We present a measurement of metallicity in the Galactic center Quintuplet Cluster made using quantitative spectral analysis of two Luminous Blue Variables (LBVs). The analysis employs line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared spectra containing lines of H, HeI, SiII, MgII, and FeII. We are able to break the H/He ratio vs. mass-loss rate degeneracy found in other LBVs and to obtain robust estimates of the He content of both objects. Our results indicate solar iron abundance and roughly twice solar abundance in the alpha-elements. These results are discussed within the framework of recent measurements of oxygen and carbon composition in the nearby Arches Cluster and iron abundances in red giants and supergiants within the central 30 pc of the Galaxy. The relatively large enrichment of alpha-elements with respect to iron is consistent with a history of more nucleosynthesis in high mass stars than the Galactic disk.
VLA images of the Sickle and Pistol H II regions near the Galactic center at 3.6 and 6 cm reveal six point sources in the region where the dense Quintuplet stellar cluster is located. The spectral indices of five of these sources between 6 cm and 3.6 cm have values of alpha = +0.5 to +0.8, consistent with the interpretation that the radio sources correspond to ionized stellar winds of the massive stars in this cluster. The radio source associated with the Pistol Star shows alpha = -0.4 +/- 0.2, consistent with a flat or slightly non-thermal spectrum.
We present initial results of a spectroscopic study of the Pistol and of the cocoon stars in the Quintuplet Cluster. From ISOCAM CVF 5--17 micron spectroscopy of the field of the Pistol Star, we have discovered a nearly spherical shell of hot dust surrounding this star, a probable LBV. This shell is most prominent at lambda > 12 micron, and its morphology clearly indicates that the shell is stellar ejecta. Emission line images show that most of the ionised material is along the northern border of this shell, and its morphology is very similar to that of the Pistol HII region (Yusef-Zadeh & Mor…
The Galactic center (GC) provides a unique laboratory for a detailed examination of the interplay between massive star formation and the nuclear environment of our Galaxy. Here, we present an 100 ks Chandra ACIS observation of the Arches and Quintuplet star clusters. We also report on a complementary mapping of the dense molecular gas near the Arches cluster made with the Owens Valley Millimeter Array. The number-flux relation of point-like X-ray sources indicates an over-population of relatively bright X-ray sources, which are apparently associated with the clusters. The sources in the core o…
Three very massive clusters are known to reside in the Galactic Center region, the Arches cluster, the Quintuplet cluster and the Central parsec cluster. We obtained spectroscopic observations of the Quintuplet cluster with the Integral Field Spectrograph SINFONI-SPIFFI at the ESO-VLT. The spectral range comprises the near-IR K-band from 1.94 to 2.45 micrometer. The 3D data cubes of the individual fields were flux-calibrated and combined to one contiguous cube, from which the spectra of all detectable point sources were extracted. We present a catalog of 160 stellar sources in the inner part o…
A multi-frequency, multi-configuration study has been made of the compact radio sources in the Galactic Center Quintuplet and Arches stellar clusters using the Very Large Array. Ten radio sources have been detected in the Quintuplet cluster. The majority of these radio sources have rising spectral indices and are positionally coincident with young massive stars that are known to have powerful stellar winds. We conclude that the three most compact of these sources are produced by stellar wind emission; thus, mass-loss rates can be derived and have an average value of 3 x 10^-5 solar masses/year…
The three young clusters in the Galactic Center represent the closest examples of massive starbursts and they define the upper mass limit of the Galactic cluster mass functions. In this review, I describe the characteristics and content of the Arches, Quintuplet, and Central clusters. They each are more massive than any other cluster in the Galaxy, and the Arches cluster, in particular, has a mass and age that make it ideal for studies of massive stellar evolution and dense stellar systems. A preliminary measurement indicates that the initial mass function in the Galactic center is top-heavy, …
H and K band moderate resolution and 4 $μ$m high resolution spectra have been obtained for FMM#362, a bright star in the Quintuplet Cluster near the Galactic Center. The spectral features in these bands closely match those of the Pistol Star, a luminous blue variable and one of the most luminous stars known. The new spectra and previously-obtained photometry imply a very high luminosity for FMM#362, L $\geq 10^6$ \Lsun, and a temperature of 10,000 - 13,000 K. Based on its luminosity, temperature, photometric variability, and similarities to the Pistol Star, we conclude that FMM#362 is a lumino…
We report the first detection of X-ray emission from the Quintuplet star cluster and compare its X-ray emission to that of the Arches star cluster. Four point sources are significantly detected near the core of the Quintuplet cluster with a total, absorption-corrected luminosity of ~1\times10^{33} ergs s^{-1}. Diffuse, thermal emission is also detected near the core of the Quintuplet cluster with an absorption-corrected luminosity of ~1\times10^{34} ergs s^{-1}. We analyze the diffuse and point-like emission from the Arches and Quintuplet and discuss the possibility that they are host to clust…
We consider here the collective accretion of gas by globular clusters and dwarf galaxies moving through the interstellar medium. In the limit of high velocity and/or sound speed of the ISM, the collective potential of the cluster is insufficient to accrete significant amounts of gas, and stars within the systems accrete gas individually. We show, however, that when the sound speed or the relative velocity of the ambient medium is less than the central velocity dispersion of the cluster, it is accreted into the collective potential of the cluster prior to being accreted onto the individual star…
Aliphatic hydrocarbons exhibit an absorption feature at 3.4 micron observed toward sources that sample diffuse regions of the interstellar medium. The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 micron hydrocarbon feature can be used to test the silicate core-organic refractory…
The origin and initial mass function of young stellar clusters near the Galactic center are still poorly understood. Two of the more prominent ones, the Arches and Quintuplet clusters, may have formed from a shock-induced burst of star formation, given their similar age and proximity to each other. Their unusual mass distribution, however, may be evidence of a contributing role played by other factors, such as stellar capture from regions outside the clusters themselves. Diffuse X-ray emission from these sources provides us with a valuable, albeit indirect, measure of the stellar mass-loss rat…
The Galactic center harbors some of the most massive star clusters known in the Galaxy: the Arches and the Quintuplet. Based on the Chandra observations of these clusters (PI: Wang) which recently became public, I discuss the X-ray emission from the massive stars in these clusters. Confirming the general trend for Wolf-Rayet (WR) stars being X-ray dim, none of them is detected in the Quintuplet cluster. The most massive star known in the Galaxy, the Pistol star, is also not detected, invoking questions regarding the proposed binary nature of this object. X-ray emission in the Arches cluster is…
We study the evolution and observability of young and compact star clusters near the Galactic center, such as the Arches cluster and the Quintuplet. The star clusters are modeled with a combination of techniques; using direct N-body, integration to calculate the motions of all stars and detailed stellar and binary evolution to follow the evolution of the stars. The modeled star clusters dissolve within 10 to 60 million years in the tidal field of the Galaxy. The projected stellar density in the modeled clusters drops within 5% to 70% of the lifetime to a level comparable to the projected backg…
We report Hubble Space Telescope (HST) Near-infrared Camera and Multi-object Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two extraordinary young clusters near the Galactic Center. For the first time, we have identified main sequence stars in the Galactic Center with initial masses well below 10 Msun. We present the first determination of the initial mass function (IMF) for any population in the Galactic Center, finding an IMF slope which is significantly more positive (Gamma approx -0.65) than the average for young clusters elsewhere in the Galaxy (Gamma approx -1…
We present Spitzer IRS spectra (R ~600, 10 - 38 micron) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude and spanning plus/minus 0.3 degrees in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the ``Bubble'' lying along the same line-of-sight as the molecular cloud G0.11-0.11, and the diffuse interstellar gas along the line-of-sight at higher Galactic latitudes. From measurements of the [O IV], [Ne II], [Ne III], [Si II], [S III], [S IV], [Fe II], [Fe III], and H_2 S(0), S(1), and S(2) lines we determi…
(Short version) We present an analysis of selected fine structure lines observed ISO toward the Radio Arc in the Galactic center region (GCR). We study the density of the ionized gas and the large scale ionization structure in a region of ~30x30 pc^2 by means of the NeIII 15.6 mum/NeII 12.8 mum and NIII 57 mum/NII 122 mum line ratios. The electron densities (n_e) derived from the OIII lines ratio indicate the presence of diffuse ionized material with n_e of \~10^{1.8-2.6} cm^{-3}. The NIII/NII ratio show two clear gradients, one pointing toward the Quintuplet cluster and the other pointing tow…
We report new results of a campaign to find Wolf-Rayet and O (WR/O) stars and high-mass X-ray binaries (HMXBs) in the Galactic center. We searched for candidates by cross-correlating the 2MASS catalog with a deep Chandra catalog of X-ray point sources in the Radio Arches region. Following up with K-band spectroscopy, we found two massive stellar counterparts to CXOGC J174555.3-285126 and CXOGC J174617.0-285131, which we classify as a broad-lined WR star of sub-type WN6b and an O Ia supergiant, respectively. Their X-ray properties are most consistent with those of known colliding-wind binaries …
About 2 million seconds of Chandra observing time have been devoted to the Galactic center (GC), including large-scale surveys and deep pointings. These observations have led to the detection of about 4000 discrete X-ray sources and the mapping of diffuse X-ray emission in various energy bands. In this review, I first summarize general results from recent studies and then present close-up views of the three massive star clusters (Arches, Quintuplet, and GC) and their interplay with the Galactic nuclear environment.
We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identifie…
We present Infrared Space Observatory (ISO) observations of fine structure lines toward the Radio Arc region, close to Sgr A, in the Galactic center region (GCR). The ionization at large scale is dominated by the clusters of massive stars in this region: the Quintuplet and the Arches cluster. Any other possible mechanism as interaction with the magnetic fields revealed by the Non-Thermal Filaments (NTFs) must play a minor role. The radiation in this area is relatively hard ($T_\mathrm{eff}\sim 33-35$ kK) but diluted. It reaches large distances due to the inhomogeneity of the medium.
Stars are most likely to merge or collide in regions of the highest stellar densities, and our own Galactic Center contains many stars packed into a relatively small volume -- even the ambient stellar number density in the central 50 pc is quite high, ~10^3 stars pc^{-3}. More striking, the three compact young clusters in this region have central densities as high as 10^6 stars pc^{-3}. We discuss these extreme environments and the possibility that stellar mergers and collisions have recently occured there. In particular, we predict that at least one massive star in the Arches cluster has alre…
We are using full spectrum fitting to determine the ages and metallicities of Galactic clusters (M67 and globular clusters). We find that the method is very accurate to measure the metallicity. Blue horizontal branches are well identified as a 'young' sub-component, and the age of the 'old' component is in fair agreement with CMD determinations.
We study the evolution and observability of young compact star clusters within about 200pc of the Galactic center. Calculations are performed using direct N-body integration on the GRAPE-4, including the effects of both stellar and binary evolution and the external influence of the Galaxy. The results of these detailed calculations are used to calibrate a simplified model applicable over a wider range of cluster initial conditions. We find that clusters within 200 pc from the Galactic center dissolve within about 70 Myr. However, their projected densities drop below the background density in t…
We discuss the existence of a mass-metallicity relation among galactic halo globular clusters. The lack of any luminosity-metallicity correlation in globular cluster systems has been used as an argument against self-enrichment models of cluster formation. We show that such a relation is statistically present among the galactic Old Halo globulars. This observational correlation implies that the least massive old clusters are the most metal-rich. This is in contradiction with the idea that, if globular clusters were self-enriched systems, the most metal-rich clusters would also be the most massi…
In this contribution we report our recent investigation of the gas metallicity in active galactic nuclei and its dependence on luminosity and redshift. We compile large spectroscopic datasets of broad-line and narrow-line AGNs, and compare them with the results of our photoionization models. Through the analysis of both the broad and the narrow emission-line regions, we find that: (1) for a given luminosity, there is no redshift dependence of the gas metallicity; (2) for a given redshift, there is a significant correlation between gas metallicity and luminosity; (3) the luminosity-metallicity …
We have compiled two new Galactic open cluster catalogues. The first one has 119 objects with age, distance and metallicity data, while the second one has 144 objects with both absolute proper motion and radial velocity data, of which 45 clusters also with metallicity data available. An iron radial gradient of about $-0.063\pm$0.008 \dexkpc is obtained from our sample. By dividing clusters into age groups, we show that iron gradient was steeper in the past. A disk age-metallicity relation could very probably exist based on the current sample.
The photometric estimate of the metallicity and the age of the Azzopardi et al. (1991, A&AS 88, 265) carbon stars (Ng 1997, A&A 328, 211) is revised to respectively Z~0.004 and ~0.1 Gyr. Under the hypothesis that the carbon stars are located at a distance related to the Sagittarius dwarf galaxy, the broad velocity dispersion of the stars can only be explained if they were formed out of Galactic material during a recent crossing of the Sagittarius dwarf galaxy through the Galactic plane.
We present iron abundance measurements, based on high resolution spectroscopy, and accurate distance determinations, based on near infrared photometry, for 34 Galactic Cepheids. The new data are used to constrain the Galactic iron abundance gradient in the outer disk, namely from 10 to 14 kpc. We confirm the flattening of the gradient toward the outer disk. In this region we also found an increase in the metallicity dispersion. Current data do not support the occurrence of a jump in the metallicity gradient for Galactocentric distances of the order of 10-12 kpc.
Two independent groups (Giveon et al. 2002; Martin-Hernandez et al. 2002) have recently investigated the Galactic metallicity gradient as probed by ISO observations of mid-infrared emission lines from HII regions. We show that the different gradients inferred by the two groups are due to differing source selection and differing extinction corrections. We show that both data sets in fact provide consistent results if identical assumptions are made in the analysis. We present a consistent set of gradients in which we account for extinction and variation in electron temperature across the disk.
We compare our compilation of the W' calcium index for 71 Galactic globular clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992, private communication) model atmospheres. We find our calcium ranking to be tightly correlated with each comparison set, in a non-linear and a linear fashion, respectively. By combining our calcium index information with the Zinn and West ranking, we are able to rank the globular clusters in our sample with a typical precisi…
In this paper we have compiled two new open cluster catalogues. In the first one, there are 119 objects with ages, distances and metallicities available, while in the second one, 144 objects have both absolute proper motion and radial velocity data, of which 45 clusters also with metallicity data available. Taking advantages of the large number of objects included in our sample, we present an iron radial gradient of about -0.063$\pm$0.008 dex/kpc from the first sample. By dividing clusters into age groups, we show that iron gradient was steeper in the past, which is consistent with the recent …
We present an abundance analysis based on high dispersion and high signal-to-noise ratio Keck spectra of a very highly microlensed Galactic bulge dwarf, OGLE-2007-BLG-349S, with Teff ~ 5400 K. The amplification at the time the spectra were taken ranged from 350 to 450. This bulge star is highly enhanced in metallicity with [Fe/H] = +0.51 \pm 0.09 dex. The abundance ratios for the 28 species of 26 elements for which features could be detected in the spectra are solar. In particular, there is no evidence for enhancement of any of the alpha-elements including O and Mg. We conclude that the high […
Nearly 400 Tycho-2 stars have been observed in a 720 square degree field in the direction of the North Galactic Pole with the high resolution echelle spectrograph ELODIE. Absolute magnitudes, effective temperatures, gravities and metallicities have been estimated, as well as distances and 3D velocities. Most of these stars are clump giants and span typical distances from 200pc to 800pc to the galactic mid-plane. This new sample, free of any kinematical and metallicity bias, is used to investigate the vertical distribution of disk stars. The old thin disk and thick disk populations are deconvol…
We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 by 0.8 degree field around the Galactic center. We increase the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities >4e32 erg/s (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A*. The posit…
New ages are computed for stars in the Solar Neighbourhood from the Edvardsson et al. (1993) data set. Distances derived from the Hipparcos parallaxes were adopted to obtain reliable ages (uncertainty less than 12%) for a subset of stars. There is no apparent age-metallicity relation for stars with an age less than 10 Gyr. Only if we consider older stars a slope of ~0.07 dex/Gyr appears. This relation is compared with those obtained from other methods, i.e. galactic open clusters, stellar population synthesis (star counts), and chemical evolution models.
We present mid-infrared ISO-SWS observations of the fine-structure emissions lines [NeII]12.8, [NeIII]15.6, [NeIII]36, [ArII]6.9, [ArIII]8.9, [SIII]18.7, [SIII]33.5, [SIV]10.5, and the recombination lines Br_alpha and Br_beta, in a sample of 112 Galactic HII regions and 37 nearby extra-galactic HII regions in the LMC, SMC, and M33. The Galactic sources have a wide range in galactocentric distance (0<R<18kpc), which enables us to study excitation and metallicity variations over large Galactic scales. We detect a steep rise in the [NeIII]15.6/[NeII]12.8,[ArIII]8.9/[ArII]6.9, and [SIV]10.5/[SIII]…
We present a quantitative spectral analysis of five very massive stars in the Arches cluster, located near the Galactic center, to determine stellar parameters, stellar wind properties and, most importantly, metallicity content. The analysis uses a new technique, presented here for the first time, and uses line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the cluster. It relies on the fact that massive stars reach a maximum nitrogen abundance that is related to initial metallicity when they are …
We present metallicities of 2690 RR0 Lyrae stars observed toward the MACHO Survey fields in the Galactic Bulge. These [Fe/H] values are based upon an empirically calibrated relationship that uses the Fourier coefficients of the light curve and are accurate to ~0.2 dex. The majority of the RR0 Lyrae stars in our sample are located in the Galactic Bulge, but 255 RR0 stars are associated with Sagittarius dwarf galaxy. We find that the RR0 Lyrae stars that belong to the Galactic Bulge have average metallicities [Fe/H] = -1.25, with a broad metallicity range from [Fe/H] = -2.26 to -0.15. The RR0 st…
The mere location of a Beat Cepheid model in a Period Ratio vs. Period diagram (Petersen diagram) puts very tight constraints on its metallicity Z. The Beat Cepheid Peterson diagrams are revisited with linear nonadiabatic turbulent convective models, and their accuracy as a probe for stellar metallicity is evaluated. They are shown to be largely independent of the helium content Y, and they are also only weakly dependent on the mass-luminosity relation that is used in their construction. However, they are found to show sensitivity to the relative abundances of the elements that are lumped into…
Establishing the reliability with which stellar population parameters can be measured is vital to extragalactic astronomy. Galactic GCs provide an excellent medium in which to test the consistency of Single Stellar Population (SSP) models as they should be our best analogue to a homogeneous (single) stellar population. Here we present age, metallicity and $α$-element abundance measurements for 48 Galactic globular clusters (GCs) as determined from integrated spectra using Lick indices and SSP models from Thomas, Maraston & Korn, Lee & Worthey and Vazdekis et al. By comparing our new measuremen…
The microlensing surveys, such as OGLE or MACHO, have led to the discovery of thousands of RR Lyrae stars in the Galactic Bulge and in the Magellanic Clouds, allowing for detailed investigation of these stars, specially the still mysterious Blazhko phenomenon. Higher incidence rate of Blazhko (BL) variables in the more metal-rich Galactic Bulge than in the LMC, suggests that occurance of Blazhko effect correlates with metallicity (Moskalik and Poretti 2003). To investigate this problem, we calibrate the photometric method of determining the metallicity of the RRab star (Kovacs and Zsoldos 1995…
We explore the self-enrichment hypothesis for globular cluster formation with respect to the star formation aspect. Following this scenario, the massive stars of a first stellar generation chemically enrich the globular progenitor cloud up to Galactic halo metallicities and sweep it into an expanding spherical shell of gas. This paper investigates the ability of this swept proto-globular cloud to become gravitationally unstable and, therefore, to seed the formation of second generation stars which may later on form a globular cluster. We use a simple model based on a linear perturbation theory…
This paper is part of a project to search the inner Milky Way for hidden massive clusters and to address the question of whether our Galaxy still forms clusters similar to the progenitors of the present-day globular clusters. We report high angular resolution deep near-infrared imaging of 21 cluster candidates selected from the catalogues of Bica et al. (2003) and Dutra et al.(2003) in a region around the Galactic Center. These catalogues were created from visual inspection of the 2MASS images. Seven objects appear to be genuine clusters, and for these objects we present estimates of extinctio…
We compare the metallicity distribution of Galactic Halo stars with 3D realizations of hierarchical galaxy formation. Outflows from dwarf galaxies enrich the intergalactic medium inhomogeneously, at a rate depending on the local galaxy density. Consequently, the first stars created in small early-forming galaxies are less metal-rich that the first stars formed in more massive galaxies which typically form later. As most halo stars are likely to originate in accreted dwarfs, while disk stars formed out of outflow-enriched gas, this scenario naturally generates a ``metallicity floor'' for old di…
We estimated the Galactic model parameters from the SDSS data reduced for two fields in the anti-centre direction of the Galaxy, $l= 180^{o}$, and symmetric relative to the Galactic plane, $b=+45^{o}$ and $b=-45^{o}$. The large size of each field, 60 deg$^{2}$, and the faint limiting apparent magnitude, g=22, give us the chance to determine reliable parameters for three components, thin and thick discs and halo, in the north and south hemispheres of the Galaxy, except the scalelengths for two discs which are adopted from Jurić et al. (2005). Metallicities were evaluated by a recent calibration…
A comparison is made between the age-metallicity relation obtained from four different types of studies: F and G stars in the Solar Neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique, and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be -0.09 dex/kpc and to be most likely constant in time. We do not correct for the vertical gradient, since its existence and value are not firmly established. Stars and clusters trace a similar age-metallicity relati…
We present K-band spectra of more than 110 M giants in Galactic bulge fields interior to -4 degrees and as close as 0.2 degrees of the Galactic Center. From the equivalent widths of three features in these spectra, EW(Na),EW(Ca), and EW(CO) we calculate [Fe/H] for the stars with a calibration derived from globular clusters Stephens et al (2000). The mean [Fe/H] for each field is in good agreement with the results from Frogel et al. (1999) based on the slope of the giant branch method. We find no evidence for a metallicity gradient along the minor or major axes of the inner bulge (R < 0.6 kpc).…
We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to $\sim 0.15 M_\odot$. This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a dis…
A sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation (AMR) in the Galactic thick disk. The results indicate that that there is indeed an age-metallicity relation present in the thick disk. By dividing the stellar sample into sub-groups, separated by 0.1 dex in metallicity, we show that the median age decreases by about 5-7 Gyr when going from [Fe/H]=-0.8 to [Fe/H]=-0.1. Combining our results with our newly published alpha-element trends for a local sample of thick disk stars, that show signatures from supernovae type Ia (SNIa), we ca…
Results concerning the dependence of photometric and radial velocity amplitudes on metallicity are presented based on about 200 Galactic classical Cepheids pulsating in the fundamental mode. The Galactic distribution of the [Fe/H] value of Cepheids is also studied. We show that the photometric amplitude ratio $A_\mathrm{I}/A_\mathrm{V}$ is independent of metallicity. The observed dependence of this ratio on the pulsation period does not correspond to the theoretical predictions.
Motivated by the recently improved knowledge on the kinematic and chemical properties of the Galactic metal-poor stars, we present the numerical simulation for the formation of the Galactic stellar halo to interpret the observational results. As a model for the Galaxy contraction, we adopt the currently standard theory of galaxy formation based on the hierarchical assembly of the cold dark matter fluctuations. We find, for the simulated stars with [Fe/H]$\le-1.0$, that there is no strong correlation between metal abundances and orbital eccentricities, in good agreement with the observations. M…
In this work, we study the Luminosity-Metallicity relation (LMR) and the Stellar Mass -Metallicity relation (MMR) of galactic systems in a hierarhical clustering scenario. We performed numerical hydrodynamical simulations with the chemical GADGET-2 of Scannapieco et al.(2005) in a LCDM universe. We found that our simulated galactic systems reproduce the observed local LMR and its evolution in zero point and slope. The simulated MMR is also in agreement with recent observational results. From the analysis of the evolution of the MMR, we found a characteristic mass at ~10^(10.2) M_sun / h which …
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios whic…
To study the effects of galactic winds on the stellar metallicity distributions and on the evolution of Draco and Ursa Minor dwarf spheroidal galaxies, we compared the predictions of several chemical evolution models, adopting different prescriptions for the galactic winds, with the photometrically-derived stellar metallicity distributions of both galaxies. The chemical evolution models for Draco and Ursa Minor, which are able to reproduce several observational features of these two galaxies, such as the several abundance ratios, take up-to-date nucleosynthesis into account for intermediate-ma…
We study the evolution and observability of young, compact star clusters near the Galactic center, such as the Arches and Quintuplet systems. The clusters are modeled by integrating the equations of motion of all stars while accounting for the internal evolution of stars and binaries, as well as the effect of the Galactic tidal field. We find that clusters within 150 pc of the Galactic center dissolve within ~55 Myr, but their projected densities drop below the background density in the direction of the Galactic center within only a few Myr, effectively making these clusters undetectable after…
We present an integrated study of star formation and galactic stellar mass assembly from z=0.05-1.5 and galactic metallicity evolution from z=0.05-0.9 using a very large and highly spectroscopically complete sample selected by rest-frame NIR bolometric flux in the GOODS-N. We assume a Salpeter IMF and fit Bruzual & Charlot (2003) models to compute the galactic stellar masses and extinctions. We determine the expected formed stellar mass density growth rates produced by star formation and compare them with the growth rates measured from the formed stellar mass functions by mass interval. We sho…
The purpose of this study is to carry out a thorough investigation of the changes in morphology of the red giant branch (RGB) of Galactic globular clusters (GGC) as a function of metallicity, in the V,I bands. To this aim, two key points are developed in the course of the analysis. (a) Using our photometric V,I database for Galactic globular clusters (the largest homogeneous data sample to date; Rosenberg et al. 1999) we measure a complete set of metallicity indices, based on the morphology and position of the red-giant branch. In particular, we provide here the first calibration of the S, DV_…
It is well known that the mere location of a Beat Cepheid model in a Period Ratio \vs Period diagram (Petersen diagram) puts constraints on its metallicity Z. But these bounds are sensitive to the mixture of elements that are lumped into the parameter $Z$. In this short paper we update the previous results that were based on the Grevesse-Noels solar mixture to the recent, revised Asplund, Grevesse & Sauval (2005) (AGS) solar mixture. We also examine the effect of the envelope depth on the accuracy of the computed pulsation periods. We find that for low period Cepheids with high Z the customary…
The Galactic Bulge has long been assumed to be a largely old stellar population. However, some recent studies based on observations with the HST WF/PC-1 and WFPC2 of stars in the Galactic Bulge have concluded that the old population may not make up more then 30% of the total. Other studies using HST/WFPC2 differential studies of `Bulge' globular clusters and field stars have found the bulge to be comparable in age to the Galactic Halo. A complication in all these studies is the presence of a substantial population of stars which mimic a young bulge population, but which may be, and are often a…
Using photometric data from the 2MASS second incremental release point source catalog we constructed Ks vs. (J-Ks) color-magnitude diagrams (CMDs) of Galactic globular clusters (GGCs) for which the JKs photometric data have been made available up to now. On the CMDs of 13 GGCs we identified RGB bump features and derived luminosities of bumps in Ks and in absolute Ks for 11 of them. We reconfirm the relation between abolute Ks magnitude of bump and metallicity [Fe/H]CG97 or [M/H] such that the luminosity of bump becomes brighter as metallicity [Fe/H]CG97 or [M/H] decreases. This result is simil…
