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IAUC 8949: MUTUAL EVENTS OF (136108) 2003 EL_61, S/2005 (136108) 2; V2670 Oph = N Oph 2008

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                                                  Circular No. 8949
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://www.cfa.harvard.edu/iau/cbat.html  ISSN 0081-0304
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MUTUAL EVENTS OF (136108) 2003 EL_61 AND S/2005 (136108) 2
     D. C. Fabrycky, Harvard University; D. Ragozzine and M. E.
Brown, California Institute of Technology; and M. J. Holman,
Smithsonian Astrophysical Observatory, report that orbital fits to
the relative astrometric positions of this dwarf planet (cf. IAUC
8577) and its inner satellite, S/2005 (136108) 2 (cf. IAUC 8636),
have revealed a near edge-on orbit, implying likely mutual events.
The new orbital model is based on images from the Hubble Space
Telescope (WFPC2) and the Keck telescope (LGS-AO).  Due to the
changing orientation of the earth-(136108) line-of-sight, the orbit
is moving closer to edge-on until August 2008, after which the
orbit will open up again, ending mutual events for the next century.
The current distance of closest projected approach is about 500 km,
nearly the same as the semi-minor axis of the triaxial primary
(Rabinowitz et al. 2005, Ap.J. 639, 1238).  Nominally, events in
May are grazing but get progressively more central (and more likely)
throughout June and July.  Shadowing is probably not occurring at
inferior or superior conjunction.  The duration of the events will
be between 0 and about 6 hr; ingress and egress will consist of
about 0.03-magnitude changes on a timescale of about 1 hour.  The
unocculted lightcurve has a double-peaked rotational modulation of
full amplitude 0.29 mag and period 3.9155 hr (Lacerda et al. 2008,
A.J. 135, 1749):  comparison to a template lightcurve is necessary.
Telescopes distributed in longitude are needed to follow events, as
the revised orbital period is 18.36 days.  The main body is rather
faint (V about 17.3), so high-precision photometry requires
moderate (about 1 m) collecting area.  Due to orbital eccentricity,
events in which the main body occults the satellite (labeled 'A'
below) are more likely to occur than events in which the satellite
occults the main body (labeled 'B' below).  This orbital model
predicts mid-event times as follows (add or subtract up to 3 hours
for ingress or egress times):  'B', May 31.07 +/- 0.04 UT; 'A',
June 7.74 +/- 0.05; 'B', June 18.33 +/- 0.05; 'A', June 25.96 +/-
0.06; 'B', July 6.61 +/- 0.05; 'A', July 14.31 +/- 0.05; 'B', July
24.98 +/- 0.05; 'A', Aug. 1.81 +/- 0.05.  Continual updates are
accessible at website URL http://www.cfa.harvard.edu/~fabrycky/EL61.

     N. N. Samus, Institute of Astronomy, Moscow, informs us that N
Oph 2008 (cf. IAUC 8947, 8948) is being assigned the variable-star
designation V2670 Oph.

                      (C) Copyright 2008 CBAT
2008 May 29                    (8949)            Daniel W. E. Green

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