General Disclaimer

One or more of the Following Statements may affect this Document

This document has been reproduced from the best copy furnished by the organizational source. It is being released in the interest of making available as much information as possible.

This document may contain data, which exceeds the sheet parameters. It was furnished in this condition by the organizational source and is the best copy available.

This document may contain tone-on-tone or color graphs, charts and/or pictures, which have been reproduced in black and white.

This document is paginated as submitted by the original source.

Portions of this document are not fully legible due to the historical nature of some of the material. However, it is the best reproduction available from the original submission.

Produced by the NASA Center for Aerospace Information (CASI) THE DIAMETER OF 88 THISBE FROM ITS OF SAO 187124

R. L. Millis, L. H. Wasserman, 0. G. Franz, N, M. White, and E. Sowell

Planetary Research Center, Lowell Observatory, Fla g staff, Arizona 86002

A. Klemola

Board of Studies in Astronomy and Astro p h y sics, Lick Observatory, Universit y of California at Santa Cruz, Santa Cruz, California 95064

R. C. Elliott, W. G. Smethells, and P. M. Price

Casey Observatory, Department of Physics and Astronomy, University of

Wisconsin - Eau Claire, Eau Claire, Wisconsin 54701

C. P. McKay and C. Steele

Sommers-Bausch Observatory, De p artment of Astro - Geophysics,

University of Colorado, Boulder, Colorado 80309

E. Everhart and E. M. Everhart

Chamberlin Observatory, Department of Ph y sics, University of Denver,

Denver, Colorado 80208

(NASA-CF-16 c'312) ^' r l' DTA"!F-eG " 99 7H2SB7 NO2-33285 ??0M I7S OCCULTATION CF S I C 1?7124 (Lowell Cbservatory) 4 p HC A 1/ M ° t ^ t CS7L 13F Unclas r,3 /b5 10bs

RECEIVED NASA Sil FACILliY ACCESS DEPT. ,^^' PAGE 2

Abstract

The 7 October 1981 occultation of SAO 187124 by 88

Thisbe was observed at twelve sites. The occultation observations, to g ether with information about the 's 11uht curve, gives a mean diameter for Thisbe of 232 t 10 km. This value is 10 p ercent lar g er than the p reviously published radiometric diameter of Thisbe.

Introduction

On 7 October 1981, the asteroid 88 Thisbe occulted

SAO 187124, a 9th-ma g nitude in Sa g ittarius. This occultation, originally p redicted b y Taylor (1981), #as observed in the United States at twelve sites betNeen

Wisconsin and Utah. In this paper we discuss the observations and derive the diameter of Thisbe.

Observations

Taylor's (1981) p rediction of the 7 October occultation, based solely on the catalo g position of the star and the published e p hemeris of Thisbe, g ave a ground

J PASE 3

track fallin g across western Canada. However, Plates take • i on 27 Se p tember 1981 with the O.S-m Carne g ie Double

Astrograoh on Mt. Hamilton yielded a more southerly track stretchin g from southern California to the Great Lakes

(see Fi g . 1a). An additional p late taken with the same telescooe on 4 October 1981 gave a similar g round track

(Fi g . 1b), as did plates taken with the i.SS-m

Astrometric Reflector at the Fla g staff Station of the U.

S. Naval Observatory on 6 October 1981 (Fi g . ic). The

Plates from Mt. Hamilton were measured relative to a reference frame defined b y chosen from the Perth 70 catalog. The USNO olates were measured relative to a network of fainter stars whose Positions were determined from the Mt. Hamilton p lates. It is evident from ins p ection of Fig. 1 that the three predicted ground tracks a g ree to within aporoximatel y one-half the width of the track. The corres p ondin g uncertainty in the predicted geocentric angular se p aration of the star and asteroid at closest approach is only 0.07 aresec.

At the time of the occultation, Thisbe +vas exoected to have a visual of 11.8, while SAO 187124 is nearl y three ma g nitudes brighter. Consequently, the change in bri g htness of the merged star-asteroid image at

immersion and emersion would be lar g e, makin g this occultation easily detectable y visual observers.

Assuming a diameter for Thisbe of 214 VM ]s given in the

TRIAD file (Rowell, Gehrels, and Zellner 1979), the maximum duration of the occultation was ex p ected to be 9.9 seconds. Thisbe's shadow would move alon g the 2rDund track from west to east.

The occultation of SAO 187124 was observed at th'! first twelve sites listed in Table I. The coordinates and altitudes of the sites are given in columns 3, 4, and 5.

The observed times of immersion and emersion are note9 in columns 6 and 7 along with the observers' estimates of the uncertainty in the absolute timing in column 8. Naves of the observers are given in the footnotes to the taole.

The observations at Circleville, Utah; Otter Creek

Reservoir, Utah; and Eau Claire, Wisconsin, were vade p hotoelectrically. All otters were visual.

Telesco p es used for these observations ran g ed from the 0.6-m reflector at Sommers-Bausch Observatory to relatively small p ortable instruments. The photoelectric PASE 5

observations from Circleville and Otter Creek Reservoir

were made with 0.35-m Celestron telesco p es especially

equi pp ed for use at remote sites. Figure 2 shows a tracing of the occultation li g ht curve recorded with one of these systems near Otter Creek Reservoir. In addition to the real-time stri p chart dis p lay shown in Fi g . 2, the data are recorded on magnetic ta p e for later digitization at high time resolution. The times listed in Table I for

Otter Creek and Circleville were determined from the di g ital data. The p hotoelectric observations at Eau

Claire were also made with a 0.35-m Celestron telescope.

Pulse counting electronics givin g 50 m sec integrations with 0.5 m sec dead time between integrations were used.

Analysis

The basic method of analysis employed in this oaoer has been discussed in detail by Wasserman eS a]. (1979) and Millis and Elliot (1979). In essence, each observed time of immersion or emersion defines a ooint on the limb of the asteroid. The asteroid's aooarent limb profile is then represented by a circle or, if the data warrant, an elliose fitted by least squares to all the observational P1► IP*E 6

points. In this way the effecti ve diameter of the face of the asteroid seen at the time of the occultation can be derived. The occultation result can then be combined with information about the asteroid's bri g htness variation with rotation and as p ect to q ive the best p ossible estimate of

Its overall mean diameter.

we have plotted in Fi q . 3 the chords across Thisbe which one derives if the observed times of immersion and emersion in Table 1 are taken at face value. The solid lines recresent p hotoelectric observations; the visual observations are indicated by dashed lines. Note that all the visuall y determined chords are displaced westward relative to the p hotoelectric chords. (Displacements in this direction imply that the visual observers reported tines which were systematicall y late relative to the photoelectric times, as would be ex p ected it res p onse time corrections were not made or were underestimated.

while the observed times of immersion and emersion re p orted by a visual observer are sublect to sionificant systematic error, the elapsed interval between the two times--that is to say, the duration of the occultation- -is determined much more accuratel y . Therefore, the visual chords in Fi g . 3 are likely to have ver y nearly the correct lengths, but they are dis p laced laterall y (i.e., in time) from their pro per p ositions. Consequently, in our least-s q uares solution for the best-fittin g circular limb p rofile of Thisbe, we have included the response times of individual visual observers as free parameters.

This aooroach has the effect of allowing the visual chords to slide p arallel to the p hotoelectric chords. Other free parameters in the solutions are the asteroid's diameter and corrections to the right ascension and declination of

Thisbe. we have arbitraril y assumed that all positional error is in Thisbe's ephemeris. The resulting solution is illustrated in Fi g . 4. All observations were given equal weight in the solutions. The best-fittin g circular limb profile has a diameter of 221.8 ± 1.4 km, while the resultin q corrections to ri g ht ascension and declination are -0.051 ± 0.00003 sec and -0.799 t 0.0013 aresec, resoectively. The diagonal line across the bottom of the figure indicates the constraint placed on the solution by photoelectric observations at Erwin Fick Observatory near

Boone, Iowa, which showed no occultation (Beavers 1981).

Other neuative photoelectric observations were recorded PA3E 8

somewhat further south of the track at Lowell Observatory and Braeside Observatory in Fla q staff, Arizona, and at the

Mt. Wilson Observatory (see Table I). To our knowledge, no observations, either photoelectric or visual, were lade north of the track.

The res p onse time corrections determined for each of the visual observers from the least-s q uares solution are listed in column 9 of Table I. They range from just over

2 seconds at Chamberlin Observatory, where clock error nay have been a problem, to sli g htl y less than 0.3 sec at

Sommers-Bausch Observatorv, where WWV time signals and the observers' res p onses were recorded simultaneously on an oscillo g raphic recorder. Ignorin g these two extreme cases and removin g the p ersonal e q uation corrections already a p plied to the Cheyenne and Cambrid g e observations, we obtain a mean res p onse time of 1.1 seconds. while. this value is larger than one might have expected, it is less than that determined from visual observations of the occultation of A15+0'1022 by Juno (Millis et al. 1981).

The last two columns of Table I contain the radial resiluals (observed sinus com p uted radius) for the

-- ^L PA32E 9

solution shown in Figure 4. These values yield an rms residual p er degree of freedom of 3.5 km, similar to the results obtained for other well-observed stellar by (Wasserman et al. 1979= Millis et 1. 1981).

The actual occultation track as determined from our analysis is shown in Fi g . 5. Sites where the occultation was successfully observed are marked by filled circles; barred circles denote sites where photoelectric observations show that no occultation occurred. The predicted tracks shown in Fig, i depart from the actual track by no more than one-half the width of the track.

Discussion

Our analysis of the occultation observations has given a value of 221.8 t 1 4 Km for the diameter of that face of Thisbe seen at the time of the occultation. The quoted standard error, however, reflects only the forral uncertainty in fitting a circle through the shifted data p oints. The true uncertainty in the diameter is larder because of the effects of errors in the observed durations M

PAGE 10

and because the true limb p rofile may de p art from a

circle.

The magnitude of the errors in the observed durations

can be evaluated by com p arin g results from observers near

the same chord. For the visual observers one finds in

these instances a peak-to- p eak scatter of about ;L0.25 sec,

giving a mean chord length which 1s accurate to better

than 2 percent. The photoelectric chords have

si g nificantly less uncertainty. Conse q uently, we estimate

that the observational errors can be fully allowed for by

increasin q the quoted formal uncertainty by no more Zhan a

factor of two.

The more serious source of error, and one that is

difficult to evaluate precisel y , is the p ossibility that

the true limb p rofile de p arts markedly from a circle.

Observational coverage of Thisbe's s^uthern hemis p here was

quite limited during the 7 October occultation.

Obviously, the existence of lar g e limb irreaularities in

that re g ion cannot be ruled out. Wo do note that in the

re g ion well covered by the observations (see Fig. 4),

limb irreaularities have scales of onl y a few kilometers PAGE 11

comparable to those seen on other well-observed israe

asteroids (Wasserman !j al. 19791 Millis A! al. 19x1).

Another possioility to be considered is that tle limb p rofile may be elli p tical rather than circular, we nave not p resented a fit of an elli p se to the data because, in our o p inion, the data do not have sufficient covera g e of the asteroid to distinguish reaningfully between an elli p se and a circle. If one does fit an ellipse, however, the observations from.Chevenne and Cambridge near the northern edge of the track and the negative result from Erwin Fick Observator y constrain the effective diameter ab) of the elli p se to fall within 3 percent of the diameter derived from the circular solution.

As has been mentioned earlier, the diameter derived from the occultation pertains only to the f6 -e of Thisbe seen at the time of the occultation. In order to derive an overall mean diameter, one must assess the variation in the asteroid's aonarent cross-section with rotation ..id as p ect by ins p ection of the object's li g ht curve. In the case of Thisbe, the available data are limited. Schooer,

Scaltriti, and Za pplle (1979) from observations on four P45E 12

nights in 1977 found the bri g htness of Thisbe to vary with

a period of 610422 and a full am p litude of 0.19 nag.

Photometry by J. w. Youn g at Table Mountain 0b8,ervatory

on 23 October 1981 is consistent with these values , althou g h a full cycle of the was not covered (Harris 1981). Young's observations, combined with the

period measured by Schober, Scaltritt, and Zappala,

indicate that the 7 October occultation occurred near the

time of minimum li g ht and, therefore, minimum

cross-sectional area. At maximum bri g htness, assurin g a

li g ht curve am p litude of 0.19 maa, one would expect the effective diameter of Thisbe to be a pp roximately 9 oercent

larger than the value determined from the occultation.

The mean diameter will be near the avera g e of these two.

Accordin g ly, we adopt 232 :L 10 km as the occultation diameter of Thisbe where the error has been estimated conservatively to allow for the various sources of uncertaint y discussed above. This value for Thisoe's diameter is 10 p ercent lar g er than the radiovetric diameter q uoted by Morrison and Zellner (1979).

No secondar y occultations attributable to possi5le satellites of Thisbe were re p orted durin g the 7 Dctooer PACE 13

198J event.

We thank the staff of the Fla g staff Station of the U.

S. Naval Observatory for takin g p lates used in refining the predictions for this occultation. The Lick

Observatory astrogra p h plates were taken by E. Harlan.

D. Dunham kindly p rovided an accurate e p hemeris for

Thisbe and transmitted visual timings by several merbers of the International Occultation Timin g Association

(IOTA). We thanK S. Schultz, L. Allred, and J. Fox for permission to use their un p ublished observations. This research was su pp orted at Lowell Observatory by NASA Srant

NSC-7603 and at Lick Observatory by NSF Grant AST

79-09098. The Lowell computin g facility , used in tnis work, was obtained with g enerous g rants from the Digital

Eaui p ment Cor p oration and the National Science Foundation and with further hel p from Mrs. R. L. Putnam, the

Perkin Fund, the National Aeronautics and Space

Administration, and the U. S. Naval Observatory. PANE 14

REFERENCES

ers, W. I. (1991). Private communication.

11, E., Gehrels, T., and Zellner, B. (1979). In

steroid, edited ny T. Gehrels (University of

rizona Press, Tucson), o. 1108.

is, A. w. (1981). Private communication.

is, R. L., and Elliot, J. L. (1979). In Asteroids-,

Jited by T. Sehrels (University of Arizona Press,

Tucson), o. 98.

Millis, R. L., *asserTan, L. H., Bowell, E., Franz,

D. 1., white, N. M., Lockwood, G. W., Nye, R.,

Bertram, R., Klemola, A., Dunham, E., Baron, R. L.,

Elliot, J. L., Harris, A., Youni, J. W., Faulkner,

J., Stanton, R., ReitseTa, H. J., Hunbard, W. B.,

Zellner, B., Lebofsky, L., Cruikshank, D. P., MacKn1K,

L. S., Becklin, F. E., Morrison, D., Lonsdale, C. J.,

Kunkle, T. D., Lee, T., Gatley, I., A'Hearn, M. F.,

DuPuy, D. L., Nolthenius, R., Ford, H., McKenna, D.,

p lacova, Z.., Horne, K., Sandmann, W., Tavlor, G. E.,

and Tucker, R. (1981). Astron. J. 86, 306.

M orrison, D., and 7ellner, B. (1979). In Asteroids_,

edited by T. Sehrels (university of Arizona Press,

Tucson), p. 1090. Schober, H. J., Scaltriti, F., and Za p oala, V. (1979).

Astron. Astro p h y s. Suopl. 36, 1.

Taylor, G. (1981). Handbook of the British Astronomical

Association, p . 26. wasserran, L. H., Millis, R. L., Franz, D. G., Bowell,

E., white, H. M., Gicles, H. L., Martin, L. J.,

Elliot, J. L., nunhan, E., Mink, D., Baron, R.,

Honeycutt, R. K., Henden, A. A., Keohart, J. E.,

A'Hearn, M. F., Reitsera, H. J., Radick, R., and Taylor, G. E. (1979). Astron. J. U, 259.

N C to O to r M MOMM Q1 Npll^d dOd i O N ^(D 04 CD X00 LndN E 1 1 1 1 1+ W Z ORIGINAL PAGE IS ^ E ^ Y OF POOR QUALITY rH v OC tY N MOOMMrwtMr^d ddd i r Or' C 4 N) r 00 LAN N 1 1 1 1 1 1 .aO N IN 1`•1 r W 1-- i-) O 4- Q1 00 N n tD 00 N CO 00 N CO t OOr-O 1 1 ^p0 O I N N 1 1 1 1 1 1 1 1 1 >1 U .a ar d W -4 I N N 41 p1 •rn M n N O O In r- O O CO r r 4J aJ O O O O O ^ .a +^ a! N O dO U C N I C7 O r 4- O d clo %0 4J O COtOLOLOM N to- NMO V U C 4J O C O NU•)tl:n^ b 1010 M MN Lo to O LnNNN0iNLoN LP) LoMLn L (A E o Ln b 0r-N 0 N0 N0 r-0 r-0 ^N0 0 0 O ON O C C C C U E 0000 O r W N N N N N N N N N N N N -^ -^ • •r 04-- 7 4••1 +1 4J .4-) r O U d O Ln t0 to O to U C 01 1^ Ln Ol 00 N 00 M Q1 Lr) +•) +3 +) +J Rf C O O Ndr 00ntD1*ltDt0 N M'-d 7 3 7 7 CT •r N d^ r- ter- - d r d d N d U U U U w i) co s- U 01 NNNr-'rr-N'- r-N^ OO OO ^ a) 0 0 0 0 0 0 0 0 0 C) 0 C) r0 L III G 0000 C L N i ...... NN ==== O O to aJ yf U L Mt0 O aJ ^ 41 "O N 0-0 it v 7 L (DkoCD IM tOON W Lnww NNt-^O O +-) a COOt000Lnc* tDOd Nt0/^ Md00 W + N - - a.•1 01 N N C.1 00 - d M tO I- N tO M I^ N N to n +-) W - N- r N N r- N N rO (T7 r E U t Q ^ C t0 ICON 00 Od C) U-) r _O V b d a aJ d-O 1-t001COM Oct CO CDC)%C)W N O NOd CD qr MdMNLndd O M F^1 7 N N aJ N NMOOt000dwC) Nr^ LO CD U-) •r L •r U QI O C •-- M M Ln r r- N O O Ln CT " C) r Q +^ a! tO 0, aJ Y O +•) CL L.> ZN C Rf -LnddC000ct0 C11d01NdLoLn 'C r d a) J dd d dM M d d d M d m -c*- m MM b 4-^ C Cl t0 d -1- Ln M d O z O N a 4) -•- L O OTOOLnt0 Lor M01 CDWm COMLoko aJ N or u a OdP- OLo 00 a)m 1C OCTdLOd Cif CO C F^ N C LL .L^ m m CD CD d LO O M L(l d d r MO •r U ^? r L O E ` V r 07 C11NN Nm WLOr- Lo- LoN%DtO C N a) > Q C Ln LO r- r N- ON O O O ^ Lo N N a s W O t C N E L.O rt U . J tDkotD%Di`n tDtDn nt0ntDl*lt^^ O L •^ •- L O C3 j.) h^j q^ L •- w +•) r 4J W r d W Q E J cc N^ aiz mY v wN m > LL w Mi co X N • W C9 Q U 31]^^ i +•^ J F-- 000 3 C N^ a) Y Z 3 L O ^ •r '- a C1 M: (a 0 0 W C -:.0 C O 0 C7 •r a) r `r O U CO +) U O .0 O • W +) N ^ aJ ^ O +J b L • L O 0) 0 L ` 1 M . - Ln a) O ?) •- r- aJ •r N U r- O L W a/ z7 w b C-V CL :Z >U O Ln > ^ b LLL^^ U aJ 7 +^ r +) L aJ L •- i> C a) > U C •r (O (z a) +•) L L. C - a) •r •r r r r X W- •r .- O a1 a) ,Q•J r- w to I- O O L Odr L to Cl (uU -D C3 Nr I- b O •^ U •r •r +J C r- > O O L J +J 2: WL+••Ln32:0aNW W WM=U- C) v E c • L +O-) aaii c -0 'rU 3(0 • 3 • M t b •- +-1 •.- -p L M 0c) O "0 L S. v L UUM:CNUOCD7:LN C7 W CJ ui2:mJ O CL '7 1=V) cr- O CD CL' 0: 3 ]L OC R 41O t ^ N fM d to ko r- 01 0 M d Ln tO r- NMdLo to r^00M O CVMctLnto CL r-• ^ r r- r- r r- ZO1 r r- r .-- r r- PAGsE 17

FIGURE CAPTIONS

Figure 1. The predicted ground track of the 7 October

1981 occultation of SAO IB7124 by 88 Thisbe based on (a) plates taken at Lick Observatory on 27 September 1981, (b) a plate taken at Lick on 4 October 1981, and (c) plates taken at the Fla g staff Station of the U. S. Naval

Observatory on 6 October 1981.

Fi g ure 2. The light curve of the 7 October 1981 occultation of SAO 187124 by 88 Thisbe observed near Otter

Creek Reservoir in central Utah.

Figure 3. Chords across Thisbe, uncorrected for the res p onse times of visual observers. Solid lines represent photoelectric observations; dashed lines are chords derived from visual observations. Table I lists the observing sites correspondin g to the chords, beginning with the northernmost chord.

Fi g ure 4. A circular limb p rofile fitted by least squares to the occultation observations. This solution yiells a diameter for the face of Thisbe seen at the time of the occultation of 221.8 + 1.4 km. The res p onse tires of PA4-E 18

visual observers were included as free oarameters in the least-squares solution. The dia g onal line near the bottom of the figure re p resents the constraint placed on the solution by ne g ative observations at Erwin Fick

Observatory.

Figure 5. The actual ground track of the 7 October 1981 occultation as determined from the least-s q uares solution shown in Fi g ure 4. Filled circles denote sites where the occultation was observed. The other symbols indicate sites where photoelectric observations showed no occultation. OR QUALITY

Figure 1. 4

PAC:Z7 IS OF POOR QUALITY

SE7

-^"- - --_ _ - _ - =tea ORIGINAL P^^uTir F POOR

(IMMERSION) N IEMEMS! ONl

O

Y E Z

S

Figure 3. v Lu v+

LA- ORIGINAL PAGE IS OF POOR QUALITY

3

Z

uJ vs W