C 2019 Y4 ATLAS

Electronic Telegram No. 4751

Central Bureau for Astronomical Telegrams
Mailing address: Hoffman Lab 209; Harvard University;
20 Oxford St.; Cambridge, MA 02138; U.S.A.
e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network

Further to CBET 4744, CCD images obtained by many observers have shown
that the nucleus of this comet has fragmented, causing the comet to lose its
sharp central condensation and spread out in clumps of diffuse light that is
typical of comets falling apart. Some of the reports received by the
Central Bureau are given below the following analysis by Sekanina.

In response to a request from the Central Bureau, Z. Sekanina, Jet
Propulsion Laboratory, writes: "Although no quantitative fragmentation model
can be formulated for C/2019 Y4 at this time, a rudimentary, yet self-
consistent, scenario is offered below, based on the extremely limited
ground-based imaging database that is available at this time (April 11). It
shows, in the least, five separate condensations (see, e.g., a 3D-intensity
distribution 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 studies
of countless split comets, is that the westernmost condensation, here denoted
C1, contains the primary, most-massive fragment of the pre-split comet. This
choice is corroborated by the very narrow profile of C1 on the 3D-intensity
distribution (comets-ml, No. 28515). An unusual but important property of C1
is its position plainly off the axis of the tail, to the north of it, unlike
the other condensations. As the primary, C1 was subjected to only a minor
momentum change in the course of the fragmentation process, and its motion
should closely mimic the motion of the pre-split comet. Astrometric positions
of the comet could possibly be combined with those of C1 to extend the orbital
arc. 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 Bortle
limit in a paper posted at URL https//arxiv.org/pdf/1903.06300.pdf). On the
other hand, since C/2019 Y4 itself is nearly certainly a fragment of comet
C/1844 Y1 that separated thousands of years ago, its a priori propensity for
disintegration is rather high. Yet, if any fragment should survive, C1 is
expected to be the one.
The off-axis position of C1 is consistent with the onset of the
fragmentation process near or somewhat before mid-March, at a heliocentric
distance of about 1.8 AU. This timing is also in line with the comet's
steep brightening that terminated on March 17 according to A. Kammerer (see
website 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 same
date according to J. P. Navarro Pina (comets-ml, No. 28390). Early after
separation, a companion is moving away from the primary essentially along the
extended radius vector. For C/2019 Y4, the relevant position angle was 175
degrees 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 from
C1, under a presumably moderate, outgassing-driven nongravitational
acceleration, in a south-southeast direction, toward the observed tail's
axis. This motion determined the position of C2 as the westernmost
condensation on the tail's axis, whose position angle in the Apr. 8-9 images
is estimated at about 95 deg, slightly south of the antisolar direction.
Next, C2 is proposed to have undergone a breakup in late March or the
first days of April. Again, this time coincides approximately with the
observed 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 the
expected motion of any secondary fragment was in the vicinity of 110 deg,
slightly south of the east. The most prominent product of this event was
the brightest and most sizable condensation, C3, which is the best candidate
for 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 been
much higher than that separating C2 from C1, and it was perhaps generated
by significant contributions from both the outgassing-driven momentum and
solar radiation pressure. The C2 and C3 condensations appear to have been
active in the images taken on Apr. 8-9.
C3 was the brightest (but hardly the only) cluster of particulate debris
scattered along the tail's axis. Guido's (ibid.) 3D-intensity distribution
displays C4 and C5, two additional, much smaller peaks down the tail's axis,
which may have originated from C2 either in separate fragmentation events
preceding the C3 event or together with C3 but under still higher
accelerations. Some images show another possible condensation between C2 and
C3. It is likely that high-resolution imaging of the comet will unveil
further 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 of
microscopic grains subjected to major effects of solar radiation pressure.
This is a very preliminary picture of what C/2019 Y4 may have lived
through in the past several weeks. If only a fraction of the narrative turns
out to realistically describe what actually happened, it will provide another
powerful argument in favor of the cometary process of cascading fragmentation."

E. Guido, Castellammare di Stabia, Italy, reports on observations made
by 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-m
and 0.84-m reflectors show the presence of a secondary feature embedded in
the tail of comet C/2019 Y4, about 6".6 in p.a. 98 degrees with respect to
the "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 degrees
with respect to the "central condensation" and about 9".1 in p.a. 98 degrees
from the second feature. A fourth feature was found on images taken on
Apr. 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 condensation
on thirty stacked 60-s exposures taken on Apr. 9.85 UT with a 0.4-m f/3.8
reflector, measuring it to be 10" from the photometric center of the
elongated "main condensation" of the comet toward p.a. 287 degrees.
Contributors of images showing indication of the comet's breaking up
that have ben posted on the "ICQ Comet Observations" forum on Facebook in
the 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, 20x80

NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
superseded by text appearing later in the printed IAU Circulars.

(C) Copyright 2020 CBAT
2020 April 13 (CBET 4751) Daniel W. E. Green


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