the Sgr dSph.Comment: 9 pages, 9 eps figures (some degraded in resolution), in press in
Astronomy and Astrophysics. Abstract here below has been shortened to fit i
The old open cluster NGC 2112: updated estimates of fundamental parameters based on a membership analysis
We report on a new, wide field (), multicolor
(), photometric campaign in the area of the nearby old open cluster NGC
2112. At the same time, we provide medium-resolution spectroscopy of 35 (and
high-resolution of additional 5) Red Giant and Turn Off stars. This material is
analyzed with the aim to update the fundamental parameters of this
traditionally difficult cluster, which is very sparse and suffers from heavy
field star contamination. Among the 40 stars with spectra, we identified 21
{\it bona fide} radial velocity members which allow us to put more solid
constraints on the cluster's metal abundance, long suggested to be as low as
the metallicity of globulars. As indicated earlier by us on a purely
photometric basis (Carraro et al. 2002), the cluster [Fe/H] abundance is
slightly super-solar ([Fe/H] =0.160.03) and close to the Hyades value, as
inferred from a detailed abundance analysis of 3 of the 5 stars with higher
resolution spectra. Abundance ratios are also marginally super solar. Based on
this result, we revise the properties of NGC 2112 using stellar models from the
Padova and Yale-Yonsei groups. For this metal abundance, we find the cluster's
age, reddening, and distance values are 1.8 Gyr, 0.60 mag, and 940 pc,
respectively. Both the Yale-Yonsei and Padova models predict the same values
for the fundamental parameters within the errors. Overall, NGC 2112 is a
typical solar neighborhood, thin disk star cluster, sharing the same chemical
properties of F-G stars and open clusters close to the Sun. This investigation
outlines the importance of a detailed membership analysis in the study of disk
star clusters.Comment: 12 pages, 10 eps figure, in press in MNRA
The VVV-SkZ pipeline: an automatic PSF-fitting photometric pipeline for the VVV survey
We present the VVV-SkZ_pipeline, a DAOPHOT-based photometric pipeline,
created to perform PSF-fitting photometry of "VISTA Variables in the V\'ia
L\'actea" (VVV) ESO Public Survey data. The pipeline replaces the user avoiding
repetitive interaction in all the operations, retaining all of the benefits of
the power and accuracy of the DAOPHOT suite. The pipeline provides an
astrometrized photometric catalog reliable up to more than 2 magnitudes
brighter than the saturation limit, where other techniques fail. It also
produces deeper and more accurate photometry. These achievements allow the
VVV-SkZ_pipeline to produce data well anchored to the selected standard
photometric system and analyze important phenomena (i.e. TRGB, RGB slope, HB
morphology, RR Lyrae), that other methods are not able to manage.Comment: Accepted by RevMexAA for vol. 49, n.2, October 201
Near-infrared photometry of globular clusters towards the Galactic bulge : observations and photometric metallicity indicators
We present wide-field JHKS photometry of 16 Galactic globular clusters located towards the Galactic bulge, calibrated on the Two Micron All-Sky Survey photometric system. Differential reddening corrections and statistical field star decontamination are employed for all of these clusters before fitting fiducial sequences to the cluster red giant branches (RGBs). Observed values and uncertainties are reported for several photometric features, including the magnitude of the RGB bump, tip, the horizontal branch (HB) and the slope of the upper RGB. The latest spectroscopically determined chemical abundances are used to build distance- and reddening-independent relations between observed photometric features and cluster metallicity, optimizing the sample size and metallicity baseline of these relations by supplementing our sample with results from the literature.We find that the magnitude difference between the HB and the RGB bump can be used to predict metallicities, in terms of both iron abundance [Fe/H] and global metallicity [M/H], with a precision of better than 0.1 dex in all three near-IR bandpasses for relatively metal-rich ([M/H] −1) clusters. Meanwhile, both the slope of the upper RGB and the magnitude difference between the RGB tip and bump are useful metallicity indicators over the entire sampled metallicity range (−2 [M/H] 0) with a precision of 0.2 dex or better, despite model predictions that the RGB slope may become unreliable at high (near-solar) metallicities. Our results agree with previous calibrations in light of the relevant uncertainties, and we discuss implications for clusters with controversial metallicities as well as directions for further investigation
Galactic Structure in the Outer Disk: The Field in the Line of Sight to the Intermediate-Age open Cluster Tombaugh 1
We employ optical photometry and high-resolution spectroscopy to study a field toward the open cluster Tombaugh 1, where we identify a complex population mixture that we describe in terms of young and old Galactic thin disks. Of particular interest is the spatial distribution of the young population, which consists of dwarfs with spectral types as early as B6 and is distributed in a blue plume feature in the color-magnitude diagram. For the first time, we confirm spectroscopically that most of these stars are early-type stars and not blue stragglers or halo/thick-disk subdwarfs. Moreover, they are not evenly distributed along the line of sight but crowd at heliocentric distances between 6.6 and 8.2 kpc. We compare these results with present-day understanding of the spiral structure of the Galaxy and suggest that they trace the outer arm. This range of distances challenges current Galactic models adopting a disk cutoff at 14 kpc from the Galactic center. The young dwarfs overlap in space with an older component, which is identified as an old Galactic thin disk. Both young and old populations are confined in space since the disk is warped at the latitude and longitude of Tombaugh 1. The main effects of the warp are that the line of sight intersects the disk and entirely crosses it at the outer arm distance and that there are no traces of the closer Perseus arm, which would then be either unimportant in this sector or located much closer to the formal Galactic plane. Finally, we analyze a group of giant stars, which turn out to be located at very different distances and to possess very different chemical properties, with no obvious relation to the other populations.Instituto de Astrofísica de La Plat
Properties of the open cluster Tombaugh 1 from high resolution spectroscopy and uvbyCaH photometry
Open clusters can be the key to deepen our knowledge on various issues
involving the structure and evolution of the Galactic disk and details of
stellar evolution because a cluster's properties are applicable to all its
members. However the number of open clusters with detailed analysis from high
resolution spectroscopy and/or precision photometry imposes severe limitation
on studies of these objects. To expand the number of open clusters with
well-defined chemical abundances and fundamental parameters, we investigate the
poorly studied, anticenter open cluster Tombaugh 1. Using precision
uvbyCaH photometry and high resolution spectroscopy, we derive the
cluster's properties and, for the first time, present detailed abundance
analysis of 10 potential cluster stars. Using radial position from the cluster
center and multiple color indices, we have isolated a sample of unevolved
probable, single-star members of Tombaugh 1. The weighted photometric
metallicity from and is [Fe/H] = -0.10 0.02, while a match to
the Victoria-Regina Str\"{o}mgren isochrones leads to an age of 0.95 0.10
Gyr and an apparent modulus of = 13.10 0.10. Radial velocities
identify 6 giants as probable cluster members and the elemental abundances of
Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni, Y,Ba, Ce, and Nd have been derived for both
the cluster and the field stars. Tombaugh 1 appears to be a typical inner thin
disk, intermediate-age open cluster of slightly subsolar metallicity, located
just beyond the solar circle, with solar elemental abundance ratios except for
the heavy s-process elements, which are a factor of two above solar. Its
metallicity is consistent with a steep metallicity gradient in the
galactocentric region between 9.5 and 12 kpc. Our study also shows that Cepheid
XZ CMa is not a member of Tombaugh 1, and reveals that this Cepheid presents
signs of barium enrichment.Comment: 74 pages, 15 figures, 13 tables; Accepted for publication in A
In-Situ Star Formation in the Outskirts of the Large Magellanic Cloud: Gaia DR2 Confirmation
We explore the Gaia DR2 proper motions of six young, main-sequence stars,
members of the Large Magellanic Cloud (LMC) reported by Moni Bidin et al.
(2017). These stars are located in the outskirts of the disk, between 7 and 13
degrees from the LMC's center where there is very low H I content. Gaia DR2
proper motions confirm that four stars formed locally, in situ, while two are
consistent with being expelled via dynamical interactions from inner, more
gas-rich regions of the LMC. This finding establishes that recent star
formation occurred in the periphery of the LMC, where thus far only old
populations are known.Comment: Accepted for publication in Astrophysical Journal Letter
- …