arXiv subjects

Zdenek Sekanina

Publications and source records attributed to Zdenek Sekanina.

At least 19 recordsLinked to original sources

arXiv2026

Orbital-Period Determination As an Indispensable Tool to Study the Pedigree of a Sungrazing Comet

Among some 4500 Kreutz sungrazers known, the orbital period has been established to better than about +/-20 years only for C/1882 R1, C/1963 R1, C/1965 S1, C/2011 W3, and C/2026 A1. I describe solutions to a range of intriguing problems involving the orbital-period determination. A helpful, but computer-intensive routine is a detailed investigation of derived orbital periods as a function of the last observation used, which allows one to filter out effects of activity (the case of C/2026 A1) or nuclear fragmentation (the case of C/1965 S1) and thereby reliably evaluate the time of the previous perihelion, a cornerstone in the quest for a sungrazer's pedigree. Experience shows that only very massive objects, such as the original nucleus of C/1882 R1 or its main fragment B are immune to effects of this kind. Different problems are presented by a sungrazer whose nucleus falls apart shortly after perihelion (the case of C/2011 W3) or by one observed only after perihelion (the case of C/1963 R1). The documented high sensitivity of the derived orbital period to minor perturbations of a sungrazing comet's motion is exploited to advantage when the standard approach does not work or yields inconclusive results.

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arXiv2026

Orbital Motion and History of Sungrazing Comet C/2026 A1 (MAPS)

We report results of our extensive computations of the orbital motion of comet C/2026 A1, including its integration back to the fourth century and parallel integration of 1000 virtual clones. We focus on this Kreutz sungrazer's apparent association with the daylight comets in AD 363, recorded by Ammianus Marcellinus. We show that the derived time of the previous perihelion is strongly affected by erratic outgassing-driven nongravitational forces and depends on the chosen set of observations. We find that the previous perihelion was reached within about 1 sigma, or some +/-15 years, of AD 363 only when we use the observations up to 2026 February 9-12, the time of a major anomalous feature on the comet's light curve. We suggest that three bright dwarf sungrazers detected between 2026 March 31 and April 12 were almost certainly distant companions, offering evidence that C/2026 A1 was part of a larger object, which separated at perihelion in AD 363 from what appears to have been the ancestor of the Great March Comet of 1843 and then fragmented far from the Sun after aphelion. This scenario is supported by required orbital similarity and suggests that comet C/2026 A1 was member of Population I.

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arXiv2026

On the Problem of Prognostication of Bright Kreutz Sungrazers

Tidal fragmentation at perihelion and nontidal fragmentation elsewhere cause the orbital distribution of Kreutz sungrazers of all sizes to be extremely complicated and highly nonuniform. Among the features are (largely fortuitous) clusters of bright (naked-eye) objects and clumps of dwarf objects (often closely genetically related, as their detection primarily by the SOHO coronagraphs suggests) on the one hand; and both spectacular and less brilliant sibling sungrazers, whose perihelion times are scattered over centuries, on the other hand. Investigation of four fragment nuclei of the Great September Comet of 1882, the products of a perihelion breakup of the comet's original nucleus, showed that their orbital periods followed a distinct pattern, which likewise applied to other tidally split sungrazers and was characterized by a specific value of the second difference of parameter u_frg of neighboring fragments' centers of mass. The algorithm has a potential for the prognostication of bright Kreutz sungrazers over the rest of the 21st century and beyond. However, because of its as yet unverified empirical character, the utmost caution should be exercised when applying the procedure.

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arXiv2026

New Kreutz Sungrazer C/2026 A1 (MAPS): Third Time's the Charm?

This paper describes progress achieved in early investigations of the orbital motion and light curve of comet C/2026 A1 (MAPS), the third ground-based discovery of a Kreutz sungrazer in the 21st century. The highly unusual trait of the comet that has so far been ascertained is its extraordinarily long orbital period. The most recent orbital computations make it increasingly likely that the object is a fragment of one of the comets observed by Ammianus Marcellinus in AD 363, thereby strengthening evidence in support of the contact-binary hypothesis of the Kreutz system. In this context, the comet is the only second-generation fragment of Aristotle's comet that we are aware of to appear after the 12th century. It does not look like a major fragment, but rather like an outlying fragment of a much larger sungrazer. In 363 it apparently separated from a parent different from the lineage of comet Pereyra. The light curve of comet MAPS has so far been fairly smooth, without outbursts. To reach the brightness of comet Ikeya-Seki, the comet would have to follow an r^(-17) law in the coming weeks, which is unlikely.

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arXiv2025

New Insights into the Nature and Orbital Motion of Aristotle's Comet in 372 BC

Extending the investigation of the presumed primordial comet as part of continuing work on a new model of the Kreutz sungrazer system, I confront a previously derived set of orbital elements with Aristotle's remarks in his Meteorologica to test their compatibility and determine the comet's perihelion time. The two translations of the treatise into English that I am familiar with differ at one point substantially from each other. Unambiguously, the year and season of the comet's appearance was early 372 BC (or -371). From Aristotle's constraint on the comet's setting relative to sunset, I infer that the probable date of perihelion passage was January 20, a date also consistent with the vague remark on frosty weather. On the day that Aristotle claims the comet was not seen, its head may have been hidden behind the Sun's disk or in contact with it. The observation that the `comet receded as far as Orion's belt, where it dissolved' is being satisfied by the tested orbit if the perihelion was reached between January 20 and February 10. Aristotle's third statement, which describes the tail as a streak 60 degrees in length, suggests a plasma feature stretching in space over 0.8 AU. The dust tail was developing more gradually and it was all that could be seen from the comet when it was approaching Orion's belt in early April. The comet was seen over a period of more than 10 weeks. The results of this study strengthen the notion that Aristotle's comet indeed was the gigantic progenitor of Kreutz sungrazers.

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arXiv2025

The Great Comets of 1843 and 1882 at Their Previous Return to Perihelion in the Twelfth Century: One Spectacular, the Other Dull

New insights into the history of C/1843 D1 and C/1882 R1, the two celebrated Kreutz sungrazers, are provided by assessing evidence on their appearance at the previous perihelion return, known as X/1106 C1 and the Chinese comet of 1138 (Ho's No. 403), respectively. The conditions differed vastly because of disparities in geocentric distance, solar elongation, and phase correction (forward scattering), all linked to the arrival times (early February vs early August). The conclusions include: the daytime sighting of the 1106 comet by Sigebert de Gembloux is consistent with expectation and so are the accounts of an exceptionally long tail observed later in twilight; the comet reached perihelion only hours before its daytime detection; the 1138 comet could have never been sighted in daylight or discovered much earlier than it actually was, in early September, one month after perihelion; at discovery, the tail is predicted to have reached elevations of 15-30 deg, while the head was only 10 deg above horizon, when observed from moderate northern latitudes; the notion that Kreutz sungrazers at perihelion between mid-May and mid-August could not be seen from the ground except possibly in daylight is misleading; the appearance of the 1138 comet's nucleus after its tidal fragmentation at perihelion is modeled on the assumption that it consisted of five major fragments (including C/1882 R1 and C/1965 S1); and unpredictable morphological changes with time in the 1882 sungrazer's split nucleus are discussed.

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arXiv2025

Bright Sungrazing Comets in a Great Historical Controversy and Prospects for Their Appearance in the Near Future

Until the second half of the 19th century, two or more brief appearances of bright comets, such as the ones in 1668 and 1702, alike in aspect and motion, seen with a tail near the Sun, were almost universally believed to be periodic returns of a single object. It is likely that the exceptional story of Halley's comet was the compelling precedent for this school of thought. Application to sungrazers was discredited by the observed fragmentation of the nucleus of the giant sungrazer of 1882 shortly after perihelion. Generally, separations and orbital periods of the Kreutz comets are known to be governed in such events by the solar tidal force, while the range in the longitude of the nodal line is linked to the pyramidal architecture caused by nontidal, cascading fragmentation along the entire orbit and described by an updated contact-binary model. Perception of the sungrazer system was changed dramatically by coronagraphic imaging from space, which led to discovery of up to ten populations of dwarf comets. Past fragmentation patterns have been used to tentatively predict the arrivals of two bright Kreutz sungrazers -- a Population II member around 2027 (and before 2040) and a Population I member around 2050.

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arXiv2024

Comet ATLAS (C/2024 S1) -- Second Ground-Based Discovery of a Kreutz Sungrazer in Thirteen Years

Comet ATLAS (C/2024 S1) is a bright dwarf sungrazer, the second Kreutz comet discovery from the ground this century, 13 years after comet Lovejoy (C/2011 W3). The Population II membership of comet ATLAS sets it apart from the overwhelming majority of other bright dwarf sungrazers, most of them classified as members of Populations I, Pe, or Pre-I in the context of the contact-binary model. The new sungrazer might be closely related to comet du Toit (C/1945 X1), but most exciting is the possibility that it is a fragment of the parent comet of the Great September Comet of 1882 (C/1882 R1) and comet Ikeya-Seki (C/1965 S1). However, this scenario requires that the original orbital period of comet ATLAS -- rather poorly known at present -- be 886 yr. If its orbital period should turn out to be decidedly shorter, another scenario involving a 13th-century sungrazer should be preferred instead. More work on the orbit needs to be done. The apparent contradiction between the discovery of comet ATLAS and previous failures to find any dwarf Kreutz sungrazers in images taken with large ground-based telescopes at moderate heliocentric distances is explained by the propensity of Population II dwarf comets for outbursts, acting in collusion with extremely rare occurrences of these objects. Also addressed are the dynamical properties of perihelion fragmentation as well as the nature and timing of the expected 21st-century cluster of Kreutz comets, swarms of dwarf sungrazers, and related issues.

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arXiv2024

A Note on the Phase Law and Light Curve of Comet Tsuchinshan-ATLAS (C/2023 A3)

The light curve of comet Tsuchinshan-ATLAS peaked in mid-April 2024, which nearly coincided with a minimum phase angle of 2.9 deg. The question of a possible correlation between the two events has implications for the comet's overall performance. In this note I examine the light curve at times of equal phase angles to circumvent the effect and show that the comet was as bright intrinsically in late March as it was in early May. From a plot of the comet's magnitude at unit geocentric distance against the phase angle before and after its minimum on 18 April I derive a very steep phase law and a relatively flat, r^(-2.55) light curve failing to fit the recently reported magnitudes. After stalling in May and June, the comet's activity enters another surge (at a time of peak phase effect), as fragmentation continues.

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arXiv2024

Inevitable Endgame of Comet Tsuchinshan-ATLAS (C/2023 A3)

Hopes are being widely expressed that C/2023 A3 could become a naked-eye object about the time of its perihelion passage in late 2024. However, based on its past and current performance, the comet is expected to disintegrate before reaching perihelion. Independent lines of evidence point to its forthcoming inevitable collapse. The first issue, which was recently called attention to by I. Ferrin, is this Oort cloud comet's failure to brighten at a heliocentric distance exceeding 2 AU, about 160 days preperihelion, accompanied by a sharp drop in the production of dust (Afρ). Apparent over a longer period of time, but largely ignored, has been the barycentric original semimajor axis inching toward negative numbers and the mean residual increasing after the light-curve anomaly, suggesting a fragmented nucleus whose motion is being affected a nongravitational acceleration; and an unusually narrow, teardrop dust tail with its peculiar orientation, implying copious emission of large grains far from the Sun but no microscopic material recently. This evidence suggests that the comet has entered an advanced phase of fragmentation, in which increasing numbers of dry, fractured refractory solids stay assembled in dark, porous blobs of exotic shape, becoming undetectable as they gradually disperse in space.

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arXiv2024

Formation and Evolution of the Stream of SOHO Kreutz Sungrazers. II. Results and Implications of Monte Carlo Simulation

I present the results of the first comprehensive effort aimed at modeling a major component of the stream of SOHO sungrazers, 5000 of which have been detected by the onboard coronagraphs since 1996. The stream of Population I of the Kreutz system, investigated by a Monte Carlo simulation technique, is treated as a product of cascading fragmentation due to unstable rotation of a "seed," a subkilometer-sized object that separated with others from comet X/1106 C1 at perihelion. The stream's activity is predicted to last for 200 years from ~1950 to ~2150, culminating in the 2010s, when a swarm of bright SOHO sungrazers (peak mags not fainter than 3) of Population I was observed. By the end of 2023 about 42 percent of the stream had already arrived. Scatter amounts to 7 deg in the longitude of the ascending node and at most 0.2 solar radius in the perihelion distance. On its initial orbit the seed would pass perihelion in 2036, 193 years after C/1843 D1, the principal fragment of X/1106 C1. Comet C/1668 E1 is proposed as another major fragment and yet another is predicted to arrive in the 2050s or 2060s.

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arXiv2024

Formation and Evolution of the Stream of SOHO Kreutz Sungrazers. I. History and Preliminary Investigations

The nearly continuous stream of miniature comets dominated by the Kreutz sungrazers has been an unexpected bonanza for cometary science initiated by the launch of the Solar and Heliospheric Observatory (SOHO) in 1995. Over the nearly 30 years since the time, no serious attempt has been made to formulate a self-consistent model for the formation and evolution of this stream of Kreutz comets -- the goal of the present two-part investigation. Part I describes historical highlights of the research that has been relevant to the problem of SOHO sungrazers (including the major contributions by Hubbard, Kreutz, and Marsden) and furnishes preliminaries of diagnostic value that are intended to facilitate, and provide critical information for, the work in Part II. Formerly noted issues, such as the high frequency of close pairs in the SOHO database, are proposed to be products of a broader process of swarming, seen in both the nodal longitude and time. I present examples of tight swarms revealed by high arrival rates of the SOHO Kreutz sungrazers, primarily from Population I.

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arXiv2023

Spectacular Post-Perihelion Tails of Bright Kreutz Sungrazers

A vast majority of bright comets between the late 2nd century and the early 18th century, moving in potentially Kreutz orbits according to Hasegawa & Nakano (2001), was first sighted between 2 and 16 days after perihelion, thanks to the spectacular tails that they were then displaying. In this paper I examine the basic properties of the post-perihelion tails of the three brightest Kreutz sungrazers of the 19th and 20th centuries -- the Great March Comet of 1843 (C/1843 D1), the Great September Comet of 1882 (C/1882 R1), and Ikeya-Seki (C/1965 S1). As the pre-perihelion tail of a sungrazer sublimates completely at perihelion, the development of its post-perihelion tail starts from scratch. In the early days after perihelion, the tail length grows rapidly on account of the plasma component. At some point the dust takes over, reaching a peak length weeks later. As the geocentric distance continues to increase and the surface brightness to decline, the tail's shortening sets in. The dust tails of Ikeya-Seki and the 1843 sungrazer contained grains subjected to solar radiation pressure accelerations not exceeding 0.6-0.7 the solar gravitational acceleration, the dust tail of the 1882 sungrazer was more complex. For weeks this comet appeared like a comet in a comet, a result of disintegration of a distant companion near perihelion. Evening Kreutz sungrazers are found to have longer tails than morning ones because of geometry. Other issues are discussed and extensive sets of tail data are provided.

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arXiv2023

The Odd Comet 157P/Tritton and Its Misunderstood Fragmentation

Comet 157P is a faint object with a history of being prone to unfortunate situations, circumstances, and/or coincidences. Several weeks after its 1978 discovery the comet disappeared and remained lost nonstop for twenty five years. Rediscovered in 2003 as a new comet, it was about 500 times brighter than in 1978, caught apparently in one of its outbursts. The comet was not detected 200 days after its 2016 perihelion, being fainter than mag 20, but 80 days later it was mag 16 and gradually fading back to mag 20 over a period of four months. The comet did not miss the opportunity to have a close encounter with Jupiter, having approached it to less than 0.3 AU on 2020 February 10. The 2017 outburst or surge of activity appears to have accompanied an event of nuclear fragmentation. The birth of a second companion is dated to the months following the Jupiter encounter. The series of weird episodes culminated near the 2022 perihelion, when one companion brightened to become observable for two weeks and after another two weeks the other flared up to be seen for the next two weeks. Unnoticed, this incredible coincidence fooled some experts into believing that a single object, designated 157P-B, was involved, even though its orbit left large residuals. I now offer representative fragmentation solutions for the two companions, the mean residuals amounting to +/-0".4 and +/-1".0, respectively.

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arXiv2023

Fragmentation and History of Comet Pair C/1844 Y1 and C/2019 Y4

I call attention to extraordinary features displayed by the genetically related long-period comet pair of C/1844 Y1 (Great Comet) and C/2019 Y4 (ATLAS). The issue addressed most extensively is the fragmentation and disintegration of the latter object, itself a thousands-of-years-old fragment. Of the four fragments of C/2019 Y4 recognized by the Minor Planet Center -- A, B, C, and D -- I confirm that B was the principal mass, which stayed undetected until early April. The comet's 2020 fragmentation is proposed to have begun with a separation of B and A near 22 January, when the nuclear condensation suddenly started to brighten rapidly. From late January to early April, only Fragment A was observed. The remaining Fragments C and D split off most probably from A in mid-March, but they too were detected only in April. A new fragment, E, is proposed to have been observed on three days. Further addressed are the issues of the orbital period and antitail of C/1844 Y1 and the extreme position of this pair among the genetically related groups/pairs of long-period comets.

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arXiv2023

On the Existence of a Super-Kreutz System of Sungrazing Comets

In the context of a recently proposed contact-binary model of the Kreutz system, all its members are products of the process of cascading fragmentation of the two lobes of the parent, Aristotle's comet of 372 BC. This process presumably began with the lobes' separation from each other near aphelion. However, not every object in a Kreutz-like orbit is a Kreutz sungrazer. Any surviving sungrazer that had split off from the progenitor before the lobes separated, as well as its surviving fragments born in any subsequent tidal or nontidal event, are by definition not members of the Kreutz system. Yet, as parts of the same progenitor, they belong -- as do all Kreutz sungrazers -- to a broader assemblage of related objects, which I refer to as a super-Kreutz system. After estimating the ratio of the number of super-Kreutz members to nonmembers among potential historical sungrazers, I generate representative extended pedigree charts for both the Kreutz system and super-Kreutz system. While the fragmentation paths and relationships among the individual sungrazers or potential sungrazers in the two charts are (with at most a few exceptions) arbitrary, the purpose of the exercise is to suggest that the Kreutz system proper could in effect represent an ultimate deagglomeration stage of the super-Kreutz system.

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arXiv2023

On Elusive Observations and a Sly Companion of Comet Wirtanen (C/1956 F1)

Noting that the extensive astrometric observations of the double comet Wirtanen (C/1956 F1) made by E. Roemer have never been published, I replicate the contents of a fortuitously discovered copy of her measurement records of the companion's offsets from the main mass in 1957-1959 and use with such data by others to refine the fragmentation solution. The sublimation-driven nongravitational acceleration is shown to essentially control the companion's motion in the orbital plane. The fragmentation parameters derived by the author in 1978 have now been improved and strong disagreement with the independent results by Roemer is noted. The revised model is employed to predict the positions of the companion on the plates exposed by Roemer on 25 September 1960, which she reported to show the principal nucleus but not the companion. At my request, these plates have now been scanned and processed at the Lowell Observatory, and the companion is found to be located at the predicted position. The images of the main mass and the companion on one of the two plates are displayed.

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arXiv2022

Populations of the Kreutz Sungrazer System in a SOHO Database

Discovery of nine populations in a set of 193 select SOHO Kreutz sungrazers (Sekanina 2021) is confirmed for the first time via a histogram of the true longitudes of the ascending node, constructed for a revised set of 220 select sungrazers imaged exclusively by the SOHO's C2 coronagraph. Marsden's orbits are approximately corrected for effects of the out-of-plane nongravitational force. Population I displays two peaks in the histogram, one presumably belonging to a side branch alike to Population Pe, but with no related naked-eye sungrazer known. Swarms/clusters of objects are commonplace, providing evidence on cascading fragmentation proceeding throughout the orbit. Augmentation to all C2-only SOHO Kreutz comets, aimed at removing deliberate bias against Populations I and Pe, reduces the appearance of Populations Ia and Pre-I to bulges along the slope of the histogram because of the swollen wings of Populations I and Pe, respectively. Populations II through IV change very little or not at all. The high Population I-to-II abundance ratio, of 14:1, may be a product of temporal limitations in fragment release. A drop in the number of fragments toward the ends of the nodal-longitude distribution, especially from Population II to IV, is in line with the contact-binary model.

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