On the origin of high-velocity runaway stars

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary  50–100 M_⊙  star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of  ≳200–400 km s⁻¹  (typical of pulsars), while  3–4 M_⊙  stars can attain velocities of  ≳300–400 km s⁻¹  (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.     

Orbital eccentricity study for the spherical component of our galaxy

An analysis of the data concerning high-velocity stars from Eggen's catalogue aimed at a determination of the approximate slope of the mass function for the spherical component of our Galaxy, and at estimating the local circular velocity, as well as the local rotation velocity, as by-products, has been performed. Our conclusions are that:

1. A linear dependence of the mass on the radius is very likely;
2. the value of the limiting radius is most likely equal to (40±10) kpc;
3. the two local velocities are approximately equal to each other, being both equal to (230±30) km s^?1;
4. the local escape velocity appears to be most likely equal to (520±30) km s^?1;
5. the total mass of a corona, obtained in this way, is (5±1)×10¹¹ M _⊙.

     

Three-dimensional distribution of high-velocity stars

Numerical calculations support the conjecture that high velocity stars born in or near the rotating disk are not distributed spherically throughout the galaxy. Instead an empty funnel region forms around the rotation axis of the galaxy, where no stars reside. The funnel appears only for high velocity stars, such as old pulsars. Within about 50 kpc distance from the galactic center the funnel shape is almost not dependent on the existence of a massive corona.     

On the orbits of globular clusters of stars in the galaxy

The study of the possible shapes of orbits of globular clusters of stars is closely connected with problems of studying the processes of formation and evolution of stars. The main difficulty in studying the orbits of these objects is the incompleteness of the information about the initial conditions of the motion, in particular the nearly complete absence of observational material on the proper motions of globular clusters. For that reason the study of the motion of these objects by direct dynamic methods has not yet succeeded. In such conditions it is necessary to resort to statistical methods developed by Edmondson and Von Hoerner, which make it possible to draw certain conclusions about the possible shapes of orbits of globular clusters on the basis of the Newtonian point-mass model.The purpose of this paper is to test the results of investigations of other authors against the latest observational data on globular clusters of stars and to develop the method itself further.Translated fromAstrofizika, Vol. 37, No. 4, 1994.     

On the wind momentum problem of O-type stars in the galaxy

We have examined the wind momentum problem of O-type stars in the Galaxy. It is shown that the discrepancy between theoretical and empirical mass loss rates and terminal velocities can be reversed by using recently updated values of force multiplier parameters. With these new values, the momentum problem found by former investigators is reversed so that there now appears to be more than enough radiation force in order to accelerate the stellar winds of a sample of Galactic O-type stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interstellar clouds and the formation of stars

Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The pseudo-plasma formalism, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important.In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well pinch the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together.Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of dust (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A stellesimal accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites.The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems.     

Bose statistics and the stars

A brief account is given of the early development of a new sources. Following the chance discovery that it was unaffected by scintillation it was proposed to apply the same principle to measuring visible stars. This proposal met with vigorous opposition from physicists when it was realised that it implied that the time of arrival of photons in two mutually coherent beams of light must be correlated. Two laboratory experiments were done to demonstrate that this correlation does in fact take place. Then, after a pilot model had measured the angular size of Sirius, a full scale stellar intensity interferometer was built and installed at Narrabri in Australia. In a programme lasting 12 years it measured the angular diameters of 32 single stars in the spectral range O to F and established the first wholly empirical temperature scale for stars in that range. For the last 10 years the work has been continued by the construction of the larger and more sensitive Sydney University Stellar Interferometer called SUSI.     

Soft X-ray diagnostics of the kinematics of high-velocity clouds

We have made near-infrared photometric observations of nine β-Cephei and eight δ-Scuti stars inJ, H, andK bands. The observed fluxes are in good agreement with those expected according to their spectral types. We conclude that these stars do not have any anomalous emission in these near-infrared bands.     

Interaction of low- and high-velocity systems in the galaxy NGC 1275

Published data on gas systems of different velocities in the galaxy NGC 1275 are examined. One of the systems is associated with NGC 1275 (low-velocity system — LV); the other is approaching it at a velocity of 3000 km/sec (high-velocity system — HV). Many of the collected results obtained from spectra and from direct images in the ultraviolet, optical, red, and infrared indicate interaction of these systems. The interaction is exhibited in the same shape and spatial distribution of the gas filaments in both systems, in the elongation of some of them toward the nucleus of the galaxy, and in the increase in brightness of the HV gas near some of the clusters of young stars of the LV system. Gas of the HV system is observed at a distance of O.5 (170 pc) from the nucleus of the galaxy, while intermediate-velocity gas (IV — 600–1520 km/sec relative to the velocity of NGC 1275) is detected at distances less than 7 (2.5 kpc). We presume that the rare cases of the detection of IV gas are related to the use of H_a observations primarily: at the velocities of 600–900 km/sec, the H_a line of the IV gas blends with the [NII] 6584Å line of the LV gas.Translated fromAstrofizika, Vol. 39, No. 4, pp. 567–584, November, 1996.     

Intermediate-metallicity, high-velocity stars and Galactic chemical evolution

High signal-to-noise ratio spectra were obtained of 10 high-proper-motion stars having  −1 ≲[Fe/H] < 0  , and a comparable number of disc stars. All but two of the high-proper-motion stars were confirmed to have  [Fe/H] > −1.0  , some approaching solar metallicity, but, even so, earlier measurements overestimated the metallicities and velocities of some of these stars. Models of stellar populations were used to assign membership probabilities to the Galactic components to which the high-velocity stars might belong. Many were found to be more probably thick-disc than halo objects, despite their large space motions, and two might be associated with the inner Galaxy. It may be necessary to reassess contamination of previous halo samples, such as those used to define the metallicity distribution, to account for contamination by high-velocity thick-disc stars, and to consider possible subcomponents of the halo.
The change in [α/Fe] ratios at  [Fe/H]≃−1.0  is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. [Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-[Ti/Fe] halo stars exist; likewise high- and low-[Ti/Fe] thick-disc stars exist. We conclude that the [Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the [α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of [Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al.     

Optical observations of flare stars in the galaxy

Observations of flare stars in the galaxy are considered. UV Ceti type stars in the solar vicinity and flare stars is star clusters and associations have almost the same properties. The differences between them are connected with the age. Flare stars are one of the richest populations in the galaxy. The evolutionary path for all flare stars is the same.Published in Astrofizika, Vol. 38, No. 4, pp. 501–507, October–December, 1995.