[IAUC] CBET 2876: 20111027 : COMET C/2010 X1 (ELENIN)

[quai en eps.harvard.edu]

                                                  Electronic Telegram No. 2876
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau en eps.harvard.edu (alternate cbat en iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


COMET C/2010 X1 (ELENIN)
     Giovanni Sostero, Ernesto Guido, and Nick Howes report on their attempts
to image comet C/2010 X1 (cf. IAUC 9226) after its solar conjunction, using
several robotic scopes that were operating under excellent sky conditions in
New Mexico and at Mauna Kea on Oct. 9.5 and 10.6 UT, yielding no sign of the
comet at low altitude.  Several stacked exposures taken on Oct. 10.6 with the
2.0-m f/10 Ritchey-Chretien "Faulkes Telescope North" show no trace of the
comet within the 10' x 10' field-of-view centered on the comet's ephemeris
(limiting magnitude around 20.5).  But after stacking unfiltered CCD images
taken in moonlight by Guido, Sostero, and Howes on Oct. 21.38 and 21.48 UT
remotely using the GRAS 0.1-m f/5 APO refractor at the Mayhill station in New
Mexico (field-of-view 3.9 deg x 2.6 deg; scale 3".5 pixels), they found
something moving on the sky background via blinking the two sets that were
separated by about 2 hours:  an extremely faint and diffuse blob of tentative
size 14' x 8' (elongated toward p.a. 300 deg) with no obvious condensation
that is close to the ephemeris position (roughly 3'.5 east-southeast of the
prediction), moving apparently with the comet's motion.
     Guido, Sostero, and Howes confirmed their detection of the comet's
"cloud" in observations obtained on Oct. 23.4 with the same refractor, the
cloud being roughly 40' long with a 6' extension near the expected position
of the comet.  Images are posted at website URLs http://bit.ly/q5QCM7 and
http://bit.ly/pXxtpY (with an "X" marking the the ephemeris position in
the second image); an animation showing the motion with respect to the
X-marked movement of the expected comet's position is shown at website URL
http://bit.ly/qOx8oF.  Sostero, Guido, and Howes then subtracted the field
stars to obtain the images posted at URL http://tinyurl.com/64fbkcb.  They
note that the sunward part of the "cometary cloud" appears much sharper
compared to the anti-solar direction; the diffuse shape of the comet appears
to be somehow "conical", about 1.5 deg long overall, with a maximum
thickness of about 10' on the side toward the solar direction, and the oval
shape of the "cometary cloud" then thins significantly tailward (p.a. about
300 deg).  The "brightest" part of this extremely faint blob of light is
located about 4'.3 in p.a. 77 deg (east-northeast) when compared to the
nominal position of the MPC ephemeris.

     Z. Sekanina, Jet Propulsion Laboratory, writes that the deep CCD images
of the disintegrated comet taken in the past few days, especially the high-
quality processed image from Oct. 23.37 UT by E. Guido et al., show enough
detail to allow simple modeling and a preliminary interpretation of the
surviving tail.  The sharp cigar-shaped trail near the southern end of this
dust-ejecta cloud points at position angle 290 +/- 1 deg, which, interpreted
as a synchrone, implies a fairly brief dust-emission event centered on Aug. 16
+/- 4 days, in fair agreement with a rather sharply peaked light curve on
Aug. 13-14 and with Mattiazzi's report of the comet's notable fading from Aug.
17 on (CBET 2801).  Despite the near-zero inclination of the comet's orbital
plane to the ecliptic, the geometry from the earth has been relatively
favorable.  Because of the small geocentric distance, the earth was on the
Aug. 23 almost 7 deg below the orbit plane, when this picture was taken, and
since the sheet of dust in the plane was spreading toward the earth, it
projected essentially to the north of the trail in an approximately 160-deg-
wide fan.  The prolonged radius vector was directed at p.a. 277 deg and thus
made an angle of 13 deg with the sharp trail.
     Because of their fan-like distribution in the orbit plane, the image
shows not only the ejecta from the first half of August, but from the comet's
entire active period, starting far from the sun on the way to perihelion.
(Comets arriving from the Oort cloud are generally known to be considerably
active at large heliocentric distances on their way in; C/2010 X1 is one such
example.)  Because the bright tip of the cigar-shaped trail lies on the line-
of-variation, it apparently represents the location of the most sizable debris
ejected during the mid-August dust-emission event; relative to the ephemeris
position, the perihelion time was late by some 0.06 day (Sept. 10.79 instead
of 10.73 TT), and this deceleration is equivalent to a sudden change in the
orbital velocity of more than 50 m/s, primarily in the direction away from
the sun.  Whereas the comet's orbital motion may have non-gravitationally
decelerated even before the mid-August event (though not enough to detect
computationally), the bulk of the effect should be due to the evolution in
the past 10 weeks or so.  Since this velocity change is too high for fragments
several meters across or larger, they must have been short-lived (like in the
case of C/1999 S4; e.g., Weaver et al. 2001, Science 292, 1329) and soon must
have given birth to ever smaller fragments in a cascading fashion.  The
largest debris surviving to date is perhaps in the centimeter range.
     The trail's total length, which must of course be reckoned from the
bright tip of the trail along p.a. 290 deg, is from the image conservatively
estimated at 50 arcmin -- implying a peak acceleration by solar radiation
pressure on the particles at the anti-solar end of about 0.0035 the solar
gravitational attraction, and their typical diameter for an assumed bulk
density of 1 g/cm^3 at 0.3 mm; these are the smallest particles that we see
in the trail.  The part of the feature on the sunward side of the bright tip
-- technically an anti-tail -- consists of dust ejected from the comet long
before it disintegrated; the lengths of the anti-tail and the rest of the tail
in the directions from the east up to the west-northwest (but not including
the trail) must be reckoned from the comet's ephemeris position.  Indeed, the
cloud to the east of the ephemeris position is seen to have a fairly sharp
southern edge and to emerge from this location.  An imaginary boundary between
the sharp trail and the anti-tail, in position angle 20 deg (normal to the
trail), is populated by particles ejected from the comet more than 200 days
before perihelion, back in January 2011; the feature can in this direction be
followed for some 10 arcmin, where the smallest particles are about 0.6 mm in
diameter.  The particles in the anti-tail itself may have been ejected as long
as several years ago, at distances of about 10 AU from the sun or more -- and
at a distance of about 25 arcmin, at the end of the anti-tail, their minimum
diameter could easily exceed 1 mm.  The uncertainty is aggravated by the
ejection velocity, especially its normal component, because of the near-edge-on
viewing:  even if it does not exceed 1 m/s, its integrated effect may amount
to a few arcmin in the image.  The same effect is also responsible for much of
the minor extension of the dust cloud to the south of the sharp trail.
     The entire cloud has slowly rotated clockwise and, in the coming weeks,
the position angle of the sharp trail and its length (in units of the length
on Oct. 23.37) should be as follows (at 0h ET):  2011 Oct. 26, 283 deg, 1.22;
Nov. 5, 267 deg, 1.51; Nov. 15, 260 deg, 1.36; Nov. 25, 256 deg, 1.14; Dec. 5,
254 deg, 0.95.  Because of the continuing expansion in space and the
increasing heliocentric and geocentric distances, the cloud will necessarily
be fading and may not be observed until December.  On the other hand, because
the earth's cometocentric latitude will gradually be decreasing, the cloud
will be getting narrower, which will lead to a higher optical depth.  By the
end of October, the anti-tail will disappear because of the changing geometry,
and the position of the early ejecta will gradually fold toward the sharp
trail, extending only slightly to the north of it.  The tip of the sharp trail
should then be at the very sunward end of the whole feature.
     The long survival of the dust-ejecta cloud of C/2010 X1 is not surprising,
and it compares favorably with a number of similar instances in the past; e.g.,
the surviving tail of comet C/1925 X1 (Ensor) was photographed in Bergedorf
until 72 days after the time of disinegration (see Sekanina 1984, Icarus 58,
81).  The amount of the dust ejecta in the cloud of C/2010 X1 cannot be
determined because of the lack of photometry.  A very crude estimate of the
total cross-sectional area of the dust particles is only provided by the
visual magnitude 10.2 that was estimated by J. J. Gonzalez on Oct. 21; the
result is about 480 km^2.  The mass of the cloud comes out to be on the order
of 10^(12) grams.


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

                         (C) Copyright 2011 CBAT
2011 October 27                  (CBET 2876)              Daniel W. E. Green