Kinematics and age of 15 stars-photometric solar analogs

The radial and space velocities are inferred for 15 stars that are photometric analogs of the Sun. The space velocity components (U, V, W) of most of these stars lie within the 10–60 km/s interval. The star HD 225239, which in our previous papers we classified as a subgiant, has a space velocity exceeding 100 km/s, and belongs to the thick disk. The inferred fundamental parameters of the atmospheres of solar analogs are combined with published evolutionary tracks to estimate the masses and ages of the stars studied. The kinematics of photometric analogs is compared to the data for a large group of solar-type stars.     

Kinematics of 11 Bidelman's weak‐lined stars

Radial velocities obtained with the CORAVEL‐type spectrometer are presented for 11 Bidelman's weak‐lined stars, classified from moderate dispersion objective prism spectra. Standard errors of the measurements are smaller than 1 km/s. For five of these stars, measured in the Vilnius photometric system, only small metal deficiency is established. Kinematically, most of the stars belong to the thin disk and only two stars, HD 71185 and BD +75° 641, are consistent with membership of the thick disk population.     

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.     

Chemical and physical properties of nearby population I stars

An analysis of some photometric and kinematic data for nearby Population I stars has been carried out. The main results show that a good correlation exists between the dynamic state and age; but a non-negligible chemical inhomogeneity seems to be present in the disk, independently of the age.     

银河系厚盘恒星的年龄-金属度关系

对太阳邻域内银河系盘星、特别是厚盘恒星年龄-金属度关系的研究进展作了较为详细的回顾和评述；说明目前对盘星是否确实存在年龄-金属度关系还存在争议，而厚盘恒星可能存在这种关系。同时指出，要得出明晰的结论，在观测资料的基础上取得合理的恒星样本至关重要。     

Mechanism for cyclical activity of the Herbig Ae star BF Ori

The photometric activity of the star BF Ori, which belongs to the family of UX Ori-type stars, is studied. New data obtained with the ASAS robotic telescope confirm the existence of a cyclical component in the light curve for this star with a duration of roughly 11-12 years. The form of the photometric cycle resembles the theoretical light curves for a young binary system with a low-mass secondary component, and which is accreting matter from a surrounding common disk. Alternative mechanisms for the cyclical variability of UX Ori stars are also discussed.     

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.     

银河系铝元素丰度研究

恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关.     

Programme of the Galactic meridian: Star counts and Galactic models 2. Distribution of absolute proper motions

A comparison of the observed distribution of absolute proper motions with a kinematical model of the Galaxy is presented. Proper motions with respect to galaxies were obtained for about 40 000 stars along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The Galaxy is considered as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. For each subsystem, spatial velocity components and their dispersions were computed. The distribution of kinematical parameters were modelled for stars located in different directions of the Galaxy.     

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.     

Information from the Kinematics of F and G Stars in the Solar Neighborhood

We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp     

Future Large-Scale Surveys of `Interesting' Stars in the Halo and Thick Disk of the Galaxy

The age of slow, methodical, star-by-star, single-slit spectroscopic observations of rare stars in the halo and thick disk of the Milky Way has come to an end. The use of new spectroscopic survey techniques which obtain large sky coverage to faint magnitudes will enable substantially better `return on investment' in the near future. We review the present state of surveys for low metallicity, field horizontal-branch, and sdM stars in the Galaxy, and describe new lines of attack which should open the way to a more than hundredfold increase in the numbers of interesting stars with available spectroscopic information. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinematics of nearby Subdwarfs

A sample of subdwarfs with accurate space velocities and standarized metallicities is presented. This was constructed by combining Hipparcos parallaxes and proper motions with radial velocities and metallicities from Carney et al. (1994; CLLA). The accurate Hipparcos parallaxes lead to an – upward – correction factor of 11% of the photometric distance scale of CLLA. The kinematical behaviour of the subdwarfs is discussed in particular in relation to their metallicities. Most of the stars turn out to be thick disk stars, but the sample contains also many genuine halo stars. While the extreme metal poor halo does not rotate, a population of subdwarfs with metallicities in the range −1.6≤ [Fe/H] ≤ −1.0 dex appears to rotate around the galactic center with a mean rotation speed of about 100 km s^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Expansion of the stellar subsystems of disk galaxies

Based on archival Hubble Space Telescope images, we have performed stellar photometry for eight edge-on spiral and irregular galaxies. We have identified stars with ages of 20, 50, 80, 160, and 500 Myr in the derived Hertzsprung-Russell diagrams and constructed their number density distributions perpendicularly to the plane of the galactic disk. We have determined the sizes of the stellar subsystems and constructed the size-age diagrams for the stars constituting these subsystems. The stellar subsystems have been found to expand in all of the investigated galaxies within the range of ages studied (from 20 to 500 Myr). The expansion velocity of the subsystems decreases as one recedes from the galactic plane. The subsystems with ages of 1.5 and 6 Gyr also exhibit an increase in their sizes with age. The sizes of these subsystems approach those of the thick disk consisting of red giants. Our results confirm the model of thick-disk formation in irregular and low-mass spiral galaxies through thin-disk expansion.     

Rotation effects in classical T Tauri stars

Surface temperature inhomogeneities in classical T Tauri stars (CTTS) induced by magnetic activity andmass accretion lead to rotationalmodulation of both photometric and spectroscopic parameters of these stars. Using the extended photometric catalogue byGrankin et al., we have derived the periods and amplitudes of the rotational modulation of brightness and color for 31 CTTS; for six of them, the periods have been revealed for the first time. The inclinations of the rotation axis and equatorial rotational velocities of CTTS have been determined. We show that the known periods of brightness variations for some of the CTTS are not the axial rotation periods but are the Keplerian periods near the inner boundary of the dusty disk. We have found that the angular velocity of CTTS with a mass of 0.3?3M _?? in the Taurus-Auriga complex remains constant in the age range 1?C10 Myr. CTTS on radiative evolutionary tracks rotate faster than completely convective CTTS. The specific angular momentum of CTTS depends on the absolute luminosity in the H?? line.     

Stellar structure of the dwarf irregular galaxy DDO 216

Observations with the 6-mBTA telescope and archival Hubble Space Telescope data were used for the photometry of stars in the dwarf edge-on irregular galaxy DDO 216 (Peg DIG). We determined the change in the number density of stars of various ages along the major and minor axes of the galaxy. We found that the young stars of the galaxy concentrate toward the center, while its old stars, red giants, form an extended thick disk 5 kpc in diameter and 2 kpc in thickness around the galaxy.     

Optical photometry of GM Cep: evidence for UXor type of variability

Results from optical photometric observations of the pre-main sequence star GM Cep are reported in the paper. The star is located in the field of the young open cluster Trumpler 37—a region of active star formation. GM Cep shows a large amplitude rapid variability interpreted as a possible outburst from EXor type in previous studies. Our data from BVRI CCD photometric observations of the star are collected from June 2008 to February 2011 in Rozhen observatory (Bulgaria) and Skinakas observatory (Crete, Greece). A sequence of sixteen comparison stars in the field of GM Cep was calibrated in the BVRI bands. Our photometric data for a 2.5 years period show a high amplitude variations ($\Delta V \sim2\mbox{$\Delta V \sim2\mbox{) and two deep minimums in brightness are observed. The analysis of collected multicolor photometric data shows the typical of UX Ori variables a color reversal during the minimums in brightness. On the other hand, high amplitude rapid variations in brightness typical for the Classical T Tauri stars also present on the light curve of GM Cep. Comparing our results with results published in the literature, we conclude that changes in brightness are caused by superposition of both: (1) magnetically channeled accretion from the circumstellar disk, and (2) occultation from circumstellar clouds of dust or from features of a circumstellar disk.     

Magnetically active stars in Taurus-Auriga: Evolutionary status

We have analyzed a sample of 74 magnetically active stars toward the Taurus-Auriga starforming region. Based on accurate data on their basic physical parameters obtained from original photometric observations and published data on their proper motions, X-ray luminosities, and equivalent widths of the Hα and Li lines, we have refined the evolutionary status of these objects. We show that 50 objects are young stars with ages of 1–40 Myr and belong to the Taurus-Auriga star-forming region. Other 20 objects have a controversial evolutionary status and can belong to both Taurus-Auriga starforming region and the Gould Belt. The remaining four objects with ages of 70–100 Myr belong to the zero-age main sequence. We have analyzed the relationship between the rotation period, mass, and age for 50 magnetically active stars. The change in the angular momentum of the sample stars within the first 40 Myr of their evolution has been investigated. An active star-protoplanetary disk interaction is shown to occur on a time scale from 0.7 to 10Myr.     

Red giant clump in the Tycho-2 catalogue

The Tycho-2 proper motions and Tycho-2 and 2MASS photometry are used to select 97348 red giant clump (RGC) stars. The interstellar extinction and photometric distance are calculated for each of the stars. The selected stars are shown to form a selection-unbiased sample of RGC stars within about 350 pc of the Sun with the addition of more distant stars. The distribution of the selected stars in space and their motion are consistent with the assumption that the RGC contains Galactic disk stars with various ages and metallicities, including a significant fraction of stars younger than 1 Gyr with masses of more than 2M _⊙. These young stars show differences of their statistical characteristics from those of older RGC stars, including differences in the variations of their distribution density with distance from the Galactic plane and in the dispersion of their velocities found using radial velocities and proper motions. The Sun has been found to rise above the Galactic plane by 13 ± 1 pc. The distribution density of the stars under consideration in space is probably determined by the Local Spiral Arm and the distribution of absorbing matter in the plane of the Gould Belt.     

Noncircular outer disks in unbarred S0 galaxies: NGC 502 and NGC 5485

Highly noncircular outer stellar disks have been detected in two SA0 (unbarred) galaxies by comparing the spectroscopic data on the rotation of stars and the photometric data on the shape and orientation of isophotes. In NGC 502, the oval distortion of the disk is manifested in the shape of the inner and outer elliptical rings occupying wide radial zones between the bulge and the disk and at the outer disk edge; such a structure can be a consequence of the so-called “dry minor merger,” multiple cannibalization of gas-free satellites. In NGC 5485, the stellar kinematics is absolutely unrelated to the orientation of isophotes in the disk region, and for this galaxy the conclusion about its global triaxial structure is unavoidable.