Where are the rings of Neptune?

Stable rings can exist at inclinations of 0–15°, 165°–180°, or ～90° to Neptune's equator, but perturbations due to the massive satellite Triton would produce a severe “warping” of the ring plane. If Neptune possesses rings, they may not lie in the plane of its equator.     

The rings of saturn: A frost-coated semiconductor?

Optical complex indices of refraction for the iron-sulfur semiconductors troilite and pyrrhotite have been measured between 0.3 and 1.1 μm, and at 1.5 and 3.0 cm in the microwave region. The behavior of the absorptive and refractive components of the index in the visual and ultraviolet regions suggests a reasonable match to published data on Saturn's rings. A combination of the iron sulfur with water ice and the orthopyroxene enstatite is consistent with an equilibrium condensation model for the formation of the solar system; the water ice and an orthopyroxene are also suggested by near-infrared observational data. A combination of these materials could explain all spectral features seen in the ring spectra to date. The microwave behavior of a small quantity of troilite embedded in water ice at low temperatures is consistent with the radar reflectivity and radio emissivity data.     

Did Saturn's rings form during the Late Heavy Bombardment?

The origin of Saturn's massive ring system is still unknown. Two popular scenarios—the tidal splitting of passing comets and the collisional destruction of a satellite—rely on a high cometary flux in the past. In the present paper we attempt to quantify the cometary flux during the Late Heavy Bombardment (LHB) to assess the likelihood of both scenarios. Our analysis relies on the so-called “Nice model” of the origin of the LHB [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461; Morbidelli, A., Levison, H.H., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465; Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A., 2005. Nature 435, 466-469] and on the size distribution of the primordial trans-neptunian planetesimals constrained in [Charnoz, S., Morbidelli, A., 2007. Icarus 188, 468-480]. We find that the cometary flux on Saturn during the LHB was so high that both scenarios for the formation of Saturn rings are viable in principle. However, a more detailed study shows that the comet tidal disruption scenario implies that all four giant planets should have comparable ring systems whereas the destroyed satellite scenario would work only for Saturn, and perhaps Jupiter. This is because in Saturn's system, the synchronous orbit is interior to the Roche Limit, which is a necessary condition for maintaining a satellite in the Roche Zone up to the time of the LHB. We also discuss the apparent elimination of silicates from the ring parent body implied by the purity of the ice in Saturn's rings. The LHB has also strong implications for the survival of the saturnian satellites: all satellites smaller than Mimas would have been destroyed during the LHB, whereas Enceladus would have had from 40% to 70% chance of survival depending on the disruption model. In conclusion, these results suggest that the LHB is the “sweet moment” for the formation of a massive ring system around Saturn.     

Gamma-ray pulsars: polar cap or outer gap emission?

The power law spectra of the six known-ray pulsars are seen to cut off at energies < 1 TeV so that no steady pulsed TeV component is observed from any known-ray pulsar. In some cases we show that the cutoff is steeper than exponential, which is consistent with magnetic pair production above the polar cap region. The small upper limits on the ratio of TeV energy flux to optical - GeV energy flux is again consistent with the polar cap model, but appears to be inconsistent with inverse Compton controlled outer gaps.     

Are solar cycles predictable?

Various methods (or recipes) have been proposed to predict future solar activity levels – with mixed success. Among these, some precursor methods based upon quantities determined around or a few years before solar minimum have provided rather high correlations with the strength of the following cycles. Recently, data assimilation with an advection-dominated (flux-transport) dynamo model has been proposed as a predictive tool, yielding remarkably high correlation coefficients. After discussing the potential implications of these results and the criticism that has been raised, we study the possible physical origin(s) of the predictive skill provided by precursor and other methods. It is found that the combination of the overlap of solar cycles and their amplitude-dependent rise time (Waldmeier's rule) introduces correlations in the sunspot number (or area) record, which account for the predictive skill of many precursor methods. This explanation requires no direct physical relation between the precursor quantity and the dynamo mechanism (in the sense of the Babcock-Leighton scheme or otherwise). (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Are QSOs local objects?

The similarities of the spectra of QSOs with those of Wolf-Rayet stars are pointed out. The emission spectrum of the earliest discovered QSO, 3C 273, in the ultraviolet and visible regions is interpreted as that of an object deficient in hydrogen like Wolf-Rayet stars but havingno redshift. The visible emission spectra of two other QSOs, 3C 48 and 3C 280.1, are also similarly interpreted. It is further assumed that the absorption lines of QSOs are produced in an expanding atmosphere so that they are violet shifted as in Wolf-Rayet stars. Fifty-four out of 55 narrow absorption lines of the QSO Q 1246-057 are interpreted on the assumption that the average velocity of the absorbing ions is 500 km s^–1, although the redshift theory can explain only 23 lines by invoking six different redshifts: Four of the five emission lines of the same object can be identified assuming no shift. Since the QSOs are here assumed to be comparatively local objects, the problems of energy supply, superluminal velocities, etc., raised by the conventional explanation do not arise in this case.Presently at the Institut für Physikalische und Theoretische Chemie, Universität Erlangen, F.R.G.     

UGC 5119: A galaxy with a stellar polar ring?

We present our photometric BV R_c observations of UGC 5119, a candidate polar-ring galaxy. We have determined its absolute magnitude, \(M_{0,B} = - 20\mathop m\limits_. 3\), and total color indices, \((B - V)_t^0 = + 0\mathop m\limits_. 73 \pm 0\mathop m\limits_. 10\) and \((V - R_c )_t^0 = + 0\mathop m\limits_. 54 \pm 0\mathop m\limits_. 10\). A Fourier analysis of the shape of its isophotes shows that UGC 5119 is most likely an elliptical galaxy with a disk component in the central part and a “boxy” feature on the periphery. At distances larger than 8″, the galaxy exhibits a turn of its major axis and a change in the phase of the fourth harmonic. Assuming the position angle of the major axis to be constant, a stellar ringlike structure is distinguished in the galaxy. The age of the ring stars is the same as that of the stars in the host galaxy. The distinguished ringlike structure cannot be attributed to typical polar rings rich in gas and in young stars.     

Strike-slip fault patterns on Europa: Obliquity or polar wander?

Variations in diurnal tidal stress due to Europa’s eccentric orbit have been considered as the driver of strike-slip motion along pre-existing faults, but obliquity and physical libration have not been taken into account. The first objective of this work is to examine the effects of obliquity on the predicted global pattern of fault slip directions based on a tidal-tectonic formation model. Our second objective is to test the hypothesis that incorporating obliquity can reconcile theory and observations without requiring polar wander, which was previously invoked to explain the mismatch found between the slip directions of 192 faults on Europa and the global pattern predicted using the eccentricity-only model. We compute predictions for individual, observed faults at their current latitude, longitude, and azimuth with four different tidal models: eccentricity only, eccentricity plus obliquity, eccentricity plus physical libration, and a combination of all three effects. We then determine whether longitude migration, presumably due to non-synchronous rotation, is indicated in observed faults by repeating the comparisons with and without obliquity, this time also allowing longitude translation. We find that a tidal model including an obliquity of 1.2°, along with longitude migration, can predict the slip directions of all observed features in the survey. However, all but four faults can be fit with only 1° of obliquity so the value we find may represent the maximum departure from a lower time-averaged obliquity value. Adding physical libration to the obliquity model improves the accuracy of predictions at the current locations of the faults, but fails to predict the slip directions of six faults and requires additional degrees of freedom. The obliquity model with longitude migration is therefore our preferred model. Although the polar wander interpretation cannot be ruled out from these results alone, the obliquity model accounts for all observations with a value consistent with theoretical expectations and cycloid modeling.     

Are stars with planets anomalous?

The chemical-dynamical properties of stars with giant planets are compared to those of a nearby star sample within the framework of a stellar orbital diffusion model. The stars-with-planets sample includes recently discovered extrasolar planets and the Sun. We find that the planet-bearing stars, 14 Her, ρ ¹ Cnc and τ Boo, are much more metal-rich than stars of similar age and this cannot be easily explained by orbital diffusion. We also confirm previous claims that the motion of the Sun relative to the local standard of rest is very small compared to other G dwarfs of similar age, and we offer a possible explanation for this apparent anomaly.     

How Linear Are Typical CCDs?

The typical linearity achievable with CCDs was questioned at the previous ESO Optical Detector Workshop in 1993. This paper describes the efforts at CTIO since then to develop a technique for measuring linearity accurately, and rapidly enough to make the exploration of CCD operating parameters practical. A simple circuit has been adopted which stabilizes the intensity of a pulsed LED. It is compact enough to be installed behind the shutter so that linearity can be monitored at the telescope while the dome lights are on. Methods are described for verifying the linearity and stability of the measurement system itself. These requirements are very stringent, since CCD gain variations have typically been measured to be 0.2% or less over most of the operating range. Methods are proposed for reducing non-linearity further, and for generating each new linearity curve from a single exposure, after having taken several calibration exposures. Several discrepancies have been seen between linearity of point sources and linearity measured with flat illumination: the amplitudes were similar but the gain variations had opposite slopes over the central part of the curve, and a more pronounced signal deficit has been noted at low light levels for point sources, which is equivalent to the subtraction of few electrons from every pixel.     

Are galaxies due to accretion?

Burnt-out Population III remnants are thought to have acted as protogalactic seeds. Once tepid pancakes have formed such seeds (≈10⁶ M⊙) could be subject to considerable growing by quasi-stationary accretion.     

Are HVCs Produced in Galactic Fountains?

Three-dimensional simulations of the disk-halo interaction show the formation of a thick HI and HII gas disk with different scale heights. The thick HI disk prevents the disk gas from expanding freely upwards, unless some highly energetic event such as chimneys occurs, whereas the thick HII disk acts as a disk-halo interaction region from where the hot ionized gas flows freely into the halo. The upflowing gas reaches the maximum height at z ∼ 9.3 ± 1 kpc becoming thermally unstable due to radiative losses, and condenses into HI clouds. Because the major fraction of the gas is gravitationally bound to the Galaxy, the cold gas returns to the disk. The descending clouds will have at some height high velocities. In a period of 200 Myr of fountain evolution, some 10 percent of the total number of clouds are HVCs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Are Spicules Related to Coronal Heating?

We suggest that the waxing and waning of chromospheric and coronal heating leads to a dynamic solar atmosphere which, under the right circumstances, may produce spicules. Little is known about the heating process. However, Anderson and Athay (1989a) concluded from their study of chromospheric heating that the heating rate per gram of chromospheric matter is only a small fraction of the heating rate per gram of coronal matter. We postulate that the increased heating rate in the corona is a consequence of heating charged particles as opposed to heating neutral atoms. This leads to a specific degree of hydrogen ionization at which coronal heating begins to predominate over chromospheric heating. It also introduces the likelihood that the waxing and waning of the heating rates will have relatively large consequences in the levels where hydrogen ionization is becoming significant. It is demonstrated that changes in the heating rates are capable of inducing increases and decreases in coronal mass comparable to the mass contained in a typical spicule.     

Are There Rings Around the Sun?

The Theory of Alfven drag (Drell et al. in J Geophys Res 70: 3131–3145 1965; Anselmo and Farinella in Icarus, 58, 182–185 1983) is applied here to show that the existence of a possible solar ring structure at a radial distance of 0.02 AU (~4R _⊙ , R _⊙  = radius of the sun) predicted by earlier authors (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may not survive Alfven drag produced during even moderate solar magnetic storms which take place from time to time through the age of the sun, but a possible solar ring structure at a radial distance of 0.13 AU (~27R _⊙ ) (Brecher et al. in Nature 282, 50–52 1979; Rawal in Bull. Astr. Soc. India 6, 92–95 1978, Moon Planets 24, 407–414 1981, Moon Planets 31, 175–182 1984, J Astrophys Astr 10, 257–259 1989) may survive intense Alfven drag produced during even strong magnetic storms of magnetic field value up to 1,000 G.     

Are the clouds of venus sulfuric acid?

Water solutions of sulfuric acid, containing about 75% H₂SO₄ by weight, have a refractive index within 0.01 of the values deduced from polarimetric observations of the Venus clouds. These solutions remain liquid at the cloud temperature, thus explaining the spherical shape of the cloud particles (droplets). The equilibrium vapor pressure of water above such solutions is 0.01 that of liquid water or ice, which accounts for the observed dryness of the cloud region. Furthermore, H₂SO₄ solutions of such concentration have spectra very similar to Venus in the 8–13 μm region; in particular, they explain the 11.2 μm band. Cold sulfuric acid solutions also seem consistent with Venus spectra in the 3–4 μm region. The amount of acid required to make the visible clouds is quite small, and is consistent with both the cosmic abundance of sulfur and the degree of out-gassing of the planet indicated by known atmospheric constituents. Sulfuric acid occurs naturally in volcanic gases, along with known constituents of the Venus atmosphere such as CO₂, HCl, and HF ; it is produced at high temperature by reactions between these gases and common sulfate rocks. The great stability and low vapor pressure of H₂SO₄ and its water solutions explain the lack of other sulfur compounds in the atmosphere of Venus—a lack that is otherwise puzzling.Sulfuric acid precipitation may explain some peculiarities in Venera and Mariner data. Because sulfuric acid solutions are in good agreement with the Venus data, and because no other material that has been proposed is even consistent with the polarimetric and spectroscopic data, H₂SO₄ must be considered the most probable constituent of the Venus clouds.     

Are extrasolar oceans common throughout the Galaxy?

Light and cold extrasolar planets such as OGLE 2005‐BLG‐390Lb, a 5.5 Earth masses planet detected via microlensing, could be frequent in the Galaxy according to some preliminary results from microlensing experiments. These planets can be frozen rocky‐ or ocean‐planet, situated beyond the snow line and, therefore, beyond the habitable zone of their system. They can nonetheless host a layer of liquid water, heated by radiogenic energy, underneath an ice shell surface for billions of years, before freezing completely. These results suggest that oceans under ice, like those suspected to be present on icy moons in the Solar system, could be a common feature of cold low‐mass extrasolar planets. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Are Some High Redshift Galaxies Actually Blueshifting?

An appreciable number of galaxies have been discovered in recent years which are assigned high redshifts. High redshift galaxies (HRGs) are used as cosmological tools for probing early universe and knowledge of star formation. However, literature search revealed some inconsistencies in identification of emission lines used for redshift determination in some of the HRGs. We re-examined the spectra of 15 HRGs, as examples, that are available in the published literature and show that the high values of redshift may have been assigned to these objects due to the misidentification of the observed lines. We demonstrate that the observed emission lines can be identified with alternative search lines at higher wavelengths which are blueshifted instead of the lines used for redshift measurements. Observed blueshifts can be interpreted in a model of multiple expanding universes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Are pre-main-sequence stars older than we thought?

We fit the colour–magnitude diagrams of stars between the zero-age main-sequence and terminal-age main sequence in young clusters and associations. The ages we derive are a factor of 1.5–2 longer than the commonly used ages for these regions, which are derived from the positions of pre-main-sequence stars in colour–magnitude diagrams. From an examination of the uncertainties in the main-sequence and pre-main-sequence models, we conclude that the longer age scale is probably the correct one, which implies that we must revise upwards the commonly used ages for young clusters and associations. Such a revision would explain the discrepancy between the observational lifetimes of protoplanetary discs and theoretical calculations of the time to form planets. It would also explain the absence of clusters with ages between 5 and 30 Myr.
We use the  τ²  statistic to fit the main-sequence data, but find that we must make significant modifications if we are to fit sequences which have vertical segments in the colour–magnitude diagram. We present this modification along with improvements to the methods of calculating the goodness-of-fit statistic and parameter uncertainties.
Software implementing the methods described in this paper is available from http://www.astro.ex.ac.uk/people/timn/tau-squared/ .