Flyby Encounters Research Articles

Habitability in Different Milky Way Stellar Environments: A Stellar Interaction Dynamical Approach

Every Galactic environment is characterized by a stellar density and a velocity dispersion. With this information from literature, we simulated flyby encounters for several Galactic regions, numerically calculating stellar trajectories as well as orbits for particles in disks; our aim was to understand the effect of typical stellar flybys on planetary (debris) disks in the Milky Way Galaxy. For the solar neighborhood, we examined nearby stars with known distance, proper motions, and radial velocities. We found occurrence of a disturbing impact to the solar planetary disk within the next 8 Myr to be highly unlikely; perturbations to the Oort cloud seem unlikely as well. Current knowledge of the full phase space of stars in the solar neighborhood, however, is rather poor; thus we cannot rule out the existence of a star that is more likely to approach than those for which we have complete kinematic information. We studied the effect of stellar encounters on planetary orbits within the habitable zones of stars in more crowded stellar environments, such as stellar clusters. We found that in open clusters habitable zones are not readily disrupted; this is true if they evaporate in less than 10(8) yr. For older clusters the results may not be the same. We specifically studied the case of Messier 67, one of the oldest open clusters known, and show the effect of this environment on debris disks. We also considered the conditions in globular clusters, the Galactic nucleus, and the Galactic bulge-bar. We calculated the probability of whether Oort clouds exist in these Galactic environments.

A possible solution to the [α/Fe]-σ problem in early-type galaxies within a hierarchical galaxy formation model

Abstract The most massive elliptical galaxies apparently formed the fastest, because the ratio of α elements (such as oxygen) to iron is the smallest. In fact, iron is mainly produced from type Ia supernovae on a time-scale of ∼0.1–1 Byr, while the α elements come from massive stars on time-scales of a few tens of million years. Reproducing such a α/Fe correlation has long been a severe problem for cosmological theories of galaxy formation, which envisage massive galaxies to assemble gradually from smaller progenitors, and to be characterized by a star formation history too much extended towards late cosmic times. While it has recently become clear that feedback from Active Galactic Nuclei (AGNs) activity plays a role in the late quenching of star formation, and that early star formation history in the galaxy progenitors affect the α/Fe ratio, major mergers alone cannot enhance the star formation in the high-redshift progenitors to the levels required to match the steepness of the observed α/Fe correlation. Here we report that the inclusion of the effects of fly-by ‘harassments’, that trigger lower level starbursts, combined with the AGN quenching of the starburst activity, considerably enhances the capability to account for the observed α/Fe ratio in ellipticals within cosmological galaxy formation models. The critical difference between the earlier work and the present result is the effect of starbursts driven by fly-by encounters that would have been very common amongst the high-redshift progenitors of massive galaxies and which would have boosted star formation in the first 2 Byr after the big bang, combined with quenching of the burst activity within the first 3–4 Gyr.

A counter-rotating core in the dwarf elliptical galaxy VCC 510

Aims. We present optical long-slit spectra of the Virgo dwarf elliptical galaxy VCC 510 at high spectral (σ ∼ 30 km s−1) and spatial resolution. The principal aim is to unravel its kinematical and stellar population properties. Methods. Heliocentric velocities and velocity dispersions as functions of galaxy radius are derived by deconvolving line-of-sight velocity distributions. The luminosity-weighted stellar population parameters age and element abundances are obtained by comparison of Lick absorption-line indices with stellar population models. Results. A maximum rotation vrot = 8 ± 2.5 kms−1 inside half the effective radius (re ≈ 20) and a mean, radially flat velocity dispersion σ = 44 ± 5 kms−1 are measured. The core extending over the inner 2 (∼140 pc) is found to rotate in the opposite sense with vcore rot ≈ −1/2 vrot. VCC 510 (MB ∼ −15.7) is therefore by far the faintest and smallest galaxy with a counter-rotating core known. From the main body rotation and the velocity dispersion profile we deduce that VCC 510 is anisotropic and clearly not entirely supported by rotation. We derive an old luminosity-weighted age (10 ± 3 Gyr) and sub-solar metallicity ([Z/H] = −0.4 ± 0.1) inside the effective radius. There is tentative evidence that the counter-rotating core might be younger and less α/Fe enhanced. From the stellar population parameters we obtain a total stellar massto- light ratio of ∼3.6 (M/LB)⊙ which is significantly lower than a rough dynamical estimate obtained from the kinematics through the virial theorem (∼15). This discrepancy hints toward the possible presence of dark matter in the centre of VCC 510. Conclusions. We discuss the origin of the counter-rotating core and exclude fly-by encounters as a viable possibility. Gas accretion or galaxy merging provide more likely explanations. VCC 510 is therefore the direct observational evidence that such processes do occur in cluster satellite galaxies on dwarf galaxy scales.

The regeneration of stellar bars by tidal interactions: numerical simulations of fly-by encounters

We study the regeneration of stellar bars triggered by a tidal interaction, using numerical simulations of either purely stellar or stellar+gas disc galaxies. We find that interactions which are sufficiently strong to regenerate the bar in the purely stellar models do not lead to a regeneration in the dissipative models, owing to the induced gas inflow in those models. In models in which the bar can be regenerated, we find a tight correlation between the strength and the pattern speed of the induced bar. This relation can be explained by a significant radial redistribution of angular momentum in the disc due to the interaction, similar to the processes and correlations found for isolated barred spirals. Furthermore, we show that the regenerated bars show the same dynamical properties as their isolated counterparts.

A stochastic Monte Carlo approach to modelling real star cluster evolution -- III. Direct integration of three- and four-body interactions

Spherically symmetric equal mass star clusters containing a large amount of primordial binaries are studied using a hybrid method, consisting of a gas dynamical model for single stars and a Monte Carlo treatment for relaxation of binaries and the setup of close resonant and fly-by encounters of single stars with binaries and binaries with each other (three- and four-body encounters). What differs from our previous work is that each encounter is being integrated using a highly accurate direct few-body integrator which uses regularized variables. Hence we can study the systematic evolution of individual binary orbital parameters (eccentricity, semi-major axis) and differential and total cross sections for hardening, dissolution or merging of binaries (minimum distance) from a sampling of several ten thousands of scattering events as they occur in real cluster evolution including mass segregation of binaries, gravothermal collapse and reexpansion, binary burning phase and ultimately gravothermal oscillations. For the first time we are able to present empirical cross sections for eccentricity variation of binaries in close three- and four-body encounters. It is found that a large fraction of three-body and four-body encounters results in merging. Previous cross sections obtained by Spitzer and Gao for strong encounters can be reproduced, while for weak encounters non-standard processes like formation of hierarchical triples occur.

Collisional Dynamics of Stellar Systems in the Northern and Southern Coalsack Regions

The visual interstellar extinction in the Northern and Southern Coalsack regions is estimated from a photographic survey of proper motion and B, V photometry of 6 × 104 stars as faint as apparent magnitude 14 over a field of view of 6° × 6° in each region. Each cloud is located at a distance of about 160 ± 50 pc from the Sun and exceeds the mean interstellar extinction by more than 0.7 ± 0.1 mag. Galactic plane orbits of 103 stars located at the same distance of 160 ± 50 pc from the Sun in these regions show a semimajor axis of 8.45 ± 0.15 kpc and a mean inclination of 03 ± 02 over the Galactic plane, suggesting a mean impact parameter of 9 ± 3 pc with diffuse clouds of relative velocity on the order of 4 ± 1 km s-1. The weak distant encounter of stars with the cold neutral medium can be explained by a mean mass of about 2500 ± 1250 M⊙ for the gas and dust content in order to account for the diffusion of stellar orbits. A mean displacement of 0012 ± 0001 yr-1 of the western limb of the Southern Coalsack is estimated in the northwest direction and permits a search for strong close stellar encounters. The transfer of angular momentum between the interstellar medium and the scattering of stellar orbits resulting from the encounters suggests a dissipation rate of (2.0 ± 0.7) × 10-4 M⊙ yr-1, leading to an equipartition time of 12 ± 4 Myr. A spin rate of 0.25 ± 0.05 km s-1 pc-1 of the diffuse clouds in the Southern Coalsack is measured and interpreted as the result of the perturbation induced by stellar systems during flyby encounters with a crossing time of 3 Myr, suggesting a cloud core radius of 1 pc.

Perturbations of Spherical Stellar Systems during Flyby Encounters

We study the internal response of a galaxy to an unbound encounter and present a survey of orbital parameters covering typical encounters in diUerent galactic environments. Overall, we conclude that rela- tively weak encounters by low-mass interloping galaxies can cause observable distortions in the pri- maries. The resulting asymmetries may persist long after the interloper is evident. We focus our attention on the production of structure in dark halos and in cluster elliptical galaxies. Any distortion produced in a dark halo can distort the embedded stellar disk, possibly leading to the formation of lopsided and warped disks. We show that distant encounters with pericenters in the outer regions of a halo can excite strong and persistent features in the inner regions. Features excited in an elliptical galaxy are directly observable, and we predict that asymmetries in the morphologies of these systems can be produced by relatively small perturbers. For example, a —yby on an orbit with pericenter equal to the half-mass radius of the primary system and velocity of 200 km s~1 (a value typical for groups) can result in a signi—cant dipole distortion for perturbers with mass as small as 5% of the primarys mass. We use these detailed results to predict the distribution of the A parameter de—ned by Abraham et al. (sensitive to lopsidedness) and the shift between the center of mass of the primary system and the posi- tion of the peak of density for a range of environments. We —nd that high-density, low velocity disper- sion environments are more likely to host galaxies with signi—cant asymmetries. Our distribution for the A parameter is in good agreement with the range spanned by the observed values for local galaxy clus- ters and for distant galaxies in the Medium Deep Survey and in the Hubble Deep Field. Assuming that primordial galaxies were located in dense environments with previrialized low velocity dispersions, our conclusions are consistent with the observational results showing a systematic trend for galaxies at larger redshifts to be more asymmetric than local galaxies. Finally, we propose a generalized asymmetry parameter A(r) which provides detailed information on the radial structure of the asymmetry produced by the mechanism explored in our work. Subject headings: celestial mechanics, stellar dynamicsgalaxies: clusters: generalgalaxies: halos ¨ galaxies: interactionsgalaxies: kinematics and dynamicsgalaxies: structure

Models of star formation in interacting and merging disk galaxies

We present a series of models of interactions between star-forming disk galaxies. The models employ an N-body code to calculate the gravitational dynamics, a discrete cloud model to govern the ISM dynamics, and a modified Schmidt law to describe star formation. We model both merging galaxies and more distant flyby encounters, with differing disk orientations. We find that the global star formation rates (SFRs) in merging galaxies can be increased by an order of magnitude for several hundred million years, with the vast majority of the star formation arising in the central regions of the merging galaxies

The comet Halley dust and gas environment

Quantitative descriptions of environments near the nucleus of comet P/Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, we review the relevant data from prior Halley apparitions and from recent cometary research. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halley's position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbation. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. We expect that the comet's activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.

Possible flyby measurements of Galilean satellite interior structure

Flyby encounters of the Galilean satellites from a Jupiter orbiter spacecraft could yield information about the second-degree gravity harmonics of these satellites. We have calculated the expected values of these harmonics for a range of plausible interior models in hydrostatic equilibrium. Because the satellites respond to comparable perturbations from rotation and tides, an independent test of hydrostatic equilibrium is feasible. For Io and Ganymede, the expected measurement accuracy from a nominal encounter should make possible an excellent discrimination from the ensemble of interior models. For Europa, a qualitative distinction between near-uniform and centrally condensed models seems feasible. Only for Callisto is the proposed experiment of marginal value.

Voyager mission description

The Voyager Project, managed by the Jet Propulsion Laboratory, involves the lauching of two advanced spacecraft to explore the Jovian and Saturnian systems, as well as interplanetary space. The one-month lauch period opens on August 20, 1977, with arrivals at Jupiter in March and July of 1979, and at Saturn in November of 1980 and August of 1981. Gravity-assist swingbys of Jupiter are utilized in order to reduce the lauch energy demands needed to reach Saturn. In addition, a gravity-assist targeting option at Saturn will be maintained on the second-arriving Voyager for a possible continuation on to Uranus, with arrival in January of 1986. Flight through the Jovian and Saturnian systems will achieve close to moderate flyby encounters with several of the natural satellites, including special flyby geometry conditions for Io and Titan, as well as an Earth occultation of the spacecraft's radio signal by the rings of Saturn. The purpose of this paper is to describe the Voyager mission characteristics in order to establish a framework upon which to better understand the objectives and goals of the eleven scientific investigations which are described in subsequent papers.

Acquisition and description of Mariner 10 television science data at Mercury

The Mariner 10 television science subsystem was an improved version of the Mariner 9 system, using 1500-mm-focal-length optics. An elaborate picture-taking sequence resulted in transmission of over 4000 frames back to earth during two flyby encounters with Mercury. These sequences utilized a real-time data rate of 117.6 kbit/s, resulting in coverage of about 75% of the lighted portion of Mercury's surface at a resolution of better than 2 km. The complete set of useful images, which amounted to about 3000 frames, was processed with three different types of digital image-processing enhancements.