Direct determination of the spiral pattern rotation speed of the Galaxy

The rotation velocity of the spiral pattern of the Galaxy is determined by direct observation of the birthplaces of open clusters of stars in the galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, like the existence of a given number of spiral arms, or the presence of a bar in the central regions. This study became possible due to the recent completion of a large database on open clusters by our group. The birthplaces of the clusters are determined by two methods, one that assumes that the orbits are circular, and the other by integrating the orbits in the Galactic potential for a time equal to the age of the clusters. We selected in the database a sample of 212 clusters for which proper motions, radial velocities, distances and ages are available, or of 612 clusters that have ages and distances available. We tested different assumptions concerning the rotation curve and the radius $R_0$ of the solar orbit. Our results confirm that a dominant fraction of the open clusters are formed in spiral arms, and that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves. We find that the corotation radius $R_c$ is close to the solar galactic orbit ($R_c/R_0 = 1.08 \pm 0.08$). This proximity has many potentially interesting consequences, like a better preservation of life on the Earth, and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the galactic disk.Comment: 9 pages, 5 figures, submitted to Ap

Corotation: its influence on the chemical abundance pattern of the Galaxy

A simple theory for the chemical enrichment of the Galaxy which takes into account the effects of spiral arms on heavy elements output was developed. In the framework of the model with the corotation close to the position of the Sun in the Galaxy the observed abundance features are explained.Comment: LaTeX, 6 pages, 5 jpg figures, uses aastex.sty, submitted to ApJ Let

Global Spiral Modes in NGC 1566: Observations and Theory

We present an observational and theoretical study of the spiral structure in galaxy NGC 1566. A digitized image of NGC 1566 in I-band was used for measurements of the radial dependence of amplitude variations in the spiral arms. We use the known velocity dispersion in the disk of NGC 1566, together with its rotation curve, to construct linear and 2D nonlinear simulations which are then compared with observations. A two-armed spiral is the most unstable linear global mode in the disk of NGC 1566. The nonlinear simulations are in agreement with the results of the linear modal analysis, and the theoretical surface amplitude and the velocity residual variations across the spiral arms are in qualitative agreement with the observations. The spiral arms found in the linear and nonlinear simulations are considerably shorter than those observed in the disk of NGC 1566. We argue therefore, that the surface density distribution in the disk of the galaxy NGC 1566 was different in the past, when spiral structure in NGC 1566 was linearly growing.Comment: 41 pages, 20 figures, to be published in the Astrophysical Journa

A New Model for the Spiral Structure of the Galaxy. Superposition of 2+4-armed patterns

We investigate the possibility of describing the spiral pattern of the Milky Way in terms of a model of superposition 2- and 4-armed wave harmonics (the simplest description, besides pure modes). Two complementary methods are used: a study of stellar kinematics, and direct tracing of positions of spiral arms. In the first method, the parameters of the galactic rotation curve and the free parameters of the spiral density waves were obtained from Cepheid kinematics, under different assumptions. To turn visible the structure corresponding to these models, we computed the evolution of an ensemble of N-particles, simulating the ISM clouds, in the perturbed galactic gravitational field. In the second method, we present a new analysis of the longitude-velocity (l-v) diagram of the sample of galactic HII regions, converting positions of spiral arms in the galactic plane into locii of these arms in the l-v diagram. Both methods indicate that the ``self-sustained'' model, in which the 2-armed and 4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively) is a good description of the disk structure. An important conclusion is that the Sun happens to be practically at the corotation circle. As an additional result of our study, we propose an independent test for localization of the corotation circle in a spiral galaxy: a gap in the radial distribution of interstellar gas has to be observed in the corotation region.Comment: 17 pages, 9 figures, Latex, uses aas2pp4.st

Anisotropy of dark matter annihilation with respect to the Galactic plane

We describe the anisotropy of dark matter clump distribution caused by tidal destruction of clumps in the Galactic disk. A tidal destruction of clumps with orbit planes near the disk plane occurs more efficiently as compared with destruction of clumps at near-polar orbits. A corresponding annihilation of dark matter particles in small-scale clumps produces the anisotropic gamma-ray signal with respect to the Galactic disk. This anisotropy is rather small, 9%, and superimposed on that due to off-centering position of the Sun in the Galaxy. The anisotropy of annihilation signal with respect to the Galactic disk provides a possibility to discriminate dark matter annihilation from the diffuse gamma-ray backgrounds of other origin.Comment: Accepted for publication in JCAP (minor changes

Dark energy from instantons

We show that in imaginary time quantum metric fluctuations of empty space form a self-consistent de Sitter gravitational instanton that can be thought of as describing tunneling from "nothing" into de Sitter space of real time (no cosmological constant or scalar fields are needed). For the first time, this mechanism is activated to give birth to a flat inflationary Universe. For the second time, it is turned on to complete the cosmological evolution after the energy density of matter drops below the threshold (the energy density of instantons). A cosmological expansion with dark energy takes over after the scale factor exceeds this threshold, which marks the birth of dark energy at a redshift $1+z\approx 1.3$ and provides a possible solution to the "coincidence problem". The number of gravitons which tunneled into the Universe must be of the order of $10^{122}$ to create the observed value of the Hubble constant. This number has nothing to do with vacuum energy, which is a possible solution to the "old cosmological constant problem". The emptying Universe should possibly complete its evolution by tunneling back to "nothing". After that, the entire scenario is repeated, and it can happen endlessly.Comment: 11 two-column pages, 1 figure, V2 is updated pre-publication version. The title is changed. Section 4 (Birth of Dark Energy) is extended. Misprints are fixe

Galactic Rotation Parameters from Data on Open Star Clusters

Currently available data on the field of velocities Vr, Vl, Vb for open star clusters are used to perform a kinematic analysis of various samples that differ by heliocentric distance, age, and membership in individual structures (the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the Galactic rotation parameters Wo =-26.0+-0.3 km/s/kpc, W'o = 4.18+-0.17 km/s/kpc^2, W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1 km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo = 0+-1 degrees. The Galactocentric distance Ro in the model used has been found to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7 kpc and 5.6+-0.3 kpc for the samples of young (50 Myr) clusters, respectively. Our study of the kinematics of young open star clusters in various spiral arms has shown that the kinematic parameters are similar to the parameters obtained from the entire sample for the Carina-Sagittarius and Perseus arms and differ significantly from them for the Orion arm. The contraction effect is shown to be typical of star clusters with various ages. It is most pronounced for clusters with a mean age of 100 Myr, with the contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table

On the Internal Structure of Relativistic Jets

A magnetohydrodynamic model is constructed for a cylindrical jet immersed in an external uniform magnetic field. It is shown that, as in the force-free case, the total electric current within the jet can be zero. The particle energetics and the magnetic field structure are determined in a self-consistent way; all jet parameters depend on the physical conditions in the external medium. In particular, we show that a region with subsonic flow can exist in the central jet regions. In actual relativistic jets, most of the energy is transferred by the electromagnetic field only when the magnetization parameter is sufficiently large, $\sigma>10^6$. We also show that, in general, the well-known solution with a central core, $B_z = B_0/(1+\varpi^2/\varpi_c^2)$, can not be realized in the presence of an external medium.Comment: 19 pages, 2 figure

Habitable Zones in the Universe

Habitability varies dramatically with location and time in the universe. This was recognized centuries ago, but it was only in the last few decades that astronomers began to systematize the study of habitability. The introduction of the concept of the habitable zone was key to progress in this area. The habitable zone concept was first applied to the space around a star, now called the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable zones have been proposed. We review the historical development of the concept of habitable zones and the present state of the research. We also suggest ways to make progress on each of the habitable zones and to unify them into a single concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and Evolution of Biospheres; table slightly revise

Ionospheric gas dynamics of satellites and diagnostic probes

The gas dynamics of interactions of a tenuous ionosphere with moving satellites and probes that have bearings on the diagnostics of the ionosphere are discussed. Emphasis is on the cases where the body is moving at mesothermal speeds, namely intermediate between the thermal speeds of ions and electrons of the ambient ionosphere. Methods of collision-free plasma kinetics with self-consistent field are used. The development of the topics for discussion starts with stationary Langmuir probe which entails the basic mechanism of body-plasma interaction that becomes further intricated as the body moves at a higher and higher speed. Applications of the theory of plasma interaction to meteors which move in the ionosphere are also presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43801/1/11214_2004_Article_BF00212707.pd