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[en] A new theory for galactic arm formation shows the arms to be continually eminating from the galactic nucleus due to a continual influx of cosmic dust. In the neighborhood of the nucleus the problem is treated as a fluid flow and a simple solution is given using conservation of momentum. When rotational dynamics are included the spinning arm system is the result. This solution resolves the problem of the missing mass, accounts for warped disk galaxies and gives a probable source for the gravity waves measured by Weber which eminate from our galactic center. Reversal of arm direction is demonstrated and examples of such reversals are cited. An approximate theoretical estimate of the age of our Sun is found to be in good agreement with radio isotope dating. A general result shows why twin star systems are in such great abundance in a galaxy. It gives a model of galactic evolution which begins with only a single massive nucleus with the collapsing gas clouds forming the arms. (orig.)
[en] Hypervelocity (69 km/s) impact of cometary material with surfaces of the GIOTTO spacecraft will induce a number of spurious and possibly harmful phenomena. The most serious of these is likely to be spacecraft charging that results from impact-produced plasma distributions surrounding GIOTTO. The ESA Plasma Environment Working Group, whose studies are the basis for this report, finds that charging may become significant within approx. 105 km of the nucleus where potentials of approx. = +20 V are to be expected. In addition to spacecraft charging, impact produced plasma may interfere with in situ plasma measurements, particularly those of ion plasma analyzers and mass spectrometers
[en] Our original survey has been extended to include more northern and southern hemisphere stars; the wavelength coverage was extended and the precision improved for several previously observed stars. Search criteria for interstellar circular polarization based on the linear polarization and reddening have been delineated and refined. Observational data are presented which confirm the theoretical prediction that the wavelength of maximum interstellar linear polarization and the wavelength of the zero crossing of interstellar circular polarization are approximately the same. An intercomparison of the results of various authors is presented and briefly discussed
[en] We study the influence of a uniform rotation upon the equilibrium conditions of a self-gravitating interstellar cloud; the equation of state is assumed to be that of polytrope with negative index and the cloud is acted upon by an external pressure. For a given angular velocity, equilibrium is possible only when the external pressure falls between two critical values: these limits approach each other with increase of mass and no equilibrium is possible beyond a critical mass which, for example, is of the order of 800sun, when the angular velocity is 10-15 s-1. A quasi-static analysis of the evolution of a cloud having constant angular momentum shows that the centrifugal forces in the equatorial plane are such that the pressure sometimes does not reach its critical value for collapse. (orig.)
[de]Der Einfluss einer gleichfoermigen Rotation auf die Gleichgewichtsbedingungen einer Selbst-gravitierenden interstellaren Wolke wird untersucht; als Zustandsgleichung wird ein Polytropenansatz mit negativem Index angenommen und auf die Wolke wirkt ein aeusserer Druck ein. Bei gegebener Winkelgeschwindigkeit ist ein Gleichgewicht nur moeglich, wenn sich der aeussere Druck zwischen zwei kritischen Werten bewegt: diese Grenzwerte naehern sich mit zunehmender Masse und oberhalb einer bestimmten kritischen Masse, z.B. 800 Sonnenmassen bei einer Winkelgeschwindigkeit von 10-15 s-1, ist kein Gleichgewicht moeglich. Eine quasistatische Analyse der Entwicklung einer Wolke mit konstanter Winkelgeschwindigkeit zeigt, dass die Zentrifugalkraefte in der Aequatorialebene so geartet sind, dass der Druck in manchen Faellen den kritischen Wert fuer einen Kollaps nicht erreicht. (orig.)
[en] The gravitational instability of an infinite homogeneous and infinitely conducting self-gravitating gas-particle medium in the presence of a vertical magnetic field and suspended particles is considered. It is found that in the presence of suspended particles and magnetic field, Jeans' criterion determines the gravitational instability. (Auth.)
[en] Although surveys of infall motions in dense cores have been carried out for years, few surveys have focused on mapping infall across cores using multiple spectral-line observations. To fill this gap, we present IRAM 30 m telescope maps of N2H+(1–0), DCO+(2–1), DCO+(3–2), and HCO+(3–2) emission toward two prestellar cores (L492 and L694-2) and one protostellar core (L1521F). We find that the measured infall velocity varies with position across each core and choice of molecular line, likely as a result of radial variations in core chemistry and dynamics. Line-of-sight infall speeds estimated from DCO+(2–1) line profiles can decrease by 40–50 m s−1 when observing at a radial offset pc from the core's dust continuum emission peak. Median infall speeds calculated from all observed positions across a core can also vary by as much as 65 m s−1, depending on the transition. These results show that while single-pointing, single-transition surveys of core infall velocities may be good indicators of whether a core is either contracting or expanding, the magnitude of the velocities they measure are significantly impacted by the choice of molecular line, proximity to the core center, and core evolutionary state.
[en] Inner Oort cloud objects (IOCs) are trans-Plutonian for their entire orbits. They are beyond the strong gravitational influences of the known planets, yet close enough to the Sun that outside forces are minimal. Here we report the discovery of the third known IOC after Sedna and 2012 VP113, called 2015 TG387. This object has a perihelion of 65 ± 1 au and semimajor axis of 1170 ± 70 au. The longitude of perihelion angle, , for 2015 TG387 is between that of Sedna and 2012 VP113 and thus similar to the main group of clustered extreme trans-Neptunian objects (ETNOs), which may be shepherded into similar orbital angles by an unknown massive distant planet called Planet X, or Planet Nine. The orbit of 2015 TG387 is stable over the age of the solar system from the known planets and Galactic tide. When including outside stellar encounters over 4 Gyr, 2015 TG387's orbit is usually stable, but its dynamical evolution depends on the stellar encounter scenarios used. Surprisingly, when including a massive Planet X beyond a few hundred au on an eccentric orbit that is antialigned in longitude of perihelion with most of the known ETNOs, we find that 2015 TG387 is typically stable for Planet X orbits that render the other ETNOs stable as well. Notably, 2015 TG387's argument of perihelion is constrained, and its longitude of perihelion librates about 180° from Planet X’s longitude of perihelion, keeping 2015 TG387 antialigned with Planet X over the age of the solar system.
[en] The recent Atacama Large Millimeter/submillimeter Array (ALMA) Disk Substructures at High Angular Resolution Project (DSHARP) survey provided illuminating results on the diversity of substructures in planet-forming disks. These substructures trace pebble-sized grains accumulated at local pressure maxima, possibly due to planet–disk interactions or other planet formation processes. DSHARP sources are heavily biased to large and massive disks that only represent the high (dust flux) tail end of the disk population. Thus it is unclear whether similar substructures and corresponding physical processes also occur in the majority of disks that are fainter and more compact. Here we explore the presence and characteristics of features in a compact disk around GQ Lup A, the effective radius of which is 1.5–10 times smaller than those of DSHARP disks. We present our analysis of ALMA 1.3 mm continuum observations of the GQ Lup system. By fitting visibility profiles of the continuum emission, we find substructures including a gap at ∼10 au. The compact disk around GQ Lup exhibits similar substructures to those in the DSHARP sample, suggesting that mechanisms of trapping pebble-sized grains are at work in small disks as well. Characteristics of the feature at ∼10 au, if due to a hidden planet, are evidence of planet formation at Saturnian distances. Our results hint at a rich world of substructures to be identified within the common population of compact disks, and subsequently a population of solar system analogs within these disks. Such study is critical to understanding the formation mechanisms and planet populations in the majority of protoplanetary disks.