Electronic Telegram No. 5556
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


(134421) 1998 QT_2
     D. Herald (Murrumbateman, Australia), on behalf of the International
Occultation Timing Association (IOTA), reports that S. Conard (IOTA; observing
from three locations near Wellsboro, PA, USA) and G. A. Lyzenga (IOTA;
observing near Altadena, CA, USA) have reported the discovery of an apparent
satellite of the main-belt asteroid (134421), based on an occultation of the
7.3-magnitude star TYC 2765-00218-1 on 2024 Oct. 12.20 UT.  A configuration
diagram showing the four observed chords (numbered 1 to 4, with the star
"moving" right to left) is posted at the following website URL:
http://www.cbat.eps.harvard.edu/iau/cbet/005500/CBET5556_Fig1.png; the
corresponding light curves are posted at the following website URL:
http://www.cbat.eps.harvard.edu/iau/cbet/005500/CBET5556_Fig2.png.  The star's
radius as specified in the Gaia DR3 catalogue (astrophysical_parameters_supp
table, item 'radius_flame_spec') is 117 (+12/-8) solar radii.  When combined
with the star's parallax of 0".001072 +/- 0".000024, the apparent diameter of
the star is 0".00117 (+0".00015/-0".00011).  The configuration diagram
includes a representation of the star's size at the scale of the diagram.
Chord 1 was recorded by Conard using a 12.7-cm Schmidt-Cassegrain telescope
(+ Astrid camera).  There is no clear occultation event in this light curve;
the vertical arrow on the horizontal axis indicates the location nearest the
time of closest approach to the satellite.  Chord 2 was recorded by Conard
with the same equipment and records the occultation by the satellite.
Chord 3 was recorded by Lyzenga using a 20-cm Schmidt-Cassegrain reflector
(+ PC164C camera); it records a grazing occultation against the main
body of the asteroid, and the vertical arrow on the horizontal axis indicates
the location nearest the time of closest approach to the satellite.  There is
no indication of an occultation by the satellite near that location.  Chord 4
was recorded by Conard using a 35.6-cm Schmidt-Cassegrain telescope (+ Watec
910 video camera); it records an occultation by the main body.  The
configuration of the four chords excludes both a double star and a grazing
occultation by a single body from being an explanation for the observations.
The occultation events in chords 3 and 4 allow for two families of ellipses
to represent the main body -- one with the major axis aligned similarly to the
chord direction, and the other having the major axis aligned approximately
normal to the chord direction.  Constraining the ellipse size to be similar to
the weighted mean diameter of 7.5 +/- 0.6 km derived from NEOWISE and
AkariAcuA measurements of the asteroid's diameter, and using the star diameter
of 0".00117 with a limb darkening of 0.1, modeling of the light curves of
chord 3 (Lyzenga) and chord 4 (Conard) indicated that the main body's size was
approximately 7.0 x 5.9 km, with the position angle of the major axis being
approximately 98 degrees.  Using the same star diameter and limb darkening,
modeling the light curve of chord 2 gave an optimal fit of an ellipse of 1.8 x
1.0 km, with the position angle of the major axis being approximately 90
degrees and the center of the satellite being about 0.2 km north of chord 2.
This model is consistent with the absence of an occultation event at both
chords 1 and 3.  The modeling also showed that the apparent drop in the light
curve of chord 1 near the time of closest approach to the satellite was
consistent with a noise event and inconsistent with an occultation event.
The separation of the satellite from the main body was 0".0138 in p.a. 268.0
degrees, with a sky-plane separation of 17.4 km.  D. Gault, Trans-Tasman
Occultation Alliance (TTOA), and Herald (IOTA and TTOA) aided in the analysis
of these observations.


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

                         (C) Copyright 2025 CBAT
2025 May 20                      (CBET 5556)              Daniel W. E. Green