arXiv subjects

Zdenek Sekanina

Publications and source records attributed to Zdenek Sekanina.

46 records · Page 3Linked to original sources

arXiv2017

Orbital Evolution, Activity, and Mass Loss of Comet C/1995 O1 (Hale-Bopp): II. Nucleus and Companions as Compact Clusters of Massive Fragments

The prime objective is to settle a contradiction between a high nongravitational acceleration affecting the orbital motion of comet C/1995 O1 and its enormous nucleus by modeling it as a compact cluster of boulder-sized fragments held together by its own gravity. The nongravitational effect is interpreted as a perturbation of the cluster's principal, most massive fragment. This and other constraints suggest that the principal fragment was probably 8-9 km across and the entire cluster ~150 times less massive than a single-body nucleus of an equal cross-sectional area derived from the Herschel far-infrared photometry of the inactive comet detected near 30 AU from the Sun. The cross-sectional area required the smallest fragments to be a few tens of meters across under a steady-state distribution. The cluster was at most ~200 km in diameter, subject to frequent collisions and significant perturbations by the Sun near perihelion, and apparently a product of tidal fragmentation of the original nucleus, more than 20 km across, at the time of close encounter with Jupiter 4 millennia ago, if the comet's tensile strength was then as low as several Pa. Published for the first time are the results of a search for companion nuclei in three post-perihelion images taken with the HST's STIS instrument in 1997-1998. At least 29 such objects <1200 km (projected) from the primary were detected, with their signals from 2.3% to 25% of the primary's and the signal-to-noise ratios between 5:1 and 29:1, apparently also cluster-like in nature.

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arXiv2016

Study of a Potential Relationship Between Two Long-Period Comets and Rapid Inward Drifting of Aphelion Due to Orbital-Cascade Resonance

We study a potential genetic relationship of comets C/1846 O1 and C/1973 D1, whose apparent orbital similarity was tested by Kresak (1982) only statistically, using the Southworth-Hawkins (1963) criterion D. Our orbit determination for C/1846 O1 shows its period was ~500 yr, ~30 times shorter than that of C/1973 D1. Formerly unrecognized, this incongruity makes the objects' common origin less likely. Long-term orbit integration suggests that, if related, the two comets would have to have separated far away from the Sun (probably ~700 AU) 21 millennia ago and, unlike C/1973 D1, C/1846 O1 would have to have been subjected to a complex orbital evolution. Given the chance of encountering Jupiter to ~0.6 AU some 400 days after perihelion, C/1846 O1 and C/1973 D1 may have been perturbed, during their return in the 15th millennium BCE, into orbits that were, respectively, smaller and larger than was the parent's, with a net difference of more than 0.002 (AU)^{-1} in 1/a. Whereas C/1973 D1 was on the way to its 1973 perihelion, C/1846 O1 should have been subjected to recurring encounters with Jupiter, during which the orbital period continued to shorten by integral multiples of the Jovian orbital period, a process called high-order orbital-cascade resonance. While the integrated perturbation effect of C/1846 O1 by Jupiter does not appear to reduce the comet's orbital period to below ~1200 yr by the mid-19th century, we find that orbital-cascade resonance offers an attractive mechanism for rapid inward drifting of aphelion especially among dynamically new comets.

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arXiv2015

Pairs and Groups of Genetically Related Long-Period Comets and Probable Identity of the Mysterious Lick Object of 1921

We present the history of investigation of the dynamical properties of pairs and groups of genetically related long-period comets (other than the Kreutz sungrazing system). Members of a comet pair or group move in nearly identical orbits and their origin as fragments of a common parent comet is unquestionable. The only variable is the time of perihelion passage, which differs from member to member considerably due primarily to an orbital-momentum increment acquired during breakup. Meter-per-second separation velocities account for gaps of years or tens of years, thanks to the orbital periods of many millennia. The physical properties of individual members may not at all be alike, as illustrated by the trio of C/1988 A1, C/1996 Q1, and C/2015 F3. We exploit orbital similarity to examine whether the celebrated and as yet unidentified object, discovered from the Lick Observatory near the Sun at sunset on 1921 August 7, happened to be a member of such a pair and to track down the long-period comet to which it could be genetically related. Our search shows that the Lick object, which could not be a Kreutz sungrazer, was most probably a companion to comet C/1847 C1 (Hind), whose perihelion distance was ~9 R_sun and true orbital period approximately 8300 years. The gap of 74.4 years between their perihelion times is consistent with a separation velocity of ~1 m/s that set the fragments apart following the parent's breakup in a general proximity of perihelion during the previous return to the Sun in the 7th millennium BCE.

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arXiv2015

Was Comet C/1945 X1 (du Toit) a Dwarf, SOHO-Like Kreutz Sungrazer?

The goal of this investigation is to reinterpret and upgrade the astrometric and other data on comet C/1945 X1, the least prominent among the Kreutz system sungrazers discovered from the ground in the 20th century. The central issue is to appraise the pros and cons of a possibility that this object is --- despite its brightness reported at discovery --- a dwarf Kreutz sungrazer. We confirm Marsden's (1989) conclusion that C/1945 X1 has a common parent with C/1882 R1 and C/1965 S1, in line with the Sekanina-Chodas (2004) scenario of their origin in the framework of the Kreutz system's evolution. We integrate the orbit of C/1882 R1 back to the early 12th century and then forward to around 1945 to determine the nominal direction of the line of apsides and perform a Fourier analysis to get insight into effects of the indirect planetary perturbations. To better understand the nature of C/1945 X1, its orbital motion, fate, and role in the hierarchy of the Kreutz system as well as to attempt detecting the comet's possible terminal outburst shortly after perihelion and answer the question in the title of this investigation, we closely examined the relevant Boyden Observatory logbooks and identified both the photographs with the comet's known images and nearly 20 additional patrol plates, taken both before and after perihelion, on which the comet or traces of its debris will be searched for, once the process of their digitization, currently conducted as part of the Harvard College Observatory's DASCH Project, has been completed and the scanned copies made available to the scientific community.

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arXiv2015

Disintegration of Comet C/2012 S1 (ISON) Shortly Before Perihelion: Evidence from Independent Data Sets

As an Oort Cloud object with a record small perihelion distance of 2.7 Rsun and discovered more than a year before its encounter with the Sun, comet C/2012 S1 is a subject of considerable scientific interest. Its activity along the orbit's inbound leg evolved through a series of cycles. Two remarkable events preserved in SOHO's and/or STEREO's near-perihelion images of its tail were an early massive production of gravel at heliocentric distances of up to ~100 AU, evidently by the annealing of amorphous water ice on and near the nucleus' surface; and, about a week before perihelion, a rapid series of powerful explosions, from the comet's interior, of water with dust at extremely high rates, causing precipitous fragmentation of the nucleus, shattering it into a vast number of sublimating boulders, and ending up, a few days later, with a major, sudden drop in gas emission. The disintegration of the comet was completed by about 3.5 hours before perihelion, at a heliocentric distance of 5.2 Rsun, when C/2012 S1 ceased to exist as an active comet. The orbital motion in this period of time was subjected to progressively increasing outgassing-driven perturbations. A comprehensive orbital analysis results in successfully fitting the comet's observed motion from 2011 to ~7 hours before perihelion.

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arXiv2015

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, Omega, the inclination, i, and the argument of perihelion, omega --- of the Kreutz system's faint, dwarf sungrazers observed only with the SOHO/STEREO coronagraphs; their published orbits were derived using a parabolic, purely gravitational approximation. In a plot of i against Omega 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 deg. The dwarf sungrazers' perihelion longitude is statistically invariable, but their perihelion latitude increases systematically with Omega. 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 catalogued 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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arXiv2014

Temporal Correlation Between Outbursts and Fragmentation Events of Comet 168P/Hergenrother

Outbursts are known to begin with a sudden appearance and steep brightening of a "stellar nucleus" --- an unresolved image of a plume of material on its way from the comet's surface and an initial stage of an expanding halo of ejecta. Since the brightness of this feature is routinely reported, together with astrometry, by most comet observers as the "nuclear magnitude," it is straightforward to determine the onset time, a fundamental parameter of any outburst, by inspecting the chronological lists of such observations for a major jump in the nuclear brightness. Although it is inadmissible to mix nuclear magnitudes by different observers without first carefully examining their compatibility, the time constraints obtained from data sets reported by different observers can readily be combined. The intersection of these sets provides the tightest possible constraint on the outburst's onset time. Applied to comet 168P/Hergenrother during its 2012 return to perihelion, three outbursts were detected and their timing determined with good to excellent accuracy. Six fragmentation events experienced by the comet are shown to have occurred in the same period of time as the outbursts. Three companions are likely to have broken off from the primary in the first outburst, two companions in the second outburst, and one companion in the last outburst. All companions were short-lived, belonging to the class of excessively brittle fragments. Yet, the results suggest that most of the mass lost in the first outburst remained relatively intact during the liftoff, while the opposite was the case in the last outburst.

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arXiv2013

Brightness and Orbital Motion Peculiarities of Comet C/2012 S1 (ISON): Comparison with Two Very Different Comets

To gain a greater insight into the impending evolution of the physical behavior of comet C/2012 S1, its light curve and orbital properties are compared with those for C/1962 C1 (Seki-Lines) and C/2002 O4 (Hoenig). All three are likely Oort Cloud comets. C/1962 C1 survived an encounter with the Sun at less than 7 Rsun, while C/2002 O4 disintegrated near perihelion at 0.78 AU from the Sun. Less than two months before its perihelion at 2.7 Rsun, C/2012 S1 has a light curve that is much closer to C/1962 C1 than C/2002 O4. It remains to be seen whether its motion is affected by nongravitational perturbations. As new data on C/2012 S1 keep coming in, its continuing comparison with the two comets will provide information on its health by updating and adjusting its status. Strengths and weaknesses of this approach for potential future applications will eventually be assessed.

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arXiv2013

Population of SOHO/STEREO Kreutz Sungrazers and the Arrival of Comet C/2011 W3 (Lovejoy)

We examine properties of the population of SOHO/STEREO (dwarf) Kreutz sungrazing comets from 2004 to 2013, including the arrival rates, peculiar gaps, and a potential relationship to the spectacular comet C/2011 W3 (Lovejoy). Selection effects, influencing the observed distribution, are largely absent among bright dwarf sungrazers, whose temporal sequence implies the presence of a swarm, with the objects brighter at maximum than apparent magnitude 3 arriving at a peak rate of about 4.6 per year in late 2010, while those brighter than magnitude 2 at a peak rate of about 4.3 per year in early 2011, both a few times the pre-swarm rate. The entire population of SOHO/STEREO Kreutz sungrazers also peaked about one year before the appearance of C/2011 W3. Orbital data show, however, that all bright dwarf sungrazers moved in paths similar to that of comet C/1843 D1, deviating 10 deg or more from the orbit of C/2011 W3 in the angular elements. The evidence from the swarm and the overall elevated arrival rates suggests the existence of a fragmented sizable sungrazer that shortly preceded C/2011 W3 but was independent of it. On the other hand, these findings represent another warning signal that the expected 21st-century cluster of spectacular Kreutz comets is on its way to perihelion, to arrive during the coming decades. It is only in this sense that we find a parallel link between C/2011 W3 and the spikes in the population of SOHO/STEREO Kreutz sungrazers.

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arXiv2012

Comet C/2011 W3 (Lovejoy): Orbit Determination, Outbursts, Disintegration of Nucleus, Dust-Tail Morphology, and Relationship to New Cluster of Bright Sungrazers

We describe the physical and orbital properties of C/2011 W3. After surviving perihelion, the comet underwent major changes (permanent loss of nuclear condensation, formation of spine tail). The process of disintegration culminated with an outburst on December 17.6 (T+1.6 d) and this delayed response is inconsistent with the rubble pile model. Probable cause was thermal stress from the heat pulse into the nucleus after perihelion, which could also produce fragmentation of sungrazers far from the Sun. The spine tail was a synchronic feature, made up of dust released at <30 m/s. Since the nucleus would have been located on the synchrone, we computed the astrometric positions of the missing nucleus as the coordinates of the points of intersection of the spine tail's axis with lines of forced orbital-period variation, derived from orbital solutions based on preperihelion astrometry from the ground. The resulting osculating orbital period was 698+/-2 years, which proves that C/2011 W3 is the first major member of the predicted new, 21st-century cluster of bright Kreutz-system sungrazers. The spine tail's tip contained dust 1-2 mm in diameter. The bizarre appearance of the dust tail in images taken hours after perihelion with coronagraphs on SOHO and STEREO is readily understood. The disconnection of the comet's head from the preperihelion tail and the apparent activity attenuation near perihelion are both caused by sublimation of all dust at heliocentric distances smaller than ~1.8 solar radii. The tail's brightness is strongly affected by forward scattering of sunlight by dust. The longest-imaged grains had a radiation-pressure parameter beta ~ 0.6, probably submicron-sized silicate grains. The place of C/2011 W3 within the hierarchy of the Kreutz system and its genealogy via a 14th century parent suggest that it is indirectly related to the celebrated sungrazer X/1106 C1.

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