Organic synthesis in the coma of Comet Hale–Bopp?

Several organic molecules have now been detected in the coma of Hale–Bopp. These species may either emanate from the nucleus, or, as has been suggested by Bockelée–Morvan et al., could be synthesized in the coma. We have modelled the gas phase chemistry which occurred in the coma of Hale–Bopp, concentrating on the observed organic molecules HCOOH, HCOOCH₃, HC₃N and CH₃CN. We find that gas phase chemical reactions are unable to synthesize the observed abundances of these molecules, so all these species are most probably present in the nuclear ice. We briefly discuss the implications of this result for the connection between cometary and interstellar ices.     

Large cometary grains – their destruction and changes in the luminosity of comets

The catastrophic thermodynamic destruction of large cometary heterogeneous grains lying on the surface of a comet nucleus is examined. The core–mantle grain-structure model is assumed. Grain fragmentation as an explanation of sudden changes in cometary brightness is proposed. The approach presented to the problem of cometary outbursts is a development of a previous author's paper. The proposed mechanism is based on the idea of thermodynamical destruction of heterogeneous cometary grains. Numerical simulations have been carried out for a wide range of values of physical characteristics of cometary material. The results obtained are consistent with observational data. The main conclusion of this paper is that thermodynamical fragmentation of large grains can explain variations in brightness and also outbursts of comets.     

Orbit of periodic comet 153P/Ikeya–Zhang

The orbit of Comet C/2002C1 (Ikeya–Zhang) has a similarity to that of Comet C/1661C1 (Hevelius), and the numerical integration of the motion of C/2002C1 backward shows a possible linkage of those two comets. Thus, 153P/Ikeya–Zhang was designated a periodic comet. Historical records of comets in 877 and 1273 are also identified with Comet 153P/Ikeya–Zhang. The integrated orbital elements during 77 and 2362, and historical records of the comet are also presented and discussed.     

The influence of the nucleus shape on the maximum size of grains ejected from a comet by gentle sublimation and jet‐like features

This paper deals with obtaining the maximum size of cometary grains ejected from nuclei of different shapes. Two mechanisms in terms of grain ejection from comets are taken into consideration. The first one is dragging of particles by outflowing gas molecules released by gentle sublimation from the comets. The second one is related with gas jets from the cavities in a nucleus by cometary jet‐like phenomena. We focused on ellipsoidal shapes of cometary nuclei but with different flattening. Calculations have been carried out for a large range of cometary parameters. It has been shown that for fixed mass of the nucleus the maximum size of grains is an increasing function of the nucleus flattening. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal destruction of cometary materials – application to activity of comets

Destruction mechanisms connected with thermodynamical behaviour of cometary material are reviewed with a special consideration of their effects on activity of comets. Consequences of thermal stresses which occur in the interior of a comet are discussed with reference to changes in the cometary brightness. Moreover, thermal destruction of grains placed in the head of the comet as well as on the surface of the nucleus is considered. It has been shown that the destruction of the cometary material can lead to an essential increase in the activity of the comet. Calculations have been carried out for a large assumed range of cometary parameters. The obtained simulated changes in the brightness of comets are consistent with the ones observed during the real variations and outbursts of brightness. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

短周期彗星表面水冰升华分布研究

彗星是太阳系遗留的原始星子,研究彗星彗核的演化对理解太阳系其他天体的形成和演化历史具有重要意义.在太阳的辐射作用下,彗星携带的挥发性成分会发生升华,并带动尘埃运动,造成彗核物质的损失.因此,彗核的升华活动对其表面形貌甚至整体形状演化都会产生影响.从IAU (International Astronomical Union) MPC (Minor Planet Center)获取轨道数据,并考虑了彗核的自转以及进动,利用MONET (Mass lossdriven shape evolution model)形状演化模型对短周期彗星做数值模拟,计算得到了短周期彗星1P/Halley、9P/Tempel 1、 19P/Borrelly、 67P/C-G (Churyumov-Gerasimenko)、 81P/Wild 2和103P/Hartley 2在一个轨道周期内的太阳辐射能量以及表面侵蚀深度的分布,结合其动力学参数讨论了自转、进动和公转等特性对其表面水冰升华分布的影响以及造成南北侵蚀差异的可能性.     

On the origin of HNC in Comet Lee

We have modelled the chemistry occurring in the coma of Comet Lee and have critically evaluated the possible routes leading to HNC. We show that the observed levels of HNC cannot be produced by ion–molecule chemistry, or by reactions of energetic H atoms with HCN. Rather, it appears that HNC is injected into the coma following the photodestruction of an unknown precursor. We discuss the possible nature of the parent of HNC and conclude that photofragmentation of large HCN polymers, such as polyaminocyanomethylene (PACM), is responsible. The degradation of hydrogen cyanide polymers may constitute a common source of HNC in comets, accounting for HNC/HCN ratios in the range measured in Lee and Hyakutake (≈ 0.06–0.12). The high HNC/HCN ratio measured in Hale–Bopp (≈0.2) and its heliocentric variation may, however, require an additional source.     

The long-term evolution and initial size of comets 46P/Wirtanen and 67P/Churyumov–Gerasimenko

We present a new method to study the long-term evolution of cometary nuclei in order to estimate their original size, and we consider the case of comets 46P/Wirtanen (hereafter 46P) and 67P/Churyumov–Gerasimenko (hereafter 67P). We calculate the past evolution of the orbital elements of both comets over 100 000 yr using a Bulirsch–Stoer integrator and over 450 000 yr using a Radau integrator, and we incorporate a realistic model of the erosion of their nucleus. Their long-term orbital evolution is prominently chaotic, resulting from several close encounters with planets, and this result is independent of the choice of the integrator and of the presence or not of non-gravitational forces. The dynamical lifetime of comet 46P is estimated at ∼133 000 yr and that of comet 67P at ∼105 000 yr. Our erosion model assumes a spherical nucleus composed of a macroscopic mixture of two thermally decoupled components, dust and pure water ice. Erosion strongly depends upon the active fraction and the density of the nucleus. It mainly takes place at heliocentric distances <4 au and lasts for only ∼7 per cent of the lifetime. Assuming a density of 300 kg m⁻³ and an average active fraction over time of 10 per cent, we find an initial radius of ∼1.3 km for 46P and ∼2.8 km for 67P. Upper limit are obtained assuming a density of 100 kg m⁻³ and an active fraction of 100 per cent, and amounts to 21 km for 46P and 25 km for 67P. Erosion acts as a rejuvenating process of the surface so that exposed materials on the surface may only contain very little quantities of primordial materials. However, materials located just under it (a few centimetres to metres) may still be much less evolved. We will apply this method to several other comets in the future.     

A Disconnection Event of Comet Lulin†

The cometary disconnection event (DE) is the separation of the entire cometary tail or a part of it from the cometary head. It is one of the most spectacular phenomena of comets. The driving mechanism remains unclear, and at present there are many competitive theories to explain the onset of DE. However, the variable solar wind is suspected to play a major role. Comet Lulin exhibited a DE on 4th Feb. 2009. The data around this date are analyzed, and it is found that the comet Lulin had already endured a DE on 3rd Feb. 2009. By comparing the morphologies of the plasma tails in these two DEs, it is concluded that the DE which occurred on 3rd Feb. 2009 is another DE, which is distinct from that of 4th Feb. 2009. In this paper, we describe the results of analysis on the DE dated 3rd Feb. 2009. The measured velocity of disconnection motion is about 68 km/s, and the calculated onset time of this DE is 3.635 ± 0.215 Feb. 2009 in UT decimal date. Combining the orbital characteristics of Comet Lulin before and after the DE occurrence and the solar-wind data measured by the STEREO-A spacecraft, it is concluded that the DE which occurred on 3rd Feb. 2009 was probably caused by the magnetic reconnection due to the interaction between the comet and a coronal mass ejection (CME).     

Cometary outbursts – search of probable mechanisms – case of 29P/Schwassmann‐Wachmann 1

Cometary outbursts, sudden increases in luminosity have not been clearly explained so far and their source is still a mystery. Various possible mechanisms as a source of cometary outbursts at large distances from the Sun have been considered. It has been stated that plausible mechanisms are the polymerization of HCN and the amorphous water ice transformation combined with electrostatic destruction of cometary grains in the head of the comet. The calculations have been carried out for a large range of cometary parameters and it has been shown that the proposed scenario of the outburst gives a jump in the comet brightness which is consistent with the real jump observed during the 29P/Schwassmann‐Wachmann 1 outbursts.     

The outbursts of the comet 29P/Schwassmann‐Wachmann 1: A new approach to the old problem

As far as outbursts activity is concerned, the 29P/Schwassmann‐Wachmann 1 is the exceptional comet. This Centaur object shows quasi‐regular flares with periodicities of 50 days (Trigo‐Rodriguez et al. 2010). In the introductory part of the presented paper the most well‐known hypotheses which try to explain this cometary behaviour are reviewed. The second, actual part of this paper presents the new model for the outburst activity of this comet. The model is based on the idea of Ipatov (2012), according to which there are large cavities below a considerable fraction of the comet's surface containing material under high gas pressure. In favourite conditions the surface layers over the cavities are thrown away and the interior of these cavities is exposed. Consequently, an outburst of the comet's brightness may be observed. The main characteristics of an outburst of this comet, the brightness jump, is calculated. Numerical simulations were carried out for wide range of possible cometary parameters. The obtained results are in good agreement with the observations. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Collisions of comets and meteoroids: The post Stardust‐NExT discussion

The paper considers results of collisions between comets and meteoroids. We re‐discuss the five different approaches to estimate the sizes of holes created during such collisions. The results of the Deep Impact and the Stardust‐NExT missions to comet 9P/Temple 1 are applied to the estimation of these methods. We use the observed amount of ejected mass, the jump of brightness of the comet 9P/Tempel 1 as well as the diameter of the excavated crater. In the paper the simple way of estimation of impact consequences by use of the conception of the fragmentation energy of comet is also discussed. The numerical calculations were carried out for reasonable assumed values of a large range of cometary characteristics. The main conclusion of this paper confirms a general presumption that the main factor which determines the size of the impact crater on the comet 9P/Tempel 1 is the kinetic energy of impactor and strength or fragmentation energy of cometary material. In the considered case the gravitation of a comet has a minor meaning (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

From the Oort cloud to observable short-period comets – I. The initial stage of cometary capture

We investigate the first stage of the dynamical evolution of Oort cloud comets entering the planetary region for the first time. To this purpose, we integrate numerically the motions of a large number of fictitious comets pertaining to two samples, both with perihelion distances up to 5.7 au and random inclinations; the first sample is composed of comets whose orbits have at least one node close to 5.2 au, while the second is not subject to this constraint. We examine the orbits when the comets come to aphelion after their first perihelion passage within the planetary region, and find that there is a clear statistical dependence of the energy perturbations on the Tisserand parameter. There appear to be two main processes, of comparable importance, governing the shortening of semimajor axes to values of less than 1000 au, i.e. planetary close encounters, especially with Jupiter, and indirect perturbations due to the shifting of the motion from barycentric to heliocentric and back; the former process mostly affects comets crossing the ecliptic at about 5.2 au, or on low-inclination orbits, while the latter mostly affects comets of small perihelion distance. This last result may help to understand the relative paucity of Halley-type comets with perihelion distances larger than about 1.5 au.     

The effect of non-gravitational forces on the median inclination of short-period comets

The numbered Jupiter family comets (orbital periods   P < 20 yr  ) have a median orbital inclination of about     . In this paper, we integrate the orbits of these comets into the future, under the influence of both typical non-gravitational forces and planetary perturbation, using a Bulirsch–Stoer integrator. In the case where non-gravitational forces were not acting, the median inclination of those comets that remained on   P < 20 yr  orbits increased at the rate of  (1.92 ± 0.12) × 10⁻³ deg yr⁻¹  for the first 3600 yr of the integration. During this time the population of the original family decreases, such that the half-life is about 13 200 ± 800 yr. The introduction of non-gravitational forces slows down the rate of increase in inclination to a value of around  (1.23 ± 0.16) × 10⁻³ deg yr⁻¹  . This rate of increase in inclination was found to be only weakly dependent on the non-gravitational parameters used during the integration. After a few thousand years, the rate of change in inclination decreases, and after 20 000 yr the inclinations of those initial Jupiter family members that still have orbits with   P < 20 yr  become constant at about     , independent of whether non-gravitational forces are acting or not. The presently known Jupiter family of comets is losing members at the rate of one in every 67 yr. To maintain the family in equilibrium, Jupiter has to capture comets at a similar rate, and these captured comets have to be of low inclination to compensate for the pumping up of inclinations by gravitational perturbation.     

The destruction of cometary grains and changes in the luminosity of comets

One explanation of the sudden changes in the brightness of comets is proposed based on the author's earlier suggestions involving the fragmentation of cometary grains. Within the inner coma, a core‐mantle model of the structure of grains is assumed. The proposed mechanism is a combination of electrostatic stress and thermodynamical fragmentation of the cometary grains water‐ice mantle. It has been shown that the vapour pressure of volatile inclusions placed in the waterice mantle of grains can increase sufficiently to cause their fragmentation. It takes place before grains can completely sublime into the vacuum away. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. The proposed approach yields increases in cometary brightness consistent with observations of typical cometary outbursts. It is concluded that this approach can provide an explanation of the sudden change in activity of comets for a wide range of heliocentric distances (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The source of energy of the comet 29P/Schwassmann-Wachmann 1 outburst activity: the test of the summary

The comet 29P/Schwassmann-Wachmann 1 is an exceptional comet as far as cometary outbursts are concerned. Despite its large distance from the Sun (about 6 au), it shows quasi-regular outburst activity, usually once or twice a year. Up to now there has not been a generally accepted model that explains this phenomenon. In the first part of this paper, the most well-known hypotheses that attempt to explain the outburst activity of this comet are presented and critically analysed. The main aim of this paper is to present a model for the outburst activity of this comet. The model is based on the global analysis of the internal structure and physical and chemical processes that take place in the cometary nucleus. Numerical calculations were carried out for reasonable assumed values of a large range of cometary characteristics. The obtained results are consistent with observational data.     

Cometary outbursts – the post-Deep Impact outlook on collisions as possible causes

The possibility of impacts between comets belonging to the Jupiter Family and other small bodies orbiting in the main asteroid belt, and the consequences in relation to cometary activity are discussed. The probability of such events and the jumps in cometary brightness caused by impacts are examined. The results are compared with the results of the Deep Impact mission to Comet 9P/Tempel 1. The main conclusion of this paper is in agreement with previous findings, namely that an impact mechanism cannot be the main cause of the outburst activity of comets.     

New measures of the orientation of the linear structure in the sunward hemisphere of the coma of Comet 19P/Borrelly: apparitions of 1994, 2001, and 2008

The sky‐projected orientation (position angle) of the axis (line of maximum density or maximum brightness) of the long time‐known, linear structure (LS) in the sunward hemisphere of the coma of Comet 19P/Borrelly is measured on 45 photographs taken by different observers under different projection conditions and covering three consecutive apparitions (1994, 2001, and 2008) for a total time interval of 5174 days. The analysis of the results by a tomographic approach yields an LS axis constantly oriented towards a fixed point in the space, at Right Ascension 214°.4 ± 0°.5 and Declination –7°.0 ± 0°.5 (J2000), corresponding to an obliquity of 103°.5 ± 1° and an orbital longitude of 147°.2 ± 1°, throughout the relevant interval. Such coordinates are close to the ones found by other authors for the spatial orientation of the nucleus spin axis during the apparitions of 1994 and 2001. In the hypothesis of an LS orientation aligned with the nucleus spin axis, the new results confirm the previous ones and show that this orientation remained unchanged during the subsequent 2008 apparition (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)