Abundance analysis of selected cepheids and the galactic distribution of metallicity

We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming r_gc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.     

The chemical evolution of the galactic disc from the kinematics and metallicities of proper motion stars

Members of the Galaxy components are identified according to stellar ages, metallicities and galactic orbits. The local thin disk is found to have a maximum age of 11 billion years and a small abundance scatter partially controlled by the radial gradient of abundances. Metal-rich and old metal-poor stars belong to inner galactic populations and SMRs represent the ultimate star generation in the bulge. The thick disk forms a smooth transition between the halo and thin disk.     

The metallicity of galactic populations and the history of star formation

The metal abundance distribution (the metallicity function, MF) of stars and globular clusters is studied. It is found to have three gaps, near [Fe/H]=–1.0, –0.5 and –0.1. The gaps are shown to be statistically significant practically at the 100% confidence level. They divide the galactic population into four metallicity groups with the average [Fe/H] of about –1.5, –0.8, –0.25 and +0.10 (groups I, II, III and IV, respectively). The main contribution into the scatter of metallicity within the groups (apart from group I) comes from random errors in abundance measurements. So we infer that the actual MF must be essentially discrete. These results substantiate the ones obtained in our previous Papers I–IV; they support our idea on active phases in the evolution of the Galaxy, which imply the intermittent enrichment and star formation.We find that the kinematics and metallicity of groups III and IV of F and G dwarfs show a paradox: the metal-rich group (group IV) of G dwarfs turns out to be kinematically older than the group III of F dwarfs with half the metal abundance. The implication of this result for star formation is discussed. Also we show that the portion of metal poor disk population F dwarfs (group III) is the same or even larger than that of G dwarfs. This fact disagrees with the conventional idea that the young kinematics of F dwarfs owes to the absence of old F dwarfs, which are supposed to be evolved into red giants.     

The metallicity of K giant stars along the Sagittarius streams

The Large Sky-Area Multi-Object Fiber Spectroscopic Telescope(LAMOST) Data Release3 provided 341 691 K giant stars with stellar parameters. Based on the models of Law Majewski, we identified 252 K giant stars in the leading stream associated with the Sagittarius(Sgr) dwarf galaxy. We obtained 132 K giant stars belonging to the trailing arm of Sgr using the model of Belokurov et al. We studied the metallicity distribution of member stars along the streams and found a flat gradient in the first wrap of the leading stream, –(0.88±0.3) × 10~(-3)dex/(~?) in the second wrap of the leading stream and–(1.2±0.3) × 10~(-3)dex/(~?) in the first wrap of the trailing stream. Moreover, we obtained a combined metallicity gradient with our sample and data from the literature. We also analyzed the properties of an overdensity, which is located in the leading stream of the Sgr.     

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.     

Imprints of spiral arms in the oxygen distribution over the galactic disc

A theory for the oxygen abundance radial distribution formation in the galactic disc of a spiral galaxy is developed. We take into account that the main sources of oxygen are Type II supernovae (SN II), the progenitors of which are massive short-lived stars strongly concentrated in the spiral arms. Hence oxygen is the most sensitive indicator of spiral arms' influence on galactic disc enrichment by heavy elements. Various models for the spiral density waves were analysed. We predict that the imprints in the oxygen radial distribution will enable us to distinguish between different models for spiral patterns. Among other parameters, the corotation radius happens to be one of the most important.     

Cosmological origin of the lowest metallicity halo stars

We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars that are likely to be found within our Galaxy today. By PopIII we refer to stars formed at large redshift ( z >4), with low metallicities ([ Z /Z_⊙]<−2.5) and in small systems (total mass ≲ 2×10⁸ M_⊙) that are extremely sensitive to stellar feedback, and which through a prescribed merging history end up becoming part of the Milky Way today. An analytic, extended Press–Schechter formalism is used to obtain the mass functions of haloes which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modelled as a mini-galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disc instabilities, all of which determine the fraction of the baryons that are subject to star formation. The use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, and give accurate estimates of their expected numbers today and their location in L /L_⊙ versus T /K Hertzsprung–Russell (HR) diagrams. A first comparison with observational data suggests that the initial mass function (IMF) of the first stars was increasingly high-mass weighted towards high redshifts, levelling off at z ≳9 at a characteristic stellar mass scale m _s=10–15 M_⊙.     

On a possible metallicity gradient in SMR stars

The possibility of existence of a metallicity gradient in super-metal-rich (SMR) stars is discussed. It is concluded that the SMR phenomenon was more active in the past than it is now.     

The effect of metallicity on the empirical relations for RR Lyrae stars

It has empirically been shown that, for a given value of the effective temperature, the correction in (B-V) due to line blanketing varies linearly with the metallicity parameter S. Next, on the basis of considerations different from those used to obtain a similar result by Sturch (1966), a relation between reciprocal effective temperature _e (=5040/T _e ) and intrinsic line-free colour index(B-V) _o,c has been obtained based on the _e and (B-V) values for five RR Lyr variables. Relations between _e and intrinsic colour(B-V) _o for different groups of stars having a S parameter in the range 0S11 have also been obtained.     

On the Galactic disc age–metallicity relation

A comparison is made between the age–metallicity relations 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⁻¹ and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.     

An excess of very bright stars in the inner bulge

From an analysis of the stars remaining in central regions of the Galaxy after subtracting those belonging to the disc and the bulge, we deduce that the inner bulge must have an extra young population with respect to the rest of the bulge. It is shown that there is a higher ratio of very bright stars in the central bulge than there is in the outer bulge. This is interpreted as being an additional young component due to the presence of star formation regions near the Galactic Centre which is absent in the outer bulge.     

The evolution of super-metal-rich stars and the interpretation of old galactic clusters

The influence of initial helium content on the evolutionary characteristics of super-metalrich stars (Z=0.10) has been investigated. The evolution of models in the range –0.05L/L <1.0 has been followed up to the relative luminosity maximum in the subgiant branch, and under the assumptionY=0.20 orY=0.40.Comparison with previous results suggests the existence of theoretical constraints that could be adopted as metal indicators for the observed H–R diagrams of old open clusters.     

Lifetimes of stars in the main sequence and the maximum mass of stars in the galactic disk

Generalized and interconsistent approximation formulas are derived to describe the relationship between the hydrogen-burning time and the zero-age stellar mass for the mass and elemental composition ranges characteristic of stars that have been formed during the lifetime of the universe. The maximum masses of population I stars are estimated based on the known statistical relationships among stellar characteristics that agree with observational data.     

Kinematics of stars in the northern and southern galactic hemispheres

We use vector spherical harmonics for a kinematic analysis of the proper motions of stars from the Hipparcos, Tycho-2, and UCAC3 catalogues in the northern and southern Galactic hemispheres. We have found that the statistically reliable values of the Ogorodnikov-Milne model parameters M ₃₂ ⁺ and M ₃₂ ^? have different signs in different hemispheres. This is a consequence of the Galaxy??s rotational retardation with distance from the principal Galactic plane. Based on various samples of stars from the above catalogues, we have obtained the following estimate for the magnitude of the vertical gradient of Galactic rotation velocity in the solar neighborhoods: (20.1 ± 2.9) < |?V_??/?z| < (49.2 ± 0.8) km s^?1 kpc^?1. Another result that is revealed by our analysis of the parameters M ₁₃ ^? and M ₁₃ ⁺ in different Galactic hemispheres is that the vertical gradient of expansion velocity for the stellar system ?V _R /? _z is positive in the northern hemisphere and negative in the southern one. This suggests that the expansion velocity V _R increases with distance fromthe Galactic plane. We show that both these gradients give rise to an apparent acceleration of the solar motion along the x and y axes of the rectangular Galactic coordinate system. Our analysis of the parameters M ₂₁ ^? and M ₁₂ ⁺ shows no significant differences in both hemispheres and has allowed us to determine the Oort parameters, to estimate the Galactic rotation velocity and period in the solar neighborhood, and to calculate the ratio of the epicyclic frequency to the angular velocity of Galactic rotation in the solar neighborhood. The derived diagonal elements of the velocity field deformation tensor suggest that the orientation of the rectangular Galactic coordinate system in space must be determined by taking into account not only the geometrical factors but also the dynamical ones. All these results agree well with these quantities estimated over the entire sphere by various authors.