A. M. Melnik

Kinematics of OB-associations and the new reduction of the Hipparcos data

The proper motions of OB-associations computed using the old and new reductions of the Hipparcos data are in good agreement with each other. The Galactic rotation curve derived from an analysis of the line-of-sight velocities and proper motions of OB-associations is almost flat in the 3-kpc neighbourhood of the Sun. The angular rotation velocity at the solar distance is � 0 = 31 ± 1k m s −1 kpc −1 . The standard deviation of the velocities of OB-associations from the rotation curve is σ = 7.2 km s −1 . The distance scale for OB-associations should be shortened by 10–20 per cent. The residual velocities of OB-associations calculated for the new and old reductions differ, on average, by 3.5 km s −1 . The mean residual velocities of OB-associations in the stellar-gas complexes depend only slightly on the data reduction

Classical Cepheids in the Galactic outer ring R1R ′2

The kinematics and distribution of classical Cepheids within ∼3 kpc from the Sun suggest the existence of the outer ring R1R ′2 in the Galaxy. The optimum value of the solar position angle with respect to the major axis of the bar, θb, providing the best agreement between the distribution of Cepheids and model particles, is θb = 37° ±13°. The kinematical features obtained for Cepheids with negative galactocentric radial velocity VR are consistent with the solar location near the descending segment of the outer ring R2. The sharp rise of extinction toward of the Galactic center can be explained by the presence of the outer ring R1 near the Sun. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

N-body simulations in reconstruction of the kinematics of young stars in the Galaxy

Aims. We try to determine the Galactic structure by comparing the observed and modeled velocities of OB-associations in the 3 kpc solar neighborhood. Methods. We made N-body simulations with a rotating stellar bar. The galactic disk in our model includes gas and stellar subsystems. The velocities of gas particles averaged over large time intervals (∼8 bar rotation periods) are compared with the observed velocities of the OB-associations. Results. Our models reproduce the directions of the radial and azimuthal components of the observed residual velocities in the Perseus and Sagittarius regions and in the Local system. The mean difference between the model and observed velocities is ΔV = 3. 3k m s −1 . The optimal value of the solar position angle θb providing the best agreement between the model and observed velocities is θb = 45 ± 5 ◦ , in good accordance with several recent estimates. The self-gravitating stellar subsystem forms a bar, an outer ring of subclass R1, and slower spiral modes. Their combined gravitational perturbation leads to time-dependent morphology in the gas subsystem, which forms outer rings with elements of the R1 -a ndR2-morphology. The success of N-body simulations in the Local System is likely due to the gravity of the stellar R1-ring, which is omitted in models with analytical bars.

Peculiar Features of the Velocity Field of OB Associations and the Spiral Structure of the Galaxy

Some of the peculiar features of the periodic velocity-field structure for OB associations can be explained using the Roberts-Hausman model, in which the behavior of a system of dense clouds is considered in a perturbed potential. The absence of statistically significant variations in the azimuthal velocity across the Carina arm probably results from its sharp increase behind the shock front, which is easily blurred by distance errors. The existence of a shock wave in the spiral arms and, at the same time, the virtually free motion of OB associations in epicycles can be reconciled in the model of particle clouds with a mean free path of 0.2–2 kpc. The velocity field of OB associations exhibits two appreciable nonrandom deviations from an ideal spiral pattern: a 0.5-kpc displacement of the Cygnus-and Carina-arm fragments from one another and a weakening of the Perseus arm in quadrant III. However, the identified fragments of the Carina, Cygnus, and Perseus arms do not belong to any of the known types of spurs.

The Distance scale and the rotation curve of young supergiants and open clusters

Abstract The Hipparcos trigonometric parallaxes of 188 blue (O-type and BO-A3 I-II) supergiants with published uvby and Hβ photometry are compared to the photomeric parallaxes of these stars based on the Mv absolute-magnitude calibration of Dambis (1990). The photometric distance scale is found to require only a minor correction −πHip = (1.03 ± 0.04)πPhot, i.e., r Hip = (0.97 ± 0.04)r Phot, which is corroborated by applying the statistical-parallax technique to a sample of 106 blue supergiants with published radial velocities and Hipparcos proper motions (r True = (0.98 ± 0.08)r Phot). The weighted mean distance-scale correction factor can thus be estimated at r True = (0.97 ± 0.03)r Phot, thereby constraining the LMC distance modulus to DM LMC = 18.36 ± 0.08 (standard error) ±0.07 (systematic error) and providing further evidence in favor of the so-called ‘short’ distance scale. An expanded sample consisting of 106 blue supergiants with corrected photometric distances, published radial velocities, and Hi...

Kinematics of OB-associations in Gaia epoch

We use stellar proper motions from the TGAS catalog to study the kinematics of OB-associations. The TGAS proper motions of OB-associations generally agree well with the Hipparcos proper motions. The parameters of the Galactic rotation curve obtained with TGAS and Hipparcos proper motions agree within the errors. The average one-dimensional velocity dispersion inside 18 OB-associations with more than 10 TGAS stars is sigma_v=3.9 km s-1, which is considerably smaller, by a factor of 0.4, than the velocity dispersions derived from Hipparcos data. The effective contribution from orbital motions of binary OB-stars into the velocity dispersion sigma_v inside OB-associations is sigma_b=1.2 km s-1. The median virial and stellar masses of OB-associations are equal to 7.1 10^5 and 9.0 10^3 Ms, respectively. Thus OB-associations must be unbound objects provided they do not include a lot of dense gas. The median star-formation efficiency is epsilon=2.1 percent. Nearly one third of stars of OB-associations must lie outside their tidal radius. We found that the Per OB1 and Car OB1 associations are expanding with the expansion started in a small region of 11--27 pc 7--10 Myr ago. The average expansion velocity is 6.3 km s-1.

The Outer Ring of the Galaxy Revealed by Young Open Clusters

The distribution of young open clusters in the Galactic plane suggests the existence of the outer ring R1R ′ 2 in the Galaxy. The solar position angle θb providing the best agreement between the observed and model distribution is θb = 35±10 . We compared the θb values derived from three different catalogues of open cluster and they appear to be consistent within the errors.Abstract The distribution of young open clusters in the Galactic plane suggests the existence of the outer ring R1R′2 in the Galaxy. The solar position angle θb providing the best agreement between the observed and model distribution is θb = 35±10°. We compared the θb values derived from three different catalogues of open clusters and they appear to be consistent within the errors.

Straight segments in the galactic discs

We study the properties of the straight segments forming in N-body simulations of the galactic discs. The properties of these features are consistent with the observational ones summarized by Chernin at al. (2001). Unlike some previous suggestions to explain the straight segments as gas dynamical instabilities, they form in our models in the stellar system. We suggest that the straight segments are forming as a response of the rotating disc to a gravity of the regions of enhanced density (overdensities) corotating with the disc. The kinematics of stars near the prominent overdensities is consistent with this hypothesis.