Jets launched at magnetar birth cannot be ignored

I question models for powering super energetic supernovae (SESNe) with a magnetar central engine that do not include jets that are expected to be launched by the magnetar progenitor. I show that under reasonable assumptions the outflow that is expected during the formation of a magnetar can carry an amount of energy that does not fall much below, and even surpasses, the energy that is stored in the newly born spinning neutron star (NS). The rapidly spinning NS and the strong magnetic fields attributed to magnetars require that the accreted mass onto the newly born NS possesses high specific angular momentum and strong magnetic fields. These ingredients are expected, as in many other astrophysical objects, to form collimated outflows/jets. I argue that the bipolar outflow in the pre-magnetar phase transfers a substantial amount of energy to the supernova (SN) ejecta, and it cannot be ignored in models that attribute SESNe to magnetars. I conclude that jets launched by accretion disks and accretion belts are more likely to power SESNe than magnetars are. This conclusion is compatible with the notion that jets might power all core collapse SNe (CCSNe).     

The yield of ionization in critical ionization velocity space experiments

We compare the conditions in laboratory and space critical ionization velocity (CIV) experiments. One significant difference that comes to light is the rapid expansion of the neutral cloud in space experiments, which does not take place in the laboratory. This has important ramifications for the ultimate ionization yield if there is a time delay in the ignition of the CIV discharge. We find that a simple kinetic model implies that the delay time of CIV ignition is a critical factor in determining the ultimate yield of the experiment. Although the delay time is difficult to calculate precisely, we provide some estimates that predict low CIV yield for typical space experimental geometries, densities and expansion rates. We examine the possibility of the variation of experimental conditions to maximize yield, but find that natural limitations in the design of space experiments may lead to low yields in the best of circumstances. This implies that experiments to date neither prove nor disprove the relevance of the CIV process to cosmology.     

Transverse oscillations in coronal loops observed with TRACE – I. An Overview of Events,Movies, and a Discussion of Common Properties and Required Conditions

We study transverse loop oscillations triggered by 17flares and filament destabilizations; only 2 such cases have been reported in the literature until now. Oscillation periods are estimated to range over a factor of ∼15, with most values between 2 and 7 min. The oscillations are excited by filament destabilizations or flares (in 6% of the 255 flares inspected, ranging from about C3 to X2). There is no clear dependence of oscillation amplitude on flare magnitude. Oscillations occur in loops that close within an active region, or in loops that connect an active region to a neighboring region or to a patch of strong flux in the quiet Sun. Some magnetic configurations are particularly prone to exhibit oscillations: two active regions showed two, and one region even three, distinct intervals with loop oscillations. The loop oscillations are not a resonance that builds up: oscillations in loops that are excited along their entire length are likely to be near the fundamental resonance mode because of that excitation profile, but asymmetrically excited oscillations clearly show propagating waves that are damped too quickly to build up a resonance, and some cases show multiple frequencies. We discuss evidence that all oscillating loops lie near magnetic separatrices that outline the large-scale topology of the field. All magnetic configurations are more complicated than a simple bipolar region, involving mixed-polarities in the interior or vicinity of the region; this may reflect that the exciting eruptions occur only in such environments, but this polarity mixing likely also introduces the large-scale separatrices that are involved. Often the oscillations occur in conjunction with gradual adjustments in loop positions in response to the triggering event. We discuss the observations in the context of two models: (a) transverse waves in coronal loops that act as wave guides and (b) strong sensitivity to changes in the field sources for field lines near separatrices. Properties that favor model b are (1) the involvement of loops at or near separatrices that outline the large-scale topology of the field, (2) the combined occurrence of oscillations and loop translations, (3) the small period spread and similar decay time scale in a set of oscillating loops in one well-observed event, and (4) the existence of loops oscillating in antiphase with footpoints close together in two cases. All other properties are compatible with either model, except the fact that almost all of the oscillations start away from the triggering event, suggestive of an outward-pushing exciting wave more in line with model a. The spread in periods from event to event suggests that the oscillations may reflect the properties of some driver mechanism that is related to the flare or mass ejection. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014957715396     

Viscous flow properties in the transport of solar wind momentum to the Venus upper ionosphere

A comparative study of the viscous transport of solar wind momentum to the upper layers of the Venus ionosphere with that occurring within the trans-terminator flow leads to estimates of the ratio of the viscosity coefficients that are applicable to both cases. Support for viscous forces between the solar wind and the ionospheric plasma in the trans-terminator flow derives from the momentum flux balance between the momentum flux in the latter flow and the deficiency of solar wind momentum along the flanks of the ionosheath. By comparing the relative width of the viscous boundary layer in the Venus ionosheath and the width of the trans-terminator flow we find that the transport of momentum within the upper ionosphere proceeds at a rate similar to that at which momentum is delivered to the upper ionosphere from the solar wind. Comparable values are obtained for the viscosity coefficient of the solar wind that streams over the ionosphere and that implied from momentum transport within the ionospheric trans-terminator flow. It is further suggested that despite the different nature of the processes that give place to the viscous transport of the solar wind momentum to the upper ionosphere (wave-particle interactions) and those responsible for its distribution within the ionosphere (through coulombian collisions) there is a similar response in the behavior of both plasmas to momentum transport. Calculations show that with comparable values of the viscosity coefficient in the ionosheath and in the upper ionospheric plasma the mean free path suitable to wave-particle interactions in the ionosheath is of the same order of magnitude as the mean free path of the planetary O⁺ ions that interact through coulombian collisions in the upper ionosphere. The effects of this similarity are considered in the discussion.     

Protostellar angular momentum transport by spiral density waves

The gravitational collapse of molecular clouds or cloud cores is expected to lead to the formation of stars that begin their lives in a state of rapid rotation. It is known that, in at least some specific cases, rapidly rotating, slf-gravitating bodies are subject to instabilities that cause them to assume ellipsoidal shapes. In this paper we investigate the consequences of such instabilities on the angular momentum evolution of a star in the process of formation from a collapsing cloud, and surrounded by a protostellar disk, with a view toward applications to the formation of the Solar System. We use a specific model of star formation to demonstrate the possibility that such a star would become unstable, that the resulting distortion of the star would generate spiral density waves in the circumstellar disk, and that the torque associated with these waves would regulate the angular momentum of the star as it feeds angular momentum to the disk. We conclude that the angular momentum so transported to the disk would not spread the disk to, say, Solar System dimensions, by the action of the spiral density waves alone. However, a viscous disk could effectively extract stellar angular momentum and attain Solar System size. Our results also indicate that viscous disks could feed mass and angular momentum to a growing protostar in such a manner that distortions of the star would occur before gravitational torques could balance the influx of angular momentum. In other situations (in which the viscosity was small), a gap could be cleared between the disk and star.     

Are superhumps good measures of the mass ratio for AM CVn systems?

We extend recent work that included the effect of pressure forces to derive the precession rate of eccentric accretion discs in cataclysmic variables to the case of double degenerate systems. We find that the logical scaling of the pressure force in such systems results in predictions of unrealistically high primary masses. Using the prototype AM CVn as a calibrator for the magnitude of the effect, we find that there is no scaling that applies consistently to all the systems in the class. We discuss the reasons for the lack of a superhump period to mass ratio relationship analogous to that known for SU UMa systems and suggest that this is because these secondaries do not have a single valued mass–radius relationship. We highlight the unreliability of mass ratios derived by applying the SU UMa expression to the AM CVn binaries.     

A three dimensional investigation of two dimensional orbits

Orbits in the principal planes of triaxial potentials are known to be prone to unstable motion normal to those planes, so that three dimensional investigations of those orbits are needed even though they are two dimensional. We present here an investigation of such orbits in the well known logarithmic potential which shows that the third dimension must be taken into account when studying them and that the instability worsens for lower values of the forces normal to the plane. Partially chaotic orbits are present around resonances, but also in other regions. The action normal to the plane seems to be related to the isolating integral that distinguishes regular from partially chaotic orbits, but not to the integral that distinguishes partially from fully chaotic orbits.     

Contribution to the background rate of a satellite X-ray detector by spallation products in a caesium iodide crystal

The energy spectra observed in a CsI crystal in the 20 keV-2 MeV range, due to the decay of radioactive isotopes produced in the crystal by bombardment with 155 MeV protons, are presented as a function of time after irradiation.It is shown that the large number of decay products produced by spallation can account for these spectra and that these spectra are in quantitative agreement with the predictions of a semi-empirical formula due to Rudstam, which gives the numbers of different isotopes produced. This formula is used to predict the spallation that would occur in such a crystal on board a satellite due to cosmic rays and passages through the South Atlantic Anomaly. Inspection shows that the spallation produced in the latter case is well approximated by that at 155 MeV. Hence the experimental results are used to explain previously observed background rates and to predict the background rates that would occur in the U.K.5 X-ray telescope of Imperial College. Using the Rudstam formula an estimate of cosmic ray induced background is also made.The relative importance of activity resulting from neutron interactions (atmospheric albedo and spacecraft secondaries) is considered.It is suggested that the Rudstam formula provides a general method of predicting induced radio-activity in satellite materials and that observed breaks in the diffuse cosmic X-ray spectrum could be due to inadequate allowance for this source of background.     

"Quasi Satellite Orbits" to observe a possible small moon of Pallas

The purpose of this paper is to make a numerical search for natural orbits that can be used for a spacecraft to study a possible small moon of Pallas. There are many speculations about the existence of a small companion around this large asteroid, so finding and classifying orbits around this possible celestial body is an interesting problem in astrodynamics and that can be used for a spacecraft to observe this body. It is assumed that this moon has a radius that can vary from 0.125 to 1 km and that is located 750 or 500 km away from the center of Pallas. The idea is to show the effects of this parameter in the orbits around this moon. It means that the moon is much smaller than Pallas, so Keplerian orbits are not possible around it. To solve this problem, it is possible to find some special orbits that are called "Quasi Satellite Orbits" (QSO). They are orbits dominated by the gravity of Pallas, but that use the smaller perturbation from the moon to keep the spacecraft close to it. The present work searches for orbits that make the spacecraft to remain at given limits in its distance from the moon, like in the range from 3 to 50 km, the values used as an example in the present paper. This value is used because it is a good range to observe the body without getting to close to it, so reducing the risks of collisions. Each trajectory can be identified by the initial conditions of the spacecraft with respect to the moon, which means its initial position and velocity. The dynamics considers the restricted three-body problem and the influence of the solar radiation pressure, because some spacecraft may have higher values for the area-to-mass ratio, which gives a non-negligible effect in the trajectory of the spacecraft.     

Mostly Dormant Comets and their Disintegration into Meteoroid Streams: A Review

The history of associating meteor showers with asteroidal-looking objects is long, dating to before the 1983 discovery that 3200 Phaethon moves among the Geminids. Only since the more recent recognition that 2003 EH1 moves among the Quadrantids are we certain that dormant comets are associated with meteoroid streams. Since that time, many orphan streams have found parent bodies among the newly discovered Near Earth Objects. The seven established associations pertain mostly to showers in eccentric or highly inclined orbits. At least 35 other objects are tentatively linked to streams in less inclined orbits that are more difficult to distinguish from those of asteroids. There is mounting evidence that the streams originated from discrete breakup events, rather than long episodes of gradual water vapor outgassing. If all these associations can be confirmed, they represent a significant fraction of all dormant comets that are in near-Earth orbits, suggesting that dormant comets break at least as frequently as the lifetime of the streams. I find that most pertain to NEOs that have not yet fully decoupled from Jupiter. The picture that is emerging is one of rapid disintegration of comets after being captured by Jupiter, and consequently, that objects such as 3200 Phaethon most likely originated from among the most primitive asteroids in the main belt, instead. They too decay mostly by disintegration into comet fragments and meteoroid streams. The disintegration of dormant comets is likely the main source of our meteor showers and the main supply of dust to the zodiacal cloud. Editorial handling: Frans Rietmeijer.     

Advances in deep space exploration via simulators & deep learning

The NASA Starlight and Breakthrough Starshot programs conceptualize fast interstellar travel via small relativistic spacecraft that are propelled by directed energy. This process is radically different from traditional space travel and trades large and slow spacecraft for small, fast, inexpensive, and fragile ones. The main goal of these wafer satellites is to gather useful images during their deep space journey. We introduce and solve some of the main problems that accompany this concept. First, we need an object detection system that can detect planets that we have never seen before, some containing features that we may not even know exist in the universe. Second, once we have images of exoplanets, we need a way to take these images and rank them by importance. Equipment fails and data rates are slow, thus we need a method to ensure that the most important images to humankind are the ones that are prioritized for data transfer. Finally, the energy on board is minimal and must be conserved and used sparingly. No exoplanet images should be missed, but using energy erroneously would be detrimental. We introduce simulator-based methods that leverage artificial intelligence, mostly in the form of computer vision, in order to solve all three of these issues. Our results confirm that simulators provide an extremely rich training environment that surpasses that of real images, and can be used to train models on features that have yet to be observed by humans. We also show that the immersive and adaptable environment provided by the simulator, combined with deep learning, lets us navigate and save energy in an otherwise implausible way.     

The Late Miocene climate response to a modern Sahara desert

The climate cooling and vegetation changes in the Miocene/Pliocene are generally well documented by various proxy data. Some important ecosystem changes occurred at that time. Palaeobotanical evidence suggests that the Sahara desert first appeared in the Pliocene, whereas in the Miocene North Africa was green. In the present study, we investigate the Late Miocene climate response to the appearance of the Sahara desert from a climate modelling sensitivity experiment. We compare a model experiment, which includes a full set of Late Miocene boundary conditions, with another one using the same boundary conditions except that the North African vegetation refers to the present-day situation. Our sensitivity study demonstrates that the introduction of the Sahara desert leads to a cooling and an aridification in Africa. In addition, we observe teleconnection patterns related to the North African desertification at around the Miocene/Pliocene boundary. From our sensitivity experiment, we observe that the Sahara contributes to a cooling in Central Asia and in North America. As compared to hypsodonty data for Central Asia, an increased aridity is underestimated in the Sahara experiment. Finally, we observe that the introduction of the Sahara leads to a cooling in the northern high latitudes. Hence, our sensitivity experiment indicates that the appearance of the Sahara desert is one piece to better understand Late Cenozoic climate cooling being most pronounced in the high latitudes.     

The chemical compositions and ages of globular clusters

Recent technological advances have led to a dramatic improvement in the quality of photometric and spectroscopic data obtainable on stars in globular clusters. Evidence from CCD-based colour magnitude diagrams points to clear differences in age between some clusters. High dispersion spectra show that abundance variations cannot explain the observed differences. In particular, it seems that NGC 288 must be 2–3 Gyr older than NGC 362. The same spectra show that although there is a spread in some molecular band strengths in NGC 362, the total C+N+O abundance remains constant, indicating that the material has undergone varying amounts of nuclear processing. No variations are seen in the abundances of iron group elements. Lower dispersion spectra for a large sample of faint stars in 47 Tucanae, obtained with a multi-object optical fibre system, show that unevolved main sequence stars in that cluster share the same CNO variations as the bright giants. The conclusion from all these data is that the intra-cluster CNO variations are neither truly primordial nor due to evolutionary mixing. It may be that there was a sufficiently extended period of star formation for material from first generation stars to be used in later generations, or that some pollution has occurred due to mass loss. Finally, it is noted that if ‘prehistoric’ clusters exist with ages of around 50 Gyr, as hypothesised in some cosmological models, these should probably still be rather obvious and readily recognised. Paper presented at the 6th Asian-Pacific Regional Meeting on Astronomy of the International Astronomical Union, India, 1993.     

Stochasticity in N-body simulations of disc galaxies

We demonstrate that the chaotic nature of N -body systems can lead to macroscopic variations in the evolution of collisionless simulations containing rotationally supported discs. The unavoidable stochasticity that afflicts all simulations generally causes mild differences between the evolution of similar models but, in order to illustrate that this is not always true, we present a case that shows an extreme bimodal divergence. The divergent behaviour occurs in two different types of code, and is independent of all numerical parameters. We identify and give explicit illustrations of several sources of stochasticity, and also show that macroscopic variations in the evolution can originate from differences at the round-off error level. We obtain somewhat more consistent results from simulations in which the halo is set-up with great care compared with those started from more approximate equilibria, but we have been unable to eliminate diverging behaviour entirely because the main sources of stochasticity are intrinsic to the disc. We show that the divergence is only temporary and that halo friction is merely delayed, for a substantial time in some cases. We argue that the delays are unlikely to arise in real galaxies, and that our results do not affect dynamical friction constraints on halo density. Stochastic variations in the evolution are inevitable in all simulations of disc–halo systems, irrespective of how they were created, although their effect is generally far less extreme than we find here. The possibility of divergent behaviour complicates comparison of results from different workers.     

Dust masses and abundances in planetary nebulae: likely strong elemental depletion in low-abundance sources

An analysis is undertaken of the relation between dust/gas mass ratios and elemental abundances within planetary nebulae (PNe). It is found that M _DUST/ M _GAS is broadly invariant with abundance, and similar to the values observed in asymptotic giant branch (AGB)-type stars. However, it is noted that the masses of dust observed in low-abundance PNe are similar to the masses of heavy elements observed in the gas phase. This is taken to imply that levels of elemental depletion must be particularly severe, and extend to many more species than have been identified so far. In particular, given that levels of C and O depletion are likely to be large, then this probably implies that species such as Fe, S, Si and Mg are depleted as well. There is already evidence for depletion of Fe, Si and Mg in individual PNe. It follows that whilst quoted abundances may accurately reflect gas-phase conditions, they are likely to be at variance with intrinsic abundances in low Z _N nebulae.
Finally, we note that there appears to be a variation in dust/gas mass ratios with galactocentric distance, with gradient similar to that observed for several elemental abundances. This may represent direct evidence for a correlation between dust/gas mass ratios and nebular abundances.     

Mars Hesperian Magmatism as Revealed by Syrtis Major and the Circum-Hellas Volcanic Province

Review of morphologic, morphometric and compositional data from Mars suggests that volcanism in the early Hesperian Syrtis Major edifice was predominantly ultramafic, in contrast to the abundant basaltic volcanism of the Hesperian to Amazonian Tharsis and Elysium provinces. Comparisons of edifice characteristics between Syrtis Major and the large, circum-Hellas Noachian to Hesperian volcanoes suggest that these structures may also be formed by ultramafic volcanic activity. The data suggest that a global scale magma compositional change occurred on Mars during the late Hesperian. The occurrence of widespread ultramafic volcanism suggests that a high degree of partial melting in a relatively hot mantle characterized Mars?? early thermal history, conditions that may be analogous to those that prevailed in the Archean Earth.     

Chondrule collisions in shock waves

Abstract— Detailed numerical models have shown that solar nebula shock waves would be able to thermally process chondrules in a way that is consistent with experimental constraints. However, it has recently been argued that the high relative velocities that would be generated between chondrules of different sizes immediately behind the shock front would lead to energetic collisions that would destroy the chondrules as they were processed rather than preserving them for incorporation into meteorite parent bodies. Here the outcome of these collisions is quantitatively explored using a simple analytic expression for the viscous dissipation of collisional energy in a liquid layer. It is shown that molten chondrules can survive collisions at velocities as high as a few hundred meters per second. It is also shown that the thermal evolution of chondrules in a given shock wave varies with chondrule size, which may allow chondrules of different textures to form in a given shock wave. While experiments are needed to further constrain the parameters used in this work, these calculations show that the expected outcomes from collisions behind shock waves are consistent with what is observed in meteorites.     

A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries

Although a single model currently exists to explain the development of curved Europan cycloids, there have been no systematic studies of the range of morphologies and quantifiable geometric parameters of cycloidal features. We address variations in geometry along individual cycloid segments, characterizing differences in cusp styles and angles, and addressing the morphologic aspects of cycloid segments and cusps. In so doing, we illustrate how geometric and morphologic evidence imply a formation mechanism that differs from the existing model in several aspects. The current model states that cycloids are initiated as tensile fractures that grow in a curved path in response to rotating diurnal tidal stresses on Europa. However, the geometry of a cycloid cusp necessitates that shear stress was resolved onto the existing cycloid segment by the rotating diurnal stresses at the instant of cusp formation. Furthermore, we observe that cycloid cusps have a strikingly similar geometry to tailcracks that developed at the tips of many ridge-like strike-slip faults on Europa in response to shearing at the fault tip. We suggest that this similarity in geometries can be attributed to an identical formation mechanism whereby cycloid cusps form by a tailcracking process. We therefore present a revised, mechanically-based model for cycloid formation that retains the basic premise that crack growth is governed by diurnal stresses, but describes the development of cycloid cusps in response to resolved shear stresses at the tips of existing cycloid segments. The ratio of normal to shear stress at the time of tailcrack formation dictates the cusp angle and, over longer time periods, influences the morphologic evolution of the cycloid segment as it is repeatedly reworked by tidal stresses.     

Amorphous silicates in the matrix of Semarkona: The first evidence for the localized preservation of pristine matrix materials in the most unequilibrated ordinary chondrites

We have investigated the fine‐grained matrix of the least‐altered unequilibrated ordinary chondrite (UOC) Semarkona (LL3.00) using different electron microscope techniques. Unlike previous studies, which found that the matrix of Semarkona was extensively altered to phyllosilicates, we have discovered the widespread occurrence of much more pristine amorphous silicates in the sample that we have studied. Detailed TEM study shows that these materials occur pervasively in the matrix as (1) continuous groundmass; (2) distinct, circular to subrounded features, which contain nanometric‐size sulfides and carbides; or (3) distinct objects containing parallel, linear features composed of sulfides and voids. These amorphous silicates have many textural and compositional similarities to the occurrences of amorphous silicates found in pristine carbonaceous chondrites (CCs); however, minor differences were also identified. Most of the textural and chemical differences suggest that these materials formed at different times and locations in the solar nebula, compared to matrix materials in CCs. Nevertheless, their occurrence suggests that the amorphous silicates in Semarkona formed by similar processes to those proposed for amorphous silicates in CCs, that is, rapid cooling that favored disequilibrium condensation of material evaporated during chondrule‐forming events. In addition, the occurrence of minimally altered amorphous silicates in Semarkona demonstrates that the effects of aqueous alteration, which have been widely described in this meteorite, are not pervasive. Instead, our new observations demonstrate that aqueous alteration has affected Semarkona heterogeneously and that locally, regions of much more pristine matrix that have escaped extensive alteration are still preserved within this meteorite. Such materials provide significant new insights into the pristine characteristics of ordinary chondrite matrix material that has not been previously available.     

A Robotics Perspective On Human Spaceflight

There has traditionally been a dichotomy in the space community regarding the efficacy of human versus robotic exploration of space. I argue that no such dichotomy is necessary, and that there is a natural and synergistic division of labour between man and machine, and that this division of labour will evolve in symbiotic fashion. The present state-of-the-art robotics technology is insufficient to replace the human in space, but is sufficient to act as a useful, even necessary, tool in aiding the astronaut in the conduct of useful work. I further argue that as robotics technology advances, the human will be further relieved to perform tasks best suited to human decision-making and flexibility that is unlikely in the near-term to be matched by autonomous or teleoperated machines. This revised version was published online in July 2006 with corrections to the Cover Date.