Multiband photometry of Comets Kohoutek, Bennett, Bradfield, and Encke

ICARUS 23, 551--560 (1974)

Multiband Photometry of Comets Kohoutek, Bennett, Bradfield, and Encke E D W A R D P. N E Y School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455 R e c e i v e d M a y 31, 1974; r e v i s e d J u l y 23, 1974 O b s e r v a t i o n s of C o m e t s K o h o u t e k (1973f), B r a d f i e l d (1974b), a n d P / E n c k e h a v e b e e n m a d e a t a n u m b e r of w a v e l e n g t h s b e t w e e n 0.55 a n d 18/%m. T h e silicate f e a t u r e first o b s e r v e d i n C o m e t B e n n e t t (1969i) s e e m s t o b e a c o m m o n c h a r a c t e r i s t i c of c o m e t a r y m a t e r i a l . T h e c o m a s of t h e s e c o m e t s r a d i a t e i n f r a r e d w i t h a n effective t e m p e r a t u r e h i g h e r t h a n t h e b l a c k - b o d y t e m p e r a t u r e a t t h e g i v e n d i s t a n c e f r o m t h e Sun. T h e a l b e d o of t h e d u s t p a r t i c l e s is b e t w e e n 0.10 a n d 0.20. T h e p a r t i c l e s i n t h e c o m a a n d tail are s m a l l ( d i a m e t e r less t h a n 2/zm), b u t t h e p a r t i c l e s i n t h e a n t i - t a i l of C o m e t K o h o u t e k m u s t b e l a r g e r t h a n a b o u t 10/~m d i a m e t e r . T h e o b s e r v a t i o n s give a n a b s o l u t e u p p e r l i m i t t o t h e d i a m e t e r of C o m e t K o h o u t e k of 3 0 k i n . A c o n s i s t e n t i n t e r p r e t a t i o n w o u l d i n d i c a t e t h a t C o m e t s K o h o u t e k a n d B r a d f i e l d h a v e n u c l e a r d i a m e t e r s of 5 t o 1 0 k m , t h a t B e n n e t t w a s several t i m e s larger, a n d t h a t P / E n c k e is 10 t i m e s smaller. T h e p e c u l i a r b e h a v i o r o f B r a d f i e l d s h o w e d t h a t t h e c o m a of a single c o m e t c a n a b r u p t l y c h a n g e its d u s t c o m p o s i t i o n .

I. OBSERVATIONS All of the observations were made at the O'Brien Observatory. The 30-in. telescope is equipped with a wobbling secondary and projects a square 27 × 27~ beam on the sky. The throw may be varied but is usually 30~. An up-looking multifilter dewar is mounted at the eassegrain focus. A gallium doped germanium bolometer operating at 1.1K is used at all wavelengths. The present complement of filters consists of V, R, I, 1.2, 1.6, 2.2, 3.5, 4.8, 8.5, 10.6, 12.5, and 18~m. The band passes are such t h a t ~ / A ~ 10. The R filter was not available for the Kohoutek measurements but was used on Bradfield. Comet Bennett was measured with an earlier system t h a t did not have filters at shorter wavelengths t h a n 2.2~m. All of the long wave filters are cooled to 78K, but the V, R, and I filters are in an external wheel and are at ambient temperature. Because the comet magnitude depends strongly on aperture size, it is important t h a t the colors be measured with identical © 1974by AcademicPress, Inc. All rights of reproduction in any form reserved. Copyright

Printed in Great Britain

geometry. The system is detector-limited at the intermediate wavelengths, the sky noise usually exceeds detector noise at wavelengths longer than 5 ~m, and the V and R wavelengths are limited in the daytime by shot noise in the daytime sky light. To study the brightness as a function of diaphragm size, an auxiliary l 1-in. Herschelian telescope with a wobbling primary is used at 1.2-18/~m. This telescope projects a 1 by 1 arc-min beam on the sky. All of the observations are made when the comets are near the meridian and therefore during daylight. The object is acquired by scanning at the ephemeris position. The hard pointing capability of the telescope is l a r e m i n , and the comet can usually be acquired in a few minutes. Vernier rates in right ascension and declination make it possible to keep a given comet position centered in the beam for up to several hours except when differential refraction is large. Standard stars are observed for calibra-

551

552

]$DWARD 1~. N E Y

I

I

t~

¢D

•

v

°

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

•

°

°

.

MULTIBAND

PHOTOMETRY

I

OF COMETS

553

I I

I I I I I I

~°~

IIII

~°~

IIII

II

I

O

~°~

v

Q

~O

554

EDWARD P. N E Y

tion and our magnitude scale assumes t h a t Lyra is zero magnitude at all wavelengths. The calibration procedure is described by Strecker and Ney (1974). The journal of the observations of the four comets is given in Table I, which includes some previously unpublished data on Comet Bennett. The photometry is usually good to ±O.1m except for 18Fm where extinction increases the uncertainty to ~:0.2m. For most of the observations the statistical errors are negligible. The absolute responsivity of the system on the telescope is reproducible from d a y to day to 10%.

II. R~SULTS Comet Kohoutelc Comet Kohoutek (1973f) was observed on 18 days during December 1973 and J a n u a r y 1974. Figure 1 shows the relative positions of the Earth and comet for these days. Starting December 29, it was measured on 10 consecutive days. An extensive high pressure region kept the weather clear and cold for an unusual length of time. The 18Fro extinction was as low as 0.3m/air mass and the extinction at other wavelengths was between 0.08m/air mass and 0.15~/air mass. Although the comet was observed through three air masses, the calibration star a Sco was nearby and could be observed with the same extinction. On December 16.7 the planet Mercury was observed at the

same sec z and the same distance from the Sun as Comet Kohoutek. The similarity of their spectra is striking. Figure 2 shows the observations. In all of the figures the quantity log AF~ = log vF~ is plotted versus logA. On such a plot a black-body curve has a fixed shape and a horizontal line represents equal energy/octave. A direct measurement of the albedo m a y be made by comparing the (hFa)~,, for the thermal radiation with t h a t for the scattered sunlight. Both Mercury and the comet run hotter than a black body at this distance from the Sun. In the case of Mercury, it is because the dark side is cold (Murdock and Ney, 1970), and in the case of the comet, it is presumably because the coma particles are small compared with the wavelength at the Planckian maximum of the thermal radiation. Both Mercury and the comet are 20% warmer than a fast-rotating black body would be. The ratio of the power radiated toward the B i~II

15r2

R I

J

K

OMAGNITUOE I ~0EC'"6.7 ,,73 |

I

I

L

r -04

M

\ ' --~ ~

LA=114

15,3

I

I~~ i(~i4 6 0 0 o " B s

P/

r

I\

0

I ~

4,

q

,

'~r~

~?)

1615 ~

H

E

L

I

O

N 15'6

l ~ - ' j ~ " / i f 5 N ~'

/ASCENDING

Fzo. 1. The r e l a t i v e p o s i t i o n s o f t h e E a r t h a n d C o m e t K o h o u t e k (1973f) d u r i n g D e c e m b e r a n d J a n u a r y . The E a r t h - c o m e t d i s t a n c e c h a n g e s v e r y little as t h e c o m e t goes f r o m r = 0.15 t o r - - 1.0.

0.6 0.8 I

2

3 4

6 7 8 IO

2o

WAVELENGTH (MICRONS) FIG. 2. T h e plane~ M e r c u r y a n d C o m e t K o h o u t e k (1973f) a t a p p r o x i m a t e l y t h e s a m e d i s t a n c e f r o m t h e E a r t h a n d t h e Sun. B o t h o b j e c t s s h o w reflected s u n l i g h t a n d t h e r m a l reradiation. Both have a characteristic temperature higher t h a n a fast rotating black body at the s a m e d i s t a n c e f r o m t h e Sun. C o m e t K o h o u t e k (1973f) h a s t h e silicate f e a t u r e s u p e r i m p o s e d on the thermal continuum.

MULTIBAND PHOTOMETRY OF COMETS

E a r t h PR to the power reflected Ps for Mercury is (PR/Ps)~=20 and for the comet (PR/Ps)~:=4. O'Dell (1971) has shown t h a t in the optically thin case Pt~/Ps = ( 1 - ~)/~ where y is the albedo. The average optical depth of the coma is on the order of 10-3-10 -5 . The comet albedo corresponding to P R / P s = 4 is ~ = 0.2. The direct comparison with Mercury allows one to set an upper limit on the size o f the comet. The comet radiates 1/2000 as much as Mercury in a 2777 beam. The angular diameter of Mercury is 5~. Observation of magnitude versus beam size (Reike and Lee, 1974) shows t h a t the comet would have been 3m dimmer in a 5~ beam. I f it were physically similar to Mercury and a solid object, it would be 30km in diameter. Analysis of our data on K o h o u t e k and Bradfield indicates t h a t the nucleus is probably 5 times sm~ller t h a n this upper limit. On December 30 both a tail and an antitail were identified in the infrared. Figure 3 shows the geometry on J a n u a r y 1.7 at a wavelength of 3.5/~m. Scans at 10.5/xm show the same structure. The tail is fan shaped and points away from the Sun. The anti-tail is narrow and is in the orbit plane. The numbers in the figure refer to the relative surface brightness at 3.5/xm and at the indicated positions. This figure closely resembles the geometry of the sketches of the skylab astronauts, who first saw the anti-tail on December 29 7

z

555

(IAUC 2614). The tail and anti-tail were mapped in the infrared until J a n u a r y 5.7, after which the anti-tail was too dim to allow determination of its colors. Figure 4 shows the colors and brightness of the coma tail and anti-tail. The coma and the tail have the silicate signature and run warmer than the black-body temperature. The anti-tail does not have the 10/xm excess and is v e r y near the black-body temperature. This result is interpreted (Ney, 1974) to indicate t h a t the particles in the coma and tail are small and t h a t those in the anti-tail are larger, or composed of a different material. The possibility t h a t the particles in the anti-tail are large is supported b y the elegant analysis of Sekanina (1974). Elaborating on the work of Finson and Probstein (1968), Sekanina shows t h a t the particles we observe in the anti-tail of K o h o u t e k must be old and must have a low ratio of radiation pressure to gravity. More precisely, the anti-tail observed on J a n u a r y 1 should consist of particles ejected about a month earlier and for which the ratio of radiation pressure to gravity is less than 2%. Our observations give limits on the particle sizes present in the coma and the anti-tail. The opacity of silicate material at 101xm is 1000cmEgm -~. For an assumed density of 3, a grain would become optically thick at a diameter of 3/~m. The presence of the silicate signature in the coma and the tail therefore shows t h a t

--

s4

--

8

3

13

2

24 47 37

19

-I -Z

I

I

I

I

I

I

I

I

I

I

I

I

I ~ " ~

I0

9

8

7

6

5

4

3

2

I

0

I

2

MINUTES OF ARC EAST

3

I0

6

4

5

I 6

MINUTES OF ARC ,WEST

Fro. 3. T h e g e o m e t r y o f t h e i n f r a r e d emission o f C o m e t K o h o u t e k (1973f) m e a s u r e d a t 3.5/~m. The n u m b e r s on t h e coma, tail a n d a n t i - t a i l refer to t h e r e l a t i v e surface b r i g h t n e s s o f t h e c o m e t in t h e i n d i c a t e d positions. T h e d a t e was J a n u a r y 1.7, 1974.

556

EDW/LRD P. N E Y B

V

R

!

J

K

L

15 II

16m

t-

E

,J

15 '4

1(5le o.6

o.s I

2

4

WAVELENGTH

6

s

to

2o

IN MICRONS

FIG. 4. A F z x v F v i n W / o r e 2 v e r s u s w a v e l e n g t h f o r t h e t a i l , c o m a a n d a n t i - t a i l o f C o m e t K o h o u t e k ( 1 9 7 3 f ) m e a s u r e d o n J a n u a r y 1.7, 1974.

the particles have a diameter considerably less than 3/~m. I f the same grains are responsible for the scattered light, a lower limit on their size m a y also be set. The colors at short wavelength are very much like the solar colors. I f the particles were very small, 21ra/)t< 1, then Rayleigh scattering would suppress the short wavelength intensities. It therefore seems t h a t the average particle diameter is greater than 0.2/xm and less than 2/zm. The temperature excess in the coma indicates that 2rra[2 < 1 at 10tzm and confirms the upper limit on radius. For the particles in the anti-tail, the silicate signature is missing completely and the temperature is close to the greybody temperature, indicating t h a t the grains are larger than about 20/~m in diameter. Sekanina's prediction of 0.02 for the ratio of radiation pressure to gravity would imply t h a t particles of density one should have a radius of 30/zm. The strength of the silicate feature at 10/~m seems to vary somewhat but the coma of Comet Kohoutek always showed

some excess at all values of r between 0.15 and 1AU. Figure 5 shows some of the postperihelion observations between r = 0.15 and r = 0.95AU. The temperature excess over a black body seems to increase as the comet approaches the Sun and the albedo has a value of 0.18 ± 0.2 at all values o f r .

Comet Bennett Maas, Ney, and Woolf (1970) reported the first observation of the 10 and 18/~m excess in this comet. At t h a t time it had been suggested (Woolfand Ney, 1969; and Gilman, 1969) t h a t the emission feature seen in two giants and two supergiant stars was due to the condensation of magnesium and iron silicates in the stellar atmospheres. Subsequently the "dust bump" has been found in most late supergiants and in m a n y giants (Ney, 1972). Humphreys, Strecker, and Ney (1971) showed t h a t it can develop in supergiants as early as spectral type G if the luminosity is high enough. The persistent appearance of the silicate feature in comets seems to emphasize its cosmological significance.

MULTIBAND PHOTOMETRY OF COMETS S V R I

d

K

L

M

0

EU~ON

~

BV R I i~tl,

55~

J

K

L

M

0

1~,

-13

i~.= i614

1 I

I '~.1

I

I

I

I

I

\

A "25.04

~J

\

"

r--.85

,// X~JAN 342".(I.19 j516

0.6 0.8 I

:5 6 7 8 I0 WAVELENGTH (MICRONS)

20

FIG. 5. A s a m p l e o f t h e p o s t - p e r i h e l i o n o b s e r v a t i o n s o f C o m e t K o h o u t e k (1973f). T h e wavelength of m a x i m u m thermal energy shifts f r o m 3 . S F m to 1 0 ~ m as t h e c o m e t m o v e s f r o m r -- 0.15 t o r -- 0.95. T h e s t r e n g t h o f t h e silicate f e a t u r e varies, b u t it is c o n s i s t e n t l y p r e s e n t . T h e c o m a - g r a i n t e m p e r a t u r e s are i n d i c a t e d a n d t h e n u m b e r in p a r e n t h e s e s gives t h e facbor b y w h i c h this t e m p e r a t u r e exceeds t h a t of a fast-rotating black body.

Comet Bennett had a more pronounced e x c e s s t h a n C o m e t K o h o u t e k . Figure 6 s h o w s t h e c o m p a r i s o n at the s a m e distance f r o m the S u n (0.64AU). The visual magnitude for B e n n e t t is inferred f r o m visual o b s e r v a t i o n s a n d a s s u m e s the s a m e dep e n d e n c e o f brightness on angular size t h a t w a s o b s e r v e d for K o h o u t e k . W e w o u l d calculate an albedo for 1969i o f = 0.25 ± 0.1, in a g r e e m e n t w i t h O'Dell w h o o b t a i n e d 7 = 0.3 ± 0.15. In addition to h a v i n g a more p r o n o u n c e d silicate feature, B e n n e t t h a d a higher albedo and w a s hotter t h a n K o h o u t e k at the s a m e d i s t a n c e f r o m t h e Sun.

Comet Bradfield A l t h o u g h lacking K o h o u t e k ' s publicity, C o m e t Bradfield h a d more personality. Bradfield w a s first o b s e r v e d on March 21.9 a n d s e e m e d to be a t w i n o f K o h o u t e k . Figure 7 s h o w s K o h o u t e k pre- and postperihelion and Bradfield postperiheliun at

o.sas

2

3

4

678Lo

zo

WAVELENGTH (MICRONS)

FIG. 6. ~F~ for Comet Bennett and Comet K o h o u t e k a t t h e s a m e d i s t a n c e f r o m t h e Sun. C o m e t B e n n e t t is b r i g h t e r , s e e m s t o h a v e a h i g h e r albedo, h a s a m o r e p r o n o u n c e d silicate feature and runs at a higher temperature. The b l a c k - b o d y t e m p e r a t u r e for a f a s t - r o t a t i n g b l a c k b o d y 350 K is also s h o w n . Bv R I

id i'

d

K

M

o

\

,5,2

J PRE ~ ,5,3475 475--'~POST

PERIHELION" OHX'~EKOUT PERIHELION E 450 . . . . . POST PERIHELION BRADFIELD ~~"

~ ~,, .<

i(~ 14

r=.50 A = I . O \ I

":;? 5 xx[

,5 '~

i~ 16

N... 0.6 0.8

2

3 4

6 7 8 I0

2O

WAVELENGTH (MICRONS)

F r o . 7. C o m e t K o h o u t e k pre- a n d p o s t p e r i helion a n d C o m e t Brsxtfield p o s t p e r i h e l i o n . T h e d a t a are s l i g h t l y c o r r e c t e d t o r = 0 . 5 mad /1 = 1.0. T h e t e m p e r a t u r e s o f t h e c o m a g r a i n s are i n d i c a t e d . T h e t e m p e r a t u r e excess o v e r a b l a c k b o d y is a b o u t 16% :.

558

EDWARD

the same distance from the Sun (r -- 0.5). The observations are corrected to A = 1, using the magnitude vs beam size relation determined from the Herschelian data. Bo th comets seem to have approximately the same 10~m feature, albedo and temperature excess. Comet Bradfield's fluxes closely resemble the preperihelion data for Comet Kohoutek. The postperihelion brightness of Comet K o h o u t e k is about a magnitude dimmer at the long wavelengths. Between March 12.9 and April 5.8, Comet Bradfield dimmed only 0.5 m at 10.5 ~m although its distance from the Sun increased from r = 0 . 5 1 to r = 0 . 6 7 A U . However, during this time the energy distribution changed drastically as shown in Fig. 8. The silicate signature disappeared and the albedo dropped from y = 0.2 to r = 0.1. Then in the next 4 days the long wave-fluxes dropped three magnitudes and the visual appearance of the comet changed. A stellar-like image of the nucleus could be seen through the coma. The nuclear visual magnitude was estim ated to be between 10m and 10.5 m. The B v

I

J

H

L

M

,6"f i' 'J' J I \'r'

K l il I

I0 I

l0 MAGNITUDE

I

I t

BRADFIELD MARCH 21.9, 1974

i\

Lr ° '

I),o

]\

1"5,~o APRIL 5 8

0.6 0.8

\

!/

72

IX]

~,~----~v ~

2 4 6 8 I0 WAVELENGTH (MICRONS}

20

FIG. 8. Comet 1974b in late March and early April. In this period the silicate feature disappeared and the albedo decreased by a factor of two. Between these dates the R filter was added. Note the excellent fit of the short wavelength points to the solar colors.

P.

NEY

behavior suggests t h a t the dust in the coma changed from small to large particles and then virtually disappeared. The nuclear magnitude observed after the dust depletion allows a determination of the size of the nucleus. I f the nuclear albedo is assumed to be unity, the diameter of the nucleus was only 5km. Because of the similarity of Comets K o h o u t e k and Bradfield at 0.5 AU, Fig. 7, it seems likely t h a t both comets have nuclei of diameter 5-10km.

Comet Encke The observations of P/ E ncke are given in Table I. This comet was so dim t h a t it was difficult to acquire. Although the signal to noise on the chart recorder was only about one (in one Hz bandwidth), extended integration led to 10 standard, deviation measurements at 3.5~m. Sky noise at the long wavelengths made it impossible to determine whether or not the 10-~m excess is present in this comet. The infrared flux from PfEncke is 100 times less t han t h a t from K o h o u t e k and Bradfield which indicates a nuclear diameter of the order of 1 km. The maximum value of AFx is a measure of the power radiated b y the coma in the solid angle subtended b y the beam. This m ay be corrected to a standard value of E a r t h - c o m e t distance using the beam size dependence determined with the l 1-in. Herschelian. Our results for this dependence are in agreement with the observat i ons of Rieke and Lee, and together with their measurements, show t h a t coma brightness distribution is the same in the visible and infrared. Figure 9 shows (2Fx)ma~ as a function of r with the data normalized to A = 1. This figure shows t h a t Bennett was the brightest of the comets and had a steeper dependence of brightness on solar distance. I f K o h o u t e k had possessed this steep slope, it would indeed have been a spectacular comet. P/Encke is by far the dimmest and smallest of these comets. DISCUSSION The emission feature at 10 and 18~m attributed to small silicate dust particles

MULTIBAND I

~E

0 ~\ • i(~ 12 X ~ [] o ~:~DX ~

I

PHOTOMETRY

' I '1'1

BENNETT (POST) KOHOUTEK (PRE) KOHOUTEK(POST)_ SRADPIELD (POST) P/ENCKE

OF COMETS

559

indicates t h a t the e v a p o r a t i n g material m a y be rich or poor in dust in the same comet. The flaring of P / S c h w a s s m a n n W a c h m a n n m a y arise in a similar way. The v a r i a t i o n in the coma dustiness suggests t h a t comets are d u s t banded. A l t h o u g h the comets m a y form with onion-like structure, it is possible t h a t some selective e v a p o r a t i o n m e c h a n i s m could produce the d u s t y layers (Gold,

197~). i(~ J5

I LIE] ] , I I , I 0.2 0.4 O.B 0.8 I r IN ASTRONOMICAL UNITS

I 1.5

Fro. 9. A p l o t o f (AFx)max as a f u n c t i o n o f t f o r

the four comets, normalized to /I = 1, and in a 27~ diameter diaphragm. Comet Bennett is the brightest and has the steepest brightening law. Comet Encke radiates 100 times less infrared than Comets Bradfield and Kohoutek at the same distance from the Sun. has now been observed in three comets, 1969i, K o h o u t e k (1973f) a n d 1974b. The d a t a are n o t a d e q u a t e on P ] E n c k e to confirm or disprove the existence of silicates in this comet. The presence of grains of d i a m e t e r less t h a n 2 ~ m a n d g r e a t e r t h a n 0 . 2 ~ m has been observed in the c o m a a n d tail of Comet K o h o u t e k . L a r g e r grains or dust of some o t h e r c o n s t i t u e n t m a k e up the antitail. The existence of large grains in the anti-tail would be consistent with particles f r a c t u r e d from the comet, too large to be blown out b y radiation pressure, a n d orbiting in independent trajectories (Sekanina, 1974). These large particles could also be those responsible for the shower meteors which are k n o w n to be associated with c o m e t orbits. The infrared emission of Comet K o h o u t e k sets an u p p e r limit on its nuclear d i a m e t e r o f 30km. O b s e r v a t i o n o f the nucleus of Comet Bradfield indicates a nuclear dia m e t e r of 5 - 1 0 k m and, because of the similarity o f the two comets, suggests a similar nuclear size for K o h o u t e k . Comet B e n n e t t is p r o b a b l y two or three times larger a n d P / E n c k e ten times smaller. The peculiar b e h a v i o r of Comet Bradfield

The albedo of the c o m e t a r y d u s t is b e t w e e n 7 = 0.1 a n d 7 = 0.2. Most o f the observations show an albedo o f a b o u t 0.18. Observations of future comets a t all wavelengths should lead to a b e t t e r understanding of the size a n d composition o f c o m e t a r y dust a n d its relation to the interstellar material. Note added in proof. In the initial reporting of data on Comets Kohoutek, Bradfield and Encke, we believed that our square focal-plane diaphragm was ~ mm on a side. When it w a s discovered that our magnitudes were systematically brighter than other observers normalized to the same beam size, we dismantled the dewar and discovered that the focal-plane diaphragm was 1 × 1ram. The previously reported 20 x 20~ beam was really 27 × 27~. The present paper corrects this error.

ACKNOWLEDGMENTS The author is indebted to J. Stoddart who collaborated in a large number of the observations and whose engineering on the telescope made them possible. He also thanks W. F. Ney, Andrew Wardrop, Herbert Hefele and Tom Lee for their assistance. S. I~. B. Cooke contributed in many helpful discussions and obtained excellent photographs of Comet Bradfield which aided in the interpretation of the photometric data. Discussions with George Rieke led to the discovery of the beam size error discussed in the addenda. This work was supported by NASA under contract no. NsG-3014. REFERENCES

FiNso~, M. L., A ~ PROBST~IN, R. F. (1968a). A theory of dust comets. I. Model and equations. Ast/rophys. J . 154, 327. FINSON, M. L., A~D P~OBSTEI~, 1~. F. (1968b). A theory of dust comets. II. Results for Comet Arend-Roland. Astrophys. J. 154, 353.

560

EDWARD P. lqEY

GILMAZ¢, R. C. (1969). On the composition of circumstellar grains. Astrophys. J. 155, L185. GOLD, T. (1974). Private communication. I-IuMPHREYS, R. M., STRECKER, D. W., AND NEY, E. P. (1971). High-luminosity G supergiants. Astrophys. J. 167, L 35. MAAS, R., N~Y, E. P., AND WOOLF, N. J. (1970). The 10-micron emission peak of Comet Bennett 1969i. Astrophys. J. 160, L101. MURDOCK, T. L., AND N:EY, E. P. (1970). Mercury: The dark-side temperature. Science 170, 535. NEY, E. P. (1972). Infrared excessesin supergiant stars : Evidence for silicates. Publ. Astron. Soc. Pacific 84, 613. NEY, E. P. (1974). Infrared observations of

Comet Kohoutek near perihelion. Astrophys.

J. 189, L141. O'DEX~L, C. R. (1971). Nature of particulate m a t t e r in comets as determined from infrared observations. Astrophys. J. 166, 675. RIEKE, G. H., AND LEE, T. A. (1974). Photom et r y of Comet Kohoutek (1973f). Nature (Londor~) 248, 737. SEKANINA, Z. (1974). The prediction of anomalous tails of comets. Sky and Telescope 47, 374. STRECKER, D. W., AND NEY, E. P. (1974). Infrared observations of anomalous I RC sources. Astron. J., in press. WooL•, N. J., AND NEY, E. P. (1969). Circumstellar infrared emission from cool stars. Astrophys. J . 155, L181.