Massive stars in the galactic center

The relatively brief history of infrared observations and quantitative analysis of massive stars in the Galactic Center is reviewed. Current observational and the theoretical status is also reported: A new generation of NLTE wind blanketed models, together with high S/N spectra of the hot star population are allowing one, for the first time, to perform metal abundance determinations (Fe, Si, Mg, Na, etc). Metallicity studies of hot stars in the IR will provide major constraints not only on the theory of evolution of massive stars but also on our efforts to solve the puzzle of the central parsecs of the Galaxy. Preliminary results are presented.     

Harmonizing the RR Lyrae and clump distance scales – stretching the short distance scale to intermediate ranges?

I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighbourhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major assumptions: (i) that the absolute magnitude–metallicity, M _V (RR)–[Fe/H], relation for RR Lyrae stars is universal, and (ii) that absolute I magnitudes of clump giants, M _I (RC), in Baade's Window are known (e.g. can be inferred from the local Hipparcos -based calibration or theoretical modelling). A comparison between the solar neighbourhood and Baade's Window sets M _V (RR) at [Fe/H]=−1.6 in the range (0.59±0.05, 0.70±0.05), somewhat brighter than the statistical parallax solution. More luminous RR Lyrae stars imply younger globular clusters, which would be in better agreement with the conclusions from the currently favoured stellar evolution and cosmological models. A comparison between Baade's Window and the LMC sets M ^LMC(RC) _I in the range (−0.33±0.09,−0.53±0.09). The distance modulus to the LMC is μ ^LMC∈(18.24±0.08,18.44±0.07). Unlike M ^LMC(RC) _I , this range in μ ^LMC does not depend on the adopted value of the dereddened LMC clump magnitude, I ^LMC(RC)₀. I argue that the currently available information is insufficient to select the correct distance scale with high confidence.     

The chemical composition of the Small Magellanic Cloud

The Small Magellanic Cloud is a close, irregular galaxy that has experienced a complex star formation history due to the strong interactions occurred both with the Large Magellanic Cloud and the Galaxy. Despite its importance, the chemical composition of its stellar populations older than ∼ 1–2 Gyr is still poorly investigated. I present the first results of a spectroscopic survey of ∼ 200 Small Magellanic Cloud giant stars performed with FLAMES@VLT. The derived metallicity distribution peaks at [Fe/H] ∼ –0.9/–1.0 dex, with a secondary peak at [Fe/H] ∼ –0.6 dex. All these stars show [α /Fe] abundance ratios that are solar or mildly enhanced (∼+0.1 dex). Also, three metal‐poor stars (with [Fe/H] ∼ –2.5 dex and enhanced [α /Fe] ratios compatible with those of the Galactic Halo) have been detected in the outskirts of the SMC: These giants are the most metal‐poor stars discovered so far in the Magellanic Clouds. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cuspy dark matter haloes and the Galaxy

The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ∼     , assuming the inner Galaxy is barred with viewing angle ∼20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ∼     . We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii     . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy.     

Scale length of the galactic thin disk

This paper presents an analysis of the first 2MASS (The Two Micron All Sky Survey) sampler data as observed at lower Galactic latitude in our Galaxy. These new near-infrared data provide insight into the structure of the thin disk of our Galaxy, The interpretation of star counts and color distributions of stars in the near-infrared with the synthetic stellar population model, gives strong evidence that the Galactic thin disk density scale length,h _R , is rather short (2.7 ± 0.1 kpc).     

The Galactic Center: a laboratory for AGN?

Summary. This paper reviews the physical state of stars and Interstellar Matter in the Galactic Bulge (radius kpc from the dynamical center of the Galaxy), in the Nuclear Bulge (kpc) and in the Sgr A Radio and GMC Complex, i.e. the central \,pc of our Galaxy. The Galactic Bulge is devoid of cold Interstellar Matter and consists mainly of old stars, while the Nuclear Bulge accounts for of the mass of all of the Interstellar Matter in the Galaxy. A similar ratio holds for the formation rate of medium and high mass stars in Bulge and Disk. The metal abundance of the Interstellar Matter in the Galactic Bulge is found to be . The H-to-CO conversion factors to be applied to molecular gas in the Central Region are by factors 3 (Arimoto et al. 1996) to 10 (Sodroski et al. 1995) lower than in the solar vicinity. Hence, most H masses derived for the Central Region appear to be considerably overestimated. The Nuclear Bulge is pervaded by a thermal plasma (K) which is responsible for the diffuse free-free emission. Lyman continuum photon and dust IR luminosity of the Nuclear Bulge again account for of the respective total luminosities of the Galaxy. Magnetic fields in the Nuclear Bulge are strong (up to mG) as compared with the Galactic Disk (a few tens of G). The field lines are oriented parallel to the galactic plane inside giant molecular clouds and perpendicular to the plane in the intercloud medium. The compact source Sgr A* is close to or at the dynamical center of the Galaxy. Its radio spectrum with a high frequency cut-off at GHz, a low frequency turnover at GHz and a flux density dependence in between can be explained by synchrotron emission from quasi-monoenergetic relativistic electrons. Due to an extinction between Sun and Galactic Center corresponding to , an intrinsic weakness of this source in the near infrared, and a strong background emission from warm dust there are only upper limits available for the flux density of Sgr A* in the far, mid and near infrared and X-ray regime. The size of Sgr A* in the radio regime is cm, its dereddened K-band flux density is mJy, its luminosity has upper limits of (if radiation comes from an Accretion Disk) and (if black-body radiation from an object with a single temperature of K is assumed). If anyone of the soft X-ray sources detected by ROSAT actually coincides with Sgr A*, its X-ray luminosity would be less than a few . With a dark mass of Sgr A* is the best candidate for a starving black hole, although there are no observational indications for the presence of a (Standard) Accretion Disk. While the radio/IR spectrum of Sgr A* is purely nonthermal, the spectrum integrated over the central parsec resembles that of a Seyfert galaxy. Sgr A* is embedded in the Hii region Sgr A West with part of the ionized gas forming a minispiral. Sgr A West is surrounded by the Circum Nuclear Disk, an irregular shaped assembly of molecular gas which extends from pc and rotates around the Galactic Center with an estimated dynamical time scale of \,yr. The total luminosity of of the central parsec is due to the radiation of early-type stars of which have now been directly identified as luminous blue supergiants. It is still debated, however, if these stars can also account for all of the ionization of Sgr A West. In addition, the central parsec contains red giants, AGB stars, and a few super giants of which the brightest are now identified by direct imaging. These stars – together with a few million low mass main sequence stars – account for the bulk of the 2.2\,m emission. The spatial distributions of the three stellar populations in the central pc are remarkably different. Sgr A* is – along the line-of-sight – presumably located close to the center of the Hii region Sgr A West, which in turn is located in front of the extended (pc) synchrotron source Sgr A East, which appears to be the remnant of a gigantic explosion (of the order of the energy of a single supernova explosion) which took place yr ago inside the GMC Sgr A East Core. X-ray observations show within pc a pervasive hot (keV) plasma of expansion age of yr. Both phenomena – as well as the formation of the Circum Nuclear Disk – may have the same origin. Influx of material is observed within the Nuclear Bulge on all distance scales. In the Nuclear Bulge (pc) as well as in the Circum Nuclear Disk (pc) inflow towards the Galactic Center occurs primarily in the galactic plane and amounts to a few . The accretion rate into the central Black Hole, deduced from the luminosity of Sgr A*, however, appears to be lower by at least five orders of magnitude (assuming standard disk accretion). But in an equilibrium state only part of the infalling mass which is not accreted by the Black Hole can be consumed by star formation. A mass inflow rate varying with time is a more natural explanation. Comparing the physical state of the Center of our Galaxy with that of Active Galactic Nuclei derived from observations and modelling, we find that most of the basic characteristics of an AGN are also present in the Galactic Center. Lacking are, however, both the evidence for a standard Accretion Disk and a hard UV spectrum with accompanying high excitation emission lines in the Galactic Center which are characteristic for AGN. The luminosity of the central parsec, , amounts to only of the total luminosity of the Galaxy of . Seen from a distance of M31 (kpc) with an angular resolution of (corresponding to a linear size of pc) the Center of our Galaxy would appear as a mildly active nucleus with some starburst activity and would probably be classified as a weak Seyfert galaxy. The synchrotron spectrum of Sgr A*, however, would be completely masked by reprocessed stellar light (i.e. free-free and dust emission). Received: October 21, 1996     

Programme of the Galactic meridian: Star counts and galactic models. 1. Distributions of stellar magnitudes and colours

A comparison of observed stellar distributions with a three-component model of the Galaxy is presented. The analysis is based on photometric and photoelectric data obtained along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The assumed model considers the Galaxy as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. To model the observed colour distribution, we distinguish main sequence stars and disk red giants as the disk subsystem; white dwarfs, subdwarfs and intermediate giants as the thick disk subsystem; extreme subdwarfs, spheroid giants and horizontal branch stars as the spheroid subsystem. A statistical relation between the apparent and absolute magnitudes of stars which make the maximum contribution to the star counts for a given disk subsystem is derived. In order to achieve the best agreement between the model and observations, we fit the values of the ‘dip’ (aw) of the disk luminosity function, the correction to the absolute magnitude of disk red giants (ΔM_V^RG) and the expression for interstellar extinction. As the main result, we obtained aw = 0.6 (logarithmic scale) and ΔM_V^RG = 0.5 mag; the interstellar extinction has to be taken into account by the modified Sandage law.     

A statistical search for supermetallicity in F, G and K stars

High-dispersion and low-resolution data are combined to search for super-metal-rich (SMR) FGK stars in the solar neighbourhood and Baade's Window. The data are assessed by using statistical analysis, with their rms errors playing a key role. A star is considered to be SMR if its value of     , while 'borderline' SMR status may be assigned if     . Borderline SMR status is assigned to μ Leo and three other giants, but no full-fledged SMR giants are found in either Baade's Window or the solar neighbourhood. By contrast, the existence of SMR class     stars turns out to be well established, with values found for [Fe/H] that are as large as ∼+0.4 dex. It is concluded that this apparent contrast between class     stars and giants should not be interpreted in astrophysical terms at present because of marked shortcomings in the available data base for giants. Recommendations are made about future research that may cure this problem and extend present knowledge about SMR dwarfs.     

Recent advances in modeling stellar interiors

Advances in stellar interior modeling are being driven by new data from large-scale surveys and high-precision photometric and spectroscopic observations. Here we focus on single stars in normal evolutionary phases; we will not discuss the many advances in modeling star formation, interacting binaries, supernovae, or neutron stars. We review briefly: (1) updates to input physics of stellar models; (2) progress in two and three-dimensional evolution and hydrodynamic models; (3) insights from oscillation data used to infer stellar interior structure and validate model predictions (asteroseismology). We close by highlighting a few outstanding problems, e.g., the driving mechanisms for hybrid γ Dor/δ Sct star pulsations, the cause of giant eruptions seen in luminous blue variables such as η Car and P Cyg, and the solar abundance problem.     

Chemical evolution with radial mixing

Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and α-elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva–Copenhagen survey (GCS) of the solar neighbourhood and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age–metallicity plane, although this plane was not used in the fitting process. Although this model's star formation rate is monotonically declining, its disc naturally splits into an α-enhanced thick disc and a normal thin disc. In particular, the model's distribution of stars in the ([O/Fe], [Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration, and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high  σ _z   from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of  σ _z   with age than predicted by theory. The metallicity gradient in the interstellar medium is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cut-off in the star formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.     

The Chemical Evolution of the Galactic Bulge

In this paper we review the chemical evolution models for the Galactic bulge: in particular, we discuss the predictions of models as compared with the available abundance data and infer the mechanism as well as the time scale for the formation of the Galactic bulge. We show that good chemical evolution models reproducing the observed metallicity distribution of stars in the bulge predict that the [α/Fe] >0 over most of the metallicity range. This is a very important constraint indicating that the bulge of our Galaxy formed at the same time and even faster than the inner Galactic halo. We also discuss predictions for the evolution of light elements such as D and ⁷Li and conclude that the D astration should be maximum due to the high star formation rate required for the bulge whereas the evolution of the abundance of Li should be similar to that observed in the solar neighbourhood, but with an higher Li abundance in the interstellar medium at the present time. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thin disk

Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad _R[Fe/H] = ?0.05 ± 0.01 kpc^?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad _Z[Fe/H] = ?0.29 ± 0.06 kpc^?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad _Z[Mg/Fe] = 0.13 ± 0.02 kpc^?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood.     

Inhomogeneous Chemical Evolution of the Galaxy in the Solar Neighbourhood

N-body numerical simulations of an inhomogeneous Galactic Chemical Evolution (GCE) of the solar neighbourhood with a high temporal resolution are presented. The solar annular ring is divided into distinct spatial grids of area ~1–2 kpc². Each grid evolves distinctly in terms of star formation and nucleosynthetic yields from numerous generations of stars. The evolution of the galaxy is simulated by considering discrete episodes of star formation. Subsequent to the evolution of the simulated stars within each grid the stellar nucleosynthetic yields are homogenized within the grid rather than the traditionally adopted criteria of homogenizing over the entire solar annular ring. This provides a natural mechanism of generating heterogeneities in the elemental abundance distribution of stars. A complex chemical evolutionary history is inferred that registers episodes of time-dependent contributions from SN II+Ib/c with respect to SN Ia. It was observed that heterogeneities can remerge even after episodes of large scale homogenizations on scales larger than the grid size. However, a comparison of the deduced heterogeneities with the observed scatter in the elemental abundances of the dwarf stars suggest only a partial match, specifically, for [Fe/H] > ?0.5. The deduced heterogeneities in the case of carbon, oxygen, magnesium, silicon, sulphur, calcium and titanium can explain the observed heterogeneities for [Fe/H] < ?0.5. It may not be possible to explain the entire observed spread exclusively on the basis of the inhomogeneous GCE.     

The Pistol Star

Results of an spectroscopic investigation of the Pistol star are presented. The near-infrared spectra and photometry data are fit with stellar wind models to find that the star is extraordinarily luminous, L = 10^6.7±0.5 L_⊙, making it one of the most luminous stars known. Coupled with the relatively cool temperature, T_eff = 10 K, the star is clearly in violation of the Humphreys-Davidson limit. The derived line of sight velocity of the star assures its membership in the Quintuplet cluster. This, along with the inferred extinction, places the star at the Galactic Center. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the structure and dynamics of the stellar tails of open star clusters

We have analyzed the formation, structure, and dynamical evolution of the population of stars that escaped from open clusters by numerical simulations using S. Aarseth’s modified NBODY6 code. In the Galactic tidal field, the population of stars that escaped from a cluster is shown to be elongated along the orbit of the cluster symmetrically about its core in the form of stellar tails of increasing sizes. We analyze the parameters of stellar tails as a function of such initial simulation conditions as the number of stars, the cluster density, the eccentricity of the Galactic cluster orbit in the plane of the Galactic disk, and the z velocity component. As a result, we constructed a grid of model stellar tails of open clusters. The grid includes such time-dependent parameters of the stellar tails as the length, the cross section, the number of stars, the velocity distribution, etc. Our simulations allow us to clarify the origin of moving clusters and stellar streams and to assess the role of star clusters in forming the stellar velocity field in the solar neighborhood.     

Galactic evolution of 7LI: observational clues for models

Different stellar sources may have contributed to the ⁷Li enrichmentof the Galaxy: type-II supernovae, novae, and AGB stars. In the latter case, the interplay between the Hot Bottom Burning (HBB) process (via the Cameron-Fowler mechanism) and a very high mass-loss rate before the evolution off the AGB (the so-called ‘superwind’ phase), can lead to a significant production of ⁷Li from low- and intermediate-mass AGB stars (Travaglio et al., 2001). We have now undertaken an observational campaign aimed at constraining our stellar and Galactic models, with a twofold goal: (i) to assemble a compilation of high-resolution spectra of Galactic, unevolved (i.e. dwarfs), warm(spectral type F) stars, in a selected metallicity range (-1.0 ≤>[Fe/H] ≤ -0.3), using the ESO 1.5m telescope and the FEROS spectrograph; (ii) to carry out a Li survey among a sample of selected AGB stars, to investigate the possible correlation between⁷Li abundance (when detected) and mass-loss rate. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thick disk

The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalog of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighborhood are used to identify thick-disk objects. We analyze the relations between chemical, spatial, and kinematic parameters of F–G stars in the identified subsystem. The relative magnesium abundances in thick-disk stars are shown to lie within the range 0.0 < [Mg/Fe] < 0.5 and to decrease with increasingmetallicity starting from [Fe/H] ≈ ?1.0. This is interpreted as evidence for a longer duration of the star formation process in the thick disk. We have found vertical gradients in metallicity (grad_Z[Fe/H] = ?0.13 ± 0.04 kpc^?1) and relative magnesium abundance (grad_Z[Mg/Fe] = 0.06 ± 0.02 kpc^?1), which can be present in the subsystem only in the case of its formation in a slowly collapsing protogalaxy. However, the gradients in the thick disk disappear if the stars whose orbits lie in the Galactic plane, but have high eccentricities and low azimuthal space velocities atypical of the thin-disk stars are excluded from the sample. The large spread in relative magnesium abundance (?0.3 < [Mg/Fe] < 0.5) in the stars of the metal-poor “tail” of the thick disk, which constitute ≈8% of the subsystem, can be explained in terms of their formation inside isolated interstellar clouds that interacted weakly with the matter of a single protogalactic cloud. We have found a statistically significant negative radial gradient in relative magnesium abundance in the thick disk (grad_R[Mg/Fe] = ?0.03 ± 0.01 kpc^{? 1}) instead of the expected positive gradient. The smaller perigalactic orbital radii and the higher eccentricities for magnesium-richer stars, which, among other stars, are currently located in a small volume of the Galactic space near the Sun, are assumed to be responsible for the gradient inversion. A similar, but statistically less significant inversion is also observed in the subsystem for the radial metallicity gradient.     

Cooling Flow Star Formation and the Apparent Stellar Ages of Elliptical Galaxies

Simple theoretical arguments indicate that cooled interstellar gas in bright elliptical galaxies forms into a young stellar population having a bottom-heavy but optically luminous initial mass function extending to approximately 2 M middle dot in circle. When the colors and spectral features of this young population are combined with those of the underlying old stellar population, the apparent ages are significantly reduced, similar to the relatively young apparent ages observed in many elliptical galaxies. Galactic mergers are not required to resupply young stars. The sensitivity of continuous star formation to LB and LX&solm0;LB is likely to account for the observed spread in apparent ages among elliptical galaxies. Local star formation is accompanied by enhanced stellar Hbeta equivalent widths, stronger optical emission lines, more thermal X-ray emission, and lower apparent temperatures in the hot gas. The young stars should cause M&solm0;L to vary with galactic radius, perturbing the fundamental plane of the old stars alone.