C 2019 Y4 ATLAS
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Electronic Telegram No. 4751Central Bureau for Astronomical TelegramsMailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A.e-mail: firstname.lastname@example.org (alternate email@example.com)URL http://www.cbat.eps.harvard.edu/index.htmlPrepared using the Tamkin Foundation Computer NetworkCOMET C/2019 Y4 (ATLAS) Further to CBET 4744, CCD images obtained by many observers have shownthat the nucleus of this comet has fragmented, causing the comet to lose itssharp central condensation and spread out in clumps of diffuse light that istypical of comets falling apart. Some of the reports received by theCentral Bureau are given below the following analysis by Sekanina. In response to a request from the Central Bureau, Z. Sekanina, JetPropulsion Laboratory, writes: "Although no quantitative fragmentation modelcan be formulated for C/2019 Y4 at this time, a rudimentary, yet self-consistent, scenario is offered below, based on the extremely limitedground-based imaging database that is available at this time (April 11). Itshows, in the least, five separate condensations (see, e.g., a 3D-intensitydistribution by E. Guido, posted at https://groups.io/g/comets-ml/messages,No. 28515; referred to below as comets-ml). The fundamental thesis, supported by overwhelming evidence from studiesof countless split comets, is that the westernmost condensation, here denotedC1, contains the primary, most-massive fragment of the pre-split comet. Thischoice is corroborated by the very narrow profile of C1 on the 3D-intensitydistribution (comets-ml, No. 28515). An unusual but important property of C1is its position plainly off the axis of the tail, to the north of it, unlikethe other condensations. As the primary, C1 was subjected to only a minormomentum change in the course of the fragmentation process, and its motionshould closely mimic the motion of the pre-split comet. Astrometric positionsof the comet could possibly be combined with those of C1 to extend the orbitalarc. Perihelion survival of C/2019 Y4 is questionable but not ruled out,given that this is not an Oort Cloud comet (see my revision of the Bortlelimit in a paper posted at URL https//arxiv.org/pdf/1903.06300.pdf). On theother hand, since C/2019 Y4 itself is nearly certainly a fragment of cometC/1844 Y1 that separated thousands of years ago, its a priori propensity fordisintegration is rather high. Yet, if any fragment should survive, C1 isexpected to be the one. The off-axis position of C1 is consistent with the onset of thefragmentation process near or somewhat before mid-March, at a heliocentricdistance of about 1.8 AU. This timing is also in line with the comet'ssteep brightening that terminated on March 17 according to A. Kammerer (seewebsite URL https://fg-kometen.vdsastro.de/koj_2020/c2019y4/19y4eaus.htm)and with Af(rho) variations (cf. IAUC 7342) that peaked at 900 cm on the samedate according to J. P. Navarro Pina (comets-ml, No. 28390). Early afterseparation, a companion is moving away from the primary essentially along theextended radius vector. For C/2019 Y4, the relevant position angle was 175degrees on Mar. 5, 163 degrees on Mar. 10, and 151 degrees on Mar. 15. Thus,in the aftermath of the prime breakup, the companion C2 was moving away fromC1, under a presumably moderate, outgassing-driven nongravitationalacceleration, in a south-southeast direction, toward the observed tail'saxis. This motion determined the position of C2 as the westernmostcondensation on the tail's axis, whose position angle in the Apr. 8-9 imagesis estimated at about 95 deg, slightly south of the antisolar direction. Next, C2 is proposed to have undergone a breakup in late March or thefirst days of April. Again, this time coincides approximately with theobserved minor flare-ups, as well as another peak of 1200 cm in Af(rho)according to Navarro Pina (ibid.). At the time, the position angle of theexpected motion of any secondary fragment was in the vicinity of 110 deg,slightly south of the east. The most prominent product of this event wasthe brightest and most sizable condensation, C3, which is the best candidatefor a cluster of particulate debris, or an assemblage of smaller clusters,containing everything from boulders to pebbles down to macroscopic grains.The acceleration under which C3 receded from C2 is expected to have beenmuch higher than that separating C2 from C1, and it was perhaps generatedby significant contributions from both the outgassing-driven momentum andsolar radiation pressure. The C2 and C3 condensations appear to have beenactive in the images taken on Apr. 8-9. C3 was the brightest (but hardly the only) cluster of particulate debrisscattered along the tail's axis. Guido's (ibid.) 3D-intensity distributiondisplays C4 and C5, two additional, much smaller peaks down the tail's axis,which may have originated from C2 either in separate fragmentation eventspreceding the C3 event or together with C3 but under still higheraccelerations. Some images show another possible condensation between C2 andC3. It is likely that high-resolution imaging of the comet will unveilfurther fragments, including secondary clusters of particulate debris.This entire sizable-fragment population is enveloped by the dust tail,emanating presumably from both the C2 and C3 masses, and composed ofmicroscopic grains subjected to major effects of solar radiation pressure. This is a very preliminary picture of what C/2019 Y4 may have livedthrough in the past several weeks. If only a fraction of the narrative turnsout to realistically describe what actually happened, it will provide anotherpowerful argument in favor of the cometary process of cascading fragmentation." E. Guido, Castellammare di Stabia, Italy, reports on observations madeby members of the "Cometary Archive for Af(rho)" project (including G. Favero,M. Facchini, A. Aletti, L. Buzzi, A. Valvasori, D. Carosati, A. Mantero, G.Milani, and himself). CCD images taken during Apr. 7.80-7.86 UT with 0.80-mand 0.84-m reflectors show the presence of a secondary feature embedded inthe tail of comet C/2019 Y4, about 6".6 in p.a. 98 degrees with respect tothe "central condensation" of the comet. Additional images taken on Apr.8.80 show a brighter third feature placed about 3".9 in p.a. 98 degreeswith respect to the "central condensation" and about 9".1 in p.a. 98 degreesfrom the second feature. A fourth feature was found on images taken onApr. 9.80 ahead of the main coma of the comet, located about 4".5 in p.a.287 degrees with respect to the "central condensation" of the comet. E. Bryssinck, Kruibeke, Belgium, also noticed the off-axis condensationon thirty stacked 60-s exposures taken on Apr. 9.85 UT with a 0.4-m f/3.8reflector, measuring it to be 10" from the photometric center of theelongated "main condensation" of the comet toward p.a. 287 degrees. Contributors of images showing indication of the comet's breaking upthat have ben posted on the "ICQ Comet Observations" forum on Facebook inthe past week include E. Bryssinck, T. Chen, L. Gallagher, M. Jaeger, F.Kugel, R. Ligustri, T. Lovejoy, M. Mattiazzo, T. Scarmato, and A. Valvasori. Additional visual total-magnitude and coma-diameter estimates (cf.CBET 4744): Apr. 10.13 UT, 8.8, 8' (C. Hergenrother, Tucson, AZ, U.S.A.,10x50 binoculars); 11.19, 8.0, 13' (C. S. Morris, Fillmore, CA, USA, 20x80binoculars).NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2020 CBAT2020 April 13 (CBET 4751) Daniel W. E. Green
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