ABSTRACT
This work reports the discovery of three new globular clusters (GCs) towards the Galactic bulge – Camargo 1107, 1108, and 1109. The discovery was made using the WISE, 2MASS, VVV, and ...Gaia-DR2 photometry. The new findings are old (12.0−13.5 Gyr) and metal-poor GCs (Fe/H < −1.5 dex) located in the bulge area close to the Milky Way (MW) mid-plane. Although the old ages and low metallicities suggest that the newly discovered GCs are likely associated with the inner halo, the possibility of these clusters being part of a primordial bulge GC subpopulation cannot be ruled out. Camargo 1107, for instance, presents a metallicity of Fe/H = −2.2 ± 0.4 dex and an age of 13.5 ± 2 Gyr, which may suggest that this cluster formed just after the big bang in the very early Universe. The discovery of GCs such as the new findings is crucial to built a coherent picture of the inner Galaxy. It is likely that at least a few more dozens of GCs are still to be discovered in the bulge.
The GIRAFFE Inner Bulge Survey (GIBS) Zoccali, M; Vasquez, S; Gonzalez, O A ...
Astronomy and astrophysics (Berlin),
3/2017, Letnik:
599
Journal Article
Recenzirano
Odprti dostop
Context. Several recent studies have demonstrated that the Galactic bulge hosts two components with different mean metallicities, and possibly different spatial distribution and kinematics. As a ...consequence, both the metallicity distribution and the radial velocity of bulge stars vary across different lines of sight. Aims. We present here the metallicity distribution function of red clump stars in 26 fields spread across a wide area of the bulge, with special emphasis on fields close to Galactic plane, at latitudes b= -2degrees and b= -1degrees, that have not been explored before. Methods. This paper includes new metallicities from a sample of approximately 5000 K giant stars, observed at spectral resolution R~ 6500, in the Calcium II Triplet region. These represent the main dataset from the GIRAFFE Inner Bulge Survey. As part of the same survey we have previously published results for a sample of approximately 600 K giant stars, at latitude b~ -4degrees, derived from higher resolution spectra (R= 22 500). Results. The combined sample allows us to trace and characterize the metal poor and metal rich bulge populations down to the inner bulge. We present a density map for each of the two components. Contrary to expectations from previous works, we found the metal poor population to be more centrally concentrated than the metal rich one, and with a more axisymmetric spatial distribution. The metal rich population, on the other hand, is arranged in a boxy distribution, consistent with an edge-on bar. By coupling metallicities and radial velocities we show that the metal poor population has a velocity dispersion that varies rather mildly with latitude. On the contrary, the metal rich population has a low velocity dispersion far from the plane (b= -8.5degrees), yet has a steeper gradient with latitude, becoming higher than the metal poor one in the innermost field (b= -1degrees). Conclusions. This work provides new observational constraints on the actual chemodynamical properties of the Galactic bulge, that will help discrimination between different formation models.
Context. Much of what we know about the Milky Way disk is based on studies of the solar vicinity. The structure, kinematics, and chemical composition of the far side of the Galactic disk, beyond the ...bulge, are still to be revealed. Aims. Classical Cepheids (CCs) are young and luminous standard candles. We aim to use a well-characterized sample of these variable stars to study the present-time properties of the far side of the Galactic disk. Methods. A sample of 45 Cepheid variable star candidates were selected from near-infrared time series photometry obtained by the VVV survey. We characterized this sample using high quality near-infrared spectra obtained with VLT/X-shooter. The spectroscopic data was used to derive radial velocities and iron abundances for all the sample Cepheids. This allowed us to separate the CCs, which are metal rich and with kinematics consistent with the disk rotation, from type II Cepheids (T2Cs), which are more metal poor and with different kinematics. Results. We estimated individual distances and extinctions using VVV photometry and period-luminosity relations, reporting the characterization of 30 CCs located on the far side of the Galactic disk, plus 8 T2Cs mainly located in the bulge region, of which 10 CCs and 4 T2Cs are new discoveries. The remaining seven stars are probably misclassified foreground ellipsoidal binaries. This is the first sizeable sample of CCs in this distant region of our Galaxy that has been spectroscopically confirmed. We use their positions, kinematics, and metallicities to confirm that the general properties of the far disk are similar to those of the well-studied disk on the solar side of the Galaxy. In addition, we derive for the first time the radial metallicity gradient on the disk’s far side. Considering all the CCs with RGC < 17 kpc, we measure a gradient with a slope of −0.062 dex kpc−1 and an intercept of +0.59 dex, which is in agreement with previous determinations based on CCs on the near side of the disk.
We have combined optical and near-infrared data of known RR Lyrae (RRL) stars in the bulge in order to study the spatial distribution of its metal-poor component by measuring precise reddening values ...and distances of 7663 fundamental-mode RRL stars with high-quality photometry. We obtain a distance to the Galactic center of R sub(0) = 8.33 + or - 0.05 + or - 0.14 kpc. We find that the spatial distribution of the RRL stars differs from the structures traced by the predominantly metal-rich red clump (RC) stars. Unlike the RC stars, the RRL stars do not trace a strong bar, but have a more spheroidal, centrally concentrated distribution, showing only a slight elongation in its very center. We find a hint of bimodality in the density distribution at high southern latitudes (b < -5degrees), which needs to be confirmed by extending the areal coverage of the current census. The different spatial distributions of the metal-rich and metal-poor stellar populations suggest that the Milky Way has a composite bulge.
Context. Three dimensional interstellar extinction maps provide a powerful tool for stellar population analysis. However, until now, these 3D maps were rather limited by sensitivity and spatial ...resolution. Aims. We use data from the VISTA Variables in the Via Lactea survey together with the Besançon stellar population synthesis model of the Galaxy to determine interstellar extinction as a function of distance in the Galactic bulge covering −10°<l< 10° and −10°<b< 5°. Methods. We adopted a recently developed method to calculate the colour excess. First we constructed the H − Ks vs. Ks and J − Ks vs. Ks colour−magnitude diagrams based on the VVV catalogues that matched 2MASS. Then, based on the temperature−colour relation for M giants and the distance-colour relations, we derived the extinction as a function of distance. The observed colours were shifted to match the intrinsic colours in the Besançon model as a function of distance iteratively. This created an extinction map with three dimensions: two spatial and one distance dimension along each line of sight towards the bulge. Results. We present a 3D extinction map that covers the whole VVV area with a resolution of 6′× 6′ for J − Ks and H − Ks using distance bins of 0.5 kpc. The high resolution and depth of the photometry allows us to derive extinction maps for a range of distances up to 10 kpc and up to 30 mag of extinction in AV (3.0 mag in AKs). Integrated maps show the same dust features and consistent values as other 2D maps. We discuss the spatial distribution of dust features in the line of sight, which suggests that there is much material in front of the Galactic bar, specifically between 5−7 kpc. We compare our dust extinction map with the high-resolution 12CO maps (NANTEN2) towards the Galactic bulge, where we find a good correlation between 12CO and AV. We determine the X factor by combining the CO map and our dust extinction map. Our derived average value X = 2.5 ± 0.47 × 1020 cm-2 K-1 km-1s is consistent with the canonical value of the Milky Way. The X-factor decreases with increasing extinction.
Context. Type II Cepheids (T2Cs) are radially pulsating variables that trace old stellar populations and provide distance estimates through their period-luminosity (PL) relation. Aims. We trace the ...structure of old stellar population in the Galactic bulge using new distance estimates and kinematic properties of T2Cs. Methods. We present new near-infrared photometry of T2Cs in the bulge from the VISTA Variables in the Vía Láctea survey (VVV). We provide the largest sample (894 stars) of T2Cs with JHKs observations that have accurate periods from the Optical Gravitational Lensing Experiment (OGLE) catalog. Our analysis makes use of the Ks-band time-series observations to estimate mean magnitudes and individual distances by means of the PL relation. To constrain the kinematic properties of our targets, we complement our analysis with proper motions based on both the VVV and Gaia Data Release 2. Results. We derive an empirical Ks-band PL relation that depends on Galactic longitude and latitude: Ks0 = (10.66 ± 0.02) − (2.21 ± 0.03)·(log P−1.2)−(0.020±0.003)·l+(0.050±0.008)·|b| mag; individual extinction corrections are based on a 3D reddening map. Our targets display a centrally concentrated distribution, with solid evidence of ellipsoidal symmetry – similar to the RR Lyræ ellipsoid – and a few halo outliers up to ≳100 kpc. We obtain a distance from the Galactic center of R0 = 8.46 ± 0.03(stat.) ± 0.11(syst.) kpc. We also find evidence that the bulge T2Cs belong to a kinematically hot population, as the tangential velocity components (συl∗ = 104.2 ± 3.0kms−1 and συb = 96.8 ± 5.5kms−1) agree within 1.2σ. Moreover, the difference between absolute and relative proper motion is in good agreement with the proper motion of Sgr A✻ from VLBA measures. Conclusions. We conclude that bulge T2Cs display an ellipsoidal spatial distribution and have kinematics similar to RR Lyræ stars, which are other tracers of the old, low-mass stellar population. T2Cs also provide an estimate of R0 that agrees excellently well with the literature, taking account of the reddening law.
Near-infrared color-excess and extinction ratios are essential for establishing the cosmic distance scale and probing the Galaxy, particularly when analyzing targets attenuated by significant dust. A ...robust determination of those ratios followed from leveraging new infrared observations from the VVV survey, wherein numerous bulge RR Lyrae and Type II Cepheids were discovered, in addition to BVJHKs(3.4 → 22) μm data for classical Cepheids and O stars occupying the broader Galaxy. The apparent optical color-excess ratios vary significantly with Galactic longitude (ℓ), whereas the near-infrared results are comparatively constant with ℓ and Galactocentric distance (\hbox{$\langle E(J-\overline{3.5~\mu {\rm m}})/E(J-K_{\rm s}) \rangle =1.28\,\pm\,0.03$}⟨E(J−3.5μm)/E(J−Ks)⟩=1.28 ± 0.03). The results derived imply that classical Cepheids and O stars display separate optical trends (RV,BV) with ℓ, which appear to disfavor theories advocating a strict and marked decrease in dust size with increasing Galactocentric distance. The classical Cepheid, Type II Cepheid, and RR Lyrae variables are characterized by ⟨AJ/E(J−Ks)⟩ = ⟨RJ,JKs⟩ = 1.49 ± 0.05 (⟨AKs/AJ⟩ = 0.33 ± 0.02), whereas the O stars are expectedly impacted by emission beyond 3.6 μm. The mean optical ratios characterizing classical Cepheids and O stars are approximately ⟨RV,BV⟩ ~ 3.1 and ⟨RV,BV⟩ ~ 3.3, respectively.
In a companion work (Paper I), we detected a large population of highly variable Young Stellar Objects (YSOs) in the Vista Variables in the Via Lactea (VVV) survey, typically with class I or flat ...spectrum spectral energy distributions and diverse light-curve types. Here we present infrared spectra (0.9-2.5 ...m) of 37 of these variables, many of them observed in a bright state. The spectra confirm that 15/18 sources with eruptive light curves have signatures of a high accretion rate, either showing EXor-like emission features (... = 2 CO, Br...) and/or FUor-like features (... = 2 CO and H sub( 2)O strongly in absorption). Similar features were seen in some long-term periodic YSOs and faders but not in dippers or short-term variables. The sample includes some dusty Mira variables (typically distinguished by smooth Mira-like light curves), two cataclysmic variables and a carbon star. In total, we have added 19 new objects to the broad class of eruptive variable YSOs with episodic accretion. Eruptive variable YSOs in our sample that were observed at bright states show higher accretion luminosities than the rest of the sample. Most of the eruptive variables differ from the established FUor and EXor subclasses, showing intermediate outburst durations and a mixture of their spectroscopic characteristics. This is in line with a small number of other recent discoveries. Since these previously atypical objects are now the majority amongst embedded members of the class, we propose a new classification for them as MNors. This term (pronounced emnor) follows V1647 Ori, the illuminating star of McNeil's Nebula. (ProQuest: ... denotes formulae/symbols omitted.)
We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with broad-band photometric data from several sources as well as parallaxes from
Gaia
Data Release 2 (DR2). ...Using the Bayesian isochrone-fitting code
StarHorse
, we derived the distances, extinctions, and astrophysical parameters for around 388 815 APOGEE stars. We achieve typical distance uncertainties of ∼6% for APOGEE giants, ∼2% for APOGEE dwarfs, and extinction uncertainties of ∼0.07 mag, when all photometric information is available, and ∼0.17 mag if optical photometry is missing.
StarHorse
uncertainties vary with the input spectroscopic catalogue, available photometry, and parallax uncertainties. To illustrate the impact of our results, we show that thanks to
Gaia
DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane. We thereby provide an unprecedented coverage of the disc close to the Galactic mid-plane (|
Z
Gal
| < 1 kpc) from the Galactic centre out to
R
Gal
∼ 20 kpc. The improvements in statistics as well as distance and extinction uncertainties unveil the presence of the bar in stellar density and the striking chemical duality in the innermost regions of the disc, which now clearly extend to the inner bulge. We complement this paper with distances and extinctions for stars in other public released spectroscopic surveys: 324 999 in GALAH DR2, 4 928 715 in LAMOST DR5, 408 894 in RAVE DR6, and 6095 in GES DR3.
Context. The Milky Way bulge is the nearest galactic bulge and the most readily accessible laboratory for studies of stellar populations in spheroids based on individual stellar abundances and ...kinematics. These studies are challenged by the strongly variable and often large extinction on a small spatial scale.Aims. We use the Vista Variables in the Via Lactea (VVV) ESO public survey data to measure extinction values in the complete area of the Galactic bulge covered by the survey at high resolution.Methods. We derive reddening values using the method described in Paper I. This is based on measuring the mean (J − Ks) color of red clump giants in small subfields of 2′ × 2′ to 6′ × 6′ in the following bulge area: − 10.3° ≤ b ≤ + 5.1° and − 10.0° ≤ l ≤ + 10.4°. To determine the reddening values E(J − Ks) for each region, we measure the RC color and compare it to the (J − Ks) color of RC stars measured in Baade’s Window, for which we adopt E(B − V) = 0.55. This allows us to construct a reddening map sensitive to small-scale variations minimizing the problems arising from differential extinction.Results. The significant reddening variations are clearly observed on spatial scales as small as 2′. We find good agreement between our extinction measurements and Schlegel maps in the outer bulge, but, as already stated in the literature the Schlegel maps are unreliable for regions within |b| ≲ 6°. In the inner regions, we compare our results with maps derived from DENIS and Spitzer surveys. While we find good agreement with other studies in the corresponding overlapping regions, our extinction map is of higher quality owing to both its higher resolution and a more complete spatial coverage of the bulge. We investigate the importance of differential reddening and demonstrate the need for high spatial resolution extinction maps for detailed studies of bulge stellar populations and structure.Conclusions. We present the first extinction map covering uniformly ~315 sq. deg. of the Milky Way bulge at high spatial resolution. We consider a 30 arcmin window at a latitude of b = −4°, which corresponds to a frequently studied low extinction window, the so-called Baade’s Window, and find that its AKs values can vary by up to 0.1 mag. Larger extinction variations are observed at lower Galactic latitudes. The extinction variations on scales of up to 2′−6′ must be taken into account when analyzing the stellar populations of the Galactic bulge.
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