Context. In the last six years, the VISTA Variable in the Vía Láctea (VVV) survey mapped 562 sq. deg. across the bulge and southern disk of the Galaxy. However, a detailed study of these regions, ...which includes ~36 globular clusters (GCs) and thousands of open clusters is by no means an easy challenge. High differential reddening and severe crowding along the line of sight makes highly hamper to reliably distinguish stars belonging to different populations and/or systems. Aims. The aim of this study is to separate stars that likely belong to the Galactic GC NGC 6544 from its surrounding field by means of proper motion (PM) techniques. Methods. This work was based upon a new astrometric reduction method optimized for images of the VVV survey. Results. PSF-fitting photometry over the six years baseline of the survey allowed us to obtain a mean precision of ~0.51 mas yr-1, in each PM coordinate, for stars with Ks< 15 mag. In the area studied here, cluster stars separate very well from field stars, down to the main sequence turnoff and below, allowing us to derive for the first time the absolute PM of NGC 6544. Isochrone fitting on the clean and differential reddening corrected cluster color magnitude diagram yields an age of ~11−13 Gyr, and metallicity Fe/H =−1.5 dex, in agreement with previous studies restricted to the cluster core. We were able to derive the cluster orbit assuming an axisymmetric model of the Galaxy and conclude that NGC 6544 is likely a halo GC. We have not detected tidal tail signatures associated to the cluster, but a remarkable elongation in the galactic center direction has been found. The precision achieved in the PM determination also allows us to separate bulge stars from foreground disk stars, enabling the kinematical selection of bona fide bulge stars across the whole survey area. Conclusions. Kinematical techniques are a fundamental step toward disentangling different stellar populations that overlap in a studied field. Our results show that VVV data is perfectly suitable for this kind of analysis.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Aims. We use the VVV survey observations in bulge regions close to the Galactic plane to trace the bar inclination at the Galactic latitude b ~ ±1 and to investigate a distinct structure in the ...inner regions of the bar that was previously detected at positive latitude (b = +1). Methods. We use the (J − Ks) colors of the red clump stars to obtain reddening values on 6 × 6 arcmin scale, minimizing the problems arising from differential extinction. Dereddened magnitudes are then used to build the luminosity function of the bulge in regions of ~0.4 sq deg to obtain the mean red clump magnitudes. These are used as distance indicators to trace the bar structure. Results. The luminosity function clearly shows the red clump mean magnitude variation with longitude, as expected from a large scale bar oriented towards us at positive Galactic longitude, with a dereddened magnitude varying from Ks0 = 13.4 at l = −10° to Ks0 = 12.4 at l = +10°. We detect a change in the orientation of the bar in the central regions with |l| < 4° at b = ±1°, in agreement with results obtained at positive latitudes by other authors. Our results are based on a different dataset and at different latitude, which shows that this change in the bar orientation is real. This suggests that there is an inner structure distinct to the large-scale Galactic bar, with a different orientation angle. This inner structure could be a secondary, inner bar, with a semi-major axis of ~500 pc that is symmetric with respect to the Galactic plane.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Aims. We present the analysis of the α/Fe abundance ratios for a large number of stars at several locations in the Milky Way bulge with the aim of constraining its formation scenario. Methods. We ...obtained FLAMES-GIRAFFE spectra (R = 22 500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b = −4°, b = −6°, b = −12°. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b = −3° and 5° away from the other three fields along the major axis. Stellar parameters and metallicities for these stars were presented in Zoccali et al. (2008, A&A, 486, 177). We have also re-derived stellar parameters and abundances for the sample of thick and thin disk red giants analyzed in Alves-Brito et al. (2010, A&A, 513, A35). Therefore using a homogeneous abundance database for the bulge, thick and thin disk, we have performed a differential analysis minimizing systematic errors, to compare the formation scenarios of these Galactic components. Results. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk. In the same context, we analyze α/Fe vs. Fe/H trends across different bulge regions. The most metal rich stars, showing low α/Fe ratios at b = −4° disappear at higher Galactic latitudes in agreement with the observed metallicity gradient in the bulge. Metal-poor stars (Fe/H < −0.2) show a remarkable homogeneity at different bulge locations. Conclusions. We have obtained further constrains for the formation scenario of the Galactic bulge. A metal-poor component chemically indistinguishable from the thick disk hints for a fast and early formation for both the bulge and the thick disk. Such a component shows no variation, neither in abundances nor kinematics, among different bulge regions. A metal-rich component showing low α/Fe similar to those of the thin disk disappears at larger latitudes. This allows us to trace a component formed through fast early mergers (classical bulge) and a disk/bar component formed on a more extended timescale.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Abstract
We provide the largest and most homogeneous sample of
α
-element (Mg, Ca, Ti) and iron abundances for field RR Lyrae (RRLs; 162 variables) by using high-resolution spectra. The current ...measurements were complemented with similar abundances available in the literature for 46 field RRLs brought to our metallicity scale. We ended up with a sample of old (
t
≥ 10 Gyr), low-mass stellar tracers (208 RRLs: 169 fundamental, 38 first overtone, and 1 mixed mode) covering 3 dex in iron abundance (−3.00 ≤ Fe/H ≤ 0.24). We found that field RRLs are ∼0.3 dex more
α
poor than typical halo tracers in the metal-rich regime (Fe/H ≥ −1.2), while in the metal-poor regime (Fe/H ≤ −2.2) they seem to be on average ∼0.1 dex more
α
enhanced. This is the first time that the depletion in
α
elements for solar iron abundances is detected on the basis of a large, homogeneous, and coeval sample of old stellar tracers. Interestingly, we also detected a close similarity in the
α
/Fe trend between
α
-poor, metal-rich RRLs and red giants (RGs) in the Sagittarius dwarf galaxy as well as between
α
-enhanced, metal-poor RRLs and RGs in ultrafaint dwarf galaxies. These results are supported by similar elemental abundances for 46 field horizontal branch stars. These stars share with RRLs the same evolutionary phase and the same progenitors. This evidence further supports the key role that old stellar tracers play in constraining the early chemical enrichment of the halo and, in particular, in investigating the impact that dwarf galaxies have had in the mass assembly of the Galaxy.
We present an abundance analysis of 101 subgiant branch (SGB) stars in the globular cluster M 22. Using low-resolution FLAMES/GIRAFFE spectra we have determined abundances of the neutron-capture ...strontium and barium and the light element carbon. With these data we explore relationships between the observed SGB photometric split in this cluster and two stellar groups characterized by different contents of iron, slow neutron-capture process (s-process) elements, and the α element calcium, which we previously discovered in M 22’s red-giant stars. We show that the SGB stars correlate in chemical composition and the color–magnitude diagram position. The stars with higher metallicity and relative s-process abundances define a fainter SGB, while stars with lower metallicity and s-process content reside on a relatively brighter SGB. This result has implications for the relative ages of the two stellar groups of M 22. In particular, it is inconsistent with a broad spread in ages of the two SGBs. By accounting for the chemical content of the two stellar groups, isochrone fitting of the double SGB suggests that their agesare not different by more than ~300 Myr.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Abstract
We discuss the largest and most homogeneous spectroscopic data set of field RR Lyrae variables (RRLs) available to date. We estimated abundances using both high-resolution and low-resolution ...(
Δ
S method) spectra for fundamental (RRab) and first overtone (RRc) RRLs. The iron abundances for 7941 RRLs were supplemented with similar estimates that are available in the literature, ending up with 9015 RRLs (6150 RRab, 2865 RRc). The metallicity distribution shows a mean value of 〈Fe/H〉 = −1.51 ± 0.01, and
σ
(standard deviation) = 0.41 dex with a long metal-poor tail approaching Fe/H ≃ − 3 and a sharp metal-rich tail approaching solar iron abundance. The RRab variables are more metal-rich (〈Fe/H〉
ab
= −1.48 ± 0.01,
σ
= 0.41 dex) than RRc variables (〈Fe/H〉
c
= −1.58 ± 0.01,
σ
= 0.40 dex). The relative fraction of RRab variables in the Bailey diagram (visual amplitude versus period) located along the short-period (more metal-rich) and the long-period (more metal-poor) sequences are 80% and 20%, while RRc variables display an opposite trend, namely 30% and 70%, respectively. We found that the pulsation period of both RRab and RRc variables steadily decreases when moving from the metal-poor to the metal-rich regime. The visual amplitude shows the same trend, but RRc amplitudes are almost two times more sensitive than RRab amplitudes to metallicity. We also investigated the dependence of the population ratio (N
c
/N
tot
) of field RRLs on the metallicity and we found that the distribution is more complex than in globular clusters. The population ratio steadily increases from ∼0.25 to ∼0.36 in the metal-poor regime, it decreases from ∼0.36 to ∼0.18 for −1.8 ≤ Fe/H ≤ −0.9 and it increases to a value of ∼0.3 approaching solar iron abundance.
ABSTRACT
Like most spiral galaxies, the Milky Way contains a population of blue, metal-poor globular clusters and another of red, metal-rich ones. Most of the latter belong to the bulge, and ...therefore they are poorly studied compared to the blue (halo) ones because they suffer higher extinction and larger contamination from field stars. These intrinsic difficulties, together with a lack of low-mass bulge globular clusters, are reasons to believe that their census is not complete yet. Indeed, a few new clusters have been confirmed in the last few years. One of them is VVV CL001, the subject of the present study. We present a new spectroscopic analysis of the recently confirmed globular cluster VVV CL001, made by means of MUSE@VLT integral field data. Individual spectra were extracted for stars in the VVV CL001 field. Radial velocities were derived by cross-correlation with synthetic templates. Coupled with proper motions from the VVV (VISTA Variables in the Vía Láctea) survey, these data allow us to select 55 potential cluster members, for which we derive metallicities using the public code
The Cannon. The mean radial velocity of the cluster is Vhelio = −324.9 ± 0.8 km s−1, as estimated from 55 cluster members. This high velocity, together with a low metallicity Fe/H = −2.04 ± 0.02 dex, suggests that VVV CL001 could be a very old cluster. The estimated distance is d = 8.23 ± 0.46 kpc, placing the cluster in the Galactic bulge. Furthermore, both its current position and the orbital parameters suggest that VVV CL001 is most probably a bulge globular cluster.
Context. The Sr/Ba and Y/Ba scatter observed in some galactic halo stars that are very metal-poor and in a few individual stars of the oldest known Milky Way globular cluster NGC 6522 have been ...interpreted as evidence of early enrichment by massive fast-rotating stars (spinstars). Because NGC 6522 is a bulge globular cluster, the suggestion was that not only the very-metal poor halo stars, but also bulge stars at Fe/H ~ −1 could be used as probes of the stellar nucleosynthesis signatures from the earlier generations of massive stars, but at much higher metallicity. For the bulge the suggestions were based on early spectra available for stars in NGC 6522, with a medium resolution of R ~ 22 000 and a moderate signal-to-noise ratio. Aims. The main purpose of this study is to re-analyse the NGC 6522 stars reported previously by using new high-resolution (R ~ 45 000) and high signal-to-noise spectra (S/N > 100). We aim at re-deriving their stellar parameters and elemental ratios, in particular the abundances of the neutron-capture s-process-dominated elements such as Sr, Y, Zr, La, and Ba, and of the r-element Eu. Methods. High-resolution spectra of four giants belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. Results. Our analysis confirms a metallicity Fe/H = −0.95 ± 0.15 for NGC 6522 and the overabundance of the studied stars in Eu (with +0.2 < Eu/Fe < + 0.4) and alpha-elements O and Mg. The neutron-capture s-element-dominated Sr, Y, Zr, Ba, and La now show less pronounced variations from star to star. Enhancements are in the range 0.0 < Sr/Fe < +0.4, +0.23 < Y/Fe < +0.43, 0.0 < Zr/Fe < +0.4, 0.0 < La/Fe < +0.35, and 0.05 < Ba/Fe < +0.55. Conclusions. The very high overabundances of Y/Fe previously reported for the four studied stars is not confirmed with the new high-quality spectra. The moderate enhancement in Sr/Fe previously reported for one of the re-studied stars is confirmed, but the strong enhancements of this ratio for the other two stars are not confirmed. Despite the moderate enhancements found for the neutron-capture s-element-dominated species, none of the four stars studied here show positive values for all Sr/Ba, Y/Ba and Zr/Ba ratios. The re-studied stars are now compatible not only with the interpretation that the s-process enhancements in these very old stars are due to spinstars, but also with alternative models such as mass transfer from s-process-rich AGB stars. Note, however, that when our results are interpreted in the context of more extended datasets from the literature, the spinstar scenario still seems to be favoured.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
PDF
