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Bruno SicardyObservatoire de Paris - LESIA
Université Pierre et Marie CurieInstitut Universitaire de France
Probing remote solar system bodies with
stellar occultations
Earth-based support toGaia Solar System science27 October 2008
Morbidelli & Levison, Nature 2003
main goals of stellar occultations by
Trans-Neptunian Objects (TNO's)
! measure TNO's sizes and shapes at sub-km accuracy(NB. 1 mas ~ 20-40 km when projected @ TNO's
! derive density for bodies with satellites
! detect atmospheres @ nbar level
! improve ephemeris accuracy at mas level
main challenges
! predictions are difficult, as TNO's diameters ~ 30-100 masfor biggest bodies
! HIP and TYC catalogues degrade due to p.m. uncertainties,and "bright" stars only (mag. 11-12)
! UCAC2 catalog good to ~ 30-50 mas
consequences
! tedious predictions, measure stars 30-50 times against
100-200 UCAC2 stars, if available
" Rio de Janeiro group, M. Assafin, R. Martins et al., Praiapackage, 2.2m La Silla, 1.6m Pico dos Dias
" Geneva group, R. Behrend et al., 1.2m La Silla
! typical final accuracy 20-30 mas in J2000/ICRF position
! when event is missed: waste of grant money, waste of timeand difficulty convincing TAC of large telescopes (VLT, CFHT,
etc…)
0.89"
2UCAC 26257135(D. Herald, Aug. 2004)
an example
the Charon stellar occultationof 11 July 2005
110 mas
55 mas
occultation of HIP 106829 by Titania8 September 2001
114 mas
JPL predic.(W. Owen)
Bordeaux predic.(M. Rapaport)
portable + fixed telescopes: 0.5 ! 2.2 µm
from Widemann et al., Icarus 2008
high accuracy for size and density determination 788.4 ± 0.6 km, " = 1.711 ± 0.005 g cm-3
from Widemann et al., Icarus 2008
from P.C. Thomas, Icarus 73, 427 (1988)
r!
p= 1 µbar
p= 0.1 µbar
p= 0.01 µbar
R=1150 km
p= 0.01 µbar
R=1150 km
0.89"
2UCAC 26257135(D. Herald, Aug. 2004)
the Charon stellar occultation
of 11 July 2005
110 mas
55 mas
VLT/NACO K band, Charon occultation 11 July 2005
Pluto
star +
Charon
1"
Sicardy et al., Nature 439, 52, 2006
«!Charon"s size and an upper limit
on its atmosphere from a stellar
occultation!»
R= 603.6±1.4 km
Personet al. AJ2006
56 K - N2100 K - CH4
56 K
Some conclusions:
"Atmosphere: limit is model dependent, e.g.
CH4 with inversion layer: psurf < 15 nbar…or…
N2 isothermal 56 K: psurf < 110 nbar etc…
" Charon!s size: RC = 603.6 ± 1.4 km
"Charon!s density: "C = 1.65 ± 0.06 g cm-3
(taken from Buie et al. AJ. 132, 290, 2006, NB."P = 2.03 ± 0.06 g cm-3 for Pluto)
"Possibility to retrieve Pluto"s radius with accurate Charon"s
accurate size --> refine atmospheric model
Pluton 20 juil. 2002
P131.1 August 21, 2002
Sicardy et al.«!Large changes in Pluto"s atmosphere
as revealed by recent stellar
occultations!»
Nature, 10 July 2003
Time !
rP
B'P
rN
1988
2002
2006
galactic planeXing
rP: heliocentric range B'P: subsolar latitude ("season")
Pluto in 1988
Pluto in 2002
Sicardy et al. Nature,
July 2003
#
Imm. Eme.
central line
half light
1% drop
18 March 2007
12 May 2007
9 June 2007
14 June 2007
31 July 2007
22 June 2008 Pluto *and* Charon stellar occultation
20 cm
40 cm
40 cm
Time (sec UT)
Stockport
Blue
Mountains
Hobart
1988
2007
part most sensitiveto pressure changes
Pico do Dias1.6-m LNA telescopeBrazil
Hansen & Paige, 1996run #12
1988(Yelle & Elliot1997)
1988
2002
Hansen & Paige, 1996run #12
1988
2002
2006
Hansen & Paige, 1996run #12
1988
2002
2006
2007
Hansen & Paige, 1996run #12
1988
2002
2006
2007
Hansen & Paige, 1996run #12
2008
22 June 2008 Pluto *and* Charon stellar occultation
caramba, raté!
Charon occultation, 22 June 2008
Observatoire des Makes, La Réunion
UT (sec)
1% drop level,Australia
Pluto-Charon occultation, 22 June 2008
Charon occultation chordLa Réunion
a= 19570.3 km
(fromTholen et al.AJ 2008)
604-km radius disk
a= 19570.3 km
r= 602-608km Gulbis et al 2006Person et al 2006
Sicardy et al 2006
a= 19570.3 km
PLU027
a= 19570.3 km
Tholen et al.AJ 2008
a= 19570.3 km
3-# level
Tholen et al.AJ 2008
PLU027
50 km
July 2005 June 2008
0.0"
0.1"
-0.1"
0.0"
Pluto offset wrt
DE413 andDE421 ephemerides
right ascension
declination
Pluto offset driftwith time
caramba, encore raté!
Triton, 21 May 2008
La Réunion
La Réunion, Maïdo
Namibia, Hakos/IAS
S
Triton, 21 May 2008
3-# !
predictions 2008-2009 available on
http://calys.obspm.fr/~sicardy
Quaoar, 09 May 2009 (20 mas= 600 km…)
Eris, 18 September 2009 (20 mas= 1400 km…)
Nix, 5 June 2009 (20 mas= 450 km…)
p= 5 µbar
p= 0.01 µbar
R=1150 km
p= 0.01 µbar
R=1150 km
conclusions
sub-mas accuracy on candidate stars allows us to:
! predict events at > 90% level for largest TNO's
! deploy stations at right places with several stations in shadow
! convince large telescopes' TAC to provide time ! access tofainter stars ! increase of observable event number
