The rotation rates of accreting planetesimals

There are obtained upper limits for the relative velocity at infinity of accreting planetesimals for a nearly constant mass of the largest accreting planetesimal and also in the case of variable mass. We conclude, that while the larger planets cannot be brought to the stage of rotational instability by stochastic collisions, the asteroids could be brought. provided that the relative velocities in the asteroid belt were larger than about 2 km s^–1.     

Collisions of galaxies of different masses, forms and sizes

We study the tidal effects of close collisions between two spherical galaxies of various mass and mass distributions by numerical simulations. The galaxies are represented by polytropes of indices n=4 and n=0 which denote cases of highcentral concentration and uniform mass distribution respectively.The initial relative velocity of the galaxies is chosen to be 700 km s^-1.The results indicate that the tidal effects are quite sensitive to both mass and mass distribution of the galaxies. The dependence oftidal capture and tidal disruption on the choice of the model and mass ratio is investigated. The classification of collisionis given for each simulation. The results also indicate that in a collision between two identical galaxies relatively more spin is imparted to the galaxies ifthey are centrally concentrated than if they are homogeneous.     

Anisotropic gravitational radiation in the problems of three and four black holes

The momentum flux in merging binary black holes is rediscussed using the actual orbit integrations. The terminal velocity acquired by the centre of mass of the system is found to be greater than the estimate of Fitchett (1983) by a factor of 1.45. The actual value in km s^–1 is still uncertain but may be as high as 2000 km s^–1. The centre of mass velocity kick at a black hole merger is incorporated in the orbit integration of few black hole systems. Assuming that the symmetric break-up mode of such systems corresponds to the classical double radio sources, we determine that the centre of mass velocity kick can be about 1000 km s^–1 at most.     

A piston-driven shock in the solar corona

A solar flare that occurred on the west limb at 1981, March 25, 2038 UT generated a massive, rapidly-expanding optical coronal transient, which moved outward with an approximately constant velocity of 800 km s^–1. An associated magnetohydrodynamic shock travelled out ahead of the transient with a velocity estimated to be approximately 1000 km s^–1. The optical and radio data on the transient and shock fit well with general theories concerning piston-driven shocks and with current MHD models for propagation of such shocks through the solar corona.     

The speeds of coronal mass ejection events

The outward speeds of mass ejection events observed with the white light coronagraph experiment on Skylab varied over a range extending from less than 100 km s^–1 to greater than 1200 km s^–1. For all events the average speed within the field of view of the experiment (1.75 to 6 solar radii) was 470 km s^–1. Typically, flare associated events (Importance 1 or greater) traveled faster (775 km s^–1) than events associated with eruptive prominences (330 km s^–1); no flare associated event had a speed less than 360 km s^–1, and only one eruptive prominence associated event had a speed greater than 600 km s^–1. Speeds versus height profiles for a limited number of events indicate that the leading edges of the ejecta move outward with constant or increasing speeds.Metric wavelength type II and IV radio bursts are associated only with events moving faster than about 400 km s^–1; all but two events moving faster than 500 km ^–1 produced either a type II or IV radio burst or both. This suggests that the characteristic speed with which MHD signals propagate in the lower (1.1 to 3 solar radii) corona, where metric wavelength bursts are generated, is about 400 to 500 km s^–1. The fact that the fastest mass ejection events are almost always associated with flares and with metric wavelength type II and IV radio bursts explains why major shock wave disturbances in the solar wind at 1 AU are most often associated with these forms of solar activity rather than with eruptive prominences.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Changes in the gravitational energy of galaxies during collisions

The problem of the change in gravitational energy of a colliding galaxy due to tidal effects is considered. The change in the internal energy, the mass of escaping matter and the change in the mean radius of the test galaxy have been estimated for a relative velocity of 1000 km s^–1 for three distances of closest approach for the following four cases: (a) both galaxies centrally concentrated, (b) both galaxies homogeneous, (c) test galaxy centrally concentrated, field galaxy homogeneous, and (d) test galaxy homogeneous, field galaxy centrally concentrated. The masses and radii of the two galaxies are taken as 10¹¹ M and 10 kpc respectively. For simplicity, the galaxies are assumed to be spherically symmetric and the distribution of mass within a centrally concentrated galaxy is assumed to be that of a polytrope of indexn=4. The results also provide estimates for the minimum relative velocity a galaxy must have in order that it may not be captured by another to form a double system. It has been found that normally a relative velocity of less than about 500 km s^–1 will lead to the formation of a double galaxy by tidal capture. In the case of a head-on collision between two centrally concentrated galaxies even a relative velocity of about 1000 km s^–1 is small enough for tidal capture. The changes in the structure of the galaxies for relative velocities equal to velocity of escape are also indicated. These results show that there is no escape of matter from the test galaxy in cases (b) and (c). In the case (a) the escape of matter can be as high as 4% of the total mass. The head-on collision between galaxies are normally not accompanied by any escape of matter. All the gain in the internal energy of galaxies during such collisions results in increase in their dimensions. The fractional increase in the mean radius of the test galaxy in the head-on collision is 1.5 in the case (a), 3.2 in the case (b) and 0.01 in the case (c). In the case (d) the test galaxy will be disrupted by the tidal forces.     

On the CN (0, 0) spectrum of Comet Mrkos 1957d

We have carried out an analysis of the (0, 0) vibrational band of the CN molecule in Comet Mrkos 1957d, including the effect of collisions. We found that the sum of the squares of the residuals can be reduced by a factor of ten, if collisions account for 46±3% of the population of the lower level. A rotational temperature can be assigned to the cometary gas. The best value found was 410±40 K. The best fit for the constantR ₁ was (1.07±0.10)×10^–4. The velocity of the comet was left as a free parameter. We found for it a value of 34.38±0.10 km s^–1. This result is in disagreement with the nuclear orbital velocity of 34.74 km s^–1. The discrepancy can be explained, if the CN molecules are ejected from the cometary nucleus preferentially in the sunward direction, with a mean velocity that corresponds to the above temperature.     

A comprehensive set of simulations of high-velocity collisions between main-sequence stars

We report on a very large set of simulations of collisions between two main-sequence (MS) stars. These computations were carried out with the smoothed particle hydrodynamics method. Realistic stellar structure models for evolved MS stars were used. In order to sample an extended domain of initial parameters space (masses of the stars, relative velocity and impact parameter), more than 14 000 simulations were carried out. We considered stellar masses ranging between 0.1 and  75 M_⊙  and relative velocities up to a few thousand km s⁻¹. To limit the computational burden, a resolution of 1000–32 000 particles per star was used. The primary goal of this study was to build a complete data base from which the result of any collision can be interpolated. This allows us to incorporate the effects of stellar collisions with an unprecedented level of realism into dynamical simulations of galactic nuclei and other dense stellar clusters. We make the data describing the initial condition and outcome (mass and energy loss, angle of deflection) of all our simulations available on the Internet. We find that the outcome of collisions depends sensitively on the stellar structure and that, in most cases, using polytropic models is inappropriate. Published fitting formulae for the collision outcomes, established from a limited set of collisions, prove of limited use because they do not allow robust extrapolation to other stellar structures or relative velocities.     

A classification of galactic collisions for galaxies of unequal dimensions

The earlier work on classification of galactic collisions for identical galaxies is extended to the situations where the two colliding galaxies differ in mass and dimension. We consider collisions between galaxies of masses 10⁷, 10⁹, 10¹¹ and 10¹³ M and let their radii be 0.1, 1, 10 and 100 kpc, respectively. The density distribution in both galaxies is represented by that of a polytrope of indexn=4. The type of collision is determined for head-on collisions for initial relative velocities 500, 1000, 2000 and 3000 km s^–1. The corresponding final velocities are also indicated.     

Photospheric and chromospheric oscillations in the base of coronal holes

In this paper we carry out an analysis of the spatial–temporal line-of-sight velocity variations measured in the chromospheric (H, H) and photospheric (Fei 6569 Å, Fei 4864 Å, Nii 4857 Å) lines at the base of 17 coronal holes. Time series of a duration from 43 to 120 min were recorded with the CCD line-array and the CCD matrix. Rather frequently we observed quasi-stationary upward flows with a measured velocity of up to 1 km s^–1 in the photosphere and up to 4–5 km s^–1 in the chromosphere (equivalent radial velocity of up to 3 km s^–1 and up to 12–15 km s^–1 accordingly) near dark points on the chromospheric network boundary inside polar CH. Line-of-sight velocity fluctuation spectra contain meaningful maxima in the low-frequency region clustering around the values 0.4, 0.75, and 1 mHz. Usually, the spatial localization of these maxima mutually coincides and, in our opinion, coincides with the chromospheric network boundary. Acoustic 3- and 5-min oscillations are enhanced in the coronal hole region and reach 1 km s^–1 in the photosphere and 3–4 km s^–1 in the chromosphere. These oscillations are not localized spatially and are distinguished throughout the entire region observed.     

EUV Spectroscopic Observations of Spray Ejecta from an X2 Flare

An X2.3 class flare was reported on 10 April 2001 in AR 9415. A halo coronal mass ejection (CME) was associated with this flare. The Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO), which was running in its daily synoptic mode, recorded a very high-velocity ejection of plasma associated with this activity. The spatial scanning and spectral capabilities of CDS allow the measurement of both transverse and line-of-sight velocities. Components of the plasma, as seen in emission from Ov at around 2.5×10⁵ K, reached transverse velocities in excess of 800 km s^–1. The nature of the spectral line profiles suggests that a rotational motion of ±350 km s^–1 was superimposed upon the general outward expansion of approximately 150 km s^–1. The ejection detected using CDS was found to have a constant acceleration and is thought to be a spray of plasma with a helical structure driven by the magnetic topology.     

Lunar velocity structure and compositional and thermal inferences

Seismic data from the Apollo Passive Seismic Network stations are analyzed to determine the velocity structure and to infer the composition and physical properties of the lunar interior. Data from artificial impacts (S-IVB booster and LM ascent stage) cover a distance range of 70–1100 km. Travel times and amplitudes, as well as theoretical seismograms, are used to derive a velocity model for the outer 150 km of the Moon. TheP wave velocity model confirms our earlier report of a lunar crust in the eastern part of Oceanus Procellarum.The crust is about 60 km thick and may consist of two layers in the mare regions. Possible values for theP-wave velocity in the uppermost mantle are between 7.7 km s^–1 and 9.0 km s^–1. The 9 km s^–1 velocity cannot extend below a depth of about 100 km and must decrease below this depth. The elastic properties of the deep interior as inferred from the seismograms of natural events (meteoroid impacts and moonquakes) occurring at great distance indicate that there is an increase in attenuation and a possible decrease of velocity at depths below about 1000 km. This verifies the high temperatures calculated for the deep lunar interior by thermal history models.Paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973.     

Forming dust precursors in the inner envelopes of carbon-rich AGB stars

The formation of polycyclic aromatic hydrocarbon (PAH) molecules is studied in the inner envelope of a typical carbon-rich AGB star. The deep envelope is formed of layers of gas that experience the passage of strong periodic shocks forming close to the stellar photosphere. The parcels of gas then follow quasi-ballistic trajectories which are characterized by high gas densities. A chemical scheme based on combustion chemistry is applied to shocked layers of gas, and a PAH formation yield is calculated. PAHs up to coronene (C₂₄H₁₂) survive shocks with strengths of 10 km s^–1, and they accumulate in the gas parcel over several stellar pulsations. This result illustrates that any C-rich AGB star can nucleate dust precursors in its envelope.     

An expanding high velocity hi cloud probably ejected from the galactic nucleus

An object located approximately atl=8°,b=–4° with a mean radial velocity of –212.3 km s^–1 has been observed in the 21 cm neutral hydrogen line. The mean weighted velocity dispersion is 11.2 km s^–1 and the total mass is estimated to be 190R ² (kpc) solar masses. We discuss possible interpretations of the origin and nature of this object. The most likely interpretation is that we observe an expanding object which has been ejected from the galactic nucleus.     

The velocity structure of the lunar crust

Seismic refraction data, obtained at the Apollo 14 and 16 sites, when combined with other lunar seismic data, allow a compressional wave velocity profile of the lunar near-surface and crust to be derived. The regolith, although variable in thickness over the lunar surface, possesses surprisingly similar seismic properties. Underlying the regolith at both the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is low-velocity brecciated material or impact derived debris. Key features of the lunar seismic velocity profile are: (i) velocity increases from 100–300 m s^–1 in the upper 100 m to 4 km s^–1 at 5 km depth, (ii) a more gradual increase from 4 km s^–1 to 6 km s^–1 at 25 km depth, (iii) a discontinuity at a depth of 25 km and (iv) a constant value of 7 km s^–1 at depths from 25 km to about 60 km. The exact details of the velocity variation in the upper 5 to 10 km of the Moon cannot yet be resolved but self-compression of rock powders cannot duplicate the observed magnitude of the velocity change and the steep velocity-depth gradient. Other textural or compositional changes must be important in the upper 5 km of the Moon. The only serious candidates for the lower lunar crust are anorthositic or gabbroic rocks.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.     

The application of a Bessel transform to the determination of stellar rotational velocities

A method for analysing line profiles by means of a transform using Bessel functions is described. This yields the stellar rotational velocityv sini, to an accuracy of about ±1 km s^–1 for rotational velocities greater than about 5 km s^–1, provided that rotation is the major source of line broadening. The theory of the method is a special case of a general theory of linear transforms in data analysis, which is outlined in an appendix.     

Implications of the Galilean satellites ice envelope explosions II. The origin of the irregular satellites

The problem of the origin of the irregular satellites is solved readily in the context of a hypothesis involving explosion of the massive ice envelopes of the Galilean satellites saturated by electrolysis products. The thrown-off unexploded (primary) ice fragments of the outermost cold layers of the envelopes are also saturated by electrolysis products. In the course of explosive ejection their internal energy increases due to shock wave heating, as a result of which they will be able to detonate in subsequent sufficiently energetic collisions. The secondary fragments from new explosions may acquire additional velocity up to a few km s^–1 without breakup into small pieces.Gravitational perturbations by the parent satellites can eject the primary fragments moving near their orbits into the periphery of or beyond Jupiter's sphere of action. If such a fragment explodes in the outer zone of the sphere, then secondary fragments may become irregular satellites resulting in the so-called internal capture (the possibilities of capture considered earlier involved only bodies entering the sphere of action from outside).The mass of the primary fragment responsible for the inner (direct) group of Jupiter's irregular satellites is estimated as 10¹⁹ kg, and the additional velocity acquired by secondary fragments as 1.3 km s^–1; evaluation of the mass of the fragment responsible for the outer (retrograde) group yields 10¹⁸ kg, and that of the additional velocity of secondary fragments, 2 km s^–1.The ice envelopes of the Galilean and similar moonlike satellites should contain impurities corresponding to the composition of type C1 carbonaceous chondrites; therefore after sublimation of water ice the irregular satellites (just as type C asteroids, the Trojans and comets) exhibit spectro-photometric properties similar to those of C-type objects.     

Rotation of pre-main-sequence stars in NGC 2264 and the solar angular momentum problem

Preliminary measurements of rotational velocities of pre-main sequence stars indicate that stars evolving into early F or late A spectral type have rotational velocities which are consistent with present Main-Sequence stars of similar spectral type. Stars evolving into G type, however, have rotational velocities which are as high as 100 km s^–1 and would reach the Main Sequence with velocities of 150 km s^–1. This requires the presence of a strong stellar wind to carry off considerable angular momentum in order to slow down the Sun to its present low rotational velocity.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

The height variation of granular and oscillatory velocities

Previous observations of spatially-resolved vertical velocity variations in ten lines of Fe i spanning the height range 0 h 1000 km are re-analyzed using velocity weighting functions. The amplitudes and scale heights of granular and oscillatory velocities are determined, as well as those of the remaining unresolved velocities. I find that the optimal representation of the amplitude of the outward-decreasing granular velocities is an exponentially decreasing function of height, with a scale height of 150 km and a velocity at zero height of 1.27 km s^–1. The optimal representation of the same quantities for oscillatory velocities is an exponential increase with height, with a scale height of 1100 km and a velocity at zero height of 0.35 km s^–1. The remaining unresolved velocities decrease with height, with a scale height of 380 km and a velocity at zero height of 2.3 km s^–1.     

Nebular shock waves generated by planetesimals passing through Jovian resonances: Possible sites for chondrule formation

Abstract— The primordial asteroid belt contained at least several hundred and possibly as many as 10,000 bodies with diameters of 1000 km or larger. Following the formation of Jupiter, nebular gas drag combined with passage of such bodies through Jovian resonances produced high eccentricities (e = 0.3‐0.5), low inclinations (i < 0.5°), and, therefore, high velocities (3–10 km/s) for “resonant” bodies relative to both nebular gas and non‐resonant planetesimals. These high velocities would have produced shock waves in the nebular gas through two mechanisms. First, bow shocks would be produced by supersonic motion of resonant bodies relative to the nebula. Second, high‐velocity collisions of resonant bodies with non‐resonant bodies would have generated impact vapor plume shocks near the collision sites. Both types of shocks would be sufficient to melt chondrule precursors in the nebula, and both are consistent with isotopic evidence for a time delay of ?1‐1.5 Myr between the formation of CAIs and most chondrules. Here, initial simulations are first reported of impact shock wave generation in the nebula and of the local nebular volumes that would be processed by these shocks as a function of impactor size and relative velocity. Second, the approximate maximum chondrule mass production is estimated for both bow shocks and impact‐generated shocks assuming a simplified planetesimal population and a rate of inward migration into resonances consistent with previous simulations. Based on these initial first‐order calculations, impact‐generated shocks can explain only a small fraction of the minimum likely mass of chondrules in the primordial asteroid belt (?10²⁴‐10²⁵g). However, bow shocks are potentially a more efficient source of chondrule production and can explain up to 10–100 times the estimated minimum chondrule mass.