Probing remote solar system bodies with stellar occultations · Universit Pierre et Marie Curie...
Transcript of Probing remote solar system bodies with stellar occultations · Universit Pierre et Marie Curie...
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
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)
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
0.89"
2UCAC 26257135(D. Herald, Aug. 2004)
the Charon stellar occultation
of 11 July 2005
110 mas
55 mas
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
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
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")
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