Subsystems of RR Lyrae variable stars in our Galaxy

We have used published, high-accuracy, ground-based and satellite proper-motion measurements, a compilation of radial velocities, and photometric distances to compute the spatial velocities and Galactic orbital elements for 174 RR Lyrae (ab) variable stars in the solar neighborhood. The computed orbital elements and published heavy-element abundances are used to study relationships between the chemical, spatial, and kinematic characteristics of nearby RR Lyrae variables. We observe abrupt changes of the spatial and kinematic characteristics at the metallicity [Fe/H]≈?0.95 and also when the residual spatial velocities relative to the LSR cross the critical value V _res≈290 km/s. This provides evidence that the general population of RR Lyrae stars is not uniform and includes at least three subsystems occupying different volumes in the Galaxy. Based on the agreement between typical parameters for corresponding subsystems of RR Lyrae stars and globular clusters, we conclude that metal-rich stars and globular clusters belong to a rapidly rotating and fairly flat, thick-disk subsystem with a large negative vertical metallicity gradient. Objects with larger metal deficiencies can, in turn, be subdivided into two populations, but using different criteria for stars and clusters. We suggest that field stars with velocities below the critical value and clusters with extremely blue horizontal branches form a spherical, slowly rotating subsystem of the protodisk halo, which has a common origin with the thick disk; this subsystem has small but nonzero radial and vertical metallicity gradients. The dimensions of this subsystem, estimated from the apogalactic radii of orbits of field stars, are approximately the same. Field stars displaying more rapid motion and clusters with redder horizontal branches constitute the spheroidal subsystem of the accreted outer halo, which is approximately a factor of three larger in size than the first two subsystems. It has no metallicity gradients; most of its stars have eccentric orbits, many display retrograde motion in the Galaxy, and their ages are comparatively low, supporting the hypothesis that the objects in this subsystem had an extragalactic origin.     

The Chemical Composition of Globular Clusters of Different Nature in Our Galaxy

A catalog of Galactic globular clusters has been compiled and used to analyze relations between the chemical and kinematic parameters of the clusters. The catalog contains positions, distances, luminosities, metallicites, and horizontal-branch morphology indices for 157 globular clusters, as well as space velocities for 72 globular clusters. For 69 globular clusters, these data are suppleented with the relative abundances of 28 chemical elements produced in various nuclear-synthesis processes, taken from 101 papers published between 1986 and 2018. The tendency for redder horizontal branches in lowmetallicity accreted globular clusters is discussed. The discrepancy between the criteria for cluster membership in the thick-disk and halo subsystems based on chemical and kinematic properties is considered. This is manifest through the fact that all metal-rich ([Fe/H] > ?1.0) clusters are located close to the center and plane of the Galaxy, regardless of their kinematic membership in particular Galaxy subsystems. An exception is three accreted clusters lost by a dwarf galaxy in Sagittarius. At the same time, the fraction of more distant clusters is high among metal-poorer clusters in any kinematically selected Galactic subsystem. In addition, all metal-rich clusters whose origins are related to the same protogalactic cloud are located in the [Fe/H]–[α/Fe] diagram considerably higher than the strip populated with field stars. All metal-poor clusters (most of them accreted) populate the entire width of the strip formed by high-velocity (i.e., presumably accreted) field stars. Stars of dwarf satellite galaxies (all of them being metal-poor) are located in this diagram much lower than accreted field stars. These facts suggest that all stellar objects in the accreted halo are remnants of galaxies with higher masses than those in the current environment of the Galaxy. Differences in the relative abundances of α-process elements among stellar objects of the Galaxy and surrounding dwarf satellite galaxies confirmthat the latter have left no appreciable stellar traces in the Galaxy, with the possible exception of the low-metallicity cluster Rup 106, which has low relative abundances of α-process elements.

     

Neutron-capture elements in halo,thick-disk,and thin-disk stars: Neodymium

We have derived the LTE neodymium abundances in 60 cool stars with metallicities [Fe/H] from 0.25 to ?1.71 by applying a synthetic-spectrum analysis to spectroscopic observations of NdII lines with a resolution of λ/Δλ?60 000 and signal-to-noise ratios of 100–200. We have improved the atomic parameters of NdII and blending lines by analyzing the corresponding line pro files in the solar spectrum. Neodymium is overabundant with respect to iron in halo stars, [Nd/Fe]=0.33±0.09, with the [Nd/Fe] ratio decreasing systematically with metallicity when [Fe/H]>?1. This reflects an onset of efficient iron production in type I supernovae during the formation of the thick disk. The [Nd/Ba] and [Nd/Eu] abundance ratios behave differently in halo, thick-disk, and thin-disk stars. The observed abundance ratios in halo stars, [Nd/Ba]=0.34±0.08 and [Nd/Eu]=?0.27±0.05, agree within the errors with the ratios of the elemental yields for the r-process. These results support the conclusion of other authors based on analyses of other elements that the r-process played the dominant role in the synthesis of heavy elements during the formation of the halo. The [Nd/Ba] and [Nd/Eu] ratios for thick-disk stars are almost independent of metallicity ([Nd/Ba]=0.28(±0.03)?0.01(±0.04) [Fe/H] and [Nd/Eu]=?0.13(±0.03)+0.05(±0.04) [Fe/H]) but are smaller in absolute value than the corresponding ratios for halo stars, suggesting that the synthesis of s-process nuclei started during the formation of the thick disk. The s-process is estimated to have contributed ?30% of the neodymium produced during this stage of the evolution of the Galaxy. The [Nd/Ba] ratio decreases abruptly by 0.17 dex in the transition from the thick to the thin disk. The systematic decrease of [Nd/Ba] and increase of [Nd/Eu] with increasing metallicity of thin-disk stars point toward a dominant role of the s-process in the synthesis of heavy elements during this epoch.     

The galactic constants and rotation curve from molecular-gas observations

We obtained the photometric distances and radial velocities for the molecular gas for 270 star-forming regions and estimated the distance to the Galactic center from ten tangent points to be R₀ = 8.01 ± 0.44 kpc. Estimates of R₀ derived over the last decade are summarized and discussed; the average value is R₀ = 7.80 ± 0.33 kpc. We analyze deviations from axial symmetry of the gas motion around the Galactic center in the solar neighborhood. Assuming a flat rotation curve, we obtain Θ₀ ～ 200 km/s for the circular velocity of the Sun from regions beyond the Perseus arm. We used these Galactic constants to construct the Galactic rotation curve. This rotation curve is flat along virtually its total extent from the central bar to the periphery. The velocity jump in the corotation region of the central bar in the first quadrant is 20 km/s. We present analytical formulas for the rotation curves of the Northern and Southern hemispheres of the Galaxy for R₀ = 8.0 kpc and Θ₀ = 200 km/s.     

Evolutionary connection between low-mass detached binaries,contact W UMa systems,and blue stragglers

We analyze the distribution of close binary stars in the orbital semimajor axis—primary mass plane. The reduced spatial density of stars with semimajor axes below 10R_⊙ is confirmed. We identify the area in this plane occupied by precursors of W UMa stars, assuming that the driving force causing the components to approach each other is their magnetic stellar wind. This scenario enables us to estimate the rate of formation (0.02/year) and lifetime (10⁸ yr) of W UMa stars. We derive a theoretical estimate of the ratio of the number of blue stragglers, N _BS , and of horizontal-branch stars, N _HB , in globular clusters based on the hypothesis that all blue stragglers are the result of component mergers in W UMa contact binaries. This ratio is N _BS /N _HB =0.4, close to the observed value for 62 Galactic globular clusters. We discuss possible reasons for the considerable dispersion of the observed estimates of this ratio for different clusters in our Galaxy.     

Relationship between the velocity ellipsoids of galactic-disk stars and their ages and metallicities

The dependences of the velocity ellipsoids of F-G stars of the thin disk of the Galaxy on their ages and metallicities are analyzed based on the new version of the Geneva-Copenhagen Catalog. The age dependences of the major, middle, and minor axes of the ellipsoids, and also of the dispersion of the total residual velocity, obey power laws with indices 0.25, 0.29, 0.32, and 0.27 (with uncertainties ±0.02). Due to the presence of thick-disk objects, the analogous indices for all nearby stars are about a factor of 1.5 larger. Attempts to explain such values are usually based on modeling relaxation processes in the Galactic disk. Elimination of stars in the most numerous moving groups from the sample slightly reduces the corresponding indices (0.22, 0.26, 0.27, and 0.24). Limiting the sample to stars within 60 pc of the Sun, so that the sample can be considered to be complete, leaves both the velocity ellipsoids and their age dependences virtually unchanged. With increasing age, the velocity ellipsoid increases in size and becomes appreciablymore spherical, turns toward the direction of the Galactic center, and loses angular momentum. The shape of the velocity ellipsoid remains far from equilibrium. With increasing metallicity, the velocity ellipsoid for stars of mixed age increases in size, displays a weak tendency to become more spherical, and turns toward the direction of the Galactic center (with these changes occurring substantially more rapidly in the transition through the metallicity [Fe/H]≈−0.25). Thus, the ellipsoid changes similarly to the way it does with age; however, with decreasing metallicity, the rotational velocity about the Galactic center monotonically increases, rather than decreases (!). Moreover, the power-law indices for the age dependences of the axes depend on the metallicity, and display a maximum near [Fe/H] ≈−0.1. The age dependences of all the velocity-ellipsoid parameters for stars with equal metallicity are roughly the same. It is proposed that the appearance of a metallicity dependence of the velocity ellipsoids for thin-disk stars, recorded from the close to the Sun, is most likely due to the radial migration of stars.     

Physical parameters and dynamical properties of the multiple system ι UMa (ADS 7114)

We analyze the physical parameters, orbital elements, and dynamic stability of the multiple system ?? UMa (HD 76644 = ADS 7114). We have used the positions from the WDS catalog and our own observations on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and the 1.5-m Russian-Turkish Telescope (Antalya, Turkey). We have obtained more precise orbital parameters of the subsystems, and spectral types, absolute magnitudes, and masses of the components. The primary has Sp = F0 V?CIV, M = 1.7 ± 0.1M _??, T _eff = 7260 ± 70 K, and log g = 4.30 ± 0.07. The companion in the close Aa subsystem is most likely a white dwarf with a mass of approximately 1.0 ± 0.3M _??. The spectral types and masses of the components in the BC subsystem are M3V, M4V and 0.35 ± 0.05M _??, 0.30 ± 0.05M _??, respectively. The total mass is 3.4 ± 0.4M _??. The Aa subsystem probably has an orbital period of 4470^d = 12.2^y and an eccentricity of approximately 0.6. The outer subsystem seems to have a period of approximately 2084 yrs and an eccentricity of approximately 0.9. We have carried out simulations using the stability criteria and shown that for all possible variations in the component parameters, the multiple system is unstable on a time scale of less than 10⁶ years with a probability exceeding 0.98. Possible reasons for this instability are discussed.     

Neutron-capture elements in halo, thick-disk, and thin-disk stars. Strontium, yttrium, zirconium, cerium

We derived Sr, Y, Zr, and Ce abundances for a sample of 74 cool dwarfs and subgiants with iron abundances, [Fe/H], between 0.25 and ?2.43. These estimates were obtained using synthetic spectra, assuming local thermodynamic equilibrium (LTE) for Y, Zr, and Ce, allowing for non-LTE conditions for Sr. We used high-resolution (λ/Δλ?40 000 and 60 000) spectra with signal-to-noise ratios between 50 and 200. We find that the Zr/Y, Sr/Y, and Sr/Zr ratios for the halo stars are the same in a wide metallicity range (?2.43 ≤ [Fe/H] ≤ ?0.90), within the errors, indicating a common origin for these elements at the epoch of halo formation. The Zr/Y ratios for thick-disk stars quickly decrease with increasing Ba abundance, indicating a lower rate of production of Zr compared to Y during active thick-disk formation. The thick-disk and halo stars display an increase in the [Zr/Ba] ratio with decreasing Ba abundance and a correlation of the Zr and Eu overabundances relative to Ba. The evolutionary behavior of the abundance ratios found for the thick-disk and halo stars does not agree with current models for the Galaxy’s chemical evolution. The abundance ratios of Y and Zr to Fe and Ba for thin-disk stars, as well as the abundance ratios within each group, are, on average, solar, though we note a slight decrease of Zr/Ba and Zr/Y with increasing Ba abundance. These results provide evidence for a dominance of asymptotic-giant-branch stars in the enrichment of the interstellar medium in heavy elements during the thin-disk epoch, in agreement with the predictions of the nucleosynthesis theory for the main s-process component.     

Chemical composition of stars in the galactic halo

The chemical compositions of the atmospheres of six metal-poor stars are analyzed. Spectra with signal-to-noise ratios of no less than 100 and a resolution of R≈17 000 were obtained using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na, K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-process elements (Y, Ba) are derived. The star G251-54 ([Fe/H]=?1.55, T _eff=5541 K, logg=3.58) is deficient in some elements compared to both stars with similar metallicities and the Sun. The atmosphere of G251-54 has the following elemental abundances relative to iron: [O/Fe]=+0.47, [α/Fe]≈?0.3, [Na/Fe]=?0.60, [Sc/Fe]=?0.57, [Cr, Ni, Fe]≈0, [Zn/Fe]=+0.16, [Cu/Fe]=?0.66, [Y/Fe]=?0.70, and [Ba/Fe]=?1.35. The remaining five stars have metallicities in the range ?1.6<[Fe/H]相似文献   

The OSACA database and a kinematic analysis of stars in the solar neighborhood

We transformed radial velocities compiled from more than 1400 published sources, including the Geneva-Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of ±1 km/s. We use these radial velocities together with the stars’ coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm Catalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (-r ≈ 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation, ω_o = ?25.7 ± 1.2 kms^?1 kpc^?1, and the vertex deviation, l = 13° ± 2°, and detected a negative K effect. This negative K effect is most conspicuous in the motion of A0–A5 giants and is equal to K = ?13.1 ± 2.0 kms^?1 kpc^?1.     

Evolution of the velocity ellipsoids in the thin disk of the Galaxy and the radial migration of stars

Data from the revised Geneva-Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative metallicity gradient in the thin disk, are responsible for the observed negative correlation between the metallicities and angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do not depend on either age or metallicity. The velocity components of the Sun relative to the Local Standard of Rest derived using data for stars born at the solar Galactocentric distance are (U _⊙, V _⊙, W _⊙) _LSR = (5.1 ± 0.4, 7.9 ± 0.5, 7.7 ± 0.2) km/s. The two coordinates of the apex of the solar motion remain equal to 〈l _⊙〉 = 70° ± 7° and 〈b _⊙〉 = 41° ± 2°, within the errors. The indices for the power-law age dependences of them ajor, middle, and minor semi-axes become 0.26±0.04, 0.32±0.03, and 0.07±0.03, respectively. As a result, with age, the velocity ellipsoid for thin-disk stars born at the solar Galactocentric distance increases only in the plane of the disk, remaining virtually constant in the perpendicular direction. Its shape remains far from equilibrium, and the direction of the major axis does not change with age: the ellipsoid vertex deviation remains constant and equal to zero within the errors (〈L〉 = 0.7° ± 0.6°, 〈B〉 = 1.9° ± 1.1°). Such a small increase in the velocity dispersion perpendicular to the Galactic plane with age can probably be explained by “heating” of the stellar system purely by spiral density waves, without a contribution from giant molecular clouds.     

Peculiarities of the abundances of neutron-capture elements in Galactic open clusters

The properties of the relative abundances of rapid and slow neutron-capture elements are studied using a catalog containing spectroscopic abundance determinations for 14 elements produced in various nuclear-synthesis processes for 90 open clusters. The catalog also contains the positions, ages, velocities, and elements of the Galactic orbits of the clusters. The relative abundances of both r-elements (Eu) and s-elements (Y, Ba, La, and Ce) in clusters with high, elongated orbits and in field stars of the Galactic thin disk display different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits, supporting the view that these objects have different natures. In young clusters, not only barium, but also the three other studied s-elements display significantly higher relative abundances than field stars of the same metallicity. The relative abundances of Eu are lower in highmetallicity clusters ([Fe/H] > -0.1) with high, elongated orbits than in field giants, on average, while the [Eu/Fe] ratios in lower-metallicity clusters are the same as those in field stars, on average, although with a large scatter. The metallicity dependence of the [O, Mg/Eu] ratios in clusters with high, elongated orbits and in field stars are substantially different. These and other described properties of the Eu abundances, together with the properties of the abundances of primary a-elements, can be understood in a natural way if clusters with high, elongated orbits with different metallicities formed as a result of interactions of two types of high-velocity clouds with the interstellar medium of the Galactic disk: low-metallicity highvelocity clouds that formed from “primordial” gas, and high-metallicity clouds with intermediate velocities that formed in “Galactic fountains.”     

Peculiarities of α-element abundances in Galactic open clusters

A catalog compiling the parameters of 346 open clusters, including their metallicities, positions, ages, and velocities has been composed. The elements of the Galactic orbits for 272 of the clusters have been calculated. Spectroscopic determinations of the relative abundances, [el/Fe], for 14 elements synthesized in various nuclear processes averaged over data from 109 publications are presented for 90 clusters. The compiled data indicate that the relative abundances of primary α elements (oxygen and magnesium) exhibit different dependences on metallicity, age, Galactocentric distance, and the elements of the Galactic orbits in clusters with high, elongated orbits satisfying the criterion (Z_max² + 4e²)^1/2 > 0.40 and in field stars of the Galactic thin disk (Z_max is the maximum distance of the orbit from the Galactic plane in kiloparsec and e is the eccentricity of the Galactic orbit). Since no systematic effects distorting the relative abundances of the studied elements in these clusters have been found, these difference suggest real differences between clusters with high, elongated orbits and field stars. In particular, this supports the earlier conclusion, based on an analysis of the elements of the Galactic orbits, that some clusters formed as a result of interactions between high-velocity,metal-poor clouds and the interstellar mediumof theGalactic thin disk. On average, clusters with high, elongated orbits and metallicities [Fe/H] < -0.1 display lower relative abundances of the primary a elements than do field stars. The low [O, Mg/Fe] ratios of these clusters can be understood if the high-velocity clouds that gave rise to them were formed of interstellar material from regions where the star-formation rate and/or the masses of Type II supernovae were lower than near the Galactic plane. It is also shown that, on average, the relative abundances of the primary a elements are higher in relatively metal-rich clusters with high, elongated orbits than in field stars. This can be understood if clusters with [Fe/H] > -0.1 formed as a result of interactions between metal-rich clouds with intermediate velocities and the interstellar medium of the Galactic disk; such clouds could form from returning gas in a so-called “Galactic fountain.”     

Equilibrium phase-space density distribution in numerical dynamical models of open clusters

In dynamical models for open clusters, virial equilibrium is not achieved over the violent relaxation time scale τ_vr. The stars form an equilibrium distribution in (?, ?_ζ, l) space, where ? and l are the energy and angular momentum per unit stellar mass in the combined field of the Galaxy and cluster and ?_ζ is the energy of motion perpendicular to the Galactic plane per unit mass of cluster stars in the gravitational field of the Galaxy. This distribution of stars changes little when t>τ_vr. The stellar phase-space distribution corresponding to this type of equilibrium and the regular cluster potential vary periodically (or quasi-periodically) with time. This phase-space equilibrium is probably possible due to an approximate balance in the stellar transitions between phase-space cells over times equal to the oscillation period for the regular cluster field.     

Proper motions of open star clusters and the rotation rate of the galaxy

The mean proper motions of 167 Galactic open clusters with radial-velocity measurements are computed from the data of the Tycho-2 catalog using kinematic and photometric cluster membership criteria. The resulting catalog is compared to the results of other studies. The new proper motions are used to infer the Galactic rotation rate at the solar circle, which is found to be ω₀=+24.6±0.8 km s^?1 kpc^?1. Analysis of the dependence of the dispersion of ω₀ estimates on heliocentric velocity showed that even the proper motions of clusters with distances r>3 kpc contain enough useful information to be used in kinematic studies demonstrating that the determination of proper motions is quite justified even for very distant clusters.     

Galactic orbits of globular clusters in the region of the Galactic bulge

Galactic orbits have been constructed over long time intervals for ten globular clusters located near the Galactic center. A model with an axially symmetric gravitational potential for the Galaxy was initially applied, after which a non-axially symmetric potential corresponding to the central bar was added. Variations in the trajectories of all these globular clusters in the XY plane due to the influence of the bar were detected. These were greatest for the cluster Terzan 4 in the meridional (RZ) plane. The globular clusters Terzan 1, Terzan 2, Terzan 4, Terzan 9, NGC 6522, and NGC 6558 always remained within the Galactic bulge, no farther than 4 kpc from the Galactic center.     

The stellar epoch in the evolution of the Galaxy

We consider the astrophysical evolution of the Galaxy over large time scales, from early stages (an age of ～10⁸ yrs) to the end of traditional stellar evolution (～10¹¹ yrs). Despite the fact that the basic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteristics of stars (their total luminosity, color, mass function, and chemical composition) are rather substantial. The interaction of the Galaxy with other stellar systems becomes an important factor in its evolution 100–1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as follows. The Galaxy forms as the result of the contraction (collapse) of a protogalactic cloud. The beginning of the Milky Way’s life—the relaxation period, which lasts about 1–2 Gyr—is characterized by active star formation and final structurization. The luminosity and colors of the Galaxy are correlated to the star formation rate (SFR). The young Galaxy intensely radiates high-energy photons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady-state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reaching 3–5M _⊙/yr at the present epoch and decreasing to 0.03M _⊙/yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Later, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M _⊙ (i.e., those able to lose matter and renew the supply of interstellar gas) comes to an end. The Galaxy enters a period of “dying”, and becomes fainter and redder. The variation of its chemical composition is manifested most appreciably in a dramatic enrichment of the interstellar gas in iron. The final “stellar epoch” in the life of the Galaxy is completed ～10¹³ yrs after its formation, when the evolution of the least massive stars comes to an end. By this time, the supplies of interstellar and intergalactic gas are exhausted, the remaining stars become dark, compact remnants, there is no further formation of new stars, and the Galactic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission.     

Galactic parameters derived from open-cluster data

The components U₀ and V₀ of the solar motion and the Oort constant A₀ are determined using the data of a homogeneous open-cluster catalog with corrected distance moduli. The results are based on a sample of 146 open clusters with known radial velocities located in the Galactic plane (b<7°) within 4 kpc of the Sun. The solar Galactocentric distance R₀ is determined using two kinematic methods. The following results are obtained: A₀=17.0±0.9 km/s kpc, U₀=10.5±1.0 km/s, V₀=11.5±1.1 km/s, R₀=8.3±0.3 pc.     

Observational manifestations of gaseous baryon dark matter

The paper considers possible observational implications of the presence of dark matter in the Galaxy in the form of dense gas clouds—clumpuscules with masses M _c ～10^?3 M _⊙ and radii R _c～3×10¹³ cm. The existence of such clouds is implied by modern interpretations of extreme scattering events—variations in quasar radio fluxes due to refraction in dense plasma condensations in the Galactic halo. The rate of collisions between these clouds is shown to be rather high: from 1 to 10M _⊙ per year is ejected into the interstellar medium as a result of such collisions. The optical continuum and 21-cm emission from hot post-collision gas could be observable. Gas clouds composed of dark matter could be formed around O stars in an H II region with radius R～30 pc and emission measure EM?20 cm^?6 pc. They could also be observable in the H_α line. The evaporation of clumpuscules by external ionizing radiation could be a substantial source of matter for the interstellar medium. Assuming that the total mass of matter entering the interstellar medium over the Hubble time does not exceed the mass of luminous matter in the Galaxy, upper limits are found for the cloud radii (R _c<3.5×10¹² cm) and the contribution of clouds to the surface density of the Galaxy (<50M _⊙pc^?2). Dissipation of the kinetic energy of matter lost by clumpuscules could provide an efficient mechanism for heating gas in the Galactic halo.     

Optical spectra of three AGB stars

The basic parameters and detailed chemical compositions of three asymptotic giant branch stars with similar effective temperatures and surface gravities have been determined using CCD spectra obtained with the échelle spectrometers of the SAO 6-m telescope. The metallicity and chemical composition of the optical counterpart of the OH/IR star IRAS 18123 + 0511 have been derived for the first time. The abundance [X/H]_⊙ of the iron group elements (V, Cr, Fe) is ?0.45 dex. An overabundance of oxygen, [O/Fe]_⊙=1.44 dex, is detected in the atmosphere of this star. The abundances of s-process heavy elements are not enhanced, and are instead underabundant with respect to the metallicity: the average value of [X/Fe]_⊙ for Y, Zr, Ba, La, Ce, Pr, Nd is ?0.25. The derived abundances confirm that IRAS 18123 + 0511 is in the AGB stage of its evolution. The metallicity of the object, together with its radial velocity V _r=78.0 km/s and Galactic latitude |b|=11°, suggest that it belongs to the old disk population. The expansion velocity of the circumstellar envelope, V _exp≈21 km/s, is derived from the positions of circumstellar absorption bands. The set of parameters obtained for the low-metallicity, highlatitude supergiants BD + 18° 2757 and BD + 18° 2890 (with iron abundances [Fe/H]_⊙=2.10 and ?1.48, respectively) confirm that they are evolved halo stars, and probably UU Her-type stars.