arXiv subjects

Zdenek Sekanina

Publications and source records attributed to Zdenek Sekanina.

At least 37 records · Page 2Linked to original sources

arXiv2022

Kreutz Sungrazers: Summary of Recent Modeling and Orbits of the SOHO Objects

I summarize and streamline the results of recent modeling of the orbital evolution and cascading fragmentation of the Kreutz sungrazers. The model starts with Aristotle's comet -- the progenitor whose nucleus is assumed to be a contact binary -- splitting near aphelion into the two lobes and concludes with the SOHO dwarf objects as the end products of the fragmentation process. The Great March Comet of 1843, a member of Population I, and the Great September Comet of 1882, a member of Population II, are deemed the largest surviving masses of the lobes. I establish that the Kreutz system consists currently of nine populations, one of which -- associated with comet Pereyra -- is a side branch of Population I. The additions to the Kreutz system proposed as part of the new model are the daylight comets of AD 363, recorded by the Roman historian Ammianus Marcellinus, and the Chinese comets of September 1041 and September 1138, both listed in Ho's catalogue. The comets of 363 are the first-generation fragments, the latter -- together with the Great Comet of 1106 -- the second-generation fragments. Attention is directed toward the populations' histograms of perihelion distance of the SOHO sungrazers and the plots of this distance as a function of the longitude of the ascending node. Arrival of bright, naked-eye Kreutz sungrazers in the coming decades is predicted.

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arXiv2022

The Great Comet of 1106, a Chinese Comet of 1138, and Daylight Comets in late 363 As Key Objects in Computer Simulated History of Kreutz Sungrazer System

We present the results of our orbital computations in support of the recently proposed contact-binary model for the Kreutz sungrazer system (Sekanina 2021, 2022). We demonstrate that comet Ikeya-Seki (C/1965 S1) previously passed perihelion decades after the Great Comet of 1106 (X/1106 C1) and argue that, like the Great September Comet of 1882 (C/1882 R1), it evidently was a fragment of the comet recorded by the Chinese in September 1138. The 1106 sungrazer appears instead to have been the previous appearance of the Great March Comet of 1843 (C/1843 D1). With no momentum exchange involved, fragments of a Kreutz sungrazer breaking up tidally near perihelion are shown to end up in orbits with markedly different periods because their centers of mass are radially shifted by a few kilometers relative to the parent. The daylight comets of AD 363, recorded by a Roman historian, are accommodated in our computations as the first appearance of the Kreutz sungrazers after their bilobed progenitor's breakup. We link the 1843-1106-363 (Lobe I) and 1882-1138-363 (Lobe II) returns to perihelion by single nongravitational orbits and gravitationally with minor center-of-mass shifts acquired in fragmentation events. We also successfully model the motion of Aristotle's comet as the rotating progenitor that at aphelion split (at a few m/s) into the two lobes, the precursors of, respectively, the 1843 and 1882 sungrazers; and provide a 1963-1041-363 link for comet Pereyra (C/1963 R1). Material fatigue could contribute to sungrazers' fragmentation throughout the orbit, including aphelion. -- Continuing problems with the nongravitational law in orbit software are noted.

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arXiv2022

Unprecedented Daylight Display of Kreutz Sungrazers in AD 363?

In the context of the recently proposed contact-binary model (Sekanina 2021), I investigate the circumstances of the first perihelion passage of the Kreutz sungrazers in orbits with barycentric periods near 735 yr, following the initial near-aphelion splitting of the presumed progenitor, Aristotle's comet of 372 BC. Given favorable conditions at this breakup and at episodes of secondary fragmentation in its aftermath, the fragments should have arrived at their first perihelion nearly simultaneously, reminiscent of the anticipated outcome for the two-superfragment model's perihelion return of AD 356 (Sekanina & Chodas 2004). The relevant case of a swarm of Kreutz sungrazers is examined to appraise possible scientific ramifications of the brief remark by Ammianus Marcellinus, a Roman historian, that "in broad daylight comets were seen" in late AD 363, only seven years later. The tested scenario, which does not contradict Ammianus' narrative and is consistent with the contact-binary model, involves a set of ten sungrazers visible in the daytime, all reaching perihelion over a period of 4.6 days. As part of this work, I comment on the role of the rapidly developing, brilliant post-perihelion tail; revise the apparent magnitude typical for the first and last naked-eye sightings; compare the visibility conditions in full daylight, in twilight, and at night; and, for the first time, present circumstantial evidence that favors comet X/1106 C1 as the parent to C/1843 D1 rather than to C/1882 R1 and C/1965 S1.

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arXiv2021

Estimating Dimensions of the Nucleus of Great September Comet of 1882 from Motions of Its Fragments

Data on perihelion fragmentation of the Great September Comet of 1882 (C/1882 R1), a prominent member of the Kreutz sungrazer system, are employed to estimate the size of the nucleus along the radius vector at the time of splitting. The prolate-spheroidal nucleus is assumed to fragment tidally at perihelion along planes normal to this direction. The relative velocities, derived by Sekanina & Chodas (2007) from revised positional-separation data on six fragments collected originally by Kreutz (1888) are interpreted as measures of the Sun's differential gravitational acceleration on the centers of mass of adjacent fragments at the time of breakup and therefore a function of heliocentric distance. Their total of 7.8 m/s is equivalent to nearly 38 km in the sum of distances of the centers of mass along the radius vector and to the nuclear size of about 60 km. The observed sheath of diffuse material, the remains of the crumbling part of the nucleus that ended up enveloping the train of the six fragments, included a population of less massive fragments. They are expected to feed, over centuries in a distant future, an influx of dwarf sungrazers reminiscent of the current stream of SOHO Kreutz comets. It is speculated that the parent comet of C/1882 R1 experienced similar fragmentation in the early 12th century and one of its major fragments -- a bright sungrazer -- may return in the mid-21st century.

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arXiv2021

The End of an Era in Cometary Astronomy: The Deceleration of Comet Encke

It is noted that effective very recently, the orbital motion of Encke's comet has become affected by a very slight nongravitational deceleration. Soon after J. F. Encke established in the early 19th century that the comet was returning to perihelion every 3.3 years, he also discovered that the object was notorious for returning to perihelion a little earlier than predicted by the Newtonian theory. The acceleration persisted over a period of two centuries, but its rate was gradually decreasing, Generations of cometary astronomers were curious to know whether or not the comet would eventually move in purely gravitational orbit. A model based on the assumption of a precession of the comet's nucleus, which predicted that the acceleration would change into a deceleration, was not published until 1979. This transition has now been documented by two independent, highly-accurate orbit determinations. The era of the comet's persevering nongravitational acceleration is finally over.

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arXiv2021

On the Detection and Distribution of Nongravitational Accelerations in the Motions of Long-Period Comets

Even though the orbital motions of most long-period comets are found to comply with the gravitational law, a rapidly increasing minority of these objects is found to display detectable outgassing-driven deviations. The systematic research of nongravitational effects in the motions of long-period comets began with Hamid & Whipple's work aimed to support Whipple's icy-conglomerate comet model in the 1950s, profited from fundamentally new results for fragments of the split comets in the 1970s, and has been expanding ever since the development of the Style II nongravitational orbit-determination model by Marsden et al. (1973). It has also thrived thanks to a dramatic improvement in the quality of astrometric observations in the past decades. A set of 78 long-period comets with derived nongravitational accelerations and perihelion distances smaller than 2 AU has been assembled and its cumulative distribution shown to mirror the distribution of nuclear dimensions, once the bias toward bright comets and comets with large nongravitational effects is accounted for. As a rule, long-period comets with detectable deviations from the gravitational law appear to have small, subkilometer-sized nuclei. Comet C/1995 O1 Hale-Bopp and perhaps some others remain a challenge.

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arXiv2021

Enigmatic Nebulous Companions to the Great September Comet of 1882 As SOHO-Like Kreutz Sungrazers Caught in Terminal Outburst

I investigate the nature of the transient nebulous companions to the sungrazing comet C/1882 R1, known as the Great September Comet. The features were located several degrees to the southwest of the comet's head and reported independently by four observers, including J. F. J. Schmidt and E. E. Barnard, over a period of ten days nearly one month after perihelion, when the comet was 0.7 AU to 1 AU from the Sun. I conclude that none of the nebulous companions was ever sighted more than once and that, contrary to his belief, Schmidt observed unrelated objects on the four consecutive mornings. Each nebulous companion is proposed to have been triggered by a fragment at most a few tens of meters across, released from the comet's nucleus after perihelion and seen only because it happened to be caught in the brief terminal outburst, when its mass was suddenly shattered into a cloud of mostly microscopic debris due possibly to rotational bursting triggered by sublimation torques. The fragment's motion was affected by a strong outgassing-driven nongravitational acceleration with a significant out-of-plane component. Although fragmentation events were common, only a small fraction of nebulous companions was detected because of their transient nature. The observed brightness of the nebulous companions is proposed to have been due mainly to C2 emissions, with a contribution from scattering of sunlight by the microscopic dust. By their nature, the fragments responsible for the nebulous companions bear a strong resemblance to the dwarf Kreutz sungrazers detected with the coronagraphs aboard the SOHO space probe. Only their fragmentation histories are different and the latter display no terminal outburst, a consequence of extremely short lifetimes of the sublimating icy and refractory material in the Sun's corona.

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arXiv2021

New Model for the Kreutz Sungrazer System: Contact-Binary Parent and Upgraded Classification of Discrete Fragment Populations

The structure of the Kreutz system of sungrazing comets is shown to be much more complex than formerly believed. Marsden's (1989) division into three subgroups (I, II, IIa) is now greatly expanded, as new evidence is being offered on nine populations of fragments -- I, Ia, II, IIa, III, Pre-I, Pe (side branch of I), IIIa, and IV, incorporating carefully screened data sets from a collection of gravitational orbits for 1500 SOHO/STEREO dwarf Kreutz comets. Tight correlations between the nominal perihelion latitude and nominal longitude of the ascending node are the result of ignored effects of an outgassing-driven acceleration on the orbital motion. The average width of a gap between adjacent populations in the corrected nodal longitude is near 9 deg; the overall range equals 66 deg. A self-consistent model postulates (i) an initial breakup, in general proximity of aphelion, of a contact-binary parent (progenitor) into its two lobes and the neck (originally linking the lobes), giving birth to, respectively, Population I (Lobe I; the main residual mass C/1843 D1), Population II (Lobe II; C/1882 R1), and Population Ia (the neck); followed by (ii) progressive fragmentation of the lobes (primarily Lobe II), mostly (but not exclusively) far from perihelion, giving successively rise to the other populations and clusters of naked-eye Kreutz sungrazers and their debris. The separation velocities were a few meters per second. Massive fragments of Populations Pre-I, IIIa, and IV are yet to be discovered. Relations among the products of cascading fragmentation are depicted in a pedigree chart. The age of the Kreutz system is estimated at two millennia and a mean orbital period of Lobe I and its main residual mass at ~740 yr. The status is reviewed of the possible historical Kreutz comets seen in AD 1106, AD 363, and 372 BC.

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arXiv2019

Sublimation of Water Ice from a Population of Large, Long-Lasting Grains Near the Nucleus of 2I/Borisov?

Potential effects of sublimation of water ice from very slowly moving millimeter-sized and larger grains, a product of activity at ~10 AU or farther from the Sun driven presumably by annealing of amorphous water ice, are investigated by comparing 2I/Borisov with a nominal Oort Cloud comet of equal perihelion distance of 2 AU. This comparison suggests that the strongly hyperbolic motion of 2I mitigates the integrated sublimation effect. The population of these grains near the nucleus of 2I is likely to have been responsible for the comet appearing excessively bright in pre-discovery images at 5-6 AU from the Sun, when the sublimation rate was exceedingly low, as well as for the prominent nuclear condensation more recently. All grains smaller then 2-3 cm across had been devolatilized by mid-October 2019 and some subjected to rapid disintegration. This left only larger chunks of the initial icy-dust halo contributing to the comet's suspected hyperactivity. Sublimation of water ice from the nucleus has been increasing since the time of discovery, but the rate has not been high enough to exert a measurable nongravitational acceleration on the orbital motion of 2I/Borisov.

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arXiv2019

Note on the HST Image of Comet 2I/Borisov Taken on 2019 October 12

The image of comet 2I/Borisov taken by the Hubble Space Telescope on October 12 shows a fan-like feature in the inner coma, which extends over a few arcseconds from the nuclear condensation, its axis pointing a little west of the north. It is proposed that this protrusion is a halo of centimeter-sized and larger dust grains or pebbles released at low velocities from the nucleus at large heliocentric distances (on the order of 10 AU or more) before perihelion and subjected to very low radiation pressure accelerations. Unlike potential microscopic dust from ongoing activity, this population of large grains should show no silicate feature near 10 microns in the infrared and its presence in 2I/Borisov can be tested observationally. The existence of material of a similar size range in Oort Cloud comets with perihelia near or beyond the snow line has long been documented by the characteristic orientation of their tails.

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arXiv2019

Are 2I/Borisov and Oort Cloud Comets Alike?

The interstellar comet 2I/Borisov bears a strong resemblance to Oort Cloud comets, judging from its appearance in images taken over the first six weeks of observation. To test the proposed affinity in more diagnostic terms, 2I is compared to Oort Cloud comets of similar perihelion distance, near 2 AU. Eight such objects are identified among the cataloged comets whose orbits have been determined with high accuracy. This work focuses on three particular characteristics: the light curve, the geometry of the dust tail, and the dust parameter Afrho. Unlike Oort Cloud comets with perihelia beyond the snow line, Oort Cloud comets with perihelia near 2 AU show strong evidence of the original halo of slowly accelerating, millimeter-sized and larger icy-dust grains only in early tail observations. The dust tail in later images is primarily the product of subsequent, water-sublimation driven activity nearer perihelion but not of activity just preceding observation, which suggests the absence of microscopic-dust ejecta. Comet 2I fits, in broad terms, the properties of the Oort Cloud comets with perihelia near 2 AU and of fairly low activity. Future tests of the preliminary conclusions are proposed.

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arXiv2019

Comments on a Recent Review Paper on Near-Sun Comets

It is noted that some results of the author's research papers cited in the comprehensive review paper "The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets" by G. H. Jones et al. (2018) have been factually misrepresented. The purpose of this note is to offer (i) revisions in order to correct the numerous errors and inaccuracies and (ii) additions where the text is confusing.

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arXiv2019

Outgassing As Trigger of 1I/`Oumuamua's Nongravitational Acceleration: Could This Hypothesis Work at All?

The question of what triggered the nongravitational acceleration of 1I/`Oumuamua continues to attract researchers' attention. The absence of any signs of activity notwithstanding, the prevailing notion is that the acceleration of the stellar, cigar-like object was prompted by outgassing. However, the Spitzer Space Telescope's failure to detect `Oumuamua not only ruled out the CO_2 and/or CO driven activity (Trilling et al. 2018), but made the cigar shape incompatible with the optical observations. Choice of water ice as the source of outgassing is shown to be flawed as well: (i) the water sublimation law is demonstrably inconsistent with the observed variations in the nongravitational acceleration derived by Micheli et al. (2018), the point that should have been assertively highlighted; and (ii) an upper limit of the production rate of water is estimated at as low as 4 x 10^(23) molecules s^(-1), requiring that, at most, only a small area of the surface be active. In this case the conservation of momentum law is satisfied only when `Oumuamua's bulk density is extremely low, <0.001 g cm^(-3), reminiscent of the formerly proposed scenario with `Oumuamua as a fragment of a dwarf interstellar comet, possibly an embryo planetesimal, disintegrating near perihelion, with the acceleration driven by solar radiation pressure (Sekanina 2019a) and no need for activity at all. High quality of astrometry and Micheli et al.'s orbital analysis, whose results were confirmed by the computations of other authors, is acknowledged.

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arXiv2019

1I/`Oumuamua and the Problem of Survival of Oort Cloud Comets Near the Sun

A 2000-2017 set of long-period comets with high-quality orbits of perihelion distance <1 AU is used to show that the objects that perish shortly before perihelion are nearly exclusively the Oort Cloud comets, especially those with perihelia within 0.6 AU of the Sun, intrinsically fainter, and dust poor. Their propensity for disintegration is much higher than predicted by Bortle's perihelion survival rule, prompting the author to propose a new synoptic index to be tested in future prognostication efforts. By their susceptibility to demise near the Sun, the nuclei of Oort Cloud comets differ dramatically from the nuclei of other long-period comets that almost always survive. In this scenario, `Oumuamua -- discovered after perihelion -- is in all probability a major piece of debris of an interstellar comet that was bound to perish near perihelion if it was similar to, though much fainter than, the known Oort Cloud comets. The nondetection of `Oumuamua by the Spitzer Space Telescope is compatible with optical data for pancake shape, but not for cigar shape, with the maximum dimension not exceeding 160 m (at an 0.1 albedo). Although the solar radiation pressure induced nongravitational acceleration requires very high porosity, `Oumuamua's estimated mass is orders of magnitude greater than for a cloud of unbound submicron-sized dust grains of equal cross section. The acceleration could have displaced `Oumuamua by 250,000 km in 50 days, scattering other potential debris over a large volume of space.

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arXiv2019

1I/`Oumuamua As Debris of Dwarf Interstellar Comet That Disintegrated Before Perihelion

Intrinsically faint comets in nearly-parabolic orbits with perihelion distances much smaller than 1 AU exhibit strong propensity for suddenly disintegrating at a time not long before perihelion, as shown by Bortle (1991). Evidence from available observations of such comets suggests that the disintegration event usually begins with an outburst and that the debris is typically a massive cloud of dust grains that survives over a limited period of time. Recent CCD observations revealed, however, that also surviving could be a sizable fragment, resembling a devolatilized aggregate of loosely-bound dust grains that may have exotic shape, peculiar rotational properties, and extremely high porosity, all acquired in the course of the disintegration event. Given that the brightness of 1I/`Oumuamua's parent could not possibly equal or exceed the Bortle survival limit, there are reasons to believe that it suffered the same fate as do the frail comets. The post-perihelion observations then do not refer to the object that was entering the inner Solar System in early 2017, as is tacitly assumed, but to its debris. Comparison with C/2017 S3 and C/2010 X1 suggests that, as a monstrous fluffy dust aggregate released in the recent explosive event, `Oumuamua should be of strongly irregular shape, tumbling, not outgassing, and subjected to effects of solar radiation pressure, consistent with observation. The unknown timing of the disintegration event may compromise studies of the parent's home stellar system. Limited search for possible images of the object to constrain the time of the (probably minor) outburst is recommended.

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arXiv2019

Preperihelion Outbursts and Disintegration of Comet C/2017 S3 (Pan-STARRS)

A sequence of events, dominated by two outbursts and ending with the preperihelion disintegration of comet C/2017 S3, is examined. The onset times of the outbursts are determined with high accuracy from the light curve of the nuclear condensation before it disappeared following the second outburst. While the brightness of the condensation was declining precipitously, the total brightness continued to grow in the STEREO-A's HI1 images until two days before perihelion. The red magnitudes measured in these images refer to a uniform cloud of nuclear fragments, 2200 km^2 in projected area, that began to expand at a rate of 76 m s^(-1) at the time of the second outburst. A tail extension, detected in some STEREO-A images, consisted of dust released far from the Sun. Orbital analysis of the ground-based observations shows that the comet had arrived from the Oort Cloud in a gravitational orbit. Treating positional residuals as offsets of a companion of a split comet, we confirm the existence of the cloud of radiation-pressure driven millimeter-sized dust grains emanating from the nucleus during the second outburst. We detect a similar, but compact and much fainter cloud (or a sizable fluffy dust aggregate fragment) released at the time of the first outburst. --- The debris would make a sphere of 140 m across and its kinetic energy is equivalent to the heat of crystallization liberated by 100 tons of amorphous water ice. Ramifications for short-lived companions of the split comets and for 1I `Oumuamua are discussed.

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arXiv2017

Major Outburst and Splitting of Long-Period Comet C/2015 ER61 (Pan-STARRS)

In early April 2017, five weeks before passing through perihelion, the dust-poor comet C/2015 ER61 was observed to undergo a major outburst, during which its intrinsic brightness increased briefly by about 2 mag. Evidence is presented in this paper to suggest that the flare-up was in all likelihood a product of an event of nuclear fragmentation that gave birth to a companion nucleus, which was first detected more than nine weeks later, on June 11, and remained under observation for 19 days. The companion is found to have separated from the parent nucleus with a velocity of less than 1 m/s; subsequently it was subjected to a nongravitational deceleration of 14 units of 10^{-5} the Sun's gravitational acceleration. The companion's overall dimensions can from this value be estimated al less than one hundred meters across. Unlike the primary nucleus, the companion displayed considerable fluctuations in brightness on time scales spanning at least three orders of magnitude, from a fraction of one hour to weeks. The long temporal gap between the companion's birth and first detection appears to be a corollary of the brightness variability. Evidence from the light curve suggests that the companion became detectable only because of large amounts of debris that it started shedding as well as increased activity from area newly exposed by progressive fragmentation, the same process that also resulted in the companion's impending disintegration by the end of June of in early July.

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arXiv2017

Orbital Evolution, Activity, and Mass Loss of Comet C/1995 O1 (Hale-Bopp). I. Close Encounter with Jupiter in Third Millennium BCE and Effects of Outgassing on the Comet's Motion and Physical Properties

This comprehensive study of comet C/1995 O1 focuses first on investigating its orbital motion over a period of 17.6 yr (1993-2010). The comet is suggested to have approached Jupiter to 0.005 AU on -2251 November 7, in general conformity with Marsden's (1999) proposal of a Jovian encounter nearly 4300 yr ago. The variations of sizable nongravitational effects with heliocentric distance correlate with the evolution of outgassing, asymmetric relative to perihelion. The future orbital period will shorten to ~1000 yr because of orbital-cascade resonance effects. We find that the sublimation curves of parent molecules are fitted with the type of a law used for the nongravitational acceleration, determine their orbit-integrated mass loss, and conclude that the share of water ice was at most 57%, and possibly less than 50%, of the total outgassed mass. Even though organic parent molecules (many still unidentified) had very low abundances relative to water individually, their high molar mass and sheer number made them, summarily, important potential mass contributors to the total production of gas. The mass loss of dust per orbit exceeded that of water ice by a factor of ~12, a dust loading high enough to imply a major role for heavy organic molecules of low volatility in accelerating the minuscule dust particles in the expanding halos to terminal velocities as high as 0.7 km s^{-1}. In Part II, the comet's nucleus will be modeled as a compact cluster of massive fragments to conform to the integrated nongravitational effect.

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