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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.12.004 Byline: E. Beer (a), D. Prialnik (b), M. Podolak (b) Keywords: Comets; coma; Comets; dust; Comets; composition Abstract: We use the model of grain behavior in the coma developed by Beer et al. [Beer, E.H., Podolak, M., Prialnik, P., 2006. Icarus 180, 473-486] to compute the contribution of ice grains to the brightness of the coma. The motion of an ice grain along the comet-Sun axis is computed, taking into account gas drag, the gravity of the nucleus, and radiation pressure of sunlight. The sublimation of the grains is also included. We assume that the maximum distance that a grain travels along this axis is indicative of the size of the coma, and we compute the resultant brightness as a function of heliocentric distance. The results are then compared to observations. Author Affiliation: (a) NASA Ames Research Center, Moffett Field, CA 94035-1000, USA (b) Department of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel 69978 Article History: Received 28 September 2006; Revised 21 November 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.018 Byline: E.H. Beer, M. Podolak, D. Prialnik Keywords: Comets; Ices; Photometry Abstract: We have developed a computer code (GEM -- grain evolution model) to simulate the behavior of ice grains in a comet coma. The grains are assumed to be composed of water-ice with an admixture of dark material ('dirt'). An initial size distribution of grains is assumed to be ejected from the nucleus. The ejected mass is taken to be proportional to the rate of gas production by the nucleus. The efficiency for absorption and re-radiation of sunlight is computed from Mie scattering theory. The grain temperature and sublimation rate at a given heliocentric distance is then derived from energy balance considerations. The evolution of the grain size distribution is followed as a function of distance from the nucleus. Author Affiliation: Department of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel Article History: Received 13 January 2005; Revised 26 September 2005

We describe observations of the scattered Kuiper Belt object (29981) 1999 [TD.sub.10] performed during five observing runs at two observatories, over 370 days from 2000 September to 2001 September. They show significant brightness variations that fit a double-peaked lightcurve with period 15.448 [+ or -] 0.012 h in V and R bands. The phase effect in V band, 0.09 [+ or -] 0.01 mag [deg.sup.-1], is smaller than that of Pluto but larger than that of several KBOs, while in R band it is 0.030 [+ or -] 0.005 mag [deg.sup.-1]. We find color variation between the two bands, which implies a non-homogeneous albedo distribution on the surface. Evidence of surface activity near perihelion in the form of a coma/tail is presented using radial image profiles and a 2D contour map. Keywords: Kuiper belt objects; Asteroids, Rotation

We defend the position taken in our earlier note that under certain conditions the D/H ratio measured in the coma of a comet can be much higher that the D/H ratio in to cometary ice itself. Keywords: Comet, composition; Comets, origin; Ices

A definitive determination of the progenitors of Type Ia supernovae (SNIa) has been a conundrum for decades. The single degenerate scenario - a white dwarf (WD) in a semi-detached binary system accreting mass from its secondary - is a plausible path; however, no simulation to date has shown that such an outcome is possible. In this study, we allowed aW Dwith a near Chandrasekhar mass of 1.4M☉ to evolve over tens of thousands of nova cycles, accumulating mass secularly while undergoing periodic nova eruptions. We present themass accretion limits within which an SNIa can possibly occur. The results showed, for each parameter combination within the permitted limits, tens of thousands of virtually identical nova cycles where the accreted mass exceeded the ejected mass, i.e. the WD grew slowly but steadily in mass. Finally, the WD became unstable, the maximal temperature rose by nearly two orders of magnitude, heavy element production was enhanced by orders of magnitude and the nuclear and neutrino luminosities became enormous. We also found that this mechanism leading to WD collapse is robust, with WDs in the range 1.0-1.38M☉, and an accretion rate of 5×10-7M☉ yr-1, all growing steadily in mass. These simulations of the onset of an SNIa event make observationally testable predictions about the light curves of pre-SN stars, and about the chemistry of SNIa ejecta. [ABSTRACT FROM AUTHOR]

Aims. Hydrogen cyanide (HCN) radiates effectively at infrared wavelengths in cometary atmospheres, and a new quantum-band model is needed to properly interpret high-resolution spectra. HCN spectra of comets 8P/Tuttle, C/2007 W1 (Boattini), and C/2008 Q3 (Garradd) have been recorded by our team using the high-resolution CRyogenic InfraRed Echelle Spectometer (CRIRES) at the Very Large Telescope (VLT), ultimately posing an excellent test for our newly developed model. Methods. We developed a quantum-band model for the v1 fundamental of HCN using the latest spectroscopic parameters available and with it retrieved HCN in the above mentioned three comets. For each comet, we sampled several lines of HCN in the spectral region near 3 μm, and retrieved molecular production rates, mixing ratios, and rotational temperatures. Results. When compared to other comets, 8P/Tuttle is relatively depleted in HCN, while C/2007 W1 (Boattini) appears to be enriched and C/2008 Q3 (Garradd) normal. The spatial profile of HCN observed in 8P/Tuttle is symmetric, consistent with isotropic outgassing from the nucleus, while in comet C/2007 W1 we observed an asymmetric excess of HCN in the anti-solar direction. We investigated the HCN-CN parentage by comparing our production rate ratios (HCN/H2O) with those of CN/OH derived at optical wavelengths. In comet C/2007 W1 the two mixing ratios are comparable, while in 8P/Tuttle our derived HCN abundance is too low to support the HCN molecule as the only parent of the CN radical. [ABSTRACT FROM AUTHOR]

We explore a possible mechanism that may explain the outbursts observed by the Deep Impact mission on comet 9P/Tempel 1 that appear to occur near sunrise on a particular area. Assuming that the area is covered by a porous, poorly conducting dust layer, the heat wave generated by solar radiation at local noon propagates through the dust layer towards the ice-rich layer underneath it. The heated ice sublimates and the vapour flows towards the surface. However, by the time the heat wave reaches the ice, the spot has moved out of sunlight and its temperature has started dropping. As the vapour flows outwards, the surface has become so cold that it refreezes. Thus, at night some ice accumulates in the dust layer, very close to the surface. At sunrise, it immediately evaporates, producing a short-lived surge of activity. Numerical simulations of this mechanism provide the duration and water production of such outbursts, which are compatible with the 9P/Tempel 1 observations of small outbursts. [ABSTRACT FROM AUTHOR]

We analyse UV observations with FAUST of four sky fields in the general direction of the Fourth Galactic Quadrant, in which we detect 777 UV sources. This is ∼50 per cent more than detected originally by Bowyer et al. We discuss the source detection process and the identification of UV sources with optical counterparts. For the first time in this project we use ground-based objective-prism information for two of the fields, to select the best-matching optical objects with which to identify the UV sources. Using this, and correlations with existing catalogues, we present reliable identifications for ∼75 per cent of the sources. Most of the remaining sources have assigned optical counterparts but, lacking additional information, we offer only plausible identifications. We discuss the types of objects found, and compare the observed population with predictions of our UV Galaxy model. [ABSTRACT FROM AUTHOR]

Focusing on recurrent novae (RN), we explore white dwarf (WD) masses and accretion rates (M.), to determine the combinations that will retain some of the accreted mass at the end of each cycle. Thus, the WD may grow toward the Chandrasekhar mass (MCh) and explode as a Type Ia Supernova (SNIa). We discuss limits imposed by the secondary mass (Ms) and evolution time scales. We also discuss observables which can assist in detecting potential progenitors of SNIa. [ABSTRACT FROM AUTHOR]

Observational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in Extreme Horizontal Branch (EHB) and subsequent evolutionary phases.Using our new stellar evolution code – a code that follows through complete evolutionary tracks, Pre-MS to cooling WD – without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs.HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 − 1.1 M⊙) and for metallicities covering both populations I and II (Z = 0.01 − 0.001). The [M,Z] combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr. [ABSTRACT FROM PUBLISHER]

We report on the development of a new stellar evolution code, and provide a taste of results, showing its capability to calculate full evolutionary tracks for a wide range of masses and metalicities. The code is fast and efficient, and is capable of following through all evolutionary phases, including core/shell flashes and thermal pulses, without any interruption or intervention. It is meant to be used also in the context of modeling the evolution of dense stellar systems, for performing live calculations for both ‘normal’ ZAMS/PRE-MS models, but mainly for ‘non-canonical’ stellar configurations (i.e. merger-products). We show a few examples of evolutionary calculations for stellar populations I and II, and for masses in the range 0.25–64 M⊙. [ABSTRACT FROM PUBLISHER]

We present a new stellar evolution code and a set of results, showing its capability to calculate full evolutionary tracks for a wide range of masses and metalicities. The code is meant to be used also in the context of modeling the evolution of dense stellar systems, for performing live evolutionary calculations both for ‘normal’ ZAMS/PRE-MS models, but mainly for ‘non-canonical’ (i.e. merger-products) stellar configurations. For such tasks, it has to be robust and efficient, capable to run through all phases of stellar evolution without interruption or intervention. Here we show a few examples of evolutionary calculations for stellar populations I and II, and for masses in the range 0.25–64 M⊙. [ABSTRACT FROM PUBLISHER]

We present a new stellar evolution code and a set of results, showing its capability to calculate full evolutionary tracks for a wide range of masses and metalicities. The code is meant to be used also in the context of modeling the evolution of dense stellar systems, for performing live evolutionary calculations both for 'normal' ZAMS/PRE-MS models, but mainly for 'non-canonical' (i.e. merger-products) stellar configurations. For such tasks, it has to be robust and efficient, capable to run through all phases of stellar evolution without interruption or intervention. Here we show a few examples of evolutionary calculations for stellar populations I and II, and for masses in the range 0.25–64 M ⊙. [ABSTRACT FROM AUTHOR]

Abstract: Durda et al. (Durda, D.D., Bottke, W.F., Enke, B.L., Merline, W.J., Asphaug, E., Richardson, D.C., Leinhardt, Z.M. [2004]. Icarus 170, 243–257), using numerical models, suggested that binary asteroids with large separation, called Escaping Ejecta Binaries (EEBs), can be created by fragments ejected from a disruptive impact event. It is thought that six binary asteroids recently discovered might be EEBs because of the high separation between their components (∼100> a/Rp >∼20). However, the rotation periods of four out of the six objects measured by our group and others and presented here show that these suspected EEBs have fast rotation rates of 2.5–4h. Because of the small size of the components of these binary asteroids, linked with this fast spinning, we conclude that the rotational-fission mechanism, which is a result of the thermal YORP effect, is the most likely formation scenario. Moreover, scaling the YORP effect for these objects shows that its timescale is shorter than the estimated ages of the three relevant Hirayama families hosting these binary asteroids. Therefore, only the largest (D ∼19km) suspected asteroid, (317) Roxane, could be, in fact, the only known EEB. In addition, our results confirm the triple nature of (3749) Balam by measuring mutual events on its lightcurve that match the orbital period of a nearby satellite in addition to its distant companion. Measurements of (1509) Esclangona at different apparitions show a unique shape of the lightcurve that might be explained by color variations. [Copyright &y& Elsevier]

'Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun 1 . Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state 1-4 and that the physical properties of its surface resemble those of cometary nuclei 5,6 , even though it showed no evidence of cometary activity 1,5,7 . The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing 8 . Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population 9 . Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of 'Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r -2 or r -1 (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from 'Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that 'Oumuamua has thermal properties similar to comets.

The evolution of a comet nucleus is investigated, taking into account the crystallization process by which the gas trapped in the ice is released to flow through the porous ice matrix. The equations of conservation of the energy and of the masses of ice and gas are solved throughout the nucleus, to obtain the evolution of the temperature, gas pressure and density profiles. A spherical nucleus composed of cold, porous amorphous ice, with 10% of CO trapped in it, serves as initial model. Several values of density (porosity) and pore size are considered. For each combination of parameters the model is evolved for 20-30 revolutions in comet P/Halley's orbit. Two aspects of the release of gas upon crystallization are analyzed and discussed: (a) the resulting continuous outward flux with high peaks at the time of crystallization, which is a cyclic process in the low-density models and sporadic in the high-density ones; (b) the internal pressures obtained down to depths of a few tens to approximately 200 m (depending on parameters), that are found to exceed the compressional strength of cometary ice. As a result, both cracking and explosions of the overlying ice layer and ejection of gas and ice/dust grains are expected to follow crystallization. They should appear as outbursts or sudden brightening of the comet. The model of 0.2 g cm-3 density is found to reproduce quite well many of the light-curve and activity characteristics of comet P/Halley.

The thermal evolution of a spherical cometary nucleus (initial radius of 2.5 km), composed initially of very cold amorphous ice and moving in comet Halley's orbit, is simulated numerically for 280 revolutions. It is found that the phase transition from amorphous to crystalline ice constitutes a major internal heat source. The transition does not occur continuously, but in five distinct rounds, during the following revolutions: 1, 7, 40-41, 110-112, and 248-252. Due to the (slow) heating of the amorphous ice between crystallization rounds, the phase transition front advances into the nucleus to progressively greater depths: 36 m on the first round, and then 91 m, 193 m, 381 m, and 605 m respectively. Each round of crystallization starts when when the boundary between amorphous and crystalline ice is brought to approximately 15 m below the surface, as the nucleus radius decreases due to sublimation. At the time of crystallization, the temperature of the transformed ice rises to 180 K. According to experimental studies of gas-laden amorphous ice, a large fraction of the gas trapped in the ice at low temperatures is released. Whereas some of the released gas may find its way out through cracks in the crystalline ice layer, the rest is expected to accumulate in gas pockets that may eventually explode, forming "volcanic calderas." The gas-laden amorphous ice thus exposed may be a major source of gas and dust jets into the coma, such as those observed on comet Halley by the Giotto spacecraft. The activity of new comets and, possibly, cometary outbursts and splits may also be explained in terms of explosive gas release following the transition from amorphous to crystalline ice.

The properties of the outer layers of comets considered for the future Comet Rendezvous and Asteroid Flyby and Comet Nucleus Sample Return missions are studied, by following numerically the thermal evolution of spherically symmetric models of the nucleus, in the orbit of Comet P/Tempel-1. The evolution starts from isothermal (10 degrees K) and homogeneous nuclei, composed of amorphous ice and dust. The crystallization of amorphous ice at 137 degrees K is taken into account. As the ice sublimates, a permanent dust mantle is allowed to accumulate, at a rate which is proportional to the sublimation rate. Evolutionary sequences are computed for different values of the density, the dust/ice mass ratio, and the (constant) fraction of the dust which is not carried away with the sublimating ice. The main conclusions are (a) the temperatures at the outer and inner surfaces of the dust mantle are not very sensitive to changes in the parameters; (b) although the dust is assumed permeable to water vapor the rate of erosion of the nucleus slows down as the dust mantle grows and its insulating effect increases; (c) the temperature at a depth of 10 m is approximately 160 degrees K for all models considered and hence, the ice at this depth is crystalline; (d) the total thickness of the crystalline ice layer, between the dust mantle and the amorphous ice core, varies from 40 to 240 m, depending on the parameters assumed. Consequently, it should be difficult for the probes of the two comet missions to sample pristine amorphous ice, unless they are aimed at the bottom of an active crater.