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The possible Varuna stellar occultation
of 6/7 December 2008
On 7 December 2008 around 02:05 -- 02:20 UT (night of Saturday December 6h to Sunday December 7th, 2008),
the Trans-Neptunian-Object Varuna might occult the star UCAC2 40846256, with Rmag~ 14.5.
Such an event, if observed at various sites, could allow us to:
- pin down Varuna's radius to km-accuracy, while its diameter is presently poorly constrained between ~ 500 and 1000 km.
- provide its shape, if non-spherical
- detect a possible faint atmosphere around that body.
Details, updates, discussions and maps are available on:
the IOTA (European Section) page.
Related predictions are posted on the
Asteroid Occultation site (Steve Preston).
Independent predictions are posted on:
the MIT page.
See below the predicted shadow tracks, posted in chronological order.
(NB. on all diagrams, the Horizons JPL ephemeris is used as the reference ephemeris)
Click on images for enhancement
click |
Prediction using the UCAC2 star position and **no** offset on Varuna ephemeris.
Black lines: Varuna shadow, dotted lines: typical uncertainty band, as of Nov. 22, 2008.
The dots are plotted every mn, shadows move from right to left. The larger dot is geocentric
closest approach, occurring at the UT time indicated at the bottom of the figure.
Click on images for enhancement
click |
Update as of
25 Novembre 2008
by Marcelo Assafin et al. (Rio group),
using a recent star position derived from observations made at Pic du Midi:
RA= 07h 29mn 48. 1720sec
DE= +25d 40' 07.741" (J2000/UCAC2)
and the following offset for Varuna (from 18 positions of Varuna, also from observations made at Pic du Midi):
off_RA = +94mas
off_DE = -279mas.
The dotted lines are typical errors on path, roughly +/-2000 km, or about +/-65 mas.
Update as of
27 Novembre 2008
by Raoul Behrend (Geneva),
using a recent star position derived from the same observations:
RA= 07h 29mn 48. 1720sec
DE= +25d 40' 07.748" (J2000/UCAC2)
and the following offset for Varuna (from 56 images):
off_RA = +23mas
off_DE = -185mas.
Formal errors on this prediction, taken from rms and number of observations, are about 25 mas, or +/- 800 km,
as shown by the dotted lines.
The red star symbols are potential observing sites.
Update as of
29 Novembre 2008
by Marcelo Assafin et al. (Rio group),
using a recent star position derived from 62 observations of the
star at Pic du Midi (T1M François Colas, T60 Bernard Trégon), St Sulpice (T60, Bernard Christophe) and
Sozzago (T40, Frederico Manzini):
RA= 07h 29mn 48. 1725sec
DE= +25d 40' 07.740" (J2000/UCAC2)
and the following offset for Varuna (from 32 images taken at Pic du Midi T1M):
off_RA = +42mas
off_DE = -194mas.
Formal errors on this prediction, taken from rms and number of observations, are about 25 mas, or +/- 800 km,
as shown by the dotted lines.
The red star symbols are potential observing sites.
Charts
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