TARGETED OCCULTATION STUDIES (WITH SOFIA & GROUND BASED

TELESCOPES)

Michael J. Person, MIT

(Based on Poster 39.07 J. L. Elliot & S. D. Kern)

DPS, Kuiper Belt Workshop

2003 September 1

EXAMPLES OF KBOS* WITH ATMOSPHERES

Pluto-Charon images from HST. The bodies are separated by 19,400 km (after Albrecht et al. 1994). The Triton image (left) is from Voyager 2.

Pluto enhanced

• Pluto and Triton– Similar sizes and densities

– N2 (CO, CH4)

• Atmosphere on Charon?

*physical definition

REQUIREMENTS FOR AN ATMOSPHERE

(Data from Brown & Ziegler 1980)

Temperature (K)

KrCH4

N2

Ar

CO

Pre

ssur

e (

µba

r)

0.5 flux dropfor Triton

occultations

30 35 40 45 50 55

10 3

10

1

10 –1

10 2

10 –2

10 –3

•Right temperature

–Not too warm–Volatiles escape

–Not too cold–Not enough sublimation

–Blackbody temperature for 30AU is about 50K

•Sufficiently massive–How massive?

= (GM/k)(mµ/rT)–Mass overcomes escape (e.g. Titan)

–Kr atmosphere on less massive objects?

•Appropriate surface volatiles–N2, CO, CH4, NH3, Ne, Ar, Kr

–Primordial, or replenished

OBJECTIVES FOR KBO OCCULTATIONS

• Accurate diameters– Calibrate thermal diameters

• SIRTF

• Sub-millimeter

• Search for atmospheres– The largest KBOs may retain atmospheres

• Search for binary companions– Models differ on the semi-major axis

distribution for KBO binaries

PROSPECTS FOR ATMOSPHERES

Quoar (0.10)

Varuna (0.07)

Charon (0.40)

Pluto (0.53-0.74)

Triton (0.75)0.04

0.64

Geometric Albedo (V)

Ixion N2 Atmospheres

Atmosphere?

log

radi

us (

km)

8 6 4 2 0 –2H magnitude

CHARON OCCULTATION (Walker 1980)

(after Elliot & Young 1991)

CHIRON OCCULTATION RESULTS

Adapted from Elliot et. al. 1995 Nature

TARGETED OCCULTATIONS BY KBOs*

• Charon– 1980 April 7—South Africa

• Single chord (Walker at SAAO)

• Pluto– 1988 June 9 (P8)-southwestern Pacific

• KAO and several ground-based sites, 7 successful

– 2002 July 20 (P126A)-South America• Many attempts, only 2 successful

– 2002 August 21 (P131.1) - Hawaii and western US

• 9 telescopes successful

• (2060) Chiron– 1993 November 7 (Ch02) - western US

• Two telescopes successful

– 1994 March 9 (Ch08) - Brazil• KAO successful

• Triton– 1993 July 10 (Tr60)

• one chord with KAO; 4 ground-based stations, none successful

– 1995 August 9 (Tr148A, Tr148B)• 9 light curves from 6 sites,

including the KAO; central flash from IRTF

– 1997 July 18 (Tr176)• 2 chords with C14's; amateur

chords too

– 1997 November 4 (Tr180)• central flash with FGS on HST; 1

ground-based site miss; 2 ground-based sites cloudy; Lear jet had problems

– 2001 August 25 (Tr231)• light curve with Sutherland 1.9-m

*physical definition

CHALLENGES FOR OCCULTATIONSBody Mag

(R)Da(AU) Radius

(km)Radius(arcsec)

Occ.Obs.

Triton 13.4 30.1 1438 ± 17c 0.067 5

Pluto 14.0 30.6 1214 ± 20d 0.055 4

Charon 16.0 30.6 621 ± 21e 0.028 1

(2060) Chiron 17.1 12.1 90 ± 7f 0.010 2

(5145) Pholus 19.2 17.9 95 ± 13f 0.007 0

(10199) Chariklo 17.5 13.1 137 ± 15f 0.014 0

(20000) Varuna 19.8 43.2 450 ± 65g 0.014 0

(28978) Ixion 19.1 42.9 ~600 0.019 0

(50000) Quaoar 18.6 43.4 610 ± 65h 0.019 0

Canonical KBO 20.5 40.0 300 0.010 0

•aheliocentric distance as of 2003 May 21

• Strategy #1: Small telescopes (0.36 m; fixed or mobile?) – Limiting stellar magnitude 16.2– Set up network in western US (or Chile & Australia)– At least two occultation chords needed (m too large for

astrometry?)• Strategy #2: Chase with SOFIA (2.7 m)

– Limiting stellar magnitude 18.4– Need accurate prediction (miss is expensive!)– Second occultation chord or astrometry (from another telescope)

• Strategy #3: Large telescope (6.5->10 m ) – Limiting stellar magnitude 19.3– Initial prediction accurate to 0.025 arcsec (=> 1/5 success rate)– Single occultation chord & astrometry with the same telescope

KBO RADII FROM OCCULTATIONS

EVENTS PER YEAR

Telescope Strategy

(aperture, m)

Limiting Stellar R

Mag

Events per Year per KBO, R

Location Factor,

fl

Weather Factor,

fw

Combined Factor,

Observable Events per Yearb, Nobs

Portable (0.36)

16.2 1 0.17 0.75 0.13 ~6

Airborne (2.5)

18.4 12 0.58 0.95 0.55 ~200

Fixed (6.5)

19.3 30 0.0067 0.60 0.0040 ~4

bfor a single telescope and the current sample of 29 KBOs brighter than an H magnitude of 5.2.

KBO OCCULTATIONS

• Objectives– Establish accurate diameters– Search for close companions– Search for atmospheres

• Approach– Brightest KBOs as targets– Three strategies

(portable, airborne, large fixed)• Needs

– Discovery of more bright KBO’s in dense star fields• Star density in the galactic plane

~ 10 times the average• More events per good orbit

– Improved orbits, high astrometric precision• 17 orbits better than 0.1"; 79 better than 1"• How much large telescope time to use for predictions vs. observations?

Ch08 prediction, 1994 March 9: observations made with the KAO&

from South Africa (SAAO)