arXiv subjects

M. Zoccali

Publications and source records attributed to M. Zoccali.

At least 73 records · Page 4Linked to original sources

arXiv2018

Weighing the two stellar components of the Galactic Bulge

Recent spectroscopic surveys of the Galactic bulge have unambiguously shown that the bulge contains two main components, that are best separated in their iron content, but also differ in spatial distribution, kinematics, and abundance ratios. The so-called metal poor (MP) component peaks at [Fe/H]~-0.4, while the metal rich (MR) one peaks at [Fe/H]$~+0.3. The total metallicity distribution function is therefore bimodal, with a dip at [Fe/H]~0. The relative fraction of the two components changes significantly across the bulge area. We provide, for the first time, the fractional contribution of the MP and MR stars to the stellar mass budget of the Galactic bulge, and its variation across the bulge area. We find that MP stars make up 48% of the total stellar mass of the bulge, within the region |l|<10, |b|<9.5, with the remaining 52% made up of MR stars. The latter dominate the mass budget at intermediate latitudes |b|~4, but become marginal in the outer bulge (|b|>8). The MP component is more axisymmetric than the MR one, and it is at least comparable, and possibly slightly dominant in the inner few degrees. As a result, the MP component, which does not follow the main bar, is not marginal in terms of the total mass budget as previously thought, and this new observational evidence must be included in bulge models. While the total radial velocity dispersion has a trend that follows the total stellar mass, when we examine the velocity dispersion of each component individually, we find that MP stars have higher velocity dispersion where they make up a smaller fraction of the stellar mass, and viceversa. This is due to the kinematical and spatial distribution of the two metallicity component being significantly different, as already discussed in the literature.

astro-ph.GA↗

arXiv2018

Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy

In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of the early-type dwarf systems, we study one of the few isolated dwarf spheroidal galaxies (dSphs) of the Local Group (LG): Cetus. We obtained VLT/FORS2 spectra ($R\sim2600$) in the region of the nIR CaII triplet lines for 80 candidate red giant branch stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. The kinematic analysis shows that Cetus is a mainly pressure-supported ($σ_v = 11.0_{-1.3}^{+1.6}$ km/s), dark-matter-dominated system ($M_{1/2}/L_V = 23.9_{-8.9}^{+9.7} M_\odot/L_\odot$) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant [Fe/H] spread (median [Fe/H] = -1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dSphs of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream. This study represents a first comprehensive analysis of Cetus chemo-kinematic properties. Our results add Cetus to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similar systems inhabiting different environments may hint at metallicity gradients in LG early-type dwarfs being driven by internal mechanisms.

astro-ph.GA↗

arXiv2018

New variable stars in the Galactic Bulge: I. The bright regime

We report the detection of 1143 variable stars towards the Galactic bulge, including 320 previously uncatalogued variables, using time-series photometry extracted from data obtained with the VIMOS imager at the Very Large Telescope. Observations of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field in the Galactic Bulge were taken over 2 years between March and October at a cadence of $\sim$ 4 days, enabling the detection of variables with periods up to $\sim$100 days. Many of these were already known, but we detected a significant number of new variables, including 26 Cepheids, a further 18 Cepheid candidates, and many contact binaries. Here we publish the catalog of the new variables, containing coordinates, mean magnitudes as well as periods and classification; full light curves for these variables are also made available electronically.

astro-ph.SR↗

arXiv2018

The central velocity dispersion of the Milky Way bulge

Recent spectroscopic and photometric surveys are providing a comprehensive view of the MW bulge stellar population properties with unprecedented accuracy. This in turn allows us to explore the correlation between kinematics and stellar density distribution, crucial to constraint the models of Galactic bulge formation. GIBS revealed the presence of a velocity dispersion peak in the central few degrees of the Galaxy by consistently measuring high velocity dispersion in three central most fields. Due to suboptimal distribution of these fields, all being at negative latitudes and close to each other, the shape and extension of the sigma peak is poorly constrained. In this study we address this by adding new observations distributed more uniformly and in particular including fields at positive latitudes that were missing in GIBS.MUSE observations were collected in four fields at (l,b)=(0,+2),(0,-2),(+1,-1),(-1,+2). Individual stellar spectra were extracted for a number of stars comprised between ~500 and ~1200, depending on the seeing and the exposure time. Velocity measurements are done by cross-correlating observed stellar spectra in the CaT region with a synthetic template, and velocity errors obtained through Monte Carlo simulations, cross-correlating synthetic spectra with a range of different metallicities and different noise characteristics. We measure the central velocity dispersion peak within a projected distance from the Galactic center of ~280pc, reaching sigma~140 km/s at b=-1. This is in agreement with the results obtained previously by GIBS at negative longitude. The central sigma peak is symmetric with respect to the Galactic plane, with a longitude extension at least as narrow as predicted by GIBS. As a result of the Monte Carlo simulations we present analytical equations for the Rvel measurement error as a function of metallicity and SNR for giant and dwarf stars.

astro-ph.GA↗

arXiv2018

Oxygen and zinc abundances in 417 Galactic bulge red giants

Oxygen and zinc in the Galactic bulge are key elements for the understanding of the bulge chemical evolution. Oxygen-to-iron abundance ratios provide a most robust indicator of the star formation rate and chemical evolution of the bulge. Zinc is enhanced in metal-poor stars, behaving as an $α$-element, and its production may require nucleosynthesis in hypernovae. Most of the neutral gas at high redshift is in damped Lyman-alpha systems (DLAs), where Zn is also observed to behave as an alpha-element. The aim of this work is the derivation of the alpha-element oxygen, together with nitrogen, and the iron-peak element zinc abundances in 417 bulge giants, from moderate resolution (R~22,000) FLAMES-GIRAFFE spectra. For stars in common with a set of UVES spectra with higher resolution (R~45,000), the data are intercompared. The results are compared with literature data and chemodynamical models.

astro-ph.SR↗

arXiv2018

On the chemical abundances of Miras in clusters: V1 in the metal-rich globular NGC 5927

We present the first spectroscopic abundance determination of iron, alpha-elements (Si, Ca and Ti) and sodium for the Mira variable V1 in the metal-rich globular cluster NGC 5927. We use high-resolution (R~ 28,000), high signal-to-noise ratio (~200) spectra collected with WINERED, a near-infrared (NIR) spectrograph covering simultaneously the wavelength range 0.91--1.35 micron. The effective temperature and the surface gravity at the pulsation phase of the spectroscopic observation were estimated using both optical (V) and NIR time-series photometric data. We found that the Mira is metal-rich ([Fe/H]=-0.55 \pm 0.15) and moderately alpha-enhanced ([alpha/Fe]=0.15 \pm 0.01, sigma=0.2). These values agree quite well with the mean cluster abundances based on high-resolution optical spectra of several cluster red giants available in the literature ([Fe/H]=-0.47 \pm 0.06, [alpha/Fe]=+0.24 \pm 0.05). We also found a Na abundance of +0.35 \pm 0.20 that is higher than the mean cluster abundance based on optical spectra (+0.18 \pm 0.13). However, the lack of similar spectra for cluster red giants and that of corrections for departures from local-thermodynamical equilibrium prevents us from establishing whether the difference is intrinsic or connected with multiple populations. These findings indicate a strong similarity between optical and NIR metallicity scales in spite of the difference in the experimental equipment, data analysis and in the adopted spectroscopic diagnostics.

astro-ph.SR↗

arXiv2018

On the RR Lyrae stars in globulars: V. the complete Near-Infrared (JHKs) census of omega Centauri RR Lyrae variables

We present a new complete Near-Infrared (NIR, $JHK_s$) census of RR Lyrae stars (RRLs) in the globular $ω$ Cen (NGC 5139). We collected 15,472 $JHK_s$ images with 4-8m class telescopes over 15 years (2000-2015) covering a sky area around the cluster center of 60x34 arcmin$^2$. These images provided calibrated photometry for 182 out of the 198 cluster RRL candidates with ten to sixty measurements per band. We also provide new homogeneous estimates of the photometric amplitude for 180 ($J$), 176 ($H$) and 174 ($K_s$) RRLs. These data were supplemented with single-epoch $JK_s$ magnitudes from VHS and with single-epoch $H$ magnitudes from 2MASS. Using proprietary optical and NIR data together with new optical light curves (ASAS-SN) we also updated pulsation periods for 59 candidate RRLs. As a whole, we provide $JHK_s$ magnitudes for 90 RRab (fundamentals), 103 RRc (first overtones) and one RRd (mixed--mode pulsator). We found that NIR/optical photometric amplitude ratios increase when moving from first overtone to fundamental and to long-period (P>0.7 days) fundamental RRLs. Using predicted Period-Luminosity-Metallicity relations, we derive a true distance modulus of 13.674$\pm$0.008$\pm$0.038 mag (statistical error and standard deviation of the median)---based on spectroscopic iron abundances---and of 13.698$\pm$0.004$\pm$0.048 mag---based on photometric iron abundances. We also found evidence of possible systematics at the 5-10% level in the zero-point of the PLs based on the five calibrating RRLs whose parallaxes had been determined with HST

astro-ph.SR↗

arXiv2017

The Magic Star of Exceptional Periodicity

We present a periodic infinite chain of finite generalisations of the exceptional structures, including e8, the exceptional Jordan algebra (and pair), and the octonions. We demonstrate that the exceptional Jordan algebra is part of an infinite family of finite-dimensional matrix algebras (corresponding to a particular class of cubic Vinberg's T-algebras). Correspondingly, we prove that e8 is part of an infinite family of algebras (dubbed "Magic Star" algebras) that resemble lattice vertex algebras.

hep-th↗

arXiv2017

Proper motions in the VVV Survey: Results for more than 15 million stars across NGC 6544

Context: In the last six years, the VVV survey mapped 562 sq. deg. across the bulge and southern disk of the Galaxy. However, a detailed study of these regions, which includes $\sim 36$ 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: Photometry over the six years baseline of the survey allowed us to obtain a mean precision of $\sim0.51$ mas/yr, 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 $\sim$ 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. Our results show that VVV data is perfectly suitable for this kind of analysis.

astro-ph.GA↗

arXiv2017

Stellar Populations in a semi-analytic model I: bulges of Milky Way-like galaxies

We study the stellar populations of bulges of Milky Way-like (MW-like) galaxies with the aim of identifying the physical processes involved in the formation of the bulge of our Galaxy. We use the semi-analytic model of galaxy formation and evolution SAG adapted to this aim; this kind of models can trace the properties of galaxies and their components like stellar discs, bulges and halos, but resolution limits prevent them from reaching the scale of stellar populations (SPs). Properties of groups of stars formed during single star formation events are stored and tracked in the model and results are compared with observations of stars in the galactic bulge. MW-like galaxies are selected using two different criteria. One of them considers intrinsic photo-metric properties and the second is focused on the cosmological context of the local group of galaxies (LG). We compare our model results with spectroscopic and photometric stellar metallicity distributions. We find that 87% of stars in bulges of MWtype galaxies in our model are accreted and formed in starbursts during disc instability events. Mergers contribute to 13% of the mass budget of the bulge and are responsible for the low metallicity tail of the distribution. Abundance ratios of α elements with respect to iron, [α/Fe], are measured in SPs of model galaxies. The patterns found in the model for SPs with different origins help to explain the lack of a gradient of [α/Fe] ratios in observed stars along the minor axis of the bulge.

astro-ph.GA↗

arXiv2017

Galactic Bulge Population II Cepheids in the VVV Survey: Period-Luminosity Relations and a Distance to the Galactic Center

We present the near-infrared observations of population II Cepheids in the Galactic bulge from VVV survey. We identify 340 population II Cepheids in the Galactic bulge from VVV survey based on their match with OGLE-III Catalogue. The single-epoch $JH$ and multi-epoch $K_s$ observations complement the accurate periods and optical $(VI)$ mean-magnitudes from OGLE. The sample consisting of BL Herculis and W Virginis subtypes is used to derive period-luminosity relations after correcting mean-magnitudes for the extinction. Our $K_s$-band period-luminosity relation, $K_s = -2.189(0.056)~[\log(P) - 1] + 11.187(0.032)$, is consistent with published work for BL Herculis and W Virginis variables in the Large Magellanic Cloud. We present a combined OGLE-III and VVV catalogue with periods, classification, mean magnitudes and extinction for 264 Galactic bulge population II Cepheids having good-quality $K_s$-band light curves. The absolute magnitudes for population II Cepheids and RR Lyraes calibrated using Gaia and Hubble Space Telescope parallaxes, together with calibrated magnitudes for Large Magellanic Cloud population II Cepheids, are used to obtain a distance to the Galactic center, $R_0=8.34\pm0.03{\mathrm{(stat.)}}\pm0.41{\mathrm{(syst.)}}$, which changes by $^{+ 0.05}_{-0.25}$ with different extinction laws. While noting the limitation of small number statistics, we find that the present sample of population II Cepheids in the Galactic bulge shows a nearly spheroidal spatial distribution, similar to metal-poor RR Lyrae variables. We do not find evidence of the inclined bar as traced by the metal-rich red-clump stars. The number density for population II Cepheids is more limited as compared to abundant RR Lyraes but they are bright and exhibit a wide range in period that provides a robust period-luminosity relation for an accurate estimate of the distance to the Galactic center.

astro-ph.SR↗

arXiv2017

Microlensing constraints on the mass of single stars from HST astrometric measurements

We report on the first results from a large-scale observing campaign aiming to use astrometric microlensing to detect and place limits on the mass of single objects, including stellar remnants. We used the Hubble Space Telescope to monitor stars near the Galactic Center for 3 years, and we measured the brightness and positions of $\sim$2 million stars at each observing epoch. In addition to this, we monitored the same pointings using the VIMOS imager on the Very Large Telescope. The stars we monitored include several bright microlensing events observed from the ground by the OGLE collaboration. In this paper, we present the analysis of our photometric and astrometric measurements for 6 of these events, and derive mass constraints for the lens in each of these. Although these constraints are limited by the photometric precision of ground-based data, and our ability to determine the lens distance, we were able to constrain the size of the Einstein ring radius thanks to our precise astrometric measurements, the first routine measurements of this type from a large-scale observing program. This demonstrates the power of astrometric microlensing as a tool to constrain the masses of stars, stellar remnants, and, in the future, of extrasolar planets, using precise ground- and space-based observations.

astro-ph.SR↗

arXiv2017

The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations

Abridged: We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge chemistry, spatial distribution, kinematics, and to compare it chemically with the thin and thick disks. The sample consist on ~2500 red clump stars in 11 bulge fields ($-10^\circ\leq l\leq+8^\circ$ and $-10^\circ\leq b\leq-4^\circ$), and a set of ~6300 disk stars selected for comparison. The bulge MDF is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. Metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. With similar Mg-enhancement levels, the position of the metal-poor bulge sequence "knee" is observed at [Fe/H]$_{knee}=-0.37\pm0.09$, being 0.06 dex higher than that of the thick disk. It suggests a higher SFR for the bulge than for the thick disk. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast ($<1$ Gyr) intense burst of stellar formation at early epochs. We associate metal-rich stars with the B/P bulge formed from the secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and $α$-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.

astro-ph.GA↗

arXiv2017

The Gaia-ESO Survey: low-alpha element stars in the Galactic Bulge

We take advantage of the Gaia-ESO Survey iDR4 bulge data to search for abundance anomalies that could shed light on the composite nature of the Milky Way bulge. The alpha-elements (Mg, Si, and whenever available, Ca) abundances, and their trends with Fe abundances have been analysed for a total of 776 bulge stars. In addition, the aluminum abundances and their ratio to Fe and Mg have also been examined. Our analysis reveals the existence of low-alpha element abundance stars with respect to the standard bulge sequence in the [alpha/Fe] vs. [Fe/H] plane. 18 objects present deviations in [alpha/Fe] ranging from 2.1 to 5.3 sigma with respect to the median standard value. Those stars do not show Mg-Al anti-correlation patterns. Incidentally, this sign of the existence of multiple stellar populations is reported firmly for the first time for the bulge globular cluster NGC 6522. The identified low-alpha abundance stars have chemical patterns compatible with those of the thin disc. Their link with massive dwarf galaxies accretion seems unlikely, as larger deviations in alpha abundance and Al would be expected. The vision of a bulge composite nature and a complex formation process is reinforced by our results. The used approach, a multi-method and model-driven analysis of high resolution data seems crucial to reveal this complexity.

astro-ph.GA↗

arXiv2016

Abundances of disk and bulge giants from hi-res optical spectra: II. O, Mg, Ca, and Ti in the bulge sample

Determining elemental abundances of bulge stars can, via chemical evolution modeling, help to understand the formation and evolution of the bulge. Recently there have been claims both for and against the bulge having a different [$α$/Fe] vs. [Fe/H]-trend as compared to the local thick disk possibly meaning a faster, or at least different, formation time scale of the bulge as compared to the local thick disk. We aim to determine the abundances of oxygen, magnesium, calcium, and titanium in a sample of 46 bulge K-giants, 35 of which have been analyzed for oxygen and magnesium in previous works, and compare them to homogeneously determined elemental abundances of a local disk sample of 291 K-giants. We use spectral synthesis to determine both the stellar parameters as well as the elemental abundances of the bulge stars analyzed here. The method is exactly the same as was used for analyzing the comparison sample of 291 local K-giants in Paper I of this series. Compared to the previous analysis of the 35 stars in our sample, we find lower [Mg/Fe] for [Fe/H]>-0.5, and therefore contradict the conclusion about a declining [O/Mg] for increasing [Fe/H]. We instead see a constant [O/Mg] over all the observed [Fe/H] in the bulge. Furthermore, we find no evidence for a different behavior of the alpha-iron trends in the bulge as compared to the local thick disk from our two samples.

astro-ph.GA↗

arXiv2016

The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields

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 line of sights. 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=-2 and b=-1, that were not explored before. This paper includes new metallicities from a sample of about 5000 K giant stars, observed at spectral resolution R=6500, in the Calcium II Triplet region. They are the main dataset of the GIRAFFE Inner Bulge Survey. As part of the same survey we have previously published results for a sample of about 600 K giant stars, at latitude b=-4 , 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 the 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=-8.5), but it has a steeper gradient with latitude, becoming higher than the metal poor one in the innermost field (b=-1). [abridged]

astro-ph.GA↗

arXiv2016

Twin Supergravities from Yang-Mills Squared

We consider `twin supergravities' - pairs of supergravities with $\mathcal{N}_+$ and $\mathcal{N}_-$ supersymmetries, $\mathcal{N}_+>\mathcal{N}_-$, with identical bosonic sectors - in the context of tensoring super Yang-Mills multiplets. It is demonstrated that the pairs of twin supergravity theories are related through their left and right super Yang-Mills factors. This procedure generates new theories from old. In particular, the matter coupled $\mathcal{N}_-$ twins in $D=3,5,6$ and the $\mathcal{N}_-=1$ twins in $D=4$ have not, as far as we are aware, been obtained previously using the double-copy construction, adding to the growing list of double-copy constructible theories. The use of fundamental matter multiplets in the double-copy construction leads us to introduce a bi-fundamental scalar that couples to the well-known bi-adjoint scalar field. It is also shown that certain matter coupled supergravities admit more than one factorisation into left and right super Yang-Mills-matter theories.

hep-th↗

arXiv2016

Discovery of RR Lyrae Stars in the Nuclear Bulge of the Milky Way

Galactic nuclei, like the one of the Milky Way, are extreme places with high stellar densities and, in most cases, hosting a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is by merging of primordial globular clusters (Capuzzo-Dolcetta 1993). An implication of this model is that this region should host stars characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae ab-type stars in the vicinity of the Galactic center, i.e. in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to building-up the high stellar density in the nuclear stellar bulge of the Milky Way.

astro-ph.GA↗