Standard stars for photometry of comets

ICARUS

88,

228-245

(1990)

Standard

Stars for Photometry

of Comets

WAYNE H. OSBORN’*2 Physics

Department,

Centrul

MICHAEL Astronomy

Program.

ROBERT Lowell

Michigan

F. A’HEARN University

L. MILLIS’

Observatory,

University, AND

ctf Maryland,

Mt. Pleasant,

Michigun

48859;

URI CARSENTY3 College

Park,

Maryland

20742;

and DAVID G. SCHLEICHER’

Mars Hill Road-400

West,

Flagst&

Arizona

86002.

PETER V. BIRCH’ Perth

Observatory,

Bickley,

Western

Australia

6076, Australia;

AND

HUGO MORENO’ Departamento

de Astronomia. Received

January

and A. GUTIERREZ-MORENO Universidad

de Chile,

31, 1990; revised

Cusilla 36-D, Chile

May 29, 1990

A set of standard stars for photometric observations of comets has been established. Magnitudes for 63 stars in 10 bandpasses are presented. The bandpasses are those of the nine filters recommended by the International Astronomical Union for comet photometry, which measure the C,, C3, CN, CO+, HzO+ and OH emission and three continuum points, plus a 10th filter which measures NH emission. The stars observed include those recommended by the International Halley Watch for use in photometric observations of Comet Halley during the 1985-86 apparition. 0 1990Academic PWS, IIIC.

1. INTRODUCTION

Much progress in understanding the physical nature of comets has been achieved in the past two decades. This progress is due largely to the establishment, by several observers, of programs of systematic observa’ Visiting astonomer, Cerro To1010 Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. ’ Visiting observer, Lowell Observatory, Flagstaff, Arizona. ’ Now at DLR, Oberpfaffenhofen NE-OE-PE, D803 1 Wessling/Obb., Germany.

0019-1035/90 Copyright

$3.00

C 1990 by Academic Press. Inc.

All rights of reproduction

in any form reserved.

tions of cometary spectra. These programs were designed to obtain, in a uniform manner, observations both of many different comets and of the variations in individual cometary spectra over time. As examples, one may mention the recent studies of A’Hearn and Millis (1980), Newburn and Spinrad (1984, 1985, 1989), and Cochran (1987). The ideal technique for monitoring changes in a comet’s spectrum would be to acquire a series of medium-resolution spectra with a variety of slit positions. In practice, however, such observations generally are not feasible. Spectroscopic observations generally require a large-aperture telescope,

STANDARD STARS FOR COMET PHOTOMETRY and observations at different slit positions require significant telescope time. Furthermore, comets often appear suddenly and many studies require measurements made over an extended time period. These facts usually preclude intensive spectroscopic monitoring because of the heavy subscription rates of large-aperture telescopes. A suitable alternative to spectroscopy is photoelectric photometry of comets employing filters selected to isolate spectral features of interest. A number of studies have demonstrated the utility of this technique, even when only modest-sized instruments are used. For example, A'Hearn et al. (1983, 1985) used 60-cm telescopes to derive gaseous production rates for Comet Encke and demonstrate the major seasonal differences in relative abundances. A major advantage of photometric observations is that observers can respond to comet discoveries quickly because suitable smaller instruments are usually available. Research on the physics of comets utilizing photoelectric photometry can be divided into two main areas: studies of emission bands and studies of the continuum. The emission-band studies are concerned with the production of various components of the gas, and ideally involve only measurement of the emitted flux of a particular species as a function of solar distance. Continuum studies are directed toward understanding the dust component of the comet, and ideally only involve measurement of the reflected solar spectrum. In practice, the observations for the two areas are intertwined; for example, reduction of the emission-band measures requires knowing the strength of the continuum underlying the band. A review of the possibilities and difficulties of cometry photometry has been given by A'Hearn (1983). 1I. THE IAU PHOTOMETRIC SYSTEM FOR COMETS

The value of early photoelectric photometry of comets was limited by the fact that the various researchers often monitored dif-

229

ferent features or, when the same feature was observed, used different bandpasses. Thus, it was difficult to combine observations from separate studies and the history of spectral features was often interrupted. To resolve this problem, a working group was established by IAU Commission 15 at the 1979 General Assembly to consider various options and recommend a standard system for photoelectric observations of comets. In particular, it was recognized that such a system would be essential for the planned intensive monitoring of Comet Halley during the 1985-86 apparition. The working group eventually specified a set of nine filters as those recommended for cometary photometry (A'Hearn 1984). Initially, five filters were adopted that measure the strengths of the CN, C 3, and C2 emission bands and two continuum points, one in the ultraviolet and one in the blue. These five "primary filters" were later supplemented by four "secondary filters" that measure OH, CO +, H2O+, and a red continuum point. The major characteristics of the nine filters of the IAU photometric system for comets are listed in Table I. The table also gives the characteristics of an additional filter that has at times been used to measure cometary NH emission (e.g., A'Hearn et al. 1981). The detailed specifications of these 10 filters and descriptions of the spectral features that they measure are given for reference in the Appendix. II1. STANDARD STARS The adoption of a set of filters for cometary photometry necessitates the establishment of a system of photometric standards. Two classes of standard stars are required. First, one needs flux standards, usually chosen to be stars of spectral type B because of their relatively few spectral lines, to calibrate the emission-line fluxes. Second, one must have G-type stars that mimic as closely as possible the solar spectrum for use in defining the spectral reflectance of the cometary dust. Obviously, the standard stars

230

OSBORN ET AL. TABLE l CHARACTERISTICS OF THE FILTERS

IAU Primary

filters

IAU Secondary

Non-IAU

filter

filters

~max

A~

3650 3871 4060 4845 5140

80 50 70 65 90

4260 6840 7025 3085

65 90 225 60

3365

65

must be distributed in right ascension, and it is preferable that they be equatorial stars accessible from observatories in both hemispheres. On the other hand, comets are often observable only for an hour or less near dawn or dusk and at large air masses which mandates standards close to the comet in the sky. It is therefore usually desirable to establish secondary standards located along the path of the comet. A set of primary standard stars for photometry of comets has been suggested by A'Hearn (1983) and is adopted here. Eleven stars of spectral types 0 9 through B3 were selected for flux calibrations. These stars are distributed more or less uniformly around the celestial equator ( ] 8 ] < 20°) with the exception of o n e - - e Ursae Maj o r i s - - wh ich was chosen because it is often used as a standard for ultraviolet spectrophotometry. These 11 flux standards have varying colors because of different degrees of interstellar reddening. However, all are characterized by being nearly free of strong spectral lines which insures that the flux measured through any given filter is insensitive to slight shifts in the filter bandpass. The remaining primary standards are Gtype stars. In contrast to the B-type flux standards, the solar spectrum has many strong spectral features and measurements of reflected sunlight through narrowband filters can be sensitive to the details of the filter bandpass. It is therefore important that

Spectral

Feature

ultraviolet continuum CN emission C 3 emission blue continuum C 2 emission CO + emission red continuum H2 O + e m i s s i o n OH emission NH emission

the response of the photometric system to the solar spectrum be known. In a series of papers, Hardorp (1982 and references therein) has identified seven stars whose spectra are excellent solar analogs on the basis of a detailed comparison of spectral features from 3650 to 4100 A. Furthermore, their overall flux distributions are in good agreement with that of the Sun as determined by Labs and Neckel (1968). These seven G-stars have been adopted as primary "solar analog standards." All eighteen Btype and G-type primary standard stars are identified in Table II. Any necessary secondary standards must, of course, be chosen for each comet. A large number of IAU cometary filter sets were purchased by the International Halley Watch organization and complete or partial sets distributed to observers worldwide in preparation for the 1985-86 apparition of Comet Halley. It was hoped that the wide geographic range of the observers would permit the comet to be kept under almost continual observation for the period around perihelion. A list of B- and G-stars to be used as the secondary standards for the Comet Halley observations was then selected by A'Hearn, Vanysek, and Campins (1984) and circulated to the observers. The Comet Halley secondary standards are listed for reference in Table III. The principal purpose of this paper is to provide the magnitudes in the various bandpasses for these stars. A

231

STANDARD STARS FOR COMET PHOTOMETRY TABLE II PRIMARY STANDARDS HD

Other

Identif.

3379 26912 52266 74280 89688 120086 120315 149363 164852 191263 219188

53 Psc ~ Tau BD -5"1912 ~ Hya 23 Sex BD -1"2858 N UMa BD -5"4318 96 Her BD +10"4189 BD +4*4985

28099 29461 30246 44594 105590 186427 191854

Hyades: vB Hyades: vB Hyades: vB HR 2290 BD -11"3246 16 Cyg B BD +43"3515

Note.

64 106 142

AB

R.A.

(1950)

Dec.

V

B-V

Sp.

00h34mlO.8 s 04 12 49.0 06 57 53.9 08 40 36.7 i0 18 27.1 13 44 44.2 13 45 34.3 16 31 47.9 18 00 14.7 20 06 15.1 23 ii 28.0

+14"57'24" +08 46 07 -05 45 21 +03 34 46 +02 32 31 -02 ii 40 +49 33 44 -06 01 59 +20 49 56 +04 43 29 +04 43 29

5.88 4.29 7.23 4.30 6.66 7.89 1.86 7.80

-0.15 -0.06 -0.01 -0.20 -0.08 -0.18 -0.19 +0. Ol

5.28 6.31 7.09

-0.09 -0.12 +0.06

B2.5 IV B3 IV 09 V B4 V B2.5 IV B3 III B3 IV B0.5 III B3 IV B3 V B0.5 III

04h23m47.7 s 04 36 07.6 04 43 38.9 06 18 47.1 12 06 53.2 19 40 32.0 20 08 33.7

+16"38'07" +14 00 29 +15 22 59 -48 42 50 -Ii 34 36 +50 24 03 +43 47 43

8.12 7 96 8 33 6 60 6 56 6 20 7 42

+0.66 +0.66 +0.67 +0.66 +0.66 +0.66 +0.66

G6 V C-5 V G5 V G3 V G2 V G2.5 V G5 V

3379: Possible variable (see discussion of Balona and Marang 1988) 26912: Published flux distribution (Breger 1976) 28099: variable in 0.05 mag. range (Lockwood et al, 1984) 74280: Published flux distribution (Breger 1976), variable in 0.015 mag. range (Hoffleit and Jaschek 1982) 89688: variable RS Sex, but apparently variations less than 0.01 mag. (Hoffleit and Jaschek 1982) 105590: Brightest star of triple system, var. in 0.08 mag. range (Hoffleit e t al. 1983) 120315: Published flux distribution (Breger 1976) 164852: Publ. flux (Breger 1976); var. at 0.02 mag. (Koubsky et al. 1985) 191854: Close double with a = 0.5".

few other stars were observed for one reason or another during the course of this work. For completeness, the results for these objects are included and they are identified at the end of Table III. IV. OBSERVATIONAL MATERIAL

Observations using the IAU-recommended filters have been obtained of the stars suggested by A'Hearn as primary standards for comet photometry and those stars recommended by the IHW as standards for observations of Comet Halley. Our observations were obtained during the period 1984-1986 at three observatories using a variety of equipment. A summary of the

sources of our observational material is given in Table IV. The table presents the observatory and month of observation, the telescope used, the number of nights of observation, the photomultiplier and filters employed, and the observers. Pulse counting electronics were employed in all cases. Observations of standard stars with the some of the IAU filters have been published by Wisniewski and Zellner (1985), Vanysek and Wolf (1985, Wolf and Vanysek 1987), and Pfau and Stecklum (1986). The data from these papers have not been combined with ours because of possible differences in the reductions, but they have been used as a check of the external accuracy of our re-

232

OSBORN ET AL. TABLE III SECONDARY STANDARDS FOR COMET HALLEY HD

Other Identif.

10126 13974 16397 16908 18803 22951 23464 24760 25680 26736 30455 31966 34078 36351 41753 47032 76151 84971 88725 96700 97991 99171 104337 121849 122980 124580 133955 136352 137432 148045 149438 165185 166197 175191 189340 191639 193901 198188 218687

BD +27°262 6 Tri BD +30°421 35 Ari 51 Ari 40 Per BD +22°549 • Per 39 Tau BD +23=649 BD +18°734 BD +14 804 AE Aur 33 Ori v Ori BD +4°1341 BD -4=2490 BD -2°2986 BD +3°2338 BD +6°2327 CoD -29°8875 BD -02°3312 CoD -41°6529 BD -18°3295 CoD -33°9467 X Cen CoD -44°9181 ~ Lup v Lup CoD -36°10161 CPD -56°7710 r Sco CoD -36 12214 CoD -33°12917 a Sgr BD -10°5238 BD -9°5382 BD -23°5703 BD -21°5840 BD +13°5059

olh36m45.6 s 02 13 59.6 02 35 30.9 02 40 30.7 02 59 27.7 03 39 12.0 03 43 15.2 03 54 29.4 04 02 22.4 04 ii 32.2 04 45 46.3 04 57 43.5 05 12 59.8 05 28 37.0 06 04 42.9 06 34 07.4 08 51 50.1 09 46 12.4 i0 ii 32.1 I0 35 55.1 ii 05 30.9 Ii 13 38.6 ii 21 58.1 ii 58 17.5 13 55 42.9 14 02 59.0 14 12 27.6 15 05 27.9 15 18 25.2 15 24 05.5 16 24 27.7 16 32 45.9 18 03 00.9 18 07 36.8 18 53 i0.0 19 57 04.3 20 08 27.5 20 20 38.8 20 46 21.9 23 07 27.5

+27°51'22" +33 59 48 +30 36 24 +27 29 44 +26 24 56 +33 48 22 +22 58 28 +39 52 03 +21 52 32 +23 27 02 +18 37 40 +14 18 36 +34 15 25 +03 15 21 +14 46 34 +04 44 14 -05 14 39 -02 28 50 +03 24 19 +05 38 ii -29 54 04 -03 Ii 57 -42 23 39 -19 22 50 -33 45 15 -40 56 28 -44 46 O0 -45 05 20 -48 08 06 -36 35 37 -56 34 03 -28 06 51 -36 Ol 32 -33 48 40 -26 21 38 -i0 05 25 -08 59 30 -21 31 05 -20 48 51 +14 09 22

7.74 4.87 7.32 4.66 6.62 4.97 8.66 2.89 5.90 8.07 6.97 6.76 5.96 5.46 4.42 8.83 6.00 8.65 7.76 9.90 6.54 7.41 6.12 5.26 8.17 4.36 6.31 4.05 5.65 5.45 8.81 2.82 5.95 6.16 2.02 5.88 6.49 8.62 8.14 6.55

112185 148108 186408 214680 218639

• UMa CPD -56°7716 16 Cyg A I0 Lac BD -15°6360

12 16 19 22 23

+56 -56 +50 +38 -14

1.77 8.9 5.96 4.88 6.42

Note.

R.A.

51 24 40 37 07

(1950)

50.1 51.4 28.7 00.7 11.9

Dec

13 35 24 47 46

V

51 16 31 22 55

B-V +0.73 +0.61 +0.60 -0.13 -0.01 +0.60 -0.18 0.62 0.62 +0.22 -0.18 -0.17 +0.45 +0.67 -0.17 +0.60 +0.60 -0.24 -0.18 -0.20 +0.70 -0.19 +0.60 -0.18 +0.65 -0.15 -0.25 +0.62 -0.14 -0.22 +0.58 -0.15 +0.55 +0.60

-0.02 +0.64 -0.20 +0.01

30455: suspected variable (Hoffleit et al. 1983) 112185: variable in 0.03 mag. range (Hoffleit and Jaschek 1982) 148045: possible misidentification 148108: possible misidentification 166197: suspected variable (Hoffleit and Jaschek 1982) 191639: Hoffleit and Jaschek (1982) say variable in 0.3 mag. range 218639: possible misidentification BD +6°2327 = AGK +5°1544.

Sp. G8 V G0 V GI V B3 V G8 V B0.5 V GO V B0.5 V G5 V G2 V G2 IV-V 09.5 Ve BI.5 V B3 V B0 III G3 V B5 GI V B5 G2 V B2 V B2 IV-V BI.5 V G5 V B2 V F9 V B3 V G3-5 V B4 Vp GO V BO V G5 V BI V B2.5 V F8 V BI V GO sd G5 V GO V A0p F5 V GI.5 V 09 V A0 V

STANDARD STARS FOR COMET PHOTOMETRY

233

TABLE IV SOURCES OF THE OBSERVATIONAL DATA Filters

Used

3 3 3 3 4 0 3 6 8 0 8 6 5 7 6 5 5 0 1

4 2 6 0

4 5 6 7 8 1 8 0 4 4 4 2 0 5 0 0

Observ.

Date

Tel.

No.

Pht.

Lowell

85 85

Mar Jun

0.6 i.i

2 4

i 2

a a

a a

a a

a a

a a

a a

a a a a

85 85

Jun Aug

1.8 1.8

2 4

2 2

a a

a a

a a

a a

a a

a a

a a

a a

85 85

Aug Sept

0.6 I.I

5 2

2 2

a a

a a

a a

a a a a

a a

a a

a a

85 85 85

Sept Oct Nov

1.8 1.8 i.I

2* 2* 7*

2 2 2

a a a

a a a

a a a

a a a

85

Dec

i.I

2*

2

a

a

a

85 86 87

Dec Jan Jan

0.8 I.I 0.6

2* 5* 2

2 2 I

a a a

a a a

85

Mar

0.6

8

i

85 85

Jul Aug

0.6 0.6

2 2

i i

85 86 86 86

Sept Mar Apr May

1.0 0.6 0.6 0.6

2 18" 18" 7*

I 2 2 2

a a a

86 87

Jun May

0.6 0.6

7* 3

2 3

a a

84

Nov

0.6

I

84 85

Dec Mar

0.6 0.6

5 2

CTIO

Perth

5

a a s s

Observers WO ',40 RM R_M,

PB,

WO PB,

MF

a a a a a a a a a

DS, RM. RM,

PB DS DS,

a

a

a

a

DS

a a a

a a a

a a a

a a a

a a a

a

a

a

RM, RM, WO,

DS DS, KL

a

a

a

a

a

a

a

a

a

HM,

EW

a a

a a

a a

a a

a a

a a

a a

a a

a a

HM, HM,

EW EW

a a a a

a a a a

a a a a

a a a a

a a a a

a

a

a a a

HM, RM, Ma, RM,

EW DS DS DT

a a

a a

a a

a a

a a

a a

RM, WO

LM

a

2

a

a

a

a

a

a

PB

2 2

a a

a a

a a

a a

a a

a a

PB PB

s

a

s

a

LW,

WO

PB

DT

Note. Key: Phototube: 1 = RCA C31034 (GaAs red-sensitive photocathode)

2 = EMI 6256 (S-I 1 blue-sensitive photocathode) 3 = Hamamatsu R943-02 (GaAs red-sensitive photocathode) Filters: a = filter used for all observations s = filter used for some observations Observers: EW--Erich Wenderoth (Univ. Chile) HM--Hugo Moreno (Univ. Chile) KL--Kevin Lee (Central Michigan Univ.) LM--Leonard Martin (Lowell Observatory) LW--Larry Wasserman (Lowell Observatory) MF--Michael Feierberg (Univ. Maryland) PB--Peter Birch (Perth Observatory) RM--Robert Millis (Lowell Observatory) Ma--R. Martin (Perth Observatory) DS--David Schleicher (Lowell Observatory) DT--Don Thompson (Lowell Observatory) WO--Wayne Osborn (Central Michigan Univ.) suits. A c o m p a r i s o n b e t w e e n the p r e s e n t r e s u l t s a n d t h o s e o f t h e s e a u t h o r s is g i v e n in S e c t i o n VI. v. THE REDUCTIONS The establishment of a standard system s h o u l d b e d o n e w i t h care a n d p a t i e n c e . Ide-

ally, all o b s e r v a t i o n s w o u l d be m a d e with i d e n t i c a l e q u i p m e n t . Stars w o u l d be obs e r v e d o n l y at l o w air m a s s e s a n d t h e e x t i n c t i o n c a r e f u l l y d e t e r m i n e d e a c h night. E a c h star s h o u l d b e o b s e r v e d o n s e v e r a l nights w i t h the o b s e r v a t i o n a l p r o g r a m c o n t i n u i n g o v e r a p e r i o d o f at l e a s t 18 m o n t h s to i n s u r e

234

OSBORN ET AL.

that stars in different parts of the sky are well linked. All data would be critically reviewed for errors, after which the raw data f r o m the individual nights would be reduced in a preliminary f o r m to a c o m m o n zero point. The various nights then would be combined, a v e r a g e values obtained, and the nights r e r e d u c e d iteratively until a self-consistent set of magnitudes is achieved. Caution would be exercised at each step to insure that systematic errors are avoided. In practice, factors such as weather, availability of telescopes and equipment, and the schedules of the individual observers normally p r e v e n t following the ideal procedure. Such is the case here. While we have followed the prescribed steps as closely as possible, there were unavoidable deviations. Chief a m o n g these is that the o b s e r v a t i o n s are heterogeneous, resulting from several o b s e r v e r s using a variety of instruments. Because not all o b s e r v e r s could m e a s u r e the same objects or e m p l o y all 10 filters, o b s e r v a t i o n s with the secondary filters were less frequent than with the p r i m a r y ones and s o m e stars were o b s e r v e d infrequently. As a c o n s e q u e n c e , in a few cases our results are based on only a small n u m b e r of observations. The o b s e r v e r s provided raw count rates, rather than magnitudes, to the senior author so that the data sets could be reduced in an identical manner. The one exception was that the extensive series of observations by Millis and Schleicher and their colleagues (marked with an asterisk in Table IV) had been converted to magnitudes and, except for 5 nights that were rereduced as a control, these data were used. No significant differences in the results were found for the rereduced control nights. The observers generally m a d e extinction m e a s u r e m e n t s on each night by observing stars o v e r a range of air masses. A widely used formula for the calculation of air m a s s is that of Hardie (1962). H o w e v e r , Young (1974) has pointed out that H a r d i e ' s expression is based on the apparent, i.e., refracted, zenith distance rather than the more conve-

niently obtained true zenith distance. While it is c o m m o n to use true zenith distance in the Hardie expression, we preferred the alternate expression for air mass based on true zenith distance given by Young (1974). In practice, the two expressions give air masses that differ by less than 0.01 for zenith distances less than 75 ° (an air mass of 3.8). The atmospheric extinction was found to be linear with air mass for all filters e x c e p t the filter for O H , located at 3085 A in the ultraviolet. For reference, representative extinction coefficients for a n u m b e r of observatories, arranged in order of decreasing elevation a b o v e sea level, are listed in Table V. The coefficients vary considerably, as would be expected, but inspection of the table reveals that the differential extinction v a l u e s - - t h e coefficient of a given filter compared to that of an adjacent o n e - - a r e very stable. The extinction relation for the 3085 filter is nonlinear, diurnally and seasonally variable, and dependent on the o b s e r v e d color of the object. Such an effect has been discussed by A ' H e a r n et al. (1981). Although the nonlinearity in the 1981 study was the result of a filter with a wide bandwidth and in the present case it is produced by a secondary bandpass (a " r e d l e a k " ) , the net result is the same: too wide a range of wavelengths contributing to the o b s e r v e d flux. Thus, we have adopted these authors' approach and represented the extinction for this bandpass through use of an effective air mass chosen such that the extinction varies linearly with Xe~'. We have used m o = m - k Xeff, where Xeff = X - [a + b ( m 3 6 5 0 - - m4845)]X 2 This expression was determined empirically and gave magnitudes outside the atmosphere for both B- and G-stars that were consistent for air masses up to 3.5. Typical values for the constants a and b were 0.08 and 0.025 for the Lowell O b s e r v a t o r y observations and 0.05 and 0.025 for Cerro Tololo

STANDARD STARS FOR COMET PHOTOMETRY

235

TABLE V REPRESENTATIVE EXTINCTION COEFFICIENTS FOR GOOD NIGHTS Observatory

Elev.

Mauna Kea

3365

3650

3871

4060

4260

4845

5140

6840

7025

4200:

0.40

0.32

0.29

0.24

0.17

0.15

0.08

0.07

Mt. Lemmon

2776

0.52 0.53 0.45 0.48

0.42 0.44 0.36 0.39

0.37 0.38 0.30 0.33

0.21 0.24 0.16 0.19

0.20 0.23 0.16 0.18

Lowell

- Mars

2204

0.72

0.52

0.43

0.35

0.30

0.20

0.17

0.09

0.12

Lowell

- And.

2195

0.65

0.47

0.37

0.31

0.25

0.16

0,14

Cerro Tololo

2100:

0.71

0.52

0.42

0.34

0,30

0.19

0.16

0.08

0.09

Bosque Alegre

1250

0.55

0.44

0.37

0,31

0.21

0.18

Perth

407

0.65

0.52

0.47

0.39

0.27

0.24

Jena

356

1,07

0.88

0,78

0.68

0.49

0.43

N o t e . Mauna Kea observations with USAF 0.6-m reflector (Tholen, private communication)

Mr. Lemmon observations with 1.5-m reflector (Wisniewski and Zellner 1985) Lowell-And. observations with 1. l-m reflector on Anderson Mesa (this paper) Lowell-Mars observations with 0.6-m reflector on Mars Hill (this paper) Cerro Tololo observations with 0.6-m Lowell reflector (this paper) Bosque Alegre observations with 1.5-m reflector (Claria et al. 1987) Perth observations with 0.6-m reflector (this paper) Jena observations with 0.9-m reflector (Pfau and Stecklum, private communication) Elevations are in meters.

Observatory (CTIO). As would be expected, the derived extinction coefficients, k, varied considerably. Typical values at Lowell Observatory were 2.3 in the winter and 2.5 in the summer while values in the range 2.0-2.2 were found for CTIO. We stress that the above expression for extinction with the 3085 filter is empirical and valid only for stars. The emission line nature of cometary spectra makes application of the color term in the equation for Xe~ inappropriate. In order to derive the 3085 filter extinction for comets, as well as check on our empirical results, we have calculated the theoretical absorption of the Earth's atmosphere in the 3085 bandpass. Our model, based on the approach of Hayes and Latham (1975), assumed three components of atmospheric extinction: aerosols, ozone, and Rayleigh scattering. The altitude of the observatory defined the Rayleigh scattering contribution, whereas the aerosol and ozone

components could vary within ranges expected for the site. For a given air mass, the model yielded monochromatic extinction values for various wavelengths, and these were multiplied by the spectral distribution of an object (either a B-star, a G-star or a comet) and by the transmission curve of the filter and then summed and converted to a magnitude. Calculations for various air masses gave the magnitude-air mass relation for the object. This was compared to extinction-star observations, varying the two free parameters of ozone column density and aerosol abundance until the observed data were reproduced by the theoretical relation. The extinction was then expressed, as before, in terms of an effective air mass. The theoretical calculations showed Xeff can be adequately expressed by Xett = X -

a ' X 2,

where the constant a' depends on the spec-

236

OSBORN ET AL. TABLE VI THEORETICAL AND EMPIRICAL OH EXTINCTION PARAMETERS CTIO Theoretical a'(B-stars) a'(G-stars) k (stars) kcomet/kB.star s

0.05 0.08 1.9 - 2.1 0.90

Lowell

A

0.05 0.08 2.3 - 2 . 4 0.92

Lowell

B

0.08 .... 2.35 ....

Empirical a'(B-stars) a'(G-stars) k (stars)

0.05 0.08 2 . 0 - 2.2

0.08 0.ii 2.3 - 2.5

0.08 .... 2.33

Sources of Data: CTIO --Mean values from 1986 March-April observations Lowell A--Mean values from 1985 August observations Lowell B--Both theoretical and empirical results from extinction observations of B-star HD 149363 on 1986-August-18.

trum of the star. The theoretically derived constants for the Xefr equation and the 3085 extinction coefficients are shown for three cases in Table VI. The a g r e e m e n t with the empirical values is reassuring. Furtherm o r e , b e c a u s e in c o m e t a r y p h o t o m e t r y one determines the extinction only from stars and the ratio b e t w e e n the extinction coefficients for c o m e t s and stars is required for the reduction o f c o m e t a r y observations, the theoretically derived ratios are of particular interest. H o w e v e r , we again stress that the absorption in this b a n d p a s s is highly variable and that the numerical results in Table VI should not be generalized to other seasons or places. After correction for extinction, the derived magnitudes were t r a n s f o r m e d to a zero point such that the magnitude was 5.88 in all filters for the star H D 3379. The individual nights w e r e combined, average values for the various magnitudes for each star obtained, and the s y s t e m again adjusted to m a k e the zero point 5.88 for H D 3379. This process was repeated until c o n v e r g e n c e . No significant s y s t e m a t i c effects were found between the different sets of observations and all o b s e r v a t i o n s were given equal weight. M e a s u r e s m a d e with air masses as large as 3.5 gave consistent results.

V1. RESULTS The adopted magnitudes for the standard stars are given in Table VII. The p r i m a r y standards and the C o m e t Halley s e c o n d a r y standards are listed together as both were o b s e r v e d and reduced together and are therefore on a uniform system. The table gives the identification of the star (usually the H D number) and the derived magnitude and its mean error for each of the 10 filters listed in Table I. Beneath the magnitude is the n u m b e r of observations on which the magnitude is based and the standard deviation of a single measure. As mentioned, observations of stars using filters from the I A U s y s t e m have been published by Wisniewski and Zellner (1985), V a n y s e k and Wolf (1985, Wolf and V a n y s e k 1987), and Pfau and Stecklum (1986). Wisniewski and Zellner determined magnitudes of 50 stars for the five primary filters but with a zero point different from ours. Their results contain stars of a variety of spectral types in addition to B- and G-stars and include a n u m b e r of faint stars suitable for observations using large telescopes. Vanysek and Wolf (1985) obtained preliminary magnitudes with the five primary filters for fourteen primary and Halley s e c o n d a r y standards. They later (Wolf and V a n y s e k

237

S T A N D A R D STARS FOR COMET PHOTOMETRY TABLE VII DERIVED

Star 3379

m3085

m3360

m3650

m3871

MAGNITUDES

m4060

m4260

m4865

5.880 46

.003 ,023

5.880 68

,002 ,018

5.880 68

,002 .018

5.880 67

.002 .016

5.880

10126

10.497 5

.013 ,030

9.768 5

.009 .021

9.304 10

.006 ,018

9.727 9

.010 .030

8.978 10

,004 .013

8.863 9

.008 .024

8.011 10

13974

7.069 5

.012 .026

6.494 .006 5 .012

6.141 .008 6 ,019

6.252 3

.009 ,021

5.902 .011 5 .025

5.811 4

.009 .017

5.130 .005 6 .011

5.096 .003 6 .009

4.290 4

16397

9,532 3

.021 .036

8.943 3

.014 .023

8.559 3

8.683 3

,015 ,O27

8.340 3

.014 .025

8.265 3

.013 ,023

7.596 3

.011 .019

7.566 3

16908

4735 5

,015 .034

4.742 5

,013 .027

4,725 ,009 9 .026

4.677 .004 9 ,012

4.676 9

.006 ,012

4,677 9

.007 .022

4.668 9

.006 ,018

9,349 3

.010 .018

8.600 6

,010 .023

8,149 8

,007 .020

8,565 8

.004 .013

7,826 8

,007 ,019

7,704 7

,006 .015

6,898 8

4.925 1

.019 .034

4.932 4

.020 .040

4.893 4

.016 .032

5.107 4

,016 ,032

5,161 4

.016 .032

5,146 4

,014 .029

5,017 4

10.011 6

.O07 .017

10.196 6

.009 ,023

9.723 6

.006 ,017

9,598 6

18803

22951

23464

24760

11,08 1

.... ....

10,46 1

.... ....

.020 ,035

5,880

m6840

.006 .027

66

.002

m5140

5.880 19

.016

67

.002

5.880

.013

67

.001

5.880

5.8B0

.008

11

.037

15

.031

2.11 1

.... ....

7.09 1

.... ....

.018

4.25 4

.011 .023

6.81 1

.... ....

6.87 1

.... ....

4.685 .011 9 .034

4.626 4

.021 .042

4.641 4

.022

,007 .018

6.866 8

.006 .018

6,033 4

.015 ,029

6.005 3

.029

.015 ,030

5.023 4

.011 .021

4,862 3

.009 .016

4.892 3

.023 ,039

,007 .016

8.960 .007 6 .017

8,892 6

.006 .014

........ ....

8.11 3

.007 .012

.003 .009

.012

m7025

,011

7.999 10

.G07 .023

.008 .013

.009

,011

.017

.021

2.433 2

.035 .050

2.529 2

.025 ,035

2.549 2

.021 .033

2,807 2

.027 .037

2.933 2

.027 .038

2,948 2

.020 .029

2.885 2

.024 .035

2.952 .... 1 ....

2,885 2

.001 .001

2.956 2

.030

8,289 3

.024 .042

7,665 3

,017 .030

7.277 8

.015 .044

7,484 8

,014 .038

6.987 8

,011 .032

6,868 8

.O06 .018

6,163 8

,008 .023

6.108 8

.006 .016

5.340 .008 3 .014

5.326 3

.029

.062 ,088

9.896 2

.024 .034

9.472 7

.017 .045

9.732 6

.013 .011

9.178 2

.014 ,037

9,055 7

.010 .028

8.318 7

.007 ,019

8,269 7

.007 .019

7.433 1

.... ....

7.360 1

.... ....

4.669 9

.017 .050

4.593 14

,011 .041

4.539 24

.006 .031

4.411 24

,006 .028

4.404 23

.004 .019

4.399 20

.005 .021

4.329 23

.003 .016

4.317 23

.005 .022

4.202

.012

4.202 7

.017 .044

28099

10.831 3

.016 ,028

9.958 18

.006 .024

9.547 23

.005 .024

9.823 23

.005 ,023

9.227 23

.005 .024

9.097 23

.006 .028

8.364 23

.005 .024

8.312 23

.004 ,019

7.51 1

.... ....

7,48 I

.... ....

29461

10.454 4

.027 .054

9.839 4

.018 .035

9.410 13

.005 .019

9,675 13

,008 ,030

9.097 13

.006 .020

8.973 11

.004 .014

8.243 13

.004 .015

8.196 13

.004 .005

7.38 4

.024 .054

7.345 12

.003 .012

30246

10.861

,017

10.170

,010

9.767 12

.007 .025

10.053 12

.003 .Oil

9.456 13

.004 .016

9,340 13

.006 .021

8.585 13

.003 .011

8.539 13

.011

7.68 1

.... ....

25680

26736

26912

30455

10.58 2

7

.046

I

.... ....

916

7

.026

1

.... ....

8.297 8

.014 .040

8.464 8

,012 .035

869

.003

8

.033

......... 0 ....

8,004 7

.010 .029

7.921 8

.011 .032

7.218 8

.008 .023

7.160 8

,010 029

6.36 1

.... ....

6.32 i

.016

....

31966

........ 0 ....

8.66 1

.... ....

8.245 14

007 .026

8.476 14

.007 .028

7.895 14

,006 .022

7.781 14

,007 018

7,038 1~

.006 .022

6.979 14

.005 021

6.15 1

--....

610 1

.... ....

34078

6484 6

,009 .022

6,15 3

.022 .038

6248 6

,011 .026

6.458 6

,012 .030

6.484 6

.015 .037

6.434 2

,017 025

6.144 6

,ClO .025

6063 6

,015 .030

5571 4

,018 036

5566 4

016 031

36351

5.187 6

,017 .043

5.229 4

.011 .022

5.248 18

.005 ,019

5383 18

.004 .016

5.426 18

.005 ,020

5.439 18

.003 .017

5419 18

.003 .011

5 444 18

.003 014

5.436 5

006 .014

5452 5

,007 .016

41753

443 3

.016 .027

4.41 1

.... ....

4.408 9

,007 .022

4.382 8

.006 .018

4.393 8

.006 .018

4,405 7

.005 .013

4403 8

.005 .013

4. b.lO ,004 8 .0ll

4.404 2

.003 .004

4.426 2

.004 .006

44594

9.110 3

.011 .019

8.496 2

.002 ,003

8.051 6

.005 .011

8.319 6

.003 .007

7.726 6

,004 ,010

7.613 6

.003 .006

6,889 6

.004 .010

6.832 6

.002 .006

6.011 3

.002 003

5.977 3

,001 .003

47032

9.776 4

.018 .035

9.57 1

.... ....

9.396 I0

.008 .024

9.577 I0

.006 .020

9,594 10

.007 ,022

9.516 I0

.004 .014

9,116 i0

.003 .011

8984 i0

.003 .010

8,303 3

005 008

8260 3

.005 ,008

52266

7053 16

,007 .028

2.075 37

.005 .028

7.037 80

.003 .025

7.327 80

,003 .024

7.415 80

.003 .024

7.404 79

.003 .023

7,240 80

.002 .017

7.256 80

.002 016

7.040 11

003 ,009

7.048 ii

,004 012

74280

4187 22

.005 .024

4.206 58

,003 .020

4.198 69

.002 018

4.240 70

,002 .017

4.265 69

.002 014

4.281 67

,001 .015

t~ 275 69

.002 .014

4.293 69

.001 ,012

4296 14

.006 .021

4324 11

.008 027

76151

8525 19

.009 .039

7.898 9

.007 ,020

7.462 24

.006 ,028

7.810 24

.003 .014

7.152 24

,003 ,014

7.044 23

.003 .016

6.291 24

.002 .011

6.238 24

,003 ,015

5,404 19

.002 .010

5.371 19

.004 .018

84971

8.579 16

.008 ,031

8.597 6

.009 ,022

8.583 21

.005 .021

8.620 21

.005 .022

8.665 21

.004 019

8.679 20

.004 .019

8629 21

,003 .015

8,660 21

.003 .014

8.612 14

.001 .011

8.631 14

.002 .008

88725

9.970 13

.010 .037

9.413 6

.010 .024

9.003 16

,002 .010

9.145 16

.OO& .016

8.791 16

.003 013

8.720 15

.003 .012

8.020 16

,003 .012

7,961 16

.003 .013

7152 II

.004 .012

7.117 II

004 .013

89688

6.838 20

,007 ,010

6.802 77

.003 .024

6.756 88

.002 .023

6.734 88

.002 .020

6.766 87

,002 .017

6.761 87

.002 .015

6,671 87

.001 .014

6.682 87

.002 .014

6.584 16

,006 .024

6.596 15

006 .023

.040 .089

11.792 7

.027 .070

11.382 7

.015 ,040

11.155 7

.010 .025

10.985 7

.010 .028

10.730 7

.012 032

10.591 7

.010 018

10.069 3

.010 .017

~6°2327

12.39 5

.042 ,094

12,09 5

007 .017

10064 3

96700

8.815 12

,004 014

8,242 ,015 4 .030

7.845 .004 17 .018

7.995 16

,003 011

7,569 ,O03 15 .012

7,480 ,002 15 ,008

6,803 15

.002 007

6.733 15

,002 .008

5,949 12

.001 ,004

5,917 12

,001 .O05

97991

6.943 ii

.008 .026

7.038 5

7.055 14

7.283 14

,006 .023

7.348 14

7,374 14

7.355 14

.006 .022

7.401 14

.006 .022

7.421 7

.004 .010

7446 8

.003 .010

015 ,034

.007 .026

.006 .021

.007 .025

238

OSBORN ET AL. TABLE VII--Continued

Star

m3650

m3360

m3085

m3871

m4060

m4260

m5140

m4845

m6840

m7025

99171

5.924 9

.006 .019

5989 6

.011 .026

5,975 15

,003 .010

6.087 15

.002 OO7

6.118 15

.002 .008

6.132 15

.003 .012

6.115 15

.002 .009

6.141 16

.003 .010

6.129 9

.002 .006

6,152 9

.002 .005

104337

4955 13

.009 .033

5019 30

.005 027

5.028 40

.003 .021

5193 40

,003 020

5.256 39

.003 .017

5.277 39

.003 017

5.240 39

,003 .017

5.283 39

.003 .019

8.283 1~

.006 .024

5,912 13

.004 .016

105590

9365 9

.003 .009

8727 24

.004 021

8.296 31

005 .028

8593 31

.005 .026

8.000 33

.004 023

7890 31

.004 021

7.141 33

,004 021

7.089 32

.003 017

6.268 9

,005 016

6.216 9

.006 .018

112185

323

....

1,93

....

2.00

....

1.89

....

1.59

.003

1,69

1

--

2

.071 2

120086

120315

7653 18 189 4

.100

302

.009 2

.008 .C34

7697 38

.043

1 90

087

2

013

284

.008 2

.OO& ,023

7695 48

006

1.91

.008

2

2 lo

.Oll

.003 .024

.012 017

2

7813 48 190

.013

004 .030 .001

2

1.98

018

002

1

7.855 48 191

....

.005 036 ---

l

l

---

7.862 48 1.93 1

1

.006 039 .... ---

7,845 48 1.94 1

....

,002 .017 .... ....

7.873 48

.002 017

1.94

....

1

-

7.867 19 1.86

-

005

.004 .015 .019

5

.026

2

7.888 15

023 5

10.743 8

.013 037

10024 3

.013 .023

9614 11

006 022

9.874 11

,008 026

9338 10

,005 .016

9.256 10

.006 019

8.466 I0

,005 .015

8421 10

.006 018

7.542 8

005 .014

2.506 8

122980

9.159 8

,006 .018

&161 63

003 .027

4188 71

.003 024

4 301 72

002 021

4 325 73

.002 018

4.334 22

.002 015

4 342 73

.002 016

4.388 73

.002 .016

4365 i0

.005 .018

4.386

8.593

133955

4.051

10

10 136352

.009

8.014

029

.007 021

2

4.017 41

.023 033

.002 013

7.644 .006 11

019

4035 45

.002 .014

1769 11

4062 45

.007 029

.002 .012

7 390 11

4.060 49

005 016

002 013

7.285 .004 11

4.066 48

014

6.626 ,003 11

.001 010

4.075 49

011

,001

6.550 11

4.065

.004 013

.002 .013

5.773 .003 10

4.049

,OOfl

49

3

.006 .020

8897 28

.003 .017

5069 11

.005 015

7.423 16

.004 017

7 014 26

.003 017

7223 26

.003 .016

6.755 27

.003 015

6.675 27

.003 016

5.946

1374)2

5.600 i0

.006 019

5570 102

,002 .019

5561 111

.001 016

5504 111

.001 014

5,472 109

.001 .018

5475 108

.001 .011

5 492 109

001 011

8485 I14

.001 .015

5.42 I0

148045

11.006 2

.013 019

I0 143 009 3 015

10236 3

026 046

9835 3

.008 014

9729 3

032 .023

9[00 3

005 .014

8 026 3

.005 009

10.85 5

.021 046

.011 030

9 501 7

009 023

9.080 7

008 022

8.936 7

148108

.073

1059 2

.103

......

10 104 5

.015 034

9 899 5

017 O39

9649 7

.009

I0

.005 017

.052

.008 024 .006 018

5745

.003

10

010

4.054

.016

8.065 11

28

.008

.005 .019

189

043

121849

124580

037

,019

4

039

5.034 11

.008 .027

.Ol .02

544 lO

.01 02

8.264 2

.010 013

8.230 3

.008 021

8.316 5

.002 016

8 295 5

,006 013

002 .014

7590 15

.007 ,026

7600 18

,003 .019

.005 009

I&9363

7.770 29

006 032

2.752 46

.003 022

7706 66

003 .024

7 938 66

002 019

8.027 66

0O2 .019

8.007 66

002 020

7.818 68

002 016

7823 66

149438

2.26 4

024 047

2.281 61

002 014

2 3~0 .002 63 019

2 656 63

002 .033

2 767 43

001 012

2801 62

.002 .015

2.780 63

.001 011

2 820 .002 63 .014

2.84 3

07 13

289 3

07 12

164852

5 &&3 33

005 029

5398 45

.004 028

5372 47

004 .026

5 337 47

004 025

5323 47

003 023

5.317 47

.OO& .025

5292 48

.002 .022

5 269

47

003 .019

5.186 29

.005 028

5.208 29

006 .035

8207 6

006 016

7 648 4

.025 050

7245 I0

004 013

7381 i0

.004 014

6988 12

006 022

6 87& 32

.003 .010

6212 12

.005 017

6139 12

007 .024

5 374 8

005 014

5.352 8

.008 023

166797

5.903 6

012 028

5 956 83

.003 026

5932 90

01~

6.104 90

.001 Of&

6 184 92

.002 015

6 190 90

002 015

6113 92

.003 027

6.154 92

,002 019

6.081 8

.003 ,008

6106 9

.005 .014

17519]

1955 3

007 013

.005 018

1955 ii

.004 .012

2 02 II

005 016

2045 8

008 027

2 065 12

.007 025

2088 12

007

004 013

1 92 1

.... - -

195

026

2.082 12

186408

8.45

....

7 61

---

7 05

---

6 937

6226

-

6 190

186&27

8779 12

.009 030

8093 12

.007 025

7 671 12

006 022

7 946 12

007 023

7 350 .006 12 .023

7,240 12

.007 025

6492 12

,008 .018

6451 12

.005 019

5616 9

189340

8 094 23

005 023

7518 16

008 O3O

7.170 25

.004 018

7249 25

003 016

6882 27

6768 24

.005 024

6.138 27

.004 019

6.075 27

.003 018

5.315 20

191263

6 430 21

.004 020

6 410 GO

.003 018

6,390 46

.002 015

6366 46

.002 015

6365 46

.002 014

6.364 46

,002 034

6347 46

.002 015

6346 46

.002 OlO

191639

6.099 lfl

005 022

6159 24

.006 029

6153 32

.004 .022

6 394 32

.003 019

6.486 32

.003 014

6495 32

.003 018

6423 32

.003 of&

6.473 32

191854

9982 9

.007 .022

9274 12

006 020

8864 12

.004 014

9 134 12

.006 015

8.539 12

.004 013

8433 12

.004 OlS

7671 12

.002 008

193901

10573 7

.001 004

10028 5

.006 014

9 723 .004 12 014

9775 ii

,007 032

9.585 12

.005 018

8494 II

.004 015

8903 11

198188

i0 428 II

.006 020

9 819 4

.016 033

9638

9609 15

.006 022

9153 17

.005 021

9.053 17

004 016

8394 17

.OOfl

4349

005

4834

004

4858

]65185

1

214680

218639

218687

219188

a.268

....

7

.023

2

065 092

687

8793 15 6 675 28

007 027 006 031

195 I] /78

002

7 40 1

7

....

.007 ,019

1

17 4 381 8

6493 2

027 039

6 227 2

8 211 I0

.013 040 .002 019

6723 103

-

1

.004 .017 005 .015

~ 716 8

-

015

1

8

022 030

5 778 2

013 018

5 518 2

7846 23

007 034

7 984 24

006 .031

7586 24

6727 118

.001 .015

&992 118

001 ,016

7 100 118

....

1

004 019

012

016 023 .002 033 .001 .014

7

-

.004 012

5 405 2

.013 018

7475 23 7 102 116

1

4.811 8

1

1

. . . . . . . . . . . ....

0

-

0

-

5635 9

.013 .039

.OOfl 056

5 296 20

.008 035

6.302 13

.OOfl 028

6,328 16

012 047

.003 015

6.432 16

.008 031

6 &63 17

.017 071

7.632 11

.003 009

6.804 8

.008 014

6.817 3

04 06

.006 015

8.827 ii

.003 011

8072 5

.003 008

8.049 6

004 009

003 Oil

8.325

004 015

7.589 9

002 .007

7943 5

OO& .008

.004

4.880

.004

4.898

013

4921

Oil

17

8

011

8

0

009 .028

038

8

.016 048

5 258 2

.011 036

5 099 2

008 011

035

6821 24

005 026

6160 24

005 027

6.003 11

006 019

5 976 i0

.005 .016

.001 .019

7001 118

.001 .013

7062 118

001 .013

6 992 ll

.011 036

7 023 11

0O9 030

007

. . . . . . . 0

239

STANDARD STARS FOR COMET PHOTOMETRY TABLE VIII COMPARISON WITH OTHER PUBLISHED MAGNITUDES Comparison sample

Wisniewski & Zellner

Wolf & Vanysek

Pfau & Stecklum

Am3650

Am3871

Am4060

Am4260

Am4845

Am5140

i B-star

1.081 1.346 0.764 -.089 -.130 . . . . . . . . . . . . . . . . . . . . . . . . .

6 G-stars

1.075 .034

1.352 .032

0.760 .038

5 B-stars

-.005 .015

-.044 .029

+.007 .015

7 G-stars

+.025 .050

+.057 .076

9 B-stars

+.011 .024

6 G-stars

-.050 .028

-.106 .034

-.154 .030

+.010 .016

+.016 .017

+.017 .015

+.003 .037

+.020 .057

+.011 .033

+.001 .025

+.004 .019

+.009 .031

+.010 .020

+.008 .016

+.007 .019

-.048 .030

-.036 .024

-.017 .021

-.014 .021

-.017 .021

1987) derived revised values plus results for the 4260 (CO + ) filter for these stars and two additional ones.4 Pfau and Stecklum also utilized the five primary filters plus the 4260 filter, publishing data for 20 B-stars and 7 solar analogs obtained in connection with comet observations. H o w e v e r , many of the stars had only a single observation. Both Wolf and Vanysek and Pfau and Stecklum's magnitudes nominally have the same zero point as our system. A comparison of our results with those of the above authors for the stars in c o m m o n is shown in Table VIII. The B-stars and Gstars have been listed separately. The upper line of each set gives the average differences between our results and the comparison sample, a measure of how well the zero points coincide, while the lower line gives the rms dispersion in the differences, a measure of how well the two data sets agree after any zero point adjustment. The agreement is good for the B-stars. The table indicates that for these stars the magnitudes in all filters can be r e p r o d u c e d 4 We note, however, that Wolf and Vanysek's Table 2b contains two typographical errors: rn4z~0 for HD 18803 should read 7.71 (rather than 7.91) and m4060for HD 25680 should be 6.99 (rather than 7.99).

with a precision of better than 0.02 mag. The situation is not so good for the G-stars. H e r e the disagreement averages 0.024 mag. for the Pfau and Stecklum data, 0.034 mag. for Wisniewski and Zellner's measures, and 0.045 mag. for Wolf and V a n y s e k ' s revised results. A closer look at the data reveals that the transformations between the data sets require significant color terms, particularly for the 3871 filter. For example, a least squares fit o f the Wolf and Vanysek data to our results using a transformation of the form m3871,1HW = m3871,w v + ot + /3 (m3650-m4845),

where a and /3 are constants, lowers the standard deviation of the fit for the 3871 filter from 0.076 mag. to 0.046. The large color term can be explained by the fact that there is a strong slope of the continuum across the 3871 filter for G-stars. Thus, a small change in the filter bandpass has a large effect on the effective wavelength. The I H W records show that the filter set of Wolf and Vanysek was manufactured at a different time from those of the other observers. Information on the variations among filter sets is given in the Appendix. The table indicates that Wisniewski and

240

OSBORN ET AL.

Zellner's magnitudes can be transformed to our system, with a mean error of -+0.03 mag., by adding the following values: -+1.076 for m3650, + 1.351 for m3871, +0.761 for m4060, - 0 .1 0 3 for m4845, and - 0 . 1 5 0 for m5140.

The internal agreement of our observations and the agreement with the published independent measures indicate that in general our derived magnitudes are accurate to -0.01 mag. There are three exceptions. First, in several cases the results are based on only one or two measures. These magnitudes should be considered tentative, and they are listed mainly because some of these stars were used by IHW observers. Second, the difficulty of measuring the OH filter and the complications in its reduction produced by the large and variable extinction make the magnitudes for this filter less reliable. We estimate mean errors of 0.02 mag. when m3085 was well observed. Third, three of the stars (HD 112185, HD 120315, HD 175191) are so bright they could not be observed in the standard manner. Measures with neutral density filters and with reduced gains on the phototubes gave somewhat different results, and these stars may have zero point errors in their magnitudes. In order to calibrate the reflectivity of cometary dust and nuclei, and also to predict the continuum flux underlying cometary emission bands, it is necessary to have accurate colors of the Sun in this photometric system. Colors for the seven primary solar analogs are presented in Table IX along with the mean values and their standard errors. The colors are given as the magnitude in each filter relative to that of the 4845 filter. More precise colors can be obtained by measuring the magnitudes in one filter relative to another which is nearby in wavelength and carrying out the entire reduction in terms of colors rather than magnitudes. This was in fact done, and showed that the colors obtained in this manner typically differ from those tabulated by less than 0.005 mag., i.e., less than the scatter among the solar analogs. Because most applications require col-

ors over a long wavelength baseline, we preferred to tabulate the colors relative to m4845 although they have slightly larger formal errors. A P P E N D I X - - D E T A I L E D CHARACTERISTICS OF THE FILTERS

The characteristics of the filters for the IAU Photometric System for Comets were selected through study of a representative set of comet spectra. The filters are denoted by their nominal peak-transmission wavelength in Angstroms. For reference, Fig. 1 shows the locations of these filters relative to the spectrum of a comet, Comet Tuttle (1980 XIII). The spectrum was assembled from both ground-based and IUE data by S. M. Larson (private communication). The five filters 3650, 3871,4060, 4845, and 5140 are known as the "primary filters" because they were the first ones produced and distributed. The early filters were manufactured by MicroCoatings, Inc. (Burlington, MA) during 1982 and 1983, while the later ones were produced by Andover Corp. (Lawrence, MA) in the period O ct oberDecember 1983. About 75 sets were produced. Because the two companies used different techniques to manufacture the filters, the characteristics vary somewhat from one filter set to another. Of the secondary filters, the 3085 ones were produced by SpectroFilm, Inc. (Winchester, MA) in February-March 1984, while the 4260, 6840, and 7025 filters were manufactured by Barr Associates (Westford, MA) in July-August 1983. The 3365 filter is not part of the IAU system, and the sole three filters were produced many years before the others by Spectro-Film in August 1979. The various filters are discussed below. Individual filters are identified by the nominal central wavelength, the full width at half maximum transmission (FWHM), and a serial number consisting of a letter indicating the manufacturer and the production number. The transmission characteristics for the 3085 and 3365 filters are included with the descriptions of the filters; for the other filters

241

STANDARD STARS FOR COMET PHOTOMETRY TABLE IX COLORS

Star 28099

3085

3365

3650

OF

THE SOLAR ANALOGS 3871

4060

4260

5140

2.267

1.594

1.183

1.459

0.863

0.733

-.052

29461 30246 44594 105590 186427 191854

2.211 2.276 2.221 2.224 2.287 2.311

1.596 1.585 1.607 1.586 1.601 1.603

1.167 1.182 1.162 1.155 1.179 1.193

1.432 1.468 1.430 1.452 1.454 1.463

0.854 0.871 0.837 0.859 0.858 0.868

0.730 0,755 0.724 0.749 0.748 0.762

-.047 -.046 -.057 -.052 -.041 -,039

Mean

2.257 + 14

1.596 + 3

1.174 _+ 5

1.451 _+ 6

0.859 _+ 4

0.743 + 5

-.048 4- 2

OH

CN cont

CO +

I C3

7025

- .854 - .863 878 893 876 867

-.884 - .898 - .905 -.912 -,925 -.857 - .854

872 _+ 6

- .891 + i0

filters age, the bandpasses often shift and, at the time o f this writing, shifts for s o m e filters have been reported. The effect o f a bandpass shift is m o s t serious for c o m e t a r y observations where o n e does not already k n o w the "correct" answer. The effect on standard star observations is m u c h smaller, but detectable as a difference in the transfor-

this information is presented together at the end o f the section. All filters w e r e specified to be 1.000 -+ .020 in. in diameter with a useable area greater than 0.750 in. and a thickness o f less than 0.375 in. All transmission data reported here are based on m e a s u r e m e n t s made w h e n the filters w e r e reasonably new. As interference

NH

6840

H20 +

C2

cont

1

cont 1

X

¢D

E I-

i!'i....

<

r~ EO O O .=

@ UD

a

~

N°-

~

z~

-~o

~

z

~-~ ~ ~ z

oS z ,s z

t~

z

t~

,~ z

N

r 3000

4000

5000

6000

7000

8000

Wavelength (Angstrom)

FIG. 1. The locations of the filters relative to cometary spectral features. The spectrum is of Comet Tuttle (Comet 1980 X I I I ) .

242

OSBORN ET AL.

mation to the standard s y s t e m between the B-stars and the G-stars. The aging effects of which we are aware include a significant increase in the " r e d l e a k " of one 3085 (OH) filter m a n u f a c t u r e d by Spectro-Film and a major shift toward longer wavelengths in the b a n d p a s s of several 3871 (CN) filters m a n u f a c t u r e d by MicroCoatings (no shifts have been reported in the 3871 filters made by Andover). T h e r e m a y have been other aging effects which h a v e not been reported to us. Finally, we note that about 20 sets o f s p e c trally similar, but physically larger, filters were p r o c u r e d by S. M. L a r s o n and distributed to o b s e r v e r s of the I H W N e a r Nucleus Studies N e t w o r k for imaging using CCD c a m e r a s . T h e s e imaging filters were manufactured to the s a m e spectral specifications as the p h o t o m e t r i c filters used in our work and had m u c h higher requirements for optical flatness. N e v e r t h e l e s s , there are some differences in the bandpasses. We have not explicitly evaluated the effects of these differences, but we expect that they will not affect the magnitudes of standard stars by more than a few hundredths of a magnitude. Most likely, the biggest differences between the imaging filters and the p h o t o m e t r i c ones will o c c u r in the coefficients for the linearization of the extinction for the 3085 filter and in the transformation of the c o m e t observations from magnitudes to fluxes.

b e c o m e s the dominant source of light because the atmospheric transmission is so much greater at the longer wavelength of the secondary bandpass than at that of the primary bandpass. This is partially responsible for the strong nonlinearity of the extinction with this filter. The transmission curve for the 3085 filter that was most c o m m o n l y used in this study is given in Table A1. 3365 Filter This filter was selected to m e a s u r e the Av = 0 sequence of the A3Hi - X3~ system of N H . Although the entire sequence is in the b a n d p a s s , virtually all the light is from the 0-0 band b e c a u s e the F r a n c k - C o n d o n factors are low for the other bands. The 3365 filter is not a part of the I A U system, and only three filters that were obtained for other programs exist. The transmission curves for the three filters are given in Table A2. The S1 filter was used for all of the standard star observations except that filter $2 was employed for the 1987 o b s e r v a t i o n s at Cerro Tololo. 3650 Filter This filter was designed to m e a s u r e the strength of the continuum in the ultraviolet. Weak emission bands of CO + and CO2 + are present in the wings of the filter, but their effect is not significant for most comets.

3085 Filter This filter was designed to measure the 00 band of the A252+ - X2H system of OH. Although the 1-1 band is in the wing of the filter, it does not contribute a large fraction of the total light. This filter is, in a sense, a special case because the primary bandpass is centered near 3075 A with a F W H M of about 80 /~ but there is also a secondary bandpass centered about 3580 A which is lower in transmission by two orders of magnitude but one order of magnitude wider. The s e c o n d a r y b a n d p a s s transmits a flux of a few percent of the primary bandpass for a flat s p e c t r u m , but at large air mass this

3871 Filter This filter was designed to m e a s u r e the Av = 0 sequence of the B2~, + - X2H (violet) system of CN. The heads of all bands in the sequence are included in the filter bandpass, but 90% of the flux is due to the 0-0 band because the 1-1 band is w e a k e r by a full order of magnitude. 4060 Filter This filter was designed to measure that portion of the A-X Swings system of C 3 that is least contaminated by other features, in particular the 3-0 band of CO + at the short

STANDARD

243

STARS FOR COMET PHOTOMETRY T A B L E A1

TRANSMISSION CURVE OF THE FILTER 3085/75-$2 A [A]

Trans.[~]

X [A]

Trans.[~]

A [A]

Trans.[~]

A [A]

Trans.[~]

2920 2930 2940 2950 2960 2970 2980 2990 3000 3010 3020 3030 3040 3050 3054 3060 3070 3080

0.000020 0.0000285 0.000050 0.0001075 0.000225 0,000485 0,00095 0.0020 0.0045 0.01125 0.03 0.095 0.25 0.385 0.3950 0.38875 0,3775 0.37375

3090 3100 3110 3120 3130 3140 3150 3160 3170 3180 3190 3200 3210 3220 3230 3240 3250 3260

0.36125 0.33125 0.24125 0.12125 0.0500 0.02125 0.010 0.00570 0.00325 0.002025 0.001325 0.000925 0.00065 0.000475 0.000360 0.000285 0.000215 0.000175

3270 3280 3290 3300 3320 3340 3360 3380 3400 3420 3440 3460 3480 3500 3520 3540 3560 3570

0.0001375 0.000110 0.0000975 0.00008 0.000055 0.0000475 0.0000375 0.00003375 0.0000325 0.0000375 0.000050 0.0000925 0.0001025 0.000275 0.0005375 0.0012125 0.0022875 0.0027375

3580 3590 3600 3620 3640 3660 3680 3700 3720 3740 3760 3780 3800 3820 3840 3860 3880 3990

0.002580 0.00275 0.00255 0.002275 0.002100 0.0020875 0.0020 0.0018625 0.0016625 0.001375 0.0010375 0.000750 0.0004875 0.0002875 0.00015 0.0000575 0.000015 0.0000025

w a v e l e n g t h side. The band s y s t e m extends o v e r at least 150 .~.

brational and rotational c o m p o n e n t s o f the Av = 0 s e q u e n c e o f the S w a n s y s t e m of C2.

4260 Filter

5140 Filter

This filter m e a s u r e s the strength of the 2-0 band o f the A2II - X2E + (comet-tail) s y s t e m o f CO ÷. B e c a u s e the strongest CO + feature, the 3-0 band, is inextricable from the C 3 e m i s s i o n , the filter w a s c h o s e n to c o v e r the 2-0 band w h i c h can be separated from the other features.

This filter w a s c h o s e n to measure the Av = 0 sequence o f the d3I]g - aaI-[u (Swan) system of C 2. The heads o f all bands in the sequence are included within the bandpass, with the majority of the flux due to the 0-0 band.

6840 Filter 4845 Filter This filter w a s designed to measure the strength o f the c o n t i n u u m in the blue. There is noticeable contamination by the high vi-

This filter was selected to measure the strength o f the continuum in the red. The bandpass is relatively free o f e m i s s i o n features. There is s o m e w e a k N H 2 e m i s s i o n ,

T A B L E A2 TRANSMISSION CURVES FOR THE 3365/70 FILTERS

[A]

Transmission Sl

3200 3225 3250 3275 3300 3325 3350

0.002 0.002 0.003 0.005 0.012 0.088 0.310

S2 0.002 0.002 0.003 0.005 0.010 0.082 0.300

[%]

A [A] S3

0.001 0.002 0.002 0.003 0.010 0.058 0.265

Transmission Sl

3375 3400 3425 3450 3475 3500

0.375 0.225 0.045 0.010 0.004 0.002

S2 0.368 0.240 0.050 0.011 0.003 0.002

[%] $3 0.332 0.290 0.080 0.013 0.003 0,002

244

OSBORN

ET AL.

T A B L E A3 AVERAGE CHARACTERISTICS OF THE IAU FILTERS FOR COMET PHOTOMETRY Filter (manuf.)

Peak A [A]

Transm. [%]

Width(.8) [A]

Width(.5) [~]

Width(.l) [A]

Width(.Ol) [A]

Andover

3650.1(5.5) 3644.2(4.1)

36.8(5.0) 29.5(2.9)

57.7(4.3) 68.3(1.3)

82.5(3.4) 88.1(1.7)

126 (4.8) 125 (2.9)

191 (6.5) 180 (4.0)

3871/50 Micro. Andover

3871.6(2.7) 3870.2(2.3)

26.4(3.0) 31.2(3.1)

31.3(4.4) 30.4(0.6)

44.2(3,9) 42.4(1.4)

70 (5.6) 64 (2.2)

115 (8.4) 92 (1.2)

4060/70 Micro. Andover

4057.9(2.0) 4054.1(1.8)

45.8(0.9) 48.7(2.0)

62.6(1.9) 59.1(3.5)

76.7(2.0) 70.5(2.9)

I00 (3.5) 91 (1.3)

137 (5.0) 117 (2.3)

4260/65 Barr

4259.8(5.8)

44.5(0.9)

62.1(1.4)

69.8(0.4)

76 (0.5)

79 (1.6)

4845/65 Micro. Andover

4848.1(5.6) 4849.0(1.1)

72.1(0.9) 73.9(0.8)

56.9(1.9) 52.4(1.0)

71.7(1.0) 65.0(0.6)

I00 (1.6) 90 (0.4)

143 (2.8)

5140/90 Micro. Andover

5140.4(5.2) 5141.3(4.5)

62.9(1.0) 73.9(1.7)

74.2(3.5) 71,3(2.7)

85.3(2.2) 82.6(4.2)

i08 (1.7) 106 (5.4)

142 (3.3) 156

6840/90 Barr

6840.3(15.)

78.8(1.6)

79.9(4.9)

90.9(4.4)

114 (4.2)

146 (5.4)

7025/225 Barr

7025.9(3.6)

79.1(1.3)

215 (1.3)

228 (1.2

259 (2,2)

311 (2.7)

3650/80 Micro.

but it does not appear to be a significant contributor to the flux. The filter does encompass the Fraunhofer B-band of telluric oxygen, a c o n s e q u e n c e being that the atmospheric extinction for the filter may depend on curve of growth effects. 7025 Filter

This filter was selected to measure the (0,6,0)-(0,0,0) band of H2 O+. In retrospect, it is clear that this filter was poorly specified. Because this band is very broad and the filter was designed to encompass all of it, the bandpass tends to be dominated by the continuum for many comets. There is also some contamination by the 6-0 emission of NH2. The characteristics for all the cometary photometry filters except 3085 and 3365 are summarized in Table A3. Also, although the manufacturers tried to maintain uniformity among filters, they were not entirely suc-

cessful. Therefore, the table gives, for each manufacturer, the mean and (in parentheses) the variance between sets for the peaktransmission wavelength and the full widths at 80, 50, 10, and 1% of peak transmission. One can see that for certain filters there is a systematic difference between the filters of different manufacturers that is greater than the random variations of the production. ACKNOWLEDGMENTS

Principal financial support for this work was provided by an International Halley Watch contract from the Jet Propulsion Laboratory, USA., to the University of Maryland. In addition, the authors thank the following for significant supplemental support: Central Michigan University for the award of a research fellowship and grant to W. Osborn; the National Aeronautics and Space Administration for support of R. Millis and D. Schleicher at Lowell Observatory through Grant NGR03-003-001 and for support of M. A ' H e a r n and U. Carsenty at the University of Maryland through Grant NSG-7322; the Government of Western Australia for support of P. Birch; and the Departamento de Invest-

STANDARD STARS FOR COMET PHOTOMETRY igacion y Bibliotecas of the Universidad de Chile for support of H. Moreno and A. Gutierrez-Moreno. A project of this magnitude is impossible without the assistance of many persons. Besides those observers listed in Table IV, we acknowledge the contributions of M. Berry, D. Edsall, J. Meyer, V. Vanysek, and J. Williams. REFERENCES A'HEARN, M. F. 1983. Photometry of comets. In Solar System Photoelectric Photometry (R. Genet, Ed.), pp. 3.1-3.33. Willmann-Bell, Richmond. A'HEARN, M. F. 1984. Standardized Filters for Comet Photometry. Final Technical Report on N S F Grant ASF-80-17319, Univ. Maryland, College Park. A'HEARN, M. F., AND R. L. MILLIS 1980. Abundance correlations among comets. Astron. J. 85, 1528-1537, A'HEARN, M. F., R. L. MILLIS, AND P. V. BIRCH 1981. Comet Bradfield 1979X: the gassiest comet? Astron. J. 86, 1559-1566. A'HEARN, M. F., R. L. MILLIS, AND D. T. THOMPSON 1983. The disappearance of OH from Comet P/Encke. Icarus 55, 250-258. A'HEARN, M. F., V. VANYSEK,AND H. CAMPINS 1984. Photometric standard stars. International Halley Watch Newsletter No. 4 21-24. A'HEARN, M. F., P. V. BIRCH, P. D. FELDMAN, AND R. L. MILLIS 1985. Comet Encke: Gas production and light curve. Icarus 64, 1-10. BALONA, L. A., AND F. MARANG 1988. The variability of 53 Psc. lnfo. Bull. Var. Stars 3157. BREGER, M. 1976. Catalog of spectrophotometric scans of stars. Astrophys. J. (Suppl.) 32, 7-88. CLARIA, J. J., R. F. SISTERO, AND E. LAPASSET 1987. Observaciones fotometricas en bandas moleculares del cometa Halley. Bol. Mex. Astron. Astrofis. 14, 651-659. COCrmAN, A. L. 1987. Another look at abundance correlations among comets. Astron. J. 93, 231-238. HARDORP, J. 1982. The sun among the stars. V. A second search for solar spectral analogs. Astron. Astrophys. 105, 120-132. HARDIE, R. H. 1962. Photoelectric reductions: measurement of airmass. In Astronomical Techniques, (W. A. Hiltner, Ed.), pp. 180-181. Univ. Chicago Press, Chicago. HAYES, D. S., AND S. W. LATHAM 1975. A rediscussion of the atmospheric extinction and the absolute

245

spectral-energy distribution of Vega. Astrophys. J. 197, 593-601. HOFFLEIT, D., AND C. JASCHEK 1982. The Bright Star Catalogue. Yale University Observatory, New Haven. HOFFLEIT, D., M. SALADYGA,AND P. WLASUK 1983. A Supplement to the Bright Star Catalogue. Yale University Observatory, New Haven. KOUBSKY, P., J. HORN, P. HARMANEC,C. T. BOLTON, R. W. LIONS, L. H. ILIEV, B. Z. KOVACEV, H. BozIc, AND K. PAVLOVSK! 1985. 96 Herculis: A remarkable early-type multiple system, lnfo. Bull. Var. Stars, 2778. LABS, D., AND H. NECKEL 1968. The radiation of the solar photosphere from 2000 A to 100 ~. Zeits. Astrophys. 69, 1-73. LOCKWOOD, G. W., D. T. THOMPSON, R. R. RADICK, W. H. OSBORN, W. E. BAGGETT, D. K. DUNCAN, AND L. W. HARTMANN 1984. The photometric variability of solar-type stars. IV. Detection of rotational modulation among Hyades stars. Publ. Astron. Soc. Pacific 96, 714-722. NEWBURN, R. L., AND H. SP1NRAD 1984. Spectrophotometry of 17 comets. I. The emission features. Astron. J. 89, 289-309. NEWBURN, R. L., AND H. SPINRAD 1985. Spectrophotometry of seventeen comets. II. The continuum. Astron. J. 90, 2591-2608. NEWBURN, R. L,, AND H. SPINRAD 1989. Spectrophotometry of 25 comets: Post-Halley updates for 17 comets plus new observations for eight additional comets. Astron, J. 97, 552-569. PFAU, W., AND B. STECKLUM 1986. Magnitudes of standard stars for IHW cometary photometry. Astron. Nachr. 307, 64. VANYSEK, V., AND M. WOLF 1985. Calibration of standard stars for the International Halley Watch. Bull. Astron. Inst. Czech. 36, 267-270. WISNIEWSKI, W., AND B. ZELLNER 1985. Standard stars for comet photometry. Icarus 63, 333-338. WOLF, M., AND V. VANYSEK 1987. The IHW narrowband photometry of comets Giacobini-Zinner and Halley. Bull. Astron. Inst. Czech. 38, 136-142. YOUNG, A. 1974. Atmospheric extinction: Errors in calculating air mass. In Methods o f Experimental Physics (N. Carleton, Ed., L. Marton, Ed. -in-chief) Vol. 12A, p. 146-152. Academic Press, New York.