Analytic solutions to the accretion of a rotating finite cloud towards a central object – I. Newtonian approach

We construct a steady analytic accretion flow model for a finite rotating gas cloud that accretes material to a central gravitational object. The pressure gradients of the flow are considered to be negligible, and so the flow is ballistic. We also assume a steady flow and consider the particles at the boundary of the spherical cloud to be rotating as a rigid body, with a fixed amount of inwards radial velocity. This represents a generalization to the traditional infinite gas cloud model described by Ulrich. We show that the streamlines and density profiles obtained deviate largely from the ones calculated by Ulrich. The extra freedom in the choice of the parameters on the model can naturally account for the study of protostars formed in dense clusters by triggered mechanisms, where a wide variety of external physical mechanisms determine the boundary conditions. Also, as expected, the model predicts the formation of an equatorial accretion disc about the central object with a radius different from the one calculated by Ulrich.     

A dynamical model with a new inversion technique applied to observations of Comet C/2000 WM1 (LINEAR)

Three continuum images of Comet C/2000 WM₁ (LINEAR) obtained on Nov 10, Nov 19, and Dec 03, 2001, are analyzed with the aid of a dynamical model, i.e. with a model that uses the size-dependent motion of dust grains under solar radiation pressure to determine the dust size distribution and its temporal change. The frames are photometrically calibrated in terms of the albedo filling factor product. On Nov 20.2 the Earth transited the orbital plane of the comet and an anti-tail was recognized in the image of Nov 19. For the determination of the particle fluxes describing the contribution of monodisperse particle shells to the cometary brightness the model uses a new regularization method employing Chebyshev polynomials of selected orders in emission time and particle size. It guarantees positiveness of the particle fluxes and imposes a varying degree of smoothness on their dependence on particle size and emission time. The particle emission velocities are still derived by trial and error. The dynamical model is described in detail. Results are presented for several low orders of the Chebyshev polynomials and are compared in order to understand the limitations imposed by the regularization process. The size distributions derived from the different observations do not always agree. This is particularly true for the earliest and most recent synchrones contributing to an image. In the observations of Nov 10 and Dec 03, i.e. excluding the anti-tail image, the integrated mass loss strongly decreases in the most recent time steps of the model although the comet is still approaching the Sun. This is interpreted as an artifact introduced by the overlap of the shells of large particle size emitted shortly before the observation. The model derives an increasing number of small particles released by the comet in the second half of November. This is at least in part considered as real and attributed to particle fragmentation occurring when the comet was at a heliocentric distance of about 1.4 AU.     

Effects of correlation between merging steps on the global halo formation

The excursion set theory of halo formation is modified by adopting the fractional Brownian motion, to account for possible correlation between merging steps. We worked out analytically the conditional mass function, halo merging rate and formation time distribution in the spherical collapse model. We also developed an approximation for the ellipsoidal collapse model and applied it to the calculation of the conditional mass function and the halo formation time distribution. For models in which the steps are positively correlated, the halo merger rate is enhanced when the accreted mass is less than  ∼25 M *  , while for the negatively correlated case this rate is reduced. Compared with the standard model in which the steps are uncorrelated, the models with positively correlated steps produce more aged population in small mass haloes and more younger population in large mass haloes, while for the models with negatively correlated steps the opposite is true. An examination of simulation results shows that a weakly positive correlation between successive merging steps appears to fit best. We have also found a systematic effect in the measured mass function due to the finite volume of simulations. In future work, this will be included in the halo model to accurately predict the three-point correlation function estimated from simulations.     

A strain-based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets

Numerical modelling of impact cratering has reached a high degree of sophistication; however, the treatment of porous materials still poses a large problem in hydrocode calculations. We present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation. In contrast to previous attempts (e.g., P-alpha model, snowplow model), our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure. Our new ?-alpha model requires only four input parameters and each has a physical meaning. The model is simple and intuitive and shows good agreement with a wide variety of experimental data, ranging from static compaction tests to highly dynamic impact experiments. Our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation. We present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity-scaled source size.     

An assessment of evidence for pingos on Mars using HiRISE

Pingos are small hills with cores of ice, formed by injection and freezing of pressurized water. The possibility of pingos on Mars is of particular interest because of the associated implications for liquid water. We have systematically searched for candidate pingos using images from the High Resolution Imaging Science Experiment (HiRISE) camera. Since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core, we have searched for fractured features and identified a variety of mounds. These features are confined to the martian mid-latitudes, in the bands where gullies are also most common. The observed fractured mounds have a variety of morphologies and are likely of multiple origins. Isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos, but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes, so these features should still be interpreted with caution. Other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures. Flat-topped mounds in Utopia have some characteristics (fracture pattern and latitudinal distribution) consistent with pingos but differ in other aspects such as shape and setting. While we do not rule out a pingo origin, we prefer an erosional model for these enigmatic features.     

Vertical structure of hyper-accreting disks and consequences for gamma-ray burst outflows

We discuss the physics of the power source for gamma-ray bursts (GRBs). There is a great variety of stellar systems proposed as progenitors of long and short GRBs, but any current model for the engine ultimately involves the formation of a hyper-accreting disk around a newly-formed black hole of a few solar masses. The study of such disks can shed light onto the burst composition and energy content. We present preliminary results from disk vertical structure calculations. These include neutrino transport, a height-dependent determination of the nuclear composition, and a simplified treatment of turbulent mixing. We find that vertical mixing is rapid enough to largely erase compositional gradients, and as a consequence the upper layers of the disk reflect the neutron-rich composition of the midplane close to the black hole. We review the implications of this for the nuclear composition of outflows in GRBs. Our models suggest an increasing role for pairs in the upper regions of the disk, and we speculate that a pair-driven wind could be a significant source of cooling. Elena M. Rossi is a Chandra Fellow     

Modeling global impact effects on middle-sized icy bodies: applications to Saturn's moons

Disrupted terrains that form as a consequence of giant impacts may help constrain the internal structures of planets, asteroids, comets and satellites. As shock waves and powerful seismic stress waves propagate through a body, they interact with the internal structure in ways that may leave a characteristic impression upon the surface. Variations in peak surface velocity and tensile stress, related to landform degradation and surface rupture, may be controlled by variations in core size, shape and density. Caloris Basin on Mercury and Imbrium Basin on the Moon have disturbed terrain at their antipodes, where focusing is most intense for an approximately symmetric spheroid. Although, the icy saturnian satellites Tethys, Mimas, and Rhea possess giant impact structures, it is not clear whether these structures have correlated disrupted terrains, antipodal or elsewhere. In anticipation of high-resolution imagery from Cassini, we investigate antipodal focusing during giant impacts using a 3D SPH impact model. We first investigate giant impacts into a fiducial 1000 km diameter icy satellite with a variety of core radii and compositions. We find that antipodal disruption depends more on core radius than on core density, suggesting that core geometry may express a surface signature in global impacts on partially differentiated targets. We model Tethys, Mimas, and Rhea according to their image-derived shapes (triaxial for Tethys and Mimas and spherical for Rhea), varying core radii and densities to give the proper bulk densities. Tethys shows greater antipodal values of peak surface velocity and peak surface tensile stress, indicating more surface damage, than either Mimas or Rhea. Results for antipodal and global fragmentation and terrain rupture are inconclusive, with the hydrocode itself producing global disruption at the limits of model resolution but with peak fracture stresses never exceeding the strength of laboratory ice.     

Stokes imaging of AM Her systems using 3D inhomogeneous models – I. Description of the code and an application to V834 Cen

Polars (or AM Her systems) are cataclysmic variables without a disc due to the strong magnetic field of the white dwarf. Most of their emission comes from the region where the accretion column impacts the white dwarf and cools through cyclotron and bremsstrahlung processes. We present a new code, cyclops , to model the optical emission from these systems including the four Stokes parameters. It considers a three-dimensional region with the electronic density and temperature varying following a shock-like profile and a dipolar magnetic field. The radiative transfer is solved in steps considering the solution with non-null input radiation. The footprint of the column in the white dwarf surface is determined by the threading region in the equatorial plane, i.e. the region from where the flow follows the magnetic lines. The extinction caused by the Thomson scattering above the emitting region is optionally included. The search for the model parameters that best fit an observational data set is carried out using a hybrid approach: a genetic algorithm is used to seek for the regions of the parameter space having the best models and then an amoeba code refines the search. An example of the application to multi-wavelength data of V834 Cen is presented. The fit found is consistent with previous parameter estimates and is able to reproduce the features of V834 data in three wavebands.     

Thermal infrared (8-13 μm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey

We report the results of the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a program of ground-based observations designed to characterize the 8-13 μm spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-in Hale telescope. We have measured the mid-infrared spectra of twenty-nine asteroids and have derived temperature estimates from our data that are largely consistent with the predictions of the standard thermal model. We have also generated relative emissivity spectra for the target asteroids. On only one asteroid, 1 Ceres, have we found emissivity features with spectral contrast greater than 5%. Our spectrum of 4 Vesta suggests emissivity variation at the 2-3% level. Published spectra of several of the small number of asteroids observed with ISO (six of which are also included in our survey), which appeared to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle. This may explain the evident absence of strong emissivity variation in the majority of the MIDAS spectra.     

用MapReduce框架构建虚拟天文台数据节点

MapReduce是一种大规模分布式并行处理框架,最初被用于互联网服务中的海量数据处理,并逐渐扩展到各个行业领域。目前,虚拟天文台面临着越来越多的地面及空间望远镜观测到的海量天文数据。为了提高中国虚拟天文台数据节点处理海量天文数据的能力,首次提出基于MapReduce框架构建中国虚拟天文台数据节点的方法,并以批量星表交叉认证为例描述了具体实现过程,性能评估结果证明基于MapReduce框架构建虚拟天文台数据节点,可以在性能、扩展性与成本等多方面获得收益。     

Jupiter's polar clouds and waves from Cassini and HST images: 1993-2006

For a variety of reasons, Jupiter's polar areas are probably the less observed regions of the planet. To study the dynamics and cloud vertical structure in the polar regions of the planet (latitudes 50° to 80° in both hemispheres) we have used images of Jupiter obtained from the ultraviolet to near infrared (258 to 939 nm) by the Cassini Imagining Science Subsystem (ISS) in December 2000. The temporal coverage was complemented with archived images from the Hubble Space Telescope (1993-2006) in a similar spectral range. The zonal wind velocities have been measured at three Cassini ISS wavelengths (CB2, MT3 and UV1, corresponding to 750, 890 and 258 nm) sounding different altitude levels. The three eastward jets detected in CB2 images (lower cloud) go to zero velocity when measured in the UV1 filter (upper haze). A radiative transfer analysis has been performed to characterize the vertical structure of cloud and hazes distribution at the poles. We also present a characterization (phase speed, amplitude and zonal wavenumber) of the previously detected circumpolar waves at 67° N and S at 890 nm and at about 50° N and −57° S at 258 nm that are a permanent phenomenon in Jupiter with some variability in its structure during the analyzed period. From the ensemble of data analyzed we propose the waves are Rossby waves whose dynamic behavior constrains plausible values for their meridional and vertical wavenumbers. This work demonstrates the long-term nature of Jupiter's polar waves, providing a dynamical and vertical characterization which supports a detailed analysis of these phenomena in terms of a Rossby wave model.     

Radiation-induced amorphization of crystalline ice

We study radiation-induced amorphization of crystalline ice, analyzing the results of three decades of experiments with a variety of projectiles, irradiation energy, and ice temperature, finding a similar trend of increasing resistance of amorphization with temperature and inconsistencies in results from different laboratories. We discuss the temperature dependence of amorphization in terms of the ‘thermal spike’ model. We then discuss the common use of the 1.65 μm infrared absorption band of water as a measure of degree of crystallinity, an increasingly common procedure to analyze remote sensing data of astronomical icy bodies. The discussion is based on new, high quality near-infrared reflectance absorption spectra measured between 1.4 and 2.2 μm for amorphous and crystalline ices irradiated with 225 keV protons at 80 K. We found that, after irradiation with 10¹⁵ protons cm⁻², crystalline ice films thinner than the ion range become fully amorphous, and that the infrared absorption spectra show no significant changes upon further irradiation. The complete amorphization suggests that crystalline ice observed in the outer Solar System, including trans-neptunian objects, may results from heat from internal sources or from the impact of icy meteorites or comets.     

The effects of strain localization on the formation of Ganymede’s grooved terrain

We investigate the effects of strain localization on the formation of Ganymede’s grooved terrain by numerically modeling the extension of an ice lithosphere in which the yield strength of the ice decreases as the magnitude of the plastic strain increases. We do this to more realistically model fault strength, which is expected to vary with slip during initial fault development. We find that the inclusion of strain weakening leads to the formation of periodic structures with amplitudes of 200-500 m, consistent with the observed amplitudes of Ganymede’s large-scale grooves. The morphology of the deformation that results from extension depends both on the thermal gradient, which sets the lithospheric thickness, and on the rate at which the yield strength of the ice decreases with increasing plastic strain. Slow weakening with strain leads to low-amplitude, periodic structures, whereas moderate to rapid weakening with strain leads to large-amplitude, non-periodic structures. The combined influence of the thermal gradient and the weakening rate leads to the formation of complex surface deformation and may help explain the variety of surface morphologies observed within the grooved terrain.     

The planets capture model of V838 Monocerotis: conclusions for the penetration depth of the planet(s)

V838 Mon is the prototype of a new class of objects. Understanding the nature of its multistage outburst and similar systems is challenging. So far, several scenarios have been invoked to explain this group of stars. In this work, the planets-swallowing model for V838 Mon is further investigated, taking into account the findings that the progenitor is most likely a massive B-type star. We find that the super-Eddington luminosity during the eruption can explain the fast rising times of the three peaks in the optical light curve. We used two different methods to estimate the location where the planets were consumed. There is a nice agreement between the values obtained from the luminosities of the peaks and from their rising time-scale. We estimate that the planets were stopped at a typical distance of one solar radius from the centre of the host giant star. The planets-devouring model seems to give a satisfying explanation to the differences in the luminosities and rising times of the three peaks in the optical light curve of V838 Mon. The peaks may be explained by the consumption of three planets or alternatively by three steps in the terminal falling process of a single planet. We argue that only the binary merger and the planets-swallowing models are consistent with the observations of the new type of stars defined by V838 Mon.     

Derivation of martian surface slope characteristics from directional thermal infrared radiometry

Directional thermal infrared measurements of the martian surface is one of a variety of methods that may be used to characterize surface roughness and slopes at scales smaller than can be obtained by orbital imagery. Thermal Emission Spectrometer (TES) emission phase function (EPF) observations show distinct apparent temperature variations with azimuth and emission angle that are consistent with the presence of warm, sunlit and cool, shaded slopes at typically ∼0.1 m scales. A surface model of a Gaussian distribution of azimuth independent slopes (described by θ-bar) is combined with a thermal model to predict surface temperature from each viewing angle and azimuth of the TES EPF observation. The models can be used to predict surface slopes using the difference in measured apparent temperature from 2 separate 60-70° emission angle observations taken ∼180° in azimuth relative to each other. Most martian surfaces are consistent with low to moderate slope distributions. The slope distributions display distinct correlations with latitude, longitude, and albedo. Exceptionally smooth surfaces are located at lower latitudes in both the southern highlands as well as in high albedo dusty terrains. High slopes are associated with southern high-latitude patterned ground and north polar sand dunes. There is little apparent correlation between high resolution imagery and the derived θ-bar, with exceptions such as duneforms. This method can be used to characterize potential landing sites by assuming fractal scaling behavior to meter scales. More precisely targeted thermal infrared observations from other spacecraft instruments are capable of significantly reducing uncertainty as well as reducing measurement spot size from 10s of kilometers to sub-kilometer scales.     

Small-scale dust structures in Halley's coma: II. Disintegration of large dust bodies

Small-scale dust structures, SDSs, altogether ∼35 events with extent ∼30-220 km, have been recognized owing to electric field records, mostly near the closest approach of Vega-2 to Halley's nucleus. Several (8-9) morphological forms of SDS have been identified, and all they make one family. Among the family members, the key form (with respect to which, all other forms can be regarded as degenerate) is a sequence of 3-5 dust clouds. The morphological forms represent various Vega-2 passes through SDSs at different stages of development. SDSs observable as the key form consisted of several fairly regularly spaced dust subpopulations, whose plane of symmetry was parallel to the comet orbit plane. That regularity together with specific features of morphological forms strongly constrain disintegration scenarios and dynamics of fragments, and allow to draw a number of conclusions, the main of which are: SDS parent bodies were ice-free dust aggregates lifted from the nucleus near the comet perihelion, whose masses were in the range ∼0.1-1 of the biggest emitted mass (mass of a body accelerated to the escape velocity, i.e., ∼300-1500 kg); the disintegration scenario comprised a few steps, and the first-step disintegration consisted mainly in consecutive detachments of biggest first-step fragments (BF-SFs) from the parent body; a SDS observable as the key form included the dust minitail of parent body and a few BF-SF minitails, the former one being longer than the latter ones; SDS parent bodies had a fractal-like internal structure, and the BF-SF mass was a few percent of the parent body mass; the thermal conductivity of SDS parent body was less than ∼0.4 W m⁻¹ K⁻¹ or so, while the latent heat of gluing organics was roughly 80 kJ mol⁻¹; the disintegration mechanism was a combination of sintering and sublimation of organics. The multistep disintegration of SDS parent bodies can be reconciled with the basically one-step disintegration of aggregates responsible for the dust boundary (Oberc, P., Icarus 1996, 124, 195-208). The fractal-like structure and the relation between BF-SF mass and parent body mass are in agreement with predictions from the Weidenschilling model of comet formation. Large ice-free dust bodies, in particular SDS parent bodies, can be identified with refractory boulders postulated by some comet nucleus models.     

Comet classification with new methods for gas and dust spectroscopy

We present the results of a program of comet long-slit spectroscopy with the Kast Dual Spectrograph on the 3-m Shane Telescope at Lick Observatory. A total of 26 comets, from a variety of dynamical families, were observed on 39 different nights from 1996 to 2007. A new statistical method extracted the twilight sky from comet frames, because traditional sky subtraction techniques were inadequate. Because previously published Haser model parent and daughter scale lengths did not fit the data well, unbiased ranges of scale lengths were searched for the best-fitting pairs. Coma gas production rates for OH, CN, C₂, C₃, NH, NH₂, and OH confirmed the widely reported carbon-chain depletion for a sub-class of comets, most notably high-perihelion Jupiter-family comets observed at r_h > 1.5 AU, with different behaviors for C₂ and C₃. Our long-slit spectroscopy data was also adapted for the A(θ)fρ dust production parameter. The assumption that A(θ)fρ is constant throughout the nucleus was not upheld. High dust-to-gas ratios for comets with large perihelia were not a selection effect, and suggest that the dust was released earlier in the formation of the coma than the gas. The dust-to-gas ratio did not exhibit any evolutionary traces between different comet dynamical families. The comet survey illuminates the diversity among comets, including the unusually carbon poor Comet 96P/Machholz.     

Jets from young stars

Most stars produce spectacular jets during their formation. There are thousands of young stars within 500 pc of the Sun and many power jets. Thus protostellar jets may be the most common type of collimated astrophysical outflow. Shocks powered by outflows excite many emission lines, exhibit a rich variety of structure, and motions with velocities ranging from 50 to over 500 km s⁻¹. Due to their relative proximity, proper motions and structural changes can be observed in less than a year. I review the general properties of protostellar jets, summarize some results from recent narrow-band imaging surveys of entire clouds, discuss irradiated jets, and end with some comments concerning outflows from high-mass young stellar objects. Protostellar outflows are ideal laboratories for the exploration of the jet physics.     

Heavy-element abundances in seven SC stars and several related stars

We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the ⁸⁵Rb/⁸⁷Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 10⁶ to ≳10⁸ cm⁻³ for the SC stars. For the four S stars the range is from 10⁷ to ≳10⁸ cm⁻³. Recent calculations by Gallino et al. show that neutron densities near 10⁷ cm⁻³ favour the ¹³C source for neutrons, while densities greater than 10⁸ cm⁻³ may be associated with neutrons from the ²²Ne source.