Physical studies of Apollo-Amor asteroids: UBVRI photometry of 1036 Ganymed and 1627 Ivar

ICARUS78, 363-381 (1989)

Physical Studies of Apollo-Amor Asteroids: UBVRI Photometry of 1036 Ganyrned and 1627 Ivar 1 G. HAHN AND P. MAGNUSSON Astronomiska Observatoriet, Box 515, S-751 20 Uppsala, Sweden

A. W. HARRIS, J. W. YOUNG, L. A. BELKORA, AND N. J. FICO Jet Propulsion Laboratory, 4800 Oak Grooe Drive, Pasadena, California 91109

D. F. LUPISHKO, V. G. SHEVCHENKO, AND F. P. VELICHKO Astronomical Observatory, Kharkoo University, Sumskaya str. 35, Kharkoo, 310022 USSR

R. BURCHI AND G. CIUNCI Laboratorio di Scienze Planetarie, Osseroatorio Astronomico Collurania, 1-64100 Teramo, Italy

M. DI MARTINO Osseroatorio Astronomico di Torino, 1-10025 Pino Torinese, Italy

AND

H. DEBEHOGNE Observatoire Royal de Belgique, Avenue Circulaire 3, B-1180 Bruxelles, Belgium

Received April 7, 1987; revised December 12, 1988 Photoelectric lightcurves of the A m o r asteroids 1036 Ganymed and 1627 lvar were obtained during J u n e - D e c e m b e r 1985. For Ganymed a drastic change in the shape of the lightcurve was found together with a significant increase of the synodic rotation period. This indicates a large change in the viewing conditions during the apparition and a complex interrelationship between these changes and the probably very irregular shape of the asteroid. The mean synodic period is 10.3 hr and the sense of rotation is retrograde. For lvar, a decrease of the synodic period (4.8 hr) is found, yielding a prograde sense of rotation. The phase curves for both asteroids show strong deviations from the H - G magnitude system phase function at large phase angles. Because of this fact and the lack of observations at phase angles < 1 7 ~ the determined absolute magnitude H ( 0 °) and the slope parameter G are very uncertain. For G a n y m e d H = 9.50 and G = 0.33, and for lvar H = 13.24 and G = 0.65. Pole solutions were tried for both asteroids, yielding unreliable results for Ganymed, while for lvar two independent solutions are in good mutual agreement. We emphasize the opportunity to obtain additional observations to clarify the complex nature of both asteroids during the 1989 (Ganymed) and the 1990 (Ivar) apparitions. The U B V R I color indices found are typical for S asteroids. © 19s9 Academic Press, Inc.

I. INTRODUCTION

The orbits of the two Amor-type asteroids 1036 Ganymed and 1627 Ivar are charPartly based on observations made at the European Southern Observatory, Chile.

acterized by mean-motion resonances with the terrestrial planets and their visibility from the Earth is governed by periodically repeating close approaches (Ip and Mehra 1973, Janiczek et al. 1972). 1036 Ganymed, being near the 3:13 reso363 0019-1035/89 $3.00 Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

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HAHN ET AL.

nance, comes rather close to the Earth approximately every 13 years, but not much closer than 0.35 AU during this century. Due to its large size it often becomes brighter than V - 13 mag. The 1985 apnarition was one of the more favorable, yielding the first opportunity for extensive photometric studies. Ganymed made its closest approach in October to within less than 0.65 AU, and was favorably placed for observing from June until the beginning of 1986. Its opposition brightness reached V 10.5 mag. Ganymed will not be that bright again until 1998, but already in 1989 there is another observing possibility, when the opposition brightness will reach V - 13 (Hahn 1986). Our knowledge of the physical properties of Ganymed is still rather poor, although Ganymed is the largest Apollo-Amor asteroid (AAA), almost twice the size of the much more famous and well-observed 433 Eros, the second largest member. It is classified as an S-type asteroid in the TRIAD file (BoweU et al. 1979) with an estimated diameter of 40 km. According to Tholen's classification it is an S-type asteroid, with an albedo of 0.151 and a diameter of 41 km (Tholen 1984). Results from IRAS show similar values: 0.17 -+ 0.02 and 41.0 -+ 2.3 km, respectively (Matson 1986). According to the classification system developed by Barucci et al. (1987), which is based on a G-mode analysis, Ganymed is an SO type. Spectrophotometric data by Chapman and Gaffey (1979) and its analysis by McFadden et al. (1984) allowed only marginally significant mineralogical interpretation, indicating the presence of silicates on the surface of Ganymed. Five IDS spectra (38008300 ]~) taken during 3 nights in November 1985 (G. Hahn, unpublished) at different rotational phases show good mutual agreement and give no indications of albedo variation with rotation. A comparison between the eight-color photometry data (Zellner and Tholen 1985), converted to relative reflectances, the reflectance spectra by Chap-

man and Gaffey (1979), and these IDS spectra yield similar results. Preliminary rotational data by Harris and Young (1986) indicated a rotation period of about 12 hr and an amplitude of 0.3 mag. An analysis of the lightcurves obtained at the Kharkov University Observatory alone has been published in Lupishko et al. (1987), yielding a synodic rotation period Psyn = 10.3082 +- 0.0010 hr and a prograde sense of rotation. 1627 I v a r is in the 11:28 resonance with the Earth, which leads to close approaches every 28 years. There are four such events during this century and they have been used rather efficiently to discover (1929) and recover (1957) this asteroid. The orbit of Ivar is also close to the 2:5 resonance with the Earth, so relatively close encounters do occur with 5-year intervals. The next and last favorable observing opportunity this century will be in 1990 (Hahn 1986). During the 1985 apparition, Ivar became bright enough for extensive photometric observations. It came as close as 0.2 AU in July, when it reached its greatest brightness of V - 12 mag. It was classified as an Stype asteroid by Tholen (1984) and Bowell et al. (1979) with an estimated diameter of about 7 km. According to McFadden (1983) the existing spectrophotometric data suggest "something unusual, but consistent with the high albedo of 0.23." There exist also six IDS spectra (G. Hahn, unpublished) from 2 nights in September 1985 which show no rotational variation, but they are significantly different from the reflectance spectra obtained by McFadden (1983) or those derived from Zellner and Tholen (1985). Preliminary photometric data by Harris and Young (1986) indicate a rotation period of 4.8 hr and an amplitude of 0.35 mag. Radar observations by Ostro et al. (1986) suggest an aspect for the 1985 apparition far from equatorial. This is consistent with the lower amplitude observed by Harris and Young (1986) compared to the large ampli-

PHOTOMETRY OF APOLLO-AMOR ASTEROIDS tude of >0.6 mag found in 1975 by Zellner et al. (1977). Lupishko et al. (1986) found a synodic rotation period e s y n = 4.7979 -+ 0.0001 hr and a prograde sense of rotation, based on the lightcurves from Kharkov University Observatory only. OBSERVATIONS

Photoelectric UBV(RI) measurements of the light variations of 1036 Ganymed and 1627 Ivar obtained at six observatories during a time span from June to December 1985 are combined. The details of the observations are given in Table I for Ganymed and in Table II for Ivar. All individual observations are listed, yielding a total of 34 nights for Ganymed and 25 for Ivar. The first two columns of each table contain the date and the reference time (UT) for which the following aspect data were calculated. This time is taken approximately at the middle of the range observed. The topocentric right ascension (RA) and declination (Dec) are given, followed by the ecliptic longitude and latitude of the phase angle bisector (PAB) (equinox and ecliptic of 1950.0). The next two columns give the heliocentric (r) and geocentric (A) distances, in AU, followed by the solar phase angle (a). The reduced mean V magnitudes H ( a ) (see paragraph on phase relation for the definition) are given next. The column headed " A m p l " gives the observed amplitude of the lightcurve, or part of it, during the night. The next four columns contain information about the comparison stars used and the quality of night. The identifications are given either as catalogue numbers (SAO, HD, BD) or, for uncatalogued stars, in the form of their compressed 1950 RA and Dec. The V magnitudes are given together with their standard deviation (s). The photometric system used, and finally, the observatory, the aperture of the telescope used, and the name(s) of the observer(s) are given. In Fig. 1, the motion of the phase angle bisector is plotted for both asteroids. The

365

crosses mark the reference dates of the composite lightcurves. We have no observations of Ganymed near to October 22 for which Hoffmann (1986) reports a nearly constant brightness, but both of the curves before and after this date show nonzero amplitudes. It should be mentioned that we know of one additional lightcurve of Ganymed, and several of Ivar, obtained by K. W. Zeigler (private communication). We have chosen not to include them in this analysis, because they cover times near dates of other lightcurves included in this analysis, and are in general of lesser quality due to the fact that a smaller telescope was used. Thus, they would not contribute to an improvement of the analysis presented here. RESULTS

Most of the lightcurve data are in the standard UBV system and therefore directly comparable. From some observations only relative photometry was obtained (Sep 23 and Oct 18). Corrections for light time have been applied. The observed V magnitudes have been converted to reduced magnitudes by subtracting 5 log(rA). The changes of the brightness within one rotation cycle, due to the changing phase angle and the changing distance to the Earth and the Sun, which has to be compensated for during very close encounters (Harris et al. 1987), were negligible for Ganymed and Ivar. In the plots the notation of the new photometric system for asteroid magnitudes (Bowell et al. 1988) has been used quoting the reduced V magnitude at phase angle ot as H(a). 1036 G A N Y M E D

Prior to this analysis of all available lightcurves, attempts to interpret the lightcurves covering shorter periods of time resulted in quite different values for the synodic rotation period, and very differentlooking composite lightcurves. It soon became apparent that this must be due to changes in the form of the lightcurves, as

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well as slight variations in the synodic period of rotation due to the changing aspect and a variable rate of change of the aspect over the course of the apparition. By considering all available data, we were able to reach a consistent solution for the rotation period, and to trace the change in the form of the lightcurve from month to month. Indeed, we see the lightcurve change from a fairly normal double periodic form to an unusual triply periodic form later in the apparition.

Synodic Rotation Period and Composite Lightcurves It was quite clear prior to this investigation that the synodic rotation period is about 10.3 hr, but a single value could not be fitted to all lightcurves. Therefore, composites were made joining contiguous pairs of months, e.g., June-July, July-August, August-September. The synodic period giving the best fit was determined partly

with the Fourier analysis algorithm by Deeming (1975), and partly by manual matching. The resulting periods are given in Table III, together with their expected errors. These errors were obtained from the departures from the 4th-order Fourier fit (solid curve in the composite lightcurves), with a resolution in rotational phase of 0.05 revolution, i.e., approximately representing the 5th- to 15th-order Fourier terms.

T A B L E II1 SYNODIC ROTATION PERIODS FOR 1036 GANYMED Date 1985

Psyn (hr)

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Due to a rapid change of lightcurve shape, especially between September and October, the fits were not ideal but do yield synodic periods much more accurate than could be derived from the single+month composites. We assumed these values for the periods at the mean time between adjacent months of data, and interpolated linearly in between. In Figs. 2-7 the composite lightcurves for each monthly period of observation are presented. A synodic rota-

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the lightcurve in Figs. 2-7. There is a large decrease of the amplitude between August and October, followed by a slight increase in amplitude until December. While there are two maxima and minima from June to September, three maxima and minima appear rather abruptly in October and remain in the following 2 months. These are strong indications of a significant change in the viewing conditions, coupled with an irregular shape. Similar effects have previously been detected for mainbelt asteroids (Zappal~t e t al. 1983, 1984). We can also note that the one lightcurve

from Zeigler (private communication) is entirely concordant with our analysis. Phase Relation

For the determination of the phase function we used the "reduced mean magnitudes" for each night. These data have been calculated by substracting the vertical shifts of the fitting procedure from the zeroorder Fourier coefficient of the composite lightcurve. In Table I these reduced mean V magnitudes H(ct) are tabulated together with the phase angle. Figure 8 shows the phase curve for Ganymed, which exhibits

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r e m a r k a b l e features. N o t e that the absolute brightness is larger in J u n e - J u l y than in Aug u s t - S e p t e m b e r , indicating a m o r e polar aspect in the f o r m e r case, if we assume constant albedo and rotation around a stable axis of m a x i m u m m o m e n t of inertia. This conclusion is strengthened by a larger amplitude in August than in July. The S e p t e m b e r to D e c e m b e r data have been fitted to the H - G magnitude system phase relation (Bowell et al. 1988), as shown by the drawn line, yielding an abso-

lute m e a n magnitude H - 9.50 -+ .01 and a slope p a r a m e t e r G = 0.33 --+ .02. The value of H is of course poorly determined because o f the a b s e n c e of m e a s u r e m e n t s at small p h a s e angles. T h e s e values can, however, be c o m p a r e d with the data from Tedesco (1986), H = 9.42, G = 0.31, in good a g r e e m e n t with our results. A least-squares fit to the same data determined the linear phase coefficient/3 = 0.025 --- .001 and the V(1, 0) = 9.77 -+ .02 quite well. Transforming these values into H and G, according to

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FIG. 6. S a m e as Fig. 2 but for N o v e m b e r 1985.

formula (13) in Bowell et al. (1988), give 9.65 and 0.37, respectively, consistent with the above values. Nevertheless, it seems that the real magnitude-phase relation of Ganymed is much more complicated and further observations, especially at low phase angles, are needed to solve this problem. The f o r m a l uncertainties in our determined H and G values are based on a preliminary analysis (Bowell et al. 1988) that may give unrealistically small values (E. Bowell, private communication). Sense of Rotation

It is apparent from Fig. 1 that dhPhB/dt

decreased during the 5 months of observations. Table III shows a clear increase of the synodic rotation period during the same interval. These two facts indicate that Ganymed rotates in a r e t r o g r a d e s e n s e , in contradiction to the prograde solution found by Lupishko et al. (1987). Color Indices

Mean color indices from the ESO data yielded U - B -- 0.40, B - V = 0.85, V - R = 0.48, and R - I -- 0.38, with mean errors of about 0.01 mag in all color indices. No variations with rotation or changes with phase angle have been found. Lupishko et al.

374

HAHN ET AL.

H(a)

•

I

1036

'

I

I

I

f

'

r--

I

GANYMED

p

9.9

1

''

I

1031000 hours

=

1021,~ 0.017 -0.034 -0039 -0.005 0 015 0.005 0.010 0.004

0

l 2 3 4

10.0

10.1

'

I

,o2

"

.

10.3

10.4

105

....

O0

I

i

0.I

I

,

Rotational

I

,

J

J

,

03

0.4

Phase

(zero

02

,

0.5 Phase

I

06

~

I

t

07

is at UT(t.c.)

P -

10.310000

I

,

09

1

1985

ShLft~clt

-¢ P,

0.000 ~,og.

hours

6 Dee.

[]

8 Dec.

lgOS

Sh~feed."

-O P ,

÷0,014 moO.

[]

8 Dec.

1985

ShLYted#

"9 P .

¢'O. Ol,f e,o9.

+

2~q Nov.

190'5

5hi.ft.:

+1"1 P,

- 0 , 0 1 0 "~1.

Z~

JO Nov.

198S

5h~f~edt

+12 P,

~0.010 ~o 9_

A

JO Nov.

1985

ShLfted'.

+11 P, 40.0119 ~off.

I Dee.

I905

Shtft~:

*9 P .

+0.007 mo 9.

2

19~5

5hL£~ed:

+7 P,

40,004

0~c.

,

0 ~ 0 m o n 5 Dec, 1 9 8 5 )

•

IB

l

0.8

Moo .

FIG. 7. Same as Fi=. g. 2 but for D e c e m b e r 1985.

(1988) obtained U-B = 0.45 - 0.03 and B - V = 0.84 --+ 0.01 on N o v e m b e r 10 at Oh UT. 1627 I V A R

A similar approach as in the analysis of the G a n y m e d lightcurves has also been applied to the Ivar data. Although no such drastic changes in the shape of the lightcurves has been observed, the large amount of data, covering a period of almost 5 months, allowed the determination of the change o f the synodic rotation period, and

thereby a pole solution and the sense of rotation. Synodic Rotation Period and Composite Lighteurves

A synodic rotation period of about 4.8 hr resulted from a first analysis of the lightcurves. Using the same procedure as above, bimonthly overlapping composite lightcurves were made. A monotonic decrease of the period was found, as listed in Table IV. An interpolated value for the syn-

PHOTOMETRY OF APOLLO-AMOR ASTEROIDS i

'

I

'

I

'

I

'

I

9. 40

G = 0.33 I ~

H

= 9. 50

375

'

I

0

IMO

~

KHI:I

[]

ESO

o

Kv[

'

,o.

10.60

_ o~'-~

Oct- ~

•

,o. 1 8 1 v ~ .; , . o o

joo

. so,,

J~l

(O 32)

3

11. 40 0

10

20

30

Phase

An91e

40

I

50

{dog)

FIG. 8. Phase curve for 1036 Ganymed. The symbols identify the different observatories (see Table II). The lightcurve amplitude in magnitudes is given in parenthesis for each month. The filled symbols indicate points not used in the determination of the Bowell-Harris-Lumme phase function, which is shown as a drawn line.

odic period for each o b s e r v a t i o n interval was used to p r o d u c e the c o m p o s i t e s s h o w n in Figs. 9-13. A steady increase of the amplitude f r o m June to O c t o b e r can be noted. P h a s e Relation The calculated reduced m e a n V magnitudes are tabulated in Table II and Fig. 14 shows the corresponding p h a s e curve. The data were fitted to the H - G function, but TABLE IV SYNODIC ROTATION PERIODS FOR 1627 IVAR Date 1985

Psyn (hr)

Psyn

Jun-Jul Jul-Aug Aug-Sep Sep-Oct

4.7996 4.7971 4.7961 4.7957

0.0001 0.0001 0.0001 0.0001

only those points m a r k e d with open symbols w e r e used. T h e r e is a trend similar to that of G a n y m e d for tx < 50 ° which is obviously deviating f r o m the H - G function. One T M O point at - 2 0 ° was also excluded b e c a u s e of its large deviation f r o m the other points. The fit resulted in an absolute V m e a n magnitude at zero p h a s e angle H = 13.24 +- .01 and a slope p a r a m e t e r G = 0.65 + .04. F o r a determination of the linear p h a s e relation data points up to ct = 40 ° w e r e used, resulting in fl = 0.022 -+ .001 and V(1, 0) = 13.29 -+ .04. T h e s e values c o r r e s p o n d to G = 0.49 and H = 13.17 according to f o r m u l a (12) in Bowell et al. (1988). Also for I v a r , the lack of data at low phase angles m a k e s the determined absolute magnitudes uncertain. Color Indices M e a n color indices f r o m all available observations h a v e been calculated f r o m the

376

HAHN ET AL. H(a)

'

~

'

I

r"

I"

~

I

"'1

''

1

I

1627 I V A R

13.7

'

P =

13.,9

480010

i

hours

14299

O 1 2 3 ,~

13.8

"'~ - ' ~

i

-0.125

-0.027 -0069

0 007 O 007

-0 003 0.004

0.028

I4.0 !4.1 14.2 14.3 14.4 ~tJ@

14.5 14.G

,

O0

I

01

J

•

,

I

0.2

Rotational

~

03

Phase

,I,

L

0.4

(zero P-

I

t

05 Phase

J

,

I

O.G

,

0 7

is at UT(I,c.)

•

0.8

3~0 m on

10 June

}90F:~

5hL£tede

"9 P ,

-0.003

~¢~3-

rq

It

1985

Shbfcod'.

+~ P ,

"0.005

mog.

"~

2~ June I9~5

Shl.ft.edJl

0 P,

-'{-

t

0.9 12 J u n e

1.0 1985)

4.800100 bourn

•

June

~

0.000 mo~l.

12 June I905

5hLft;ed;

-t P,

I5 June 1905

~hL?tec];

-S

A

13 Juno 198'5

5hLftod:

-6 P ,

91

I4 Juna I905

ShLfted;

-10

¢t

I4 J~n~ 1985

5hLfted:

-11 P ,

-0.020

I9 Jun~ 190¢5

ShLl'ced:

-38 P,

+0.023 mog,

P,

O. O00 mo(j. 101010 ] ~ " -0.010

twog.

P, --0.020 m ~ . ~o9.

FIG. 9, Composite lightcurve for 1627 Ivar for June 1985 (as for Ganymed).

mean of each night, U-B = 0.46, B - V = 0.89, V-R = 0.48, and R - I = 0.37, with mean errors of about 0.01 mag. No significant color variations with rotation or phase angle could be detected. Pole Position and S e n s e o f Rotation

From Fig. 1 we observe a negative second-order time derivative also for Ivar's PAB longitude, but the synodic period was

decreasing (Table IV), indicating a prograde sense o f rotation in agreement with the result by Lupishko et al. (1986). Application of the pole determination method by Magnusson (1986) to the synodic periods in Table IV and the second harmonic amplitudes o f the composite lightcurves gives the pole solution PI = (110 °, +20 °) and P2 -(320 °, +40°). Reduced X2 values of 5.0 and !.4 have been obtained for PI and P2, re-

PHOTOMETRY OF APOLLO-AMOR ASTEROIDS •

l

'

I

I

I

I

I

I

1627 IVAR

13.6

•

I ....

P =

4.79880

0

14.287

1 2 3 4

0.003 -0.076 0006 -0.001

13.7

13.8

377 •

hour's

0.030 0.113 -0.029 0.009

13.9 14.0 14.1

/

14.2, 14.3 14.4 14.5

I~_

0.0

l

0.1

,

•

,

I

0.2

03

RotatioT~al Phase

J

.... I

I

0.4

I

0.5

I

I

,

0.6

1

_,

02'

I

0.8

0.9

1.0

(Zero Phase is at UT(l.c.) 6^0 ~t on 12 July 1985) p -

,l. 798800 hours

•

12 J v t , d

1985

Sh~.ft.~ls

0 P,

0.000

t~ocj.

•

12

Jr/. 3 1005

Sh~,ft.~d:

-1 P ,

0.000

,~og.

r'l

14 JvL~ 198~

Sfikt'tede

-13 F,

-0.027

teo9 .

15 Ju~3 1905

She.fred:

-18

P, - 0 . 0 1 3

ling.

, ~ J~t~ 1905

ShL£t.edl

-TB P, -0.021 •ocj.

"+"

i

FIG. 10. Same as Fig. 9 but for July 1985.

spectively, making the P2 solution significantly more likely. The main source of error taken into account is the spread of the nights for each monthly average. Departures from the assumptions of the model used probably also contribute to the errors. The method by Velichko and Lupishko (private communication) leads to a pole Pj = (150°, +10 °) and P2 = (330°, +20°), also favoring the P2 solution. The differences between the two results give a clear indication that model errors of order 20° must be expected. The corresponding sidereal periods of rotation are found to be 4.7989 and

4.7952 hr for the two methods, respectively. CONCLUSIONS

Despite the large amount of observations spread over a long interval, the complex nature of both asteroids could not be revealed more than tentatively. Although a wide range in the phase angle was covered, the change of the aspect angle, which is obvious but of uncertain magnitude, does not allow an accurate determination of the phase curve, especially not for Ganymed, because it is difficult to separate the effects

378

HAHN ET AL. I

137

1627

I

t

I

l

r-'

I

'1

IVAR

}"

P

=

14.f88 -0.012 0.012 -0120 -0.207

1 2 3

13.9 14.0

xt

~

•

14.1

'

hours

4.797300

0

13.8

f"

0.017

O.010

-0.018

0.017

•

0@

$I

14.2 14.3 14.4 14.5 ,

14.6

00

I

Oi

,

t

,

0_2

I

~

I

0 3

ROtCLtl,0"fl, Cl,l Pttt~se

,

0_4

.I

r

0.5

I

J

I

0.6

,

0 7

}

0.8

,

l

09

10

(Zero Ph,a.se ~,s at UT(t.c.) oner~ on 1,l A~g. 1985)

P-

4.797?g~ hovr~

•

31 Rug.

19813

SALfL~r

-05 P,

•

51 Aug.

1905

Sht,ftad:

-~

•

3l

5hLktadg

-B7 P,

+0.183 ~a~j.

Ruff. I g ~

-~O.JO3 •o 9.

P, +0.103 m~g.

r-1

I

5op. 1985

5h&ted:

-90 P,

+0.173 ~og.

[]

I

Sap.

1985

SkLftedl

-91 P ,

+0.173 ~ocj.

-~-

13 Ruff. 1985

ShLfted:

+~ P,

+0.003 mix3.

4-

l,'f A~tg. 1985

5htft~l"

+3 P,

÷0.003 moO.

/x

I4 Rug.

1985

ShLft~d:

-l P,

0 . 0 0 0 ~t~j.

A

I4 /:tug, 1905

Sht,ftad:

-2, P,

~t

23 Rug,

Ig~

ShLftadc

-~6 P ,

"~0.111

mug.

~t

23 Rug.

1985

Sh~fted:

-'~7 P,

+0.1II

~ocj,

0.000 ~og,

1

9 Ruff. 19~5

ShLftedz

"21 P ,

-0.~I

moo .

i

9

1~

ShLf't~d:

+20 P ,

-0,061

ett~.

I985

Shtftsd:

-¢ P,

-0.041 meg.

Ruff.

l~ Rug,

FIG. 1 I. Same as Fig. 9 but for A u g u s t 1985.

of the changing phase angle from those of changing aspect. The sense of rotation could be derived unambiguously and it is also clear that Ganymed must be rather Jr-

regular, as judged from the shape of the lightcurve. The derived slope parameters for both asteroids turned out to be significantly higher than for typical S-type aster-

PHOTOMETRY OF APOLLO-AMOR ASTEROIDS

H(a)

,

i

(

'

t

1

r

•

i

V

1627 I V A R

13.4

'

379

i

-

1

v = 4.795900 ho~trs 13891 0.010 - 0 . 0 3 2 -0.229 -0.133 -0.003 0.007

o

1 2 3

13.5 13.6

,

o

+

o

13.7 13.8 13.9 14.0 14.1 14.2 ,

14.3

0.0

1

,

I

0.1

,

I

0.2

Rotc~t£on~L

J

0.3

Ph(~se

I

J

0.4

I

,

I

0.5

0.6

,

l

,

07

t. 795900

14 5ep.

Sh;.f~r

*10 P, +0.072 ~og.

•

I'! ~,ep. 1 0 0 5

Sh/,t'tod."

+g P ,

r-I

16 5ep. 19~:J

ShLftedi

0 P,

+O. IT¢~ moej.

I-I

16 ~ p .

1965

5h~,fced'.

-1 P,

+0.066 mog,

4-

18 Sep.

1965

ShLf'todl

-10 P,

+0.161 ,ao9.

-I-

18 5~o. 19~3

ShL£te#'.

-11 P, +0.161 n ~ .

27 . ~ .

1985

Shiftad~

-SS P, +0.1~' ~ .

ShLfted:

-SB Pp +O.J~ mog.

29 Sep, 1985

Sht,tted:

-65 P, +0.2.0,1 t~g.

29 5ep. 1985

ShLftndl

-~

111

21 5ep. 1965

5hLFted:

-25 P,

0.000 ~Jg,

•

21 5ep. 1985

Sh~ttodl

-26 P,

O.O00 ~o9.

II

2l

ShLFted:

-27 P,

0.000 mo9.

1985

,

1.0

Sep. t985)

+0.072 am9,

27 S~p. 1985

1905

I

09

l'~our,~

•

5~o.

,

( Z e r o P~,¢se ~s a t UT(t.c.)2~0 " on 16

P -

Z~

I

0.8

P,

+0.204

~mcj.

FIG. 12. S a m e as Fig. 9 b u t for S e p t e m b e r 1985.

oids (G = 0.18 - 0.02, Lagerkvist and Williams 1987) but the UBVRI color indices found are quite typical for S asteroids. Further observations, in particular at lower phase angles, already during the May 1989 (a - 6°) apparition of Ganymed may give the necessary data to establish a better phase relation and a pole solution, because

the direction of the PAB will be quite different from that in 1985. For Ivar the opposition in April 1990 will only allow observations to phase angles down to - 1 4 °, but the PAB range will be different from the 1985 apparition, providing a good chance to improve the position of the pole.

H(a)

r

I

''

"

]

I

'

I

I

I

1627 IVAR

13.3

13.5

-~ "h

~

\

~,

'

r

v = 479460 hours

13.4 ;/~,,~

~

~

~

0 13."713 , 0.015 - 0 . 0 , 5 2 -0.007 -0.294 3 0.009-0.0,3 4 - 0 . 0 3 8 0.004

13.6 13.7 13.8 13.9 14.0 14.1 J

f4.2

I

0.0

f

0.1

I

,

I

0.2

~

I

03

t

0.4

Rotational Phase

I

,

I

05

L.

0.6

I,

I

j

0 7

I

J

0.8

09

1.0

(Zero P~a.se i s a t UT(l,c.) lhOTM on 16 Oct. f 9 8 5 )

P-

4.79.tt600 bores

16 Oct. 1gO5

5hlf~edr

0 P,

0.000 e~g.

O

16 Oct. 1905

5hi,feed:

-1 P,

0.000 meg.

[]

I~ 0c~. 1985

Shl£~edt

-10

Y, +0.010 ~og.

FIG. 13. Same as Fig. 9 but for October 1985. I

13.20

l

'

I

I

G = O. G5

o

TMO

13. 2 4

"

KHIZl

c~

ESO

~

D ,.a

'

13. GO E

"0 ~J "0 14. O0 C o

| 14. 40 tO. ¢'r "r

14. 80

0

,

I

I0

~

E

20

,

E

30 Phase A n g l e

,

I

40

,

I

50

,

{deg}

FIG. 14. Phase curve for 1627 Ivar. Symbols identify different observatories. Filled points were not used in the determination of the B o w e l l - H a r r i s - L u m m e phase function, which is shown as a drawn line. 380

PHOTOMETRY

OF APOLLO-AMOR

ACKNOWLEDGMENTS G.H. and P.M. thank Prof. T. Oja for his help with the observations at Kvistaberg and with the reduction of the measurements. The work by A.H. and J.Y. at JPL was supported under contract from NASA. D.F.L., V.G.S., and F.P.V. thank V. Kazakov for his help with the observations at Kharkov Observatory. We are grateful to the referee, Dr. E. Bowell, for his valuable comments and improvements of the manuscript.

REFERENCES BARUCCI, M. A., M. T. CAPRIA, A. CORADIN1, AND M. FULCHIGNONI 1987. Classification of asteroids using G-mode analysis. Icarus 72, 304-324. BOWELL, E., T. GEHRELS, AND B. ZELLNER 1979. Magnitudes, colors, types and adopted diameters of asteroids. In Asteroids (T. Gehrels, Ed.), pp. 11081129. Univ. of Arizona Press, Tucson. BOWELL, E., A. W. HARRIS, AND K. LUMME 1988. A two-parameter magnitude system for asteroids. Preprint CHAPMAN, C. R., AND M. J. GAFFEY 1979. Reflectance spectra for 277 asteroids. In Asteroids (T. Gehrels, Ed.), pp. 655-687. Univ. of Arizona Press, Tucson. DEEMING, T. J. 1975. Fourier analysis with unequallyspaced data. Astrophys, Space Sci. 36, 137-158. HAHN, G. 1986. A data base of observing conditions for A t e n - A p o l l o - A m o r objects during the 20th century. Uppsala Astron. Observ. Rep. No. 38. HARRIS, A. W., AND J. W. YOUNG 1986. Photometric results for Earth-approaching asteroids. Bull. Amer. Astron. Soc. 17, 726. HARRIS, A. W., J. W. YOUNG, J. GOGUEN, H. HAMMEL, G. HAHN, E. F. TEDESCO, AND n . J. THOLEN 1987. Photoelectric lightcurves of the asteroid 1862 Apollo. Icarus 70, 246-256. HOFFMANN, M. 1986. Photometric observations of 1036 Ganymed. Minor Planet Bull. 13, 27. IP, W.-H., AND R. MEHRA 1973. Resonances and librations of some Apollo and Amor asteroids with the Earth. Astron. J. 78, 142-147. JANICZEK, P. M., P. K. SEIDELMANN, AND R. L. DUNCOMBE 1972. Resonances and encounters in the inner Solar System. Astron. J. 77, 764-773. LAGERKVIST, C.-I., AND I. P. WILLIAMS 1987. Physical studies of asteroids. XV. Determination of slope parameters and absolute magnitudes for 51 asteroids. Astron. Astrophys. Suppl. Set. 68, 295-315. LUPISHKO, n . F., F. P. VELICHKO, V. V. KAZAKOV,

ASTEROIDS

381

AND V. G. SHEVCHENKO 1987. Asteroid 1036 Ganymed: Lightcurves, period and sense of rotation. Kinemat. Fiz. Neb. Tel. 3, 92-93. LUPISHKO, D. F., F. P. VELICHKO, AND V. G. SHEVCHENKO 1986. Asteroid 1627 Ivar: UBV photometry, period and sense of rotation. Kinemat. Fiz. Neb. Tel 2, 39-43. LUPISHKO, D. F., F. P. VEL1CHKO, AND V. G. SHEVCHENKO 1988. Photometry of Amor asteroids 1036 Ganymed and 1139 Atami. Soy. Astron. Vestnik, in press. MAGNUSSON, P. 1986. Distribution of spin axes and senses of rotation for 20 large asteroids. Icarus 68, 1-39. MATSON, D. L. (Ed.) 1986. IRAS asteroid and comet catalogs. In Infrared Astronomical Satellite Asteroid and Comet Survey: Preprint Version No. 1, Sect. III-l, pp. 1-83. Jet Propulsion Laboratory Publication D-3698. MCFADDEN, L. A. 1983. Spectral Reflectance o f Near-Earth Asteroids: Implications for Composition, Origin and Evolution. Ph.D. thesis, Univ. of Hawaii, Honolulu. MCFADDEN, L. A., M. J. GAFFEY, AND T. B. McCORD 1984. Mineralogical-petrological characterization of near-Earth asteroids. Icarus 59, 25-40. OSTRO, S. J., D. B. CAMPBELL, AND I. I. SHAPIRO 1986. Radar properties of near-Earth asteroids. Bull. Amer. Astron. Soc. 17, 729-730. TEDESCO, E. F. 1986. Ground-based data for asteroids. In Infrared Astronomical Satellite Asteroid and Comet Survey: Preprint Version No. I (D. L. Matson, Ed.), pp. 9:1-9:42. Jet Propulsion Laboratory Publication D-3698. THOLEN, D. 1984. Asteroid taxonomy from Cluster Analysis o f Photometry. Ph.D. thesis, Univ. of Arizona, Tucson. ZAPPALA., V., M. DI MARTINO, Z. KNE~EVI~, AND G. DJURASEVI~ 1984. New evidence for the effect of phase angle on asteroid lightcurve shape: 21 Lutetia. Astron. Astrophys. 130, 208-210. ZAPPAL.~, V., M. DI MARTINO, F. SCALTRITI, R. BURCHI, L. MILANO, J. W. YOUNG, G. WAHLGREN, AND K. PAVLOVSK! 1983. Remarkable modification of lightcurves for shadowing effects on irregular surfaces: The case of the asteroid 37 Fides. Astron. Astrophys. 123, 326-330. ZELLNER, B., L. ANDERSSON, AND J. GRADIE 1977. UBV photometry of small and distant asteroids. Icarus 31, 447-455. ZELLNER, B., AND D. J. THOLEN 1985. The eight-color asteroid survey: Results for 589 minor planets. Icarus 61, 355-416.