Electronic Telegram No. 5656
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


(50142) 2000 AY_129
     S. Kidd, British Astronomical Association (BAA), reports the discovery of
an apparent satellite of the minor planet (50142) based on an occultation of
the star UCAC4 541-014802 (which has catalogued Gaia magnitudes G = 11.04 and
R = 9.98) on 2025 Dec. 3.017 UT.  The magnitude of this main-belt asteroid
was 18.6.  The observation was made near Stevenage, Hertfordshire, UK, using a
35-cm telescope (+ ASI432 CMOS camera).  A light drop of duration 0.13 s was
followed 0.29 s later by a drop of duration 0.53 s, with the corresponding
chord lengths being 1.5 and 6.2 km.  Both light drops fell to below the level
of comparison star UCAC4 541-014790 (magnitudes G = 12.71 and R = 11.85)
and close to the limiting magnitude of the recording of G = 14.0.  The
magnitude drops considered together are too large to be explained by a double
star.  The Gaia-catalogue (Gaia Data Release 3) radius of the star ("estimated
from FLAME" using parameters in DR3) is 78 solar radii, which, when combined
with the stellar parallax of 0".00045, equates to an apparent diameter of
0".00032.  This diameter is sufficiently large to affect the Fresnel
diffraction fringes, but not large enough to significantly affect the shape
of the light drops.  The NEOWISE diameter of the asteroid is 6.7 +/- 0.6 km.
The Asteroid Lightcurve Photometry Database (ALCDEF) has ten light curves for
this asteroid, with an indicated maximum magnitude variation of 0.58
magnitude.  The measurements of the asteroid's magnitude in Gaia DR3 are not
inconsistent with this variation.  The light curve was modeled on the basis
of the main body appearing as an ellipse with an apparent area equivalent to
the NEOWISE diameter, with the 1.58:1 ratio of the major and minor axes
equating to the maximum magnitude variation from ALCDEF.
     The light curve, together with a plot of the light curve overlaying the
Fresnel diffraction modeled light curve is posted at website URL
http://www.cbat.eps.harvard.edu/iau/cbet/005600/CBET5656_Fig1.png, and the
Fresnel diffraction model that gave the best agreement with the light curve
together with a sky-plane plot of the event (which also shows the diameter of
the star relative to the asteroid), are posted at the following website URL:
http://www.cbat.eps.harvard.edu/iau/cbet/005600/CBET5656_Fig2.png.  The
separation of the two bodies is too large for the light curve to be explained
by an elongate body having a size consistent with the NEOWISE diameter, and
there is nothing in the light curve to suggest a continuity of the main body
with the satellite.  The axes of the main body used in the model are 8.0 x 5.0
km, with the axes of the satellite being 2.2 x 1.2 km.  There are four
possible placements of the two bodies relative to the chord, with each body
being either above or below the chord.  With regard to these possibilities,
the separation of the satellite from the main body is 0".0060 +/- 0".0001 in
p.a. 265 +/-15 deg, with a sky-plane separation of 7.4 km.
     T. Haymes, BAA; C. Weber, European Section of the International
Occultation Timing Association (IOTA); D. Herald, Trans-Tasman Occultation
Alliance (TTOA) and IOTA; and D. Gault, TTOA, aided in the analysis, using a
Fresnel diffraction modeling tool by Bob Anderson (IOTA).


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

                         (C) Copyright 2026 CBAT
2026 January 14                  (CBET 5656)              Daniel W. E. Green