Background Of Stars Research Articles

Drag on a Satellite Moving across a Spherical Galaxy: Tidal and Frictional Forces in Short‐lived Encounters

The drag force on a satellite of mass M moving with speed V in the gravitational field of a spherically symmetric background of stars is computed. During the encounter, the stars are subject to a time-dependent force that alters their equilibrium. The resulting distortion in the stellar density field acts back to produce a force FΔ that decelerates the satellite. This force is computed using a perturbative technique known as linear response theory. In this paper, we extend the formalism of linear response to derive the correct expression for the back-reaction force FΔ that applies when the stellar system is described by an equilibrium one-particle distribution function. FΔ is expressed in terms of a suitable correlation function that couples the satellite dynamics to the unperturbed dynamics of the stars. At time t, the force depends upon the whole history of the composite system. In the formalism, we account for the shift of the stellar center of mass resulting from linear momentum conservation. The self-gravity of the response is neglected since it contributes to a higher order in the perturbation. Linear response theory applies also to the case of a satellite orbiting outside the spherical galaxy. The case of a satellite moving on a straight line, at high speed relative to the stellar dispersion velocity, is explored. We find that the satellite during its passage raises (1) global tides in the stellar distribution and (2) a wake, i.e., an overdense region behind its trail. If the satellite motion is external to the galaxy, it suffers a dissipative force that is not exclusively acting along V but acquires a component along R, the position vector relative to the center of the spherical galaxy. We derive the analytical expression of the force in the impulse approximation. In penetrating short-lived encounters, the satellite moves across the stellar distribution and the transient wake excited in the density field is responsible for most of the deceleration. We find that dynamical friction arises from a memory effect involving only those stars perturbed along the path. The force can be written in terms of an effective Coulomb logarithm that now depends upon time. The value of ln Λ is computed for two simple equilibrium density distributions; it is shown that the drag increases as the satellite approaches the denser regions of the stellar distribution and attains a maximum after pericentric passage. When the satellite crosses the edge of the galaxy, the force does not vanish since the galaxy keeps memory of the perturbation induced and declines on a time comparable to the dynamical time of the stellar system. In the case of a homogeneous cloud, we compute the total energy loss. In evaluating the contribution resulting from friction, we derive self-consistently the maximum impact parameter, which is found to be equal to the length traveled by the satellite within the system. Tides excited by the satellite in the galaxy reduce the value of the energy loss by friction; in close encounters, this value is decreased by a factor of ~1.5.

The Polarizing Power of the Interstellar Medium in Taurus

We present a study of the polarizing power of the dust in cold dense regions (dark clouds) compared to that of dust in the general interstellar medium (ISM). Our study uses new polarimetric, optical, and spectral classification data for 36 stars to carefully study the relation between polarization percentage (p) and extinction (A_V) in the Taurus dark cloud complex. We find two trends in our p-A_V study: (1) stars background to the warm ISM show an increase in p with A_V; and (2) the percentage of polarization of stars background to cold dark clouds does not increase with extinction. We detect a break in the p-A_V relation at an extinction 1.3 +/- 0.2 mag, which we expect corresponds to a set of conditions where the polarizing power of the dust associated with the Taurus dark clouds drops precipitously. This breakpoint places important restrictions on the use of polarimetry in studying interstellar magnetic fields.

The european observation network: Ground-based support for gamma-ray satellites

We present first preliminary results obtained with the European Observation Network consisting of 9 observatories in the Czech Republic, Germany and Bulgaria. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.

European Observation Network: Ground - Based Support for Gamma-Ray Satellites

We present first preliminary results obtained with the European Observation Network consisting of 9 observatories in the Czech Republic, Germany and Bulgaria. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.

Tidal Capture in Star Clusters

We review the tidal capture process and in particular the chaotic orbital evolution which follows capture. We discuss the formation of low-mass X-ray binaries in globular clusters via tidal capture and speculate on the possibility that some field low-mass X-ray binaries were formed this way in open clusters which have since dispersed, or in existing old open clusters which are not accessible to observation because of obscuration by dust or because they are indistinguishable from the rich background of galactic stars.

European Observation Network: first experience with BACODINE triggers

We present preliminary results obtained with the European Observation Network. This network consists of 9 observatories in the Czech Republic, Germany and Bulgaria and has been involved in the BACODINE activities since April 1, 1994. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.

Orbital evolution of a massive black hole pair by dynamical friction

view Abstract Citations (15) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Orbital Evolution of a Massive Black Hole Pair by Dynamical Friction Vecchio, Alberto ; Colpi, Monica ; Polnarev, Alexander G. Abstract We investigate the evolution of a massive black hole pair under the action of dynamical friction (DF) by a uniform background of light stars with isotropic velocity distribution. In our scenario, the primary black hole $M_1$ sits, at rest, in the center of the spherical star distribution (with mass $M_c$) and the secondary less massive companion $M_2$ moves along bound orbits determined by the background gravitational field; it loses energy $E$ and angular momentum $J$ by DF. We investigate mostly analytically the secular evolution of the orbital parameters when the motion of $M_2$ is determined either by the mean field generated by the uniform stellar distribution or by the gravitational field of $M_1$. We find that $J$ and $E$ are lost so as to cause the increase of the eccentricity $e$ during the orbital decay of $M_2$. When $M_2$ enters the region where the gravitational field of $M_1$ dominates, the evolution depends on the ratio between the black hole velocity $v$ and the stellar dispersion velocity $\sigma.$ We explore both cases $v<\sigma$ and $v>\sigma$. Only for $v>\sigma$ the rise of $e$ would be severe but this transition occurs close to the cusp radius below which our description becomes invalid. Energy losses by gravitational wave emission become comparable to those by dynamical friction at a critical distance that depends sensitively on the ratio $M_1/M_c$. The braking index $n$ is calculated in this transition region. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174683 arXiv: arXiv:astro-ph/9405051 Bibcode: 1994ApJ...433..733V Keywords: Astronomical Models; Binary Stars; Black Holes (Astronomy); Kinetic Friction; Orbital Mechanics; Stellar Evolution; Angular Momentum; Eccentricity; Energy; Gravitation; Velocity; Velocity Distribution; Astrophysics; STARS: BINARIES: CLOSE; BLACK HOLE PHYSICS; Astrophysics E-Print: 32 pages in LaTeX, 5 figures available on request from colpi@vaxmi.mi.infn.it full text sources arXiv | ADS |

Improved Observations of Faint Planetary Nebulae in the Magellanic Clouds

The accuracy of spectrophotometry for Magellanic Cloud planetary nebulae is limited by a number of physical effects and operational difficulties : atmospheric dispersion, wavelength-dependent seeing, pointing and guiding errors, and a background of numerous stars and diffuse emission. We describe procedures to minimize the impact of some of these. We then compare our results for both bright and faint objects with published values. There are a few exceptional cases (e.g,, LMC 89), but generally we find that bright planetary nebulae in the Clouds have been observed with high accuracy. On the other hand, observations of the faint sample from Jacoby (1980) suffer seriously from many of these effects

The First Data from the MACHO Experiment

MAssive Compact Halo Objects such as brown dwarfs, Jupiters, and black holes are prime candidates to comprise the dark halo of our galaxy. Paczynski noted that objects (dubbed MACHOS) with masses in the range 10{sup {minus}6} M{circle_dot} < M {approx_lt} 100M{circle_dot} can be detected via gravitational microlensing of stars in the Magellanic Clouds with the caveat that only about one in 10{sup 6} stars will be lensed at any given time. Our group has recently begun a search for microlensing using a refurbished 1.27 meter telescope at the Mount Stromlo Observatory in Australia. Since the summer of 1992, we have been imaging up to 10{sup 7} stars a night in the Large Magellanic Cloud using our large format two-color 3.4 {times} 10{sup 7} pixel CCD camera. Here I report on our first results based on an analysis of {approximately}10{sup 6} of these stars. Although this is not enough data to make definitive statements about the nature of the dark matter, we are able to conclude that the rate of variable star background events is not larger than the expected MACHO signal.

Binary-single-star scattering. V - Steady state binary distribution in a homogeneous static background of single stars

We compute the rate at which hard (tightly bound) equal-mass binaries form in a homogeneous stellar background, as the solution of an integral equation describing the combined effects of creation, destruction, hardening, and softening of binaries. Most hard binaries are born very soft. The rate of energy production by hard binaries can be (and has already been) directly used in Fokker-Planck models for the evolution of globular clusters past core collapse. We also calculate the survival probability for a binary as a function of its binding energy, and the steady state distribution of binding energies

Reconciliation between the Mach principle and the principle of equality of the inertial and gravitational masses

A study of kinematics of a 2-body system is used to show that the Mach principle, previously rejected by general relativity, can still serve as an alternative to the concept of absolute space, if one assumes that the background of distant stars determines both the inertial and the gravitational masses of a body.

«Apollon« och det mytologiska hos Matisse

Summary The content of Matisse's “Apollo” (fig. 1) is full of mystery. Several different motifs—a face, water, three trees, a fan, leaves in various colours, and two posts—are brought together in a symmetrical composition. Attempts have been made to regard the picture as illustrating certain passages in the Apollo myth, for instance the story of Daphne and Apollo, or of Phaethon, the son of the Sun‐god. It was the course of events in the myth that was to identify and explain the various motifs, but these attempts led to disparate and conflicting conclusions. The author of this paper endeavours to find out whether Matisse had any particular way of treating mythological themes. The two Icarus pictures he made in 1943 do not reflect the conflict situation of the myth, for Icarus is portrayed in a night scene against a background of stars instead of the sun: the vax in his wings would not melt in this setting. In actual fact the figures represent airmen on a night‐attack who were being fired at: the white fig...

Mass segregation in globular clusters

Photoelectric aperture-photometry ofω Cen inU, B, V, R andI bands has established that the cluster is bluer between 2 arcmin and 4 arcmin from the centre, than it is elsewhere. The difference inB - I colour between the centre and this blue zone is ≃ 0.45 mag. The core radius is found to be dependant on the wavelength band chosen for observation, the smallest core radius being for theI band. Equidensitometry ofω Cen inB, V and infrared bands shows a wavelength dependence with the cluster being nearly spherical in the infrared band. It shows a maximum ellipticity around 3 arcmin from the cluster centre. The blue contribution in this zone comes from both a diffuse background of unresolved stars and an increase in the relative abundance of horizontal branch (HB) stars. The similarity between the diffuse background and the HB stars has been demonstrated. A photographic subtraction technique is used to study the distribution of HB stars in the cluster.

Magnetic field structure in the vicinity of five dark cloud complexes

view Abstract Citations (263) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic field structure in the vicinity of five dark cloud complexes. Vrba, F. J. ; Strom, S. E. ; Strom, K. M. Abstract The magnetic-field geometry in the outer regions of five dark cloud complexes has been mapped by measuring the linear polarization of stars background to the cloud peripheries. These observations are aimed at establishing a relationship between field geometry and cloud morphology for cloud complexes which are active sites of star formation. For one of these complexes, Barnard 42, polarization measurements were also obtained (at a wavelength of 2.2 microns) for several heavily obscured stars in order to probe the magnetic-field geometry and grain-alignment efficiency in the dense central regions of this dark cloud. Publication: The Astronomical Journal Pub Date: November 1976 DOI: 10.1086/111976 Bibcode: 1976AJ.....81..958V Keywords: Astronomical Maps; Interstellar Magnetic Fields; Magnetic Field Configurations; Polarization Characteristics; Astronomical Catalogs; Astronomical Coordinates; Background Radiation; Cloud Physics; Clouds; Interstellar Matter; Astrophysics full text sources ADS | data products SIMBAD (320)

1.1.3 Preliminary Results of the Helios a Zodiacal Light Experiment

Helios A was launched on December 10, 1974 into a highly elliptical orbit with a perihelion of 0.31 A.U., which was reached on March 15, 1975. The zodiacal light experiment on Helios, described in the preceding paper, worked flawlessly and provided the first observations of the zodiacal light from inside the Earth’ orbit. A typical example from the raw data of the 15° – photometer is shown in Fig.1. There is a strong intensity increase towards the sun and a remarkably flat intensity distribution at large elongations. The Milky way is superimposed on the zodiacal light at longitudes 135° to 180° and 315° to 360°. Due to the orbital motion of Helios the star background is being shifted with respect to the zodiacal light, which will facilitate its separation from the total observed intensity. The peak at the right side of Fig.1 is due to the star α CMi, which we intend to use for the calibration of the instrument in addition to the ground calibrations. A preliminary evaluation showed less than 20% difference between the two calibrations. This is typical for other stars and for the other photometers, too, and gives a safe upper limit for the accuracy of the absolute calibration. Temperature effects are comparatively small and have not been corrected so far.

Attitude stabilisation of the Geos-C spacecraft

GEOS-C will be the third of a series of spacecraft designed for the National Geodetic Satellite Programme. The two earlier and successful spacecraft were designated GEOS-I and II. In these, geocentric stabilisation was acheived to optimise the usage of the radiated and reflected radio and optical power of the geodetic observations. It was the atmospheric attenuation of the optical beacons, by which the spacecraft could be photographed against a star background, which imposed the accuracy requirement of earth pointing to within 5°. Descriptions as well as performance analyses of these attitude control systems have been published. In each, the proper mass distribution was obtained through an end mass on a de Havilland extendible boom. Since initial misorientation and perturbing torques induce motion about the equilibrium orientation in a gravity-gradient system, a mechanism for energy dissipation is necessary. A Magnetically Anchored Eddy Current Damper was used for this purpose. It consists of a magnet assembly that is free to lock onto the geomagnetic field and rotate freely within a conducting spherical shell. Relative motion of the two produces eddy currents in the conductor which in turn impede the relative motion.

On-Board Optical Navigation Data from Mariner 71*

Satellite-star optical navigation, a new spacecraft navigation technique has been successfully demonstrated during Mariner 9's approach to Mars in mid-November of 1971. As Mariner 9 approached Mars, 21 TV pictures of its satellites, Phobos and Deimos, were taken and transmitted to earth. On earth, these pictures were carefully processed in order to locate the positions of the bright satellites against the relatively faint star background. These position fixes were used to obtain more accurate knowledge of Mariner 9's path with respect to Mars and its satellites. The accuracy of the new navigation technique has been confirmed by comparison with the standard radio navigation results on Mariner 9. This successful demonstration of satellite-star optical navigation is the culmination of over 2 years of development work at JPL sponsored by NASA's Office of Aeronautics and Space Technology (OAST). NASA now has developed and demonstrated a prototype of a more accurate navigation system, using onboard measurements, which is essential to meeting the challenging requirements of future missions such as the multi-planet gravity-assisted outer planet missions. This paper describes the objectives and performance of the pre-flight and real time activities leading to the successful demonstration of satellite-star navigation data.

The Investigation of Lunar Limb Structure by Means of Stellar Occultations

It has long been recognized that the analysis of occultation traces from point source stars might provide a means of investigating the structure of the lunar limb on a remarkably small scale, certainly of tens of meters, possibly on a scale of meters.The routine process of analysis of such an occultation trace produces a curve fitted to the standard model for a point source, in which the observed rate of fringe passage is matched to that computed from the rate and position angle of the relative motion of the moon with respect to the star background and the position angle of the point at which the occultation occurs. If θυ is the position angle towards which the relative motion of the lunar center takes place, θ that at which the occultation occurs, and ψ = θν — θ, then the predicted rate of the lunar limb perpendicular to itself at this point isRp = V cos ψwhere V is the velocity of the lunar center.

The application of the image orthicon television system to rocket trajectory determination

During darkness or twilight the high optical sensitivity of the Image Orthicon Television Camera tube can be used to track a light source carried on board a rocket and hence, by relating the position of the light to a star background, the rocket's trajectory can be determined. The technique is considerably simpler and potentially more accurate than standard radar and doppler methods currently in use. A method of computing trajectory from one T.V. site, in conjunction with very limited radar data is described.

The motion of Jupiter's fifth satellite

A method is described for obtaining and reducing a series of photographic observations of the fifth satellite and the disk of Jupiter against a background of stars. A discussion of these and earlier observations leads to revised values for the mean motion and the regression of the nodes.