The Light Curve of Periodic Comet Wirtanen
Teh light curve of Periodic Comet Wirtanen has been analyzed for the 1986 and 1991 apparitions, the only two monitored by visual observers.
Wirtanen comet light curve periodic comets apparit↗
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Mascon distribution techniques for asteroids and comets
The mass-concentration model is an approach that has been used to model the gravitational fields of irregularly shaped bodies such as asteroids and comets. By this approach, the body is treated as a collection of point masses. The method is conceptually simple, easy to program, valid down to the surface, and capable of modeling arbitrary density heterogeneities. How the mass concentrations are distributed as well as how mass is assigned to these concentrations is, however, nontrivial. These aspects significantly affect the accuracy and efficiency of the gravitational model. In this paper, we frame the distribution process in terms of numerical integration applied to finite volume meshes. We describe a new method using unstructured, curvilinear, finite volume meshes to significantly improve the accuracy of the mass-concentration model. We then compare the accuracy and efficiency of several variations of our distribution technique to those from literature using Asteroid Eros and Bennu as example bodies. Our results show that the mascon model can be as accurate as the analytic polyhedral model at the surface using an equivalent number of computational elements—i.e., mascon to surface facets. We report the improvement in the model’s performance can be mainly attributed to the volume mesh topology while mesh curving can provide modest case-dependent improvements.
79 ASTRONOMY AND ASTROPHYSICS↗
Comets and carbonaceous chondrites delivered noble gases to the Moon
Trapped xenon isotopes in two Apollo 17 mare basalt fragments are similar to those in primitive meteorites. Xenon would have been effectively excluded from the Moon in the aftermath of its formation in a giant impact. As such, lunar mantle xenon trapped in the mare basalts is best explained as being derived from late accretion occurring before or concurrent with the formation of the lunar crust. Here, the isotopic composition indicates that a combination of comets and carbonaceous chondrites delivered this xenon to the lunar mantle. The inferred mass of accreted cometary ice would have delivered significantly less than a part per million of water to the lunar mantle. The inferred mass of accreted carbonaceous chondrites would have supplied at least a half of a part per million of water. The data further indicate that enstatite chondrites are unlikely to have supplied the majority of late accreted mass.
Chemistry↗
Hubble space telescope investigation of main-belt comet 133P/Elst-Pizarro
We report new observations of the prototype main-belt comet (active asteroid) 133P/Elst-Pizarro taken at high angular resolution using the Hubble Space Telescope. The object has three main components: (1) a point-like nucleus; (2) a long, narrow antisolar dust tail; and (3) a short, sunward anti-tail. There is no resolved coma. The nucleus has a mean absolute magnitude H{sub V} = 15.70 ± 0.10 and a light curve range ΔV = 0.42 mag, the latter corresponding to projected dimensions 3.6 × 5.4 km (axis ratio 1.5:1) at the previously measured geometric albedo of 0.05 ± 0.02. We explored a range of continuous and impulsive emission models to simultaneously fit the measured surface brightness profile, width, and position angle of the antisolar tail. Preferred fits invoke protracted emission, over a period of 150 days or less, of dust grains following a differential power-law size distribution with index 3.25 ≤q ≤ 3.5 and with a wide range of sizes. Ultra-low surface brightness dust projected in the sunward direction is a remnant from emission activity occurring in previous orbits, and consists of the largest (≥cm-sized) particles. Ejection velocities of one-micron-sized particles are comparable to the ∼1.8 m s{sup –1} gravitational escape speed of the nucleus, while larger particles are released at speeds less than the gravitational escape velocity. The observations are consistent with, but do not prove, a hybrid hypothesis in which mass loss is driven by gas drag from the sublimation of near-surface water ice, but escape is aided by centripetal acceleration from the rotation of the elongated nucleus. No plausible alternative hypothesis has been identified.
79 ASTRONOMY AND ASTROPHYSICS↗
EXPLORING THE POTENTIAL FORMATION OF ORGANIC SOLIDS IN CHONDRITES AND COMETS THROUGH POLYMERIZATION OF INTERSTELLAR FORMALDEHYDE
Polymerization of interstellar formaldehyde, first through the formose reaction and then through subsequent condensation reactions, provides a plausible explanation for how abundant and highly chemically complex organic solids may have come to exist in primitive solar system objects. In order to gain better insight on the reaction, a systematic study of the relationship of synthesis temperature with resultant molecular structure was performed. In addition, the effect of the presence of ammonia on the reaction rate and molecular structure of the product was studied. The synthesized formaldehyde polymer is directly compared to chondritic insoluble organic matter (IOM) isolated from primitive meteorites using solid-state {sup 13}C nuclear magnetic resonance, Fourier transform infrared, and X-ray absorption near edge structure spectroscopy. The molecular structure of the formaldehyde polymer is shown to exhibit considerable similarity at the functional group level with primitive chondritic IOM. The addition of ammonia to the solution enhances the rate of polymerization reaction at lower temperatures and results in substantial incorporation of nitrogen into the polymer. Morphologically, the formaldehyde polymer exists as submicron to micron-sized spheroidal particles and spheroidal particle aggregates that bare considerable similarity to the organic nanoglobules commonly observed in chondritic IOM. These spectroscopic and morphological data support the hypothesis that IOM in chondrites and refractory organic carbon in comets may have formed through the polymerization of interstellar formaldehyde after planetesimal accretion, in the presence of liquid water, early in the history of the solar system.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Coupled escape probability for an asymmetric spherical case: Modeling optically thick comets
We have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, for use in asymmetrical spherical situations. Our model is intended specifically for use in modeling optically thick cometary comae, although not limited to such use. This method enables the accurate modeling of comets' spectra even in the potentially optically thick regions nearest the nucleus, such as those seen in Deep Impact observations of 9P/Tempel 1 and EPOXI observations of 103P/Hartley 2.
79 ASTRONOMY AND ASTROPHYSICS↗
The solar corona as probed by comet Lovejoy (C/2011 W3)
Extreme-ultraviolet images of Comet Lovejoy (C/2011 W3) from the Atmospheric Imaging Assembly show striations related to the magnetic field structure in both open and closed magnetic regions. The brightness contrast implies coronal density contrasts of at least a factor of six between neighboring flux tubes over scales of a few thousand kilometers. These density structures imply variations in the Alfvén speed on a similar scale. They will drastically affect the propagation and dissipation of Alfvén waves, and that should be taken into account in models of coronal heating and solar wind acceleration. In each striation, the cometary emission moves along the magnetic field and broadens with time. The speed and the rate of broadening are related to the parallel and perpendicular components of the velocities of the cometary neutrals when they become ionized. We use a magnetohydrodynamic model of the coronal magnetic field and the theory of pickup ions to compare the measurements with theoretical predictions, in particular with the energy lost to Alfvén waves as the cometary ions isotropize.
79 ASTRONOMY AND ASTROPHYSICS↗
ON THE POSSIBILITY OF SIGNIFICANT ELECTRON DEPLETION DUE TO NANOGRAIN CHARGING IN THE COMA OF COMET 67P/CHURYUMOV-GERASIMENKO NEAR PERIHELION
We approach the complicated phenomena of gas-dust interactions in a cometary ionosphere, focusing in particular on the possibility of significant depletion in electron number density due to grain charging. Our one-dimensional ionospheric model, accounting for grain charging processes, is applied to the subsolar direction and the diamagnetic cavity of 67P/Churyuomov-Gerasimenko, the target comet for the ESA Rosetta mission, at perihelion (∼1.25-1.30 AU). We argue on the one hand that grains with radii >100 nm are unlikely to significantly affect the overall ionospheric particle balance within this environment, at least for cometocentric distances >10 km. On the other hand, if nanograins with radii in the 1-3 nm range are ejected to the coma at a level of ∼1% with respect to the mass of the sublimated gas, a significant electron depletion is expected up to cometocentric distances of several tens of kilometers. We relate these results to the recent Cassini discoveries of very pronounced electron depletion compared with the positive ion population in the plume of Enceladus, which has been attributed to nanograin charging.
79 ASTRONOMY AND ASTROPHYSICS↗
STRONG EROSION-DRIVEN NONGRAVITATIONAL EFFECTS IN ORBITAL MOTIONS OF THE KREUTZ SUNGRAZING SYSTEM’S DWARF COMETS
We investigate the relationships among the angular orbital elements—the longitude of the ascending node Ω, the inclination i, and the argument of perihelion ω—of the Kreutz system’s faint, dwarf sungrazers observed only with the Solar and Heliospheric Observatory/STEREO coronagraphs; their published orbits were derived using a parabolic, purely gravitational approximation. In a plot of i against Ω the bright Kreutz sungrazers (such as C/1843 D1, C/1882 R1, C/1963 R1, etc.) fit a curve of fixed apsidal orientation, whereas the dwarf members are distributed along a curve that makes with the apsidal curve an angle of 15°. The dwarf sungrazers’ perihelion longitude is statistically invariable, but their perihelion latitude increases systematically with Ω. We find that this trend can be explained by a strong erosion-driven nongravitational acceleration normal to the orbit plane, confirmed for several test dwarf Kreutz sungrazers by orbital solutions with nongravitational terms incorporated directly in the equations of motion on a condition of fixed apsidal orientation. Proceeding in three steps, we first apply Marsden et al.'s standard formalism, solving for the normal acceleration only, and eventually relax additional constraints on the nongravitational law and the acceleration’s radial and transverse components. The resulting nongravitational accelerations on the dwarf sungrazers exceed the maximum for cataloged comets in nearly parabolic orbits by up to three orders of magnitude, topping in exceptional cases the Sun’s gravitational acceleration! A mass-loss model suggests that the dwarf sungrazers’ nuclei fragment copiously and their dimensions diminish rapidly near the Sun, implying the objects’ imminent demise shortly before they reach perihelion.
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INFRARED AND OPTICAL IMAGINGS OF THE COMET 2P/ENCKE DUST CLOUD IN THE 2003 RETURN
We report contemporaneous imaging observations of the short-period comet 2P/Encke in infrared and optical wavelengths during the 2003 return. Both images show the same unique morphology consisting of a spiky dust cloud near the nucleus and a dust trail extending along the orbit. We conducted a dynamical simulation of dust particles to characterize the morphology and found that dust particles were ejected intensively for a short duration (≲10 days) a few days after perihelion passage. The maximum particle size is at least on the order of 1 cm in radius following a differential power-law size distribution with an index of −3.2 to −3.6. The total mass ejected in the 2003 return is at least 1.5 × 10{sup 9}–1.2 × 10{sup 10} kg, which corresponds to 0.003%–0.03% of the nucleus mass. We derived the albedo of the dust cloud as 0.01–0.04 at a solar phase angle of 26.°2, which is consistent with or possibly greater than that of the nucleus. We suppose that impulsive activity such as an outburst is a key to understanding the peculiar appearance of 2P/Encke.
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ON THE DUST ENVIRONMENT OF MAIN-BELT COMET 313 P/Gibbs
We present observations carried out using the 10.4 m Gran Telescopio Canarias and an interpretative model of the dust environment of activated asteroid 313 P/Gibbs. We discuss three different models relating to different values of the dust parameters, i.e., dust loss rate, maximum and minimum sizes of particles, power index of the size distribution, and emission pattern. The best model corresponds to an isotropic emission of particles which started on August 1. The sizes of grains were in the range of 0.1−2000 μm, with velocities for 100 μm particles between 0.4−1.9 m s{sup −1}, with a dust production rate in the range of 0.2−0.8 kg s{sup −1}. The dust tails’ brightnesses and morphologies are best interpreted in terms of a model of sustained and low dust emission driven by water-ice sublimation, spanning since 2014 August 1, and triggered by a short impulsive event. This event produced an emission of small particles of about 0.1 μm with velocities of ∼4 m s{sup −1}. From our model we deduce that the activity of this main-belt comet continued for at least four months since activation.
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Gone in a blaze of glory: The demise of comet C/2015 D1 (SOHO)
We present studies of C/2015 D1 (SOHO), the first sunskirting comet ever seen from ground stations over the past half century. The Solar and Heliospheric Observatory (SOHO) witnessed its peculiar light curve with a huge dip followed by a flare-up around perihelion: the dip was likely caused by sublimation of olivines, directly evidenced by a coincident temporary disappearance of the tail. The flare-up likely reflects a disintegration event, which we suggest was triggered by intense thermal stress established within the nucleus interior. Photometric data reveal an increasingly dusty coma, indicative of volatile depletion. A catastrophic mass-loss rate of ∼10{sup 5} kg s{sup −1} around perihelion was seen. Ground-based Xingming Observatory spotted the post-perihelion debris cloud. Our morphological simulations of post-perihelion images find newly released dust grains of size a ≳ 10 μm in radius; however, a temporal increase in a{sub min} was also witnessed, possibly owing to swift dispersions of smaller grains swept away by radiation forces without replenishment. Together with the fading profile of the light curve, a power-law dust size distribution with index γ = 3.2 ± 0.1 is derived. We detected no active remaining cometary nuclei over ∼0.1 km in radius in post-perihelion images acquired at Lowell Observatory. Applying a radial nongravitational parameter, A{sub 1}=(1.209±0.118)×10{sup −6} AU day{sup −2}, from an isothermal water–ice sublimation model to the SOHO astrometry significantly reduces residuals and sinusoidal trends in the orbit determination. The nucleus mass ∼10{sup 8}–10{sup 9} kg and the radius ∼50–150 m (bulk density ρ{sub d} = 0.4 g cm{sup −3} assumed) before the disintegration are deduced from the photometric data; consistent results were determined from the nongravitational effects.
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THE DUST ENVIRONMENT OF MAIN-BELT COMET P/2012 T1 (PANSTARRS)
The Main-Belt Comet P/2012 T1 (PANSTARRS) has been imaged using the 10.4 m Gran Telescopio Canarias and the 4.2 m William Herschel Telescope at six epochs in the period from 2012 November to 2013 February, with the aim of monitoring its dust environment. The dust tails' brightness and morphology are best interpreted in terms of a model of sustained dust emission spanning four to six months. The total dust mass ejected is estimated at {approx}6-25 Multiplication-Sign 10{sup 6} kg. We assume a time-independent power-law size distribution function, with particles in the micrometer to centimeter size range. Based on the quality of the fits to the isophote fields, an anisotropic emission pattern is favored against an isotropic one, in which the particle ejection is concentrated toward high latitudes ({+-}45 Degree-Sign to {+-}90 Degree-Sign ) in a high-obliquity object (I = 80 Degree-Sign ). This seasonally driven ejection behavior, along with the modeled particle ejection velocities, are in remarkable agreement to those we found for P/2010 R2 (La Sagra).
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DETERMINATION OF AN UPPER LIMIT FOR THE WATER OUTGASSING RATE OF MAIN-BELT COMET P/2012 T1 (PANSTARRS)
A new Main-Belt Comet (MBC) P/2012 T1 (PANSTARRS) was discovered on 2012 October 6, approximately one month after its perihelion, by the Pan-STARRS1 survey based in Hawaii. It displayed cometary activity upon its discovery with one hypothesis being that the activity was driven by sublimation of ices; as a result, we searched for emission assumed to be driven by the sublimation of subsurface ices. Our search was of the H{sub 2}O 1{sub 10}-1{sub 01} ground state rotational line at 557 GHz from P/2012 T1 (PANSTARRS) with the Heterodyne Instrument for the Far Infrared on board the Herschel Space Observatory on 2013 January 16, when the object was at a heliocentric distance of 2.504 AU and a geocentric distance of 2.064 AU. Perihelion was in early 2012 September at a distance of 2.411 AU. While no H{sub 2}O line emission was detected in our observations, we were able to derive sensitive 3{sigma} upper limits for the water production rate and column density of <7.63 Multiplication-Sign 10{sup 25} molecules s{sup -1} and of <1.61 Multiplication-Sign 10{sup 11} cm{sup -2}, respectively. An observation taken on 2013 January 15 using the Very Large Telescope found the MBC to be active during the Herschel observation, suggesting that any ongoing sublimation due to subsurface ice was lower than our upper limit.
79 ASTRONOMY AND ASTROPHYSICS↗
THE EXTRAORDINARY MULTI-TAILED MAIN-BELT COMET P/2013 P5
Hubble Space Telescope observations of main-belt comet P/2013 P5 reveal an extraordinary system of six dust tails that distinguish this object from any other. Observations two weeks apart show dramatic morphological change in the tails while providing no evidence for secular fading of the object as a whole. Each tail is associated with a unique ejection date, revealing continued, episodic mass loss from the 0.24 ± 0.04 km radius nucleus over the last five months. As an inner-belt asteroid and probable Flora family member, the object is likely to be highly metamorphosed and unlikely to contain ice. The protracted period of dust release appears inconsistent with an impact origin, but may be compatible with a body that is losing mass through a rotational instability. We suggest that P/2013 P5 has been accelerated to breakup speed by radiation torques.
79 ASTRONOMY AND ASTROPHYSICS↗
CONSTRAINTS ON THE PHYSICAL PROPERTIES OF MAIN BELT COMET P/2013 R3 FROM ITS BREAKUP EVENT
Jewitt et al. recently reported that main belt comet P/2013 R3 experienced a breakup, probably due to rotational disruption, with its components separating on mutually hyperbolic orbits. We propose a technique for constraining physical properties of the proto-body, especially the initial spin period and cohesive strength, as a function of the body's estimated size and density. The breakup conditions are developed by combining mutual orbit dynamics of the smaller components and the failure condition of the proto-body. Given a proto-body with a bulk density ranging from 1000 kg m{sup –3} to 1500 kg m{sup –3} (a typical range of the bulk density of C-type asteroids), we obtain possible values of the cohesive strength (40-210 Pa) and the initial spin state (0.48-1.9 hr). From this result, we conclude that although the proto-body could have been a rubble pile, it was likely spinning beyond its gravitational binding limit and would have needed cohesive strength to hold itself together. Additional observations of P/2013 R3 will enable stronger constraints on this event, and the present technique will be able to give more precise estimates of its internal structure.
79 ASTRONOMY AND ASTROPHYSICS↗
HUBBLE SPACE TELESCOPE OBSERVATIONS OF THE NUCLEUS OF COMET C/2012 S1 (ISON)
We report on the analysis of several sequences of broadband visible images of comet C/2012 S1 (ISON) taken with the Wide Field Camera 3 of the Hubble Space Telescope on 2013 April 10, May 8, October 9, and November 1 in an attempt to detect and characterize its nucleus. Whereas the overwhelming coma precluded the detection of the nucleus in the first two sequences, the contrast was sufficient in early October to unambiguously retrieve the signal from the nucleus. Two images taken within a few minutes led to similar V magnitudes for the nucleus of 21.97 and 22.0 with a 1σ uncertainty of 0.065. Assuming a standard value for the geometric albedo (0.04) and a linear phase function with a coefficient of 0.04 mag deg{sup –1}, these V values imply that the nucleus radius is 0.68 ± 0.02 km. Although this result does depend on these two assumptions, we argue that the radius most likely lies in the range 0.6-0.9 km. This result is consistent with the constraints derived from the water production rates reported by Combi et al. The last sequence of images in 2013 November revealed temporal variation of the innermost coma. If attributed to a single rotating jet, this coma brightness variation suggests the rotational period of the nucleus may be close to ∼10.4 hr.
79 ASTRONOMY AND ASTROPHYSICS↗
PREVAILING DUST-TRANSPORT DIRECTIONS ON COMET 67P/CHURYUMOV–GERASIMENKO
Dust transport and deposition behind larger boulders on the comet 67P/Churyumov–Gerasimenko (67P/C–G) have been observed by the Rosetta mission. We present a mechanism for dust-transport vectors based on a homogeneous surface activity model incorporating in detail the topography of 67P/C–G. The combination of gravitation, gas drag, and Coriolis force leads to specific dust transfer pathways, which for higher dust velocities fuel the near-nucleus coma. By distributing dust sources homogeneously across the whole cometary surface, we derive a global dust-transport map of 67P/C–G. The transport vectors are in agreement with the reported wind-tail directions in the Philae descent area.
79 ASTRONOMY AND ASTROPHYSICS↗