On the classification of comet plasma tails

The investigation of plasma tails of comets is an important part of comet research. Different classifications of plasma tails of comets are proposed. Plasma acceleration in the tails is investigated in sufficient detail. Several cometary forms are explained. Plasma tails of Mars and Venus were observed during the first studies of these planets. They are associated with the capture of ionized atoms and exosphere molecules by the solar wind magnetized plasma flow. Distinct plasma tails of Mars and Venus are caused by the mass loading of the solar wind with heavy ions. It was shown that the transverse dimension of the tails of Mars, Venus, and comets can be quite accurately determined by production rate of the obstacle to the solar wind flow. While plasma tails of Mars and Venus are investigated by in situ measurements from spacecraft, observations of comet tails from the Earth make it possible to see the entire object under study and to monitor changes in its structure. A certain similarity of cometary and planetary tails can be explained by the nonmagnetic nature of both types of bodies. Thus, it is reasonable to suppose that investigations of plasma tails of comets can supplement the information obtained by in situ methods of the study of the planets. In this paper, plasma tails of comets, presumably analogous to the plasma tails of Mars and Venus, have been identified on modern photographs of comets (more than 1500 photographs viewed). Only quasi-steady laminar tails are considered. They are divided into two types: double structures and outflows. The paper attempts to define the 3D structure of double structures and to determine certain characteristics of outflows.     

Tidal tails in cold dark matter cosmologies

We study the formation of tidal tails in pairs of merging disc galaxies with structural properties motivated by current theories of cold dark matter (CDM) cosmologies. In a recent study, Dubinski, Mihos & Hernquist showed that the formation of prominent tidal tails can be strongly suppressed by massive and extended dark haloes. For the large halo-to-disc mass ratio expected in CDM cosmologies their sequence of models failed to produce strong tails like those observed in many well-known pairs of interacting galaxies. In order to test whether this effect can constrain the viability of CDM cosmologies, we construct N ‐body models of disc galaxies with structural properties derived in analogy to the recent analytical work of Mo, Mao & White. With a series of self-consistent collisionless simulations of galaxy–galaxy mergers we demonstrate that even the discs of very massive dark haloes have no problems developing long tidal tails, provided the halo spin parameter is large enough. For our class of models, the halo-to-disc mass ratio is not a good indicator of the ability to produce tails. Instead, the relative size of disc and halo or, alternatively, the ratio of circular velocity to local escape speed at the half mass radius of the disc is a more useful criterion. This result holds in all CDM models. While tidal tails can provide useful information on the structure of galaxies, it thus appears unlikely that they are able to constrain the values of the cosmological parameters within these models.     

Effects of external tidal field on the evolution of the outer regions of multi-mass star clusters

We present N -body simulations (including an initial mass function) of globular clusters in the Galaxy in order to study effects of the tidal field systematically on the properties of the outer parts of globular clusters. Using nbody6 , which correctly takes into account the two-body relaxation, we investigate the development of tidal tails of globular clusters in the Galactic tidal field. For simplicity, we have employed only the spherical components (bulge and halo) of the Galaxy, and ignored the effects of stellar evolution which could have been important in the very early phase of the cluster evolution. The total number of stars in our simulations is about 20 000, which is much smaller than the realistic number of stars. All simulations had been done for several orbital periods in order to understand the development of the tidal tails. In our scaled-down models, the relaxation time is sufficiently short to show the mass segregation effect, but we did not go far enough to see the core collapse, and the fraction of stars lost from the cluster at the end of the simulations is only ∼10 per cent. The radial distribution of extra-tidal stars can be described by a power law with a slope around −3 in surface density. The directions of tidal tails are determined by the orbits and locations of the clusters. We find that the length of tidal tails increases towards the apogalacticon and decreases towards the perigalacticon. This is an anti-correlation with the strength of the tidal field, caused by the fact that the time-scale for the stars to respond to the potential is similar to the orbital time-scale of the cluster. The escape of stars in the tidal tails towards the pericentre could be another reason for the decrease of the length of tidal tails. We find that the rotational angular velocity of tidally induced clusters shows quite different behaviour from that of initially rotating clusters.     

Analysis of the structure and dynamics of the stellar tails of open star clusters

We have analyzed the formation, structure, and dynamical evolution of the population of stars that escaped from open clusters by numerical simulations using S. Aarseth’s modified NBODY6 code. In the Galactic tidal field, the population of stars that escaped from a cluster is shown to be elongated along the orbit of the cluster symmetrically about its core in the form of stellar tails of increasing sizes. We analyze the parameters of stellar tails as a function of such initial simulation conditions as the number of stars, the cluster density, the eccentricity of the Galactic cluster orbit in the plane of the Galactic disk, and the z velocity component. As a result, we constructed a grid of model stellar tails of open clusters. The grid includes such time-dependent parameters of the stellar tails as the length, the cross section, the number of stars, the velocity distribution, etc. Our simulations allow us to clarify the origin of moving clusters and stellar streams and to assess the role of star clusters in forming the stellar velocity field in the solar neighborhood.     

Ram-pressure stripping of disc galaxies orbiting in clusters – II. Galactic wakes

We present 3D hydrodynamical simulations of ram-pressure stripping of a disc galaxy orbiting in a galaxy cluster. In this paper, we focus on the properties of the galaxies' tails of stripped gas. The galactic wakes show a flaring width, where the flaring angle depends on the gas disc's cross-section with respect to the galaxy's direction of motion. The velocity in the wakes shows a significant turbulent component of a few     . The stripped gas is deposited in the cluster rather locally, i.e. within     from where it was stripped. We demonstrate that the most important quantity governing the tail density, length and gas mass distribution along the orbit is the galaxy's mass-loss per orbital length. This in turn depends on the ram pressure as well as the galaxy's orbital velocity.
For a sensitivity limit of     in projected gas density, we find typical tail lengths of     . Such long tails are seen even at large distances (0.5 to     ) from the cluster centre. At this sensitivity limit, the tails show little flaring, but a width similar to the gas disc's size.
Morphologically, we find good agreement with the H  i tails observed in the Virgo cluster by Chung et al. 2007 . However, the observed tails show a much smaller velocity width than predicted from the simulation. The few known X-ray and Hα tails are generally much narrower and much straighter than the tails in our simulations. Thus, additional physics like a viscous intracluster medium (ICM), the influence of cooling and tidal effects may be needed to explain the details of the observations.
We discuss the hydrodynamical drag as a heat source for the ICM but conclude that it is not likely to play an important role, especially not in stopping cooling flows.     

银河系恒星潮汐流的探测进展

银晕外区存在众多星流,它们或源自银河系的矮伴星系,或源自晕族球状星团,常分别称为矮星系星流和球状星团潮汐尾。星流可以利用各类示踪星,并通过不同的途径加以探测,对若干代表性矮星系星流和球状星团潮汐尾的探测进展做了简要的介绍。     

A study of the icy tails of the distant comets

The properties of the icy-grain model, formulated recently for the nearly straight, structureless tails of a number of comets with large perihelion distances, are studied. The approach used is based on the comparison of the orientation and general profile of the tails with a set of synchrones, i.e., theoretical trajectories of particles emitted from the cometary nucleus at particular times. A number of features observed in the distant comets, such as a sharply bounded envelope around the nuclear condensation, jet activity in the coma, a slight curvature of the tail, and the absence of its appreciable broadening with increasing distance from the nucleus, are also explained by the icy-grain model. The model is further confronted with the tail-orientation, spectroscopic, and spectrophotometric data available on comets with perihelia beyond 2.2 AU. It is established that the transition region between 2 and 3 AU, where water snow starts evaporating rapidly, has a profound effect on the dynamics of the icy tails. It is suggested that the icy (or solid-hydrate) grains, constituting the tails of the distant comets, may be carriers of fine meteoric-dust particles, of microns and submicron sizes, which are set free once the grains start disintegrating by evaporation.     

Ⅰ型彗尾扭折效应及其行星际激波的某些特性

本文对十多次等离子体彗尾扭折的资料进行了测算,由此分析了行星际激波的某些特性。由太阳事件导致的行星际激波,不仅时空尺度很大、而且强度也很高;但是由别的原因导致的行星际骚扰,一般产生的,则是时空尺度较小的弱激波。根据彗尾扭折效应的分析,文章给出了行星际激波波面前后的速度差值,其大致范围在100—500km/s。     

On the characteristics of tidal structures of interacting galaxies

We present the results of our analysis of the geometrical tidal tail characteristics for nearby and distant interacting galaxies. The sample includes more than two hundred nearby galaxies and about seven hundred distant ones. The distant galaxies have been selected in several deep fields of the Hubble Space Telescope (HDF-N, HDF-S, HUDF, GOODS, GEMS) and they are at mean redshift 〈z〉 = 0.65. We analyze the distributions of lengths and thicknesses for the tidal structures and show that the tails in distant galaxies appear shorter than those in nearby ones. This effect can be partly attributed to observational selection, but, on the other hand, it may result from the general evolution of the sizes of spiral galaxies with z. The positions of interacting galaxies on the galaxy luminosity (L)-tidal tail length (l) plane are shown to be explained by a simple geometrical model, with the upper envelope of the observed distribution being \(l \propto \sqrt L\). We have solved the problem on the relationship between the observed distribution of tail flatting and the tail length in angular measure by assuming the tidal tails to be arcs of circumferences visible at arbitrary angles to the line of sight. We conclude that the angular length of the tidal tails visually distinguished in nearby and distant galaxies, on average, exceeds 180°.     

Kelvin-Helmholtz instabilities in cometary ion tails

We interpret observations of wave-like phenomena in cometary ion tails in terms of the interaction of the tails with the solar wind through Kelvin-Helmholtz helical instabilities. The calculations are applied to three comets, Comet Kohoutek (1973f), Comet Arend Roland (1957 III) and Comet Morehouse (1908c). Whilst some disagreement is found with earlier work, it is nevertheless shown that, assuming typical parameters, the growth rate of the K-H helical mode should be significant for wavelengths approximately equal to the tail radius.     

Shock waves in a dusty plasma with dust of opposite polarities

A four-component dusty plasma consisting of electrons, ions, and negative as well as positive dust particles has been considered. The basic features of shock waves that may exist in such a four-component dusty plasma have been theoretically investigated by the reductive perturbation method. The implications of our results in different regions of space (viz. cometary tails, mesosphere, Jupiter’s magnetosphere, etc.) are briefly discussed.     

哈雷彗星Ⅰ型彗尾风差角的测定

1986年3月8日哈雷彗星发生一次Ⅰ型彗尾断尾事件,新主尾与老主尾的风差角不同。本文对我们所摄哈雷彗星大尺度底片,用光度测量方法定出主尾轴的视风差角;并且用导出的公式,算出相应的真风差角。最后,对所得结果进行一些讨论。     

Helical waves in type-1 comet tails

Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field.     

Physical conditions in the plasma tail of comet C/1987 P1 bradfield

Photometric measurements of photographic images of comet C/1987 P1 Bradfield have been carried out with a flat-bed scanner equipped with a slide module. Lengthwise and transverse photometric profiles of the cometary plasma tail have been obtained. Magnetic field induction and some other physical characteristics of the cometary plasma tail observed in November 1987 have been estimated with the use of the diffusion model for a cometary tail by Shul’man and Nazarchuk (1968). It has been shown that the scanned images of comets can be used for estimating the physical characteristics of cometary tails.     

Tidal tails of star clusters

Based on recent findings of a formation mechanism of substructure in tidal tails by Küpper et al., we investigate a more comprehensive set of N -body models of star clusters on orbits about a Milky Way like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up.
The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case.
We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disc shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark matter substructures in the haloes of galaxies.     

Velocity-Space Proton Diffusion in the Solar Wind Turbulence

We study the velocity-space quasi-linear diffusion of the solar wind protons driven by oblique Alfvén turbulence at proton kinetic scales. Turbulent fluctuations at these scales possess the properties of kinetic Alfvén waves (KAWs) that are efficient in Cherenkov-resonant interactions. The proton diffusion proceeds via Cherenkov kicks and forms a quasi-linear plateau – the nonthermal proton tail in the velocity distribution function (VDF). The tails extend in velocity space along the mean magnetic field from 1 to (1.5?–?3) V _A, depending on the spectral break position, on the turbulence amplitude at the spectral break, and on the spectral slope after the break. The most favorable conditions for the tail generation occur in the regions where the proton thermal and Alfvén velocities are about equal, V _Tp/V _A≈1. The estimated formation times are within 1?–?2 h for typical tails at 1 AU, which is much shorter than the solar wind expansion time. Our results suggest that the nonthermal proton tails, observed in situ at all heliocentric distances >?0.3 AU, are formed locally in the solar wind by the KAW turbulence. We also suggest that the bump-on-tail features – proton beams, often seen in the proton VDFs, can be formed at a later evolutional stage of the nonthermal tails by the time-of-flight effects.     

Formation of gas discs in merging galaxies

Observations indicate that much of the interstellar gas in merging galaxies may settle into extended gaseous discs. Here, I present simulations of disc formation in mergers of gas-rich galaxies. Up to half of the total gas settles into embedded discs; the most massive instances result from encounters in which both galaxies are inclined to the orbital plane. These discs are often warped, many have rather complex kinematics, and roughly a quarter have counter-rotating or otherwise decoupled central components. Discs typically grow from the inside out; infall from tidal tails may continue disc formation over long periods of time.     

Coronal expansion and the transfer of solar angular momentum to the interplanetary space

We discuss the question of loss of angular momentum through coronal expansion. From a large volume of data on Type-1 cometary tails we have confirmed the presence of a tangential component in the coronal expansion, which has not only a stochastic component but also a constant component of 9.8 km/s. Through coronal expansion the Sun has lost 80% of its angular momentum since it evolved on to the main sequence and the angular velocity of the Sun is decreasing exponentially. This result should have a large effect on the dynamical evolution of the Sun.     

Tidal structures of z ∼ 1 galaxies in the Hubble Deep Field

An analysis of the images of objects in the Northern Hubble Space Telescope Deep Field has revealed twelve galaxies with tidal tails at redshifts from 0.5 to 1.5. The integrated characteristics of the newly discovered tidal structures are found to be similar to those of the tails of local interacting galaxies. The space density of galaxies with tidal tails is found to depend on z as (1+z)^4±1(q ₀=0.05), according to the data on objects with z=0.5–1.0. The exponent decreases to 3.6 if barred galaxies are included. The change in the rate of close encounters between galaxies of comparable masses (i.e., those that produce extended tidal structures) is estimated. If the rate of galactic mergers is governed by the same process, our data are indicative of the rapid evolution of galaxy merger rate toward z ～ 1.