I will review the role of massive stars in galactic evolution both from the nucleosynthesis and energetics point of view. In particular, I will highlight some important observational facts explained by means of massive stars in galaxies of different morphological type: the Milky Way, ellipticals and dwarf spheroidals. I will describe first the time-delay model and its interpretation in terms of abundance ratios in galaxies, then I will discuss the importance of mass loss in massive stars to reproduce the data in the Galactic bulge and disk. I will discuss also how massive stars can be important producers of primary nitrogen if rotation in stellar models is taken into account. Concerning elliptical galaxies, I will show that to reproduce the observed [Mg/Fe] versus Mass relation in these galaxies it is necessary to assume a more important role of massive stars in more massive galaxies and that this can be achieved by means of downsizing in star formation. I will discuss how massive stars are responsible in triggering galactic winds both in ellipticals and dwarf spheroidals. These latter systems show a low overabundance of alpha-elements relative to Fe with respect to Galactic stars of the same [Fe/H]: this is interpreted as due to a slow star formation coupled with very efficient galactic winds. Finally, I will show a comparison between the predicted Type Ib/c rates in galaxies and the observed GRB rate and how we can impose constraints on the mechanism of galaxy formation by studying the GRB rate at high redshift.
We develop a numerical chemical model allowing for radial flows of gas, with the aim to analyse the possible role of gas flows in the chemical evolution of the Galactic Disc. The dynamical effects of the Galactic Bar on the radial gas profile of the Disc are especially addressed.
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebula abundances derived from C II lambda 4267 and C III lambda lambda 1906 + 1909 lines, respectively. These observational yields are assumed in chemical evolution models for the solar vicinity and the Galactic disk. C/O values observed in stars of the solar vicinity and Galactic H II regions are compared with those predicted by chemical evolution models for the Galaxy. I conclude that the C yields derived from permitted lines are in better agreement with the observational cons…
Recent applications of PN to the study of galactic chemical evolution are reviewed, such as PN and stellar populations, abundance gradients, including their space and time variations, determination of the He/H radial gradient and of the helium-to-metals enrichment ratio, and the [O/Fe] x [Fe/H] relation in the solar neighbourhood and in the galactic bulge.
A population number synthesis code follows in detail the evolution of a population of single stars and of close binaries. We use our code to simulate the population of neutron star - neutron star and black hole - neutron star binaries. We then combine our population number synthesis code with a galactic chemical evolutionary model in order to follow the time evolution of the formation and merger rate of these double compact star binaries and the resulting chemical enrichment of r-process elements, over the whole Galactic lifetime. It can be concluded that the neutron star/black hole merger pro…
Adopting a single-zone framework, with accretion of primordial gas on a free-fall timescale, the chemical evolution of the Galactic bulge is calculated, assuming (i) a corresponding rapid timescale for star formation, and (ii) an initial mass function biased towards massive stars. We emphasise here the uncertainties associated with the underlying physics (specifically, stellar nucleosynthesis) and how those uncertainties are manifest in the predicted abundance ratio patterns in the resulting present-day Galactic bulge stellar populations.
A short overview is presented of the role that Low and Intermediate mass stars play in Galactic Chemical Evolution; their action affects key elements and isotopes, like deuterium, He-3, Li-7, carbon and nitrogen, and s-process nuclei. In all those cases, critical uncertainties still remain and are briefly discussed here.
The formalism of the simple model of galactic chemical evolution (GCE) and its main ingredients (stellar properties, initial mass function, star formation rate and gas flows) are presented in this tutorial. It is stressed that GCE is not (yet) an astrophysical theory, but it merely provides a framework in which the large body of data concerning the chemical composition of stars and gas in galaxies may be intrepreted. A few examples illustrating those concepts are provided, through studies of the Solar neighborhood and of the Milky Way's halo.
In this paper we study the impact of nova nucleosynthesis on models for the chemical evolution of the Galaxy. It is found that novae are likely to be the sources of non-negligible fractions of the 7Li and 13C observed in disc stars. Moreover, they might be responsible for the production of important amounts of 17O at late times and probably account for a major fraction of the Galactic 15N.
In the present paper, we introduce a two-component model of the Galactic disk to investigate its chemical evolution. The formation of the thick and thin disks occur in two main accretion episodes with both infall rates to be Gaussian. Both the pre-thin and post-thin scenarios for the formation of the Galactic disk are considered. The best-fitting is obtained through $χ^2$-test between the models and the new observed metallicity distribution function of G dwarfs in the solar neighbourhood (Hou et al 1998). Our results show that post-thin disk scenario for the formation of the Galactic disk shou…
Motivated by the inability of Galactic chemical evolution models to reproduce some of the observed solar neighbourhood distribution of elements (and isotopes) with atomic numbers 6 < Z < 15, we have revisited the relevant stellar and Galactic models as part of an ambitious new program aimed at resolving these long-standing discrepancies. Avoiding the use of (traditional) parametric models for low- and intermediate-mass stellar evolution, we have generated a new, physically self-consistent, suite of stellar models and integrated the nucleosynthetic outputs into GEtool, our semi-analytical galac…
For several decades now, open clusters have been used to study the structure and chemical evolution of the disk of our Galaxy. Due to the fact that their ages and metallicities can be determined with relatively good precision, and since they can be observed even at great distances, they are excellent tracers of the variations in the abundance of heavy chemical elements with age and position in the Galactic disk. In the present work we analyze the star formation history and the chemical evolution of the disk of the Galaxy using numerical simulations of the dynamical evolution of the system of o…
We describe the basic features of a Monte Carlo model specifically designed to follow the inhomogenous chemical evolution of the Galactic halo, taking into account the effects of local enrichment and mixing of the halo gas, and with particular emphasis on elements like Eu produced by r-process nucleosynthesis. We compare our results with spectroscopic data for the chemical composition of metal-poor halo stars and globular clusters like M13, M5, M92, M4, and we infer some constraints on the star formation history of the halo and the rate and mass spectrum of supernovae during the first epoch of…
While the origin of r-process nuclei remains a long-standing mystery, recent spectroscopic studies of extremely metal-poor stars in the Galactic halo strongly suggest that it is associated with core-collapse supernovae. In this article, an overview of the recent theoretical studies of the r-process is presented with a special emphasis on the astrophysical scenarios related to core-collapse supernovae. We also review a recent progress of the Galactic chemical evolution studies as well as of the spectroscopic studies of extremely metal-poor halo stars, which provide us important clues to better …
The role of planetary nebulae as probes for the galactic chemical evolution is reviewed. Their abundances throughout the Galaxy are discussed for key elements, in particular oxygen and other alpha elements. The abundance distribution derived from planetary nebulae leads to the establishment of radial abundance gradients in the galactic disk that are important constraints to model the chemical evolution of the Galaxy. The radial gradient, well determined for the solar neighborhood, is examined for distinct regions. For the galactic anticenter in particular, the observational data confirm result…
A LiBeB evolution model including Galactic Cosmic Ray nucleosynthesis, the $ν$-process, novae, AGB and C-stars is presented. We have included Galactic Cosmic Ray Nucleosynthesis (GCRN) in a complete Chemical Evolution Model that takes into account 76 stable isotopes from hydrogen to zinc. Any successful LiBeB evolution model should also be compatible with other observational constraints like the age-metallicity relation, the G-dwarf distribution or the evolution of other elements. At the same time, we have checked how different would be a model that took into account the last observations by W…
Invited Review at IAU Symp 164 on Stellar Populations. The Milky Way Galaxy offers a unique opportunity for testing theories of galaxy formation and evolution. The study of the spatial distribution, kinematics and chemical abundances of stars in the Milky Way Galaxy allows one to address specific questions pertinent to this meeting such as When was the Galaxy assembled? Is this an ongoing process? What was the merging history of the Milky Way?
In this contribution we focus on results from chemical evolution models for the solar neighbourhood obtained by varying the IMF. Results for galaxies of different morphological type are discussed as well. They argue against a universal IMF independent of star forming conditions.
We provide new nucleosynthesis yields depending on metallicity and energy (i.e., (normal supernovae and hypernovae), and show the evolution of heavy element abundances from C to Zn in the solar neighborhood. We then show the chemodynamical simulation of the Milky Way Galaxy and discuss the G-dwarf problem. We finally show the cosmological simulation and discuss the galaxy formation and chemical enrichment.
We present a stochastic model based on the Monte Carlo technique to study the inhomogeneous chemical evolution of the Galactic halo. In particular, we consider the local enrichment and subsequent mixing of the interstellar gas resulting from bursts of star formation, following explicitly the fragmentation and coalescence of interstellar gas clouds. The model takes into account the mixing of halo gas through cloud-cloud collisions and the delayed mixing of supernova ejecta into the interstellar medium. The consequences of the infall of halo gas onto the Galactic disk are also discussed. The mas…
In this review I focus on a few selected topics, where recent theoretical and/or observational progress has been made and important developments are expected in the future. They include: 1) Evolution of isotopic ratios, 2) Mixing processes and dispersion in abundance ratios, 3) Abundance gradients in the Galactic disk (and abundance patterns in the inner Galaxy), 4) The question of primary Nitrogen and 5) Abundance patterns in extragalactic damped Lyman-alpha systems (DLAs).
The abundance gradients and the radial gas profile of the Galactic disc are analysed by means of a model for the chemical evolution of galaxies. As one of the major uncertainties in models for galaxy evolution is the star formation (SF) process, various SF laws are considered,to assess the response of model predictions to the different assumptions. Only some SF laws are successful in reproducing the metallicity gradient, and only if combined with a suitable infall timescale increasing outward (inside-out formation scenario). Still, it is difficult to reproduce at the same time also the observe…
The recent introduction of high resolution/large spectral range spectrographs provided the opportunity to investigate in detail the chemical composition of classical Cepheids. This paper is focussed on new abundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galactic radial abundance gradient. The stellar effective temperatures have been determined using the method of line depth ratios, while the surface gravity and the microturbulent velocity v$_{t}$ by imposing the ionization balance between Fe I and Fe II an…
Our Galactic Center hosts over 10% of the known massive stars in the Galaxy. The majority of these stars are located in three particularly massive clusters that formed within the past 5 Myr. While these clusters are extraordinary, their formation repesents about half of the total inferred star formation rate in the Galactic Center. There is mounting evidence that the clusters are just present-day examples of the hundreds of such similar clusters that must have been created in the past, and whose stars now comprise the bulk of all stars seen in the region. I discuss the massive stellar content …
In order to resolve the overproduction problem of 11B in supernova explosions during Galactic chemical evolution, the dependence of the ejected masses of the light elements produced through the nu-process in supernova explosions on supernova neutrino parameters is investigated and constraints on the supernova neutrinos are evaluated. Detailed nucleosynthesis in a supernova explosion model corresponding to SN 1987A is calculated by postprocessing. The ejected masses of 11B and 7Li depend strongly on the temperature of mu- and tau-neutrinos and their antiparticles and are roughly proportional to…
We investigate the origin of the abundance variations observed among similarly aged F and G dwarfs in the local Galactic disk. We present arguments in support of combined infall of metal-deficient gas and sequential enrichment by successive stellar generations in the local Galactic disk ISM. We show that galactic chemical evolution models which take into account these processes simultaneously are consistent with both the observed abundance variations among similarly aged dwarfs in the solar neighbourhood and the abundances observed in the local disk ISM. For reasonable choices of parameters, t…
Chemical abundances for 23 candidate subgiant stars have been derived with the aim at exploring their usefulness for studies of galactic chemical evolution. High-resolution spectra from ESO CAT-CES and NOT-SOFIN covered 16 different spectral regions in the visible part of the spectrum. Some 200 different atomic and molecular spectral lines have been used for abundance analysis of about 30 elemental species. The wings of strong, pressure-broadened metal lines were used for determination of stellar surface gravities, which have been compared with gravities derived from Hipparcos parallaxes and i…
We find a new analytical solution for the chemical evolution equations, taking into account the delayed contribution of all low and intermediate mass stars (LIMS) as one representative star that enriches the interstellar medium.This solution is built only for star formation rate proportional to the gas mass in a closed box model. We obtain increasing C/O and N/O ratios with increasing O/H, behavior impossible to match with the Instantaneous Recycling Approximation (IRA). Our results, obtained by two analytical equations, are very similar to those found by numerical models that consider the lif…
Electron temperatures, densities, ionic and elemental abundances of helium, nitrogen, oxygen, argon, sulfur and neon were derived for a sample of bulge planetary nebulae, representative of its intermediate mass population. Using these results as constraints, a model for the chemical evolution of the galactic bulge was developed. The results indicate that the best fit is achieved using a double-infall model, where the first one is a fast collapse of primordial gas and the second is slower and enriched by material ejected by the bulge itself during the first episode.
Deuterium is the best indicator of the baryon density; however, only its present abundance is known (and only locally) and its chemical evolution is intertwined with that of $^3$He. Because galactic abundances are spatially heterogeneous, mean chemical-evolution models are not well suited for extrapolating the pre-solar D and $^3$He abundances to their primeval values. We introduce a new approach which explicitly addresses heterogeneity, and show that the decade-old big-bang nucleosynthesis concordance interval $η\approx (2 -8)\times 10^{-10}$ based on D and $^3$He is robust.
We review the yields of intermediate mass elements (from C to Zn) from massive stars and their associated uncertainties, in the light of recent theoretical results. We consider the role of those yields for our understanding of the chemical evolution of the solar neighbourhood and of the halo of our Galaxy. Current yields reproduce in a satisfactory way the solar system composition, but several problems remain concerning abundance ratios in halo stars.
The Galactic center is a hotbed of star formation activity, containing the most massive star formation site and three of the most massive young star clusters in the Galaxy. Given such a rich environment, it contains more stars with initial masses above 100 \Msun than anywhere else in the Galaxy. This review concerns the young stellar population in the Galactic center, as it relates to massive star formation in the region. The sample includes stars in the three massive stellar clusters, the population of younger stars in the present sites of star formation, the stars surrounding the central bla…
The Galactic Center contains the most massive young clusters in the Galaxy and serves as the closest example of a massive starburst region. Our recent results suggest that the Galactic Center environment produces massive clusters with relatively flat initial mass functions, as might be expected on theoretical grounds. I will discuss these recent results, along with evidence for star formation in the immediate vicinity of the super massive black hole at the Galactic Center. The results of this work might be useful in extrapolating to other galactic centers with similar conditions, as well as ot…
AIMS: We spectroscopically characterize the Galactic Bulge to infer its star formation timescale, compared to the other Galactic components, through the chemical signature on its individual stars. METHODS: We derived iron and oxygen abundances for 50 K giants in four fields towards the Galactic bulge. High resolution (R=45,000) spectra for the target stars were collected with FLAMES-UVES at the VLT. RESULTS: Oxygen, as measured from the forbidden line at 6300 Å, shows a well-defined trend with [Fe/H], with [O/Fe] higher in bulge stars than in thick disk ones, which were known to be more oxygen…
In this paper we report new observations and derive chemical abundances for a sample of 26 planetary nebulae (PN) located in the anticenter direction. Most of these nebulae are far away objects, located at galactocentric distances greater than about 8 kpc, so that they are particularly useful for the determination of the radial gradients at large distances from the galactic center. A comparison of the present results with previously determined abundances suggests that the radial abundance gradients flatten out at distances larger than about 10 kpc from the center.
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, …
We review recent advances in our understanding of massive stars through the analysis of their spectra. Improvements in model atmospheres and analysis methods are briefly discussed. Results obtained for stars in the Local Group are compared and the present status of different open questions, like the temperature scale of OB stars, the Wind Momentum-Luminosity Relation or the stellar rotation, is outlined.
We present preliminary abundance analyses of a group of high-velocity metal-poor stars. This report describes our work in progress. We examine the derived abundances in the context of previous studies of metal-poor stars with unusual abundances and/or extreme galactocentric orbits. Included in this sample are BD+80 245, a star previously known to have unusually low alpha-element abundances, and G4-36, a new low-alpha star.
The high metallicity of the intra-cluster medium (ICM) is generally interpreted on the base of the galactic wind scenario for elliptical galaxies. In this framework, we develop a toy-model to follow the chemical evolution of the ICM, formulated in analogy to chemical models for individual galaxies. Just as the ingredients for usual models are (a) the stellar yields, amount of metals newly synthesized and re-ejected by stars; (b) the Star Formation Rate and (c) the stellar Initial Mass Function (IMF), our model for clusters involves: (a') ``galactic yields'' derived from galactic wind models of…
The primary present-day observables upon which theories of galaxy evolution are based are a system's morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.
Molecular hydrogen clouds can contribute substantially to the galactic halo< dark matter and may lead to the birth of massive halo objects (MHOs) observed indirectly by microlensing. We present a method to detect these molecular clouds in the halo of M31 using the Doppler shift effect. We also consider the possibility to directly observe MHOs in the halo of M31 via their infrared emission.
In this paper we present a study of chemical abundances in six star-forming regions. Stellar parameters and metallicities are derived using high-resolution, high S/N spectra of weak-line T-Tauri stars in each region. The results show that nearby star-forming regions have a very small abundance dispersion (only 0.033dex in [Fe/H]). The average metallicity found is slightly below that of the Sun, although compatible with solar once the errors are taken into account. The derived abundances for Si and Ni show that the observed stars have the abundances typical of Galactic thin disk stars of the sa…
Masses of black holes in nearby galactic nuclei can be measured in a variety of ways, using stellar and gaseous kinematics. Reliable black hole masses are known for several dozen objects, so that demographic questions can start to be addressed with some confidence. Prospects for the near future are discussed briefly.
Halo initial mass functions (IMFs), heavily-biased toward white dwarf (WD) precursors (i.e. 1->8 M_sun), have been suggested as a suitable mechanism for explaining microlensing statistics along the line of sight to the LMC. Such IMFs can apparently be invoked without violating the observed present-day WD luminosity function. By employing a simple chemical evolution argument, we demonstrate that reconciling the observed halo Population II dwarf abundances (i.e. [C,N/O]=-0.5), with that expected from the postulated ``WD-heavy'' IMF (i.e. [C,N/O]=+0.5), is difficult.
We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic Center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H- and K-bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced respectively by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near solar Fe abundances. The Galactic Center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 +- 0.06 agrees well with previous work. The tot…
Using the output from a grid of 60 Type II supernova models (Woosley \& Weaver 1994) of varying mass (11 $\ltaprx$ M/M\sun $\ltaprx$ 40) and metallicity (0, 10$^{-4}$, 0.01, 0.1, and 1 Z\sol), the chemical evolution of 76 stable isotopes, from hydrogen to zinc, is calculated. The chemical evolution calculation employs a simple dynamical model for the Galaxy (infall with a 4 billion year $e$-folding time scale onto a exponential disk and 1/r$^2$ bulge), and standard evolution parameters, such as a Salpeter initial mass function and a quadratic Schmidt star formation rate. The theoretical result…
The massive black hole (MBH) in the Galactic Center and the stars around it form a unique stellar dynamics laboratory for studying how relaxation processes affect the distribution of stars and compact remnants and lead to close interactions between them and the MBH. Recent theoretical studies suggest that processes beyond "minimal" two-body relaxation may operate and even dominate relaxation and its consequences in the Galactic Center. I describe loss-cone refilling by massive perturbers, strong mass segregation and resonant relaxation; review observational evidence that these processes play a…
The birthrate of stars of masses > 10 solar masses is estimated from a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sun and the result extrapolated to estimate the galactic supernova rate contributed by such stars. The solar-neighborhood galactic-plane massive star birthrate is estimated at 176 stars per cubic kpc per million years. Based on a model where the galactic stellar density distribution comprises a disk plus central hole like that of the dust infrared emission (Drimmel & Spergel 2001 ApJ 556, 181) the galactic supernova rate is estimated as probably not less than 1 nor …
Recent models of galactic chemical evolution account for updated evolutionary models of massive stars (with special emphasis on stellar winds) and for the effects of intermediate mass and massive binaries. The results are summarised. We also present a critical discussion on possible effects of stellar rotation on overall galactic chemical evolutionary simulations.
The warm ionized gas in low-mass, metal-poor starforming galaxies is chemically homogeneous despite the prevalence of large H II regions which contain hundreds of evolved massive stars, supernovae, and Wolf-Rayet stars with chemically-enriched winds. Galaxies with large Wolf-Rayet star content are chemically indistinguishable from other vigorously star-forming galaxies. Furthermore, no significant localized chemical fluctuations are present in the vicinity of young star clusters, despite large expected chemical yields of massive stars. An ad-hoc fine-tuning of the release, dispersal and mixing…
We report the discovery of a hitherto undetected population of Wolf-Rayet stars in the young galactic open cluster Westerlund 1. Optical spectroscopy of the cluster identified 11 such objects; provisional classification suggests that 6 are nitrogen rich (WN) and 5 carbon rich (WC). Including the previously identified Blue, Yellow and Red Super- & Hypergiants, Westerlund 1 clearly has a very rich population of massive post-Main Sequence objects. To date, the post-MS population of Westerlund 1 is significantly larger than that of any other galactic young open cluster - with the possible exceptio…
In this review we compare the three existing sets of theoretical yields of zero metal massive stars available in the literature. We also show how each of these three different sets of yields fits the element abundance ratios observed in the extremely metal poor star CD 38245. We find that, at present, no theoretical set of yields of zero metal massive stars is able to satisfactorily reproduce the elemental ratios [X/Fe] of this star.
We investigate the possibility that present-day galactic haloes contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. Many MBHs will not reach the centre of the main halo, however, but continue to orbit within satellite subhaloes. Using a semi-analyt…
We consider the simultaneous chemical, photometric, and gaseous thermal energy evolution of elliptical galaxies. The evolution of chemical abundances in the intracluster medium (ICM) is set by the differing timescales for gas ejection, via supernovae (SNe)-driven winds, from dwarf, normal, and giant ellipticals, and is monitored concurrently. Emphasis is placed upon the influence of, and sensitivity to, the underlying stellar initial mass function (IMF), star formation efficiency, supernovae Type Ia rates, supernovae remnant (SNR) dynamics, and the most recent advances in stellar nucleosynthes…
We present a longslit optical spectrophotometric survey in the irregular "post-starburst" galaxy NGC 1569 to search for chemical gradients and inhomogeneities in the interstellar medium. Despite the presence of two massive evolved starclusters and numerous HII regions, we find no evidence for chemical gradients or inhomogeneities that may be attributed to enrichment from the recent star formation activity. The chemical properties are consistent with 12+log(O/H)=8.19+/-0.04 and log(N/O)=-1.40+/-0.05 at all locations. No localized chemical self-enrichment ("pollution") from massive star evolutio…
The enrichment of Pb in the Galaxy is followed in the framework of a detailed model of Galactic chemical evolution that already proved adequate to reproduce the chemical enrichment of O and of the elements from Ba to Eu. The stellar yields are computed through nucleosynthesis calculations in the Asymptotic Giant Branch (AGB) phase of low- and intermediate-mass stars, covering a wide range of metallicities. The physical parameters of the stellar structure were derived from full stellar evolutionary models previously computed. We show that low-mass AGB stars are the main producers of Pb in the G…
We present results from dynamical Monte Carlo simulations of dense star clusters near the Galactic center. These clusters sink toward the center of the Galaxy by dynamical friction. During their inspiral, they may undergo core collapse and form an intermediate-mass black hole through runaway collisions. Such a cluster can then reach within a parsec of the Galactic center before it completely disrupts, releasing many young stars in this region. This scenario provides a natural explanation for the presence of the young stars observed near the Galactic center. Here we determine the initial condit…
We have carried out abundance analysis for a sample of high galactic latitude supergiants in search of evolved stars. We find that HD 27381 has atmospheric parameters and an abundance pattern very similar to that of the post-AGB star HD 107369. HD 10285 and HD 25291 are moderately metal-poor and show the influence of mixing that has brought the products of NeNa cycle to the surface. The high galactic latitude B supergiant HD 137569 shows selective depletion of refractory elements normally seen in post-AGB stars. We find that the high velocity B type star HD 172324 shows moderate deficiency of …
We present a new model for the chemical evolution of elliptical galaxies taking into account SN feedback, detailed nucleosynthesis and galactic winds. We discuss the effect of galactic winds on the chemical enrichment of the ICM and compute the energy per particle injected by the galaxies into the ICM.
Anticipating forthcoming observations with the Chandra X-ray telescope, we describe the continuation of interstellar cooling flows deep into the cores of elliptical galaxies. Interstellar gas within about r = 50 parsecs from the massive black hole is heated to T > 1 keV and should be visible unless thermal heating is diluted by non-thermal pressure. Since our flows are subsonic near the massive black holes, distributed cooling continues within 300 pc from the center. Dark, low mass stars formed in this region may be responsible for some of the mass attributed to central black holes.
We present chemical abundances in K and M red-giant members of the Galactic bulge derived from high-resolution infrared spectra obtained with the Phoenix spectrograph on Gemini-South. The elements studied are carbon, nitrogen, oxygen, sodium, titanium, and iron. The evolution of C and N abundances in the studied red-giants show that their oxygen abundances represent the original values with which the stars were born. Oxygen is a superior element for probing the timescale of bulge chemical enrichment via [O/Fe] versus [Fe/H]. The [O/Fe]-[Fe/H] relation in the bulge does not follow the disk rela…
