445

ACCURATE TWO-COLOUR PHOTOMETRY AND ASTROMETRYFOR HIPPARCOS

DOUBLE STARS:



FIRST STATISTICAL RESULTS FOR A SAMPLE OF INTERMEDIATE BINARIES

1 2 3 3 1 2

E. Oblak , J. Cuyp ers ,P. Lamp ens , W. Seggewiss , M. Chareton , D. Duval

1

Observatoire de Besancon, France

2

Koninklijke Sterrenwachtvan Belgie, Belgium

3

Hoher List Observatorium, Germany

2. OBSERVATIONS, REDUCTION AND ABSTRACT

CALIBRATION

A large numb er of new astrometric and photometric

CCD measurements of intermediate visual binaries

The observations were gathered at ESO, La Silla

angular separation b etween one and fteen arcsec,

in Octob er 1991 J. Cuyp ers, in February 1992

literature values and Hipparcos data are compared.

W. Seggewiss, in August 1992, Decemb er 1993 and

The claimed mean relative p osition errors 0.02 arc-

August 1994 P. Lamp ens, in November 1992 and

sec for b oth the CCD and Hipparcos are con rmed.

Novemb er 1994 E. Oblak. The 91-cm Dutch tele-

For 8 p er cent of the systems in the sample we had

scop e was used with di erent CCD camera's with

no Hipparcos solution and in ab out 6 p er centofthe

pixel sizes in the range 0.3 to 0.5 arcsec.

cases the CCD and Hipparcos solutions were to o dif-

ferent to be used in the comparison. The disp er-

sion between literature values and CCD results for

The raw CCD images were treated in a standard way,

the joint magnitudes of the systems was 0.03 mag.

but a sp ecially develop ed pro le tting pro cedure

A comparable disp ersion is found only for the Hip-

Cuyp ers 1997 was used for extracting the relative

parcos data when colour corrections for b oth comp o-

p ositions and di erential magnitudes when the pro-

nents supplied by the CCD observations are taken

les of the comp onents overlap.

into account. In that case the mean errors on in-

dividual comp onents from Hipparcos data are ab out

Photometric observations were carried out with

0.04 mag for the primaries and 0.08 mag for the sec-

Bessel V and Gunn i lters. Observations of stan-

ondaries.

dard stars from a list compiled by Grenon 1991,

Key words: double stars, photometry, astrometry.

based on lists of Landolt 1983 and Menzies 1989,

were used to correct for extinction and to transform

the data into the standard V R I system.

1. INTRODUCTION

To calibrate the relative astrometry a set of wider

pairs taken from the lists of Brosche and Sinachop ou-

From 1991 on, a photometric observational pro-

los 1988, 1989 were measured. Stars were trailed

gramme of intermediate visual binaries angular sep-

over the frames for a rst estimate of the orienta-

aration b etween one and fteen arcseconds, di erence

tion of the CCD detector. In some campaigns op en

in magnitude less than 4 mag has b een carried out

clusters could already be used to calibrate the as-

in the framework of a Europ ean Network of Lab ora-

trometry Sinachop oulos al. 1993. This resulted in

tories Oblak et al. 1992a. The stars selected for the

a very accurate scale and orientation calculation as

programme had no complete comp onent photometry

shown b elow.

listed in the data base compiled at Besancon Kun-

dera et al. 1997.

Observations were made at di erent observatories

La Palma, Teide and Calar Alto, Spain and Haute-

Provence, France, but here we rep ort on data ob-

3. EVALUATION OF THE CCD DATA

tained solely at the Europ ean Southern Observatory

Chile, where several campaigns and a Key Pro-

gramme Oblak et al. 1992b for this purp ose were

carried out.

Since each CCD observation consists of a rep etitive



sequence of frames up to 15 for large
m and b e-

Based on observations obtained at the Europ ean Southern

Observatory, Chile and on data from the Hipparcos astrometry

cause some double stars were measured in di erent

satellite.

runs, an internal evaluation is p ossible.

446

3.1. Astrometry Toevaluate the quality of the photometric observa-

tions a comparison was made b etween the measured

and calibrated global V magnitudes of the systems

Internal p osition errors are smaller than 0.015 arcsec,

joint magnitude of comp onent A and B and the

while the errors on the means are 0.004 arcsec in gen-

values available in the catalogues Mermillio d et al.

eral. An increase of the error with larger di erential

1996, Kundera et al. 1997. Of 133 double stars of

magnitudes is obvious. There is also a degradation of

our sample a global V magnitude was available, ei-

the consistency for separations smaller than 3 arcsec

ther from Stromgren 96 or from Geneva photome-

due to seeing limitation and for separations larger

try 72. The histogram of the di erences is given in

than 9 arcsec mayb e due to loss of isoplanicity.

Figure 1. We do not know whether the slight o set

0.006 mag b etween the two sets is signi cant. The

disp ersions of the di erences are 0.028 mag for the

The mean value of the di erences in p ositions for the

Stromgren and 0.030 mag for the Geneva data. In

28 systems observed more than once was 0.024 arc-

this disp ersion the errors on the literature values are,

sec, but the median was as small as 0.013 arcsec, since

of course, also included, so 0.03 mag is undoubtedly

4 systems have di erences greater than 0.07 arcsec.

avery conservative upp er limit of the mean error on

The errors due to the calibration of the scale and ori-

the CCD magnitudes. No signi cant dep endence of

entation of the CCD for each campaign are included

the di erences on the system parameters was found.

here and some of the systems were remeasured be-

A more detailed error analysis is given in Lamp ens

cause the rst observation was uncomplete or not

et al. 1997.

satisfactory.

0.15

3.2. Photometry Hipparcos error Hipparcos − CCD

CCD errors (internal)

Internal errors on the di erential and derived com-

p onent magnitudes are very small <0.005 mag.

There is some degradation for separations smaller

3 arcsec. The standard stars measurements

than 0.10

show deviations smaller than 0.03 mag after calibra-

tion and transformation to the standard system. The

blue V I < 0:2 mag and, as exp ected, the very

red stars V I>1:2 mag sometimes deviate more.

Only a limited numb er of double stars 14 was mea-

Arcseconds The median deviation of the

sured more then once. 0.05

magnitudes was only 0.007 mag for the primaries,

0.013 mag for the secondaries and 0.012 mag for the

di erence in magnitude b etween A and B. The mean

and standard deviation of the di erences were 0.02

mag, but it is not excluded that some comp onents

are variables. 0.00 0 5 10 15

Separation (Arcseconds)

Figure2. The claimed internal errors on CCD and Hip-

parcos relative astrometry and the position di erences be-

tween CCD and Hipparcos results for 416 double stars.

4. COMPARISON WITH HIPPARCOS DATA

4.1. Astrometry

A total of 488 double star systems were used for a

detailed comparison b etween our CCD data and the

Hipparcos results the 28 systems measured twice

were considered only once. For 41 systems 8 per

cent we had no Hipparcos solution, while for 31

systems 6 per cent the p ositional di erences were

larger than 0.15 arcsec. The errors on the Hipparcos

solutions for the latter cases were well ab oveaverage.

Some of these systems had small separations and, as

a consequence, larger errors on the CCD measure-

Figure1. The distribution of the di erences between the

ments. In other cases a di erent, wide, usually faint,

global V magnitudes in literature and the CCD results.

comp onentwas considered as the secondary. In two

447

cases Hipparcos detected a very close companion, not colour information is necessary to compare the Hip-

recognized in the CCD images. For 15 other systems parcos magnitudes to the Johnson V magnitudes,

wehave no explanation for the large di erences yet. otherwise errors could be as large as 0.2 mag Fig-

ures 4a and 5a. We used our measured V I colour

For the sample of 416 double stars the b est matchbe-

index for the transformation from Hp to V magni-

tween the CCD and Hipparcos p ositions was searched

tudes following the indications in the Hipparcos cat-

in the least-squares sense for each observing cam-

alogue ESA 1997. The errors after correction are

paign separately. For some campaigns no orientation

given in Figures 4b and 5b and the distribution of

change was necessary, since it was found to b e accu-

these errors is also shown Figures 4c and 5c. The



rate the 0.1 level, due to the well de ned star trails

rms of the errors for the A comp onents is ab out

or the go o d calibration with the aid of the astromet-

0.05 mag and ab out 0.09 mag for the B comp onents.

ric standard elds. A scale correction was usually

Since we estimated the error of the CCD V magni-

necessary with a nal accuracy at the 0.1 per cent

tudes to b e 0.03 mag, in agreement with the compar-

level.

ison to the literature values, a go o d estimate of the

The di erences in p ositions are given in Figure 2, errors on V magnitudes derived from Hipparcos Hp is

together with the estimated Hipparcos errors and in- ab out 0.04 mag on average for the A comp onents and

ternal CCD errors. Estimates of the errors on the 0.08 mag for the B comp onents. Individual errors on

calibration of the CCD data are included in the his- the V magnitudes are related to the magnitudes of

togram of the distribution of the errors Figure 3.

the comp onents.

The mean error of the Hipparcos sample 0.017 arc-

sec and the mean error of the CCD data 0.021 arc-

sec are in excellent agreement with the mean value

5. CONCLUSIONS

of the p osition di erences 0.026 arcsec. Individual

errors dep end on the separation and the magnitude

di erences of the comp onents.

With the data set presented here excellent p ossibil-

ities exist to evaluate and complement the Hippar-

cos measurements of the `intermediate' double stars.

We can already state that the overall accuracy of the

ma jority 85 to 95 p er cent of the double star astro- y Hipparcos is con rmed for sepa-

150 metric solutions b Only in a few cases the

CCD errors (ext.) rations from 1 to 15 arcsec.

uch larger, the solutions are not compat- Hipparcos errors errors are m

Hipparcos − CCD

ible with the system referred to by the HIP number

or Hipparcos had no solution for the system. This

will b e investigated further.

100

On ground based CCD astrometry we can say that,

the level of accuracy with the describ ed instrumen-

tation dep ends on the p ossibility of external calibra-

tion, while the internal precision on the relative p osi-

tions can b e extremely go o d 0.007 arcsec. If either ts or an astro-

50 a set of reliable Hipparcos measuremen

metric eld e.g. a star cluster is available to do the

nal calibration, mo dest instrumentation will reach

an accuracy of 0.02 arcsec, which is comparable to the Hipparcos results.

0

CCD photometry presented here resulted in a

0.00 0.02 0.04 0.06 0.08 0.10 The

of accurate and well calibrated photo-

Arcseconds large sample

metric data for the comp onents, as indicated by the

comparison with literature values on the global pho-

Figure 3. The distribution of the errors on CCD and

tometry of the systems. Errors on the magnitudes of

the comp onents are in general smaller than 0.03 mag.

Hipparcos relative astrometry and the position di erences

between CCD and Hipparcos results for 416 double stars.

To make full use of the Hipparcos double star pho-

tometry, the colours of the individual comp onents are

necessary. In that case comp onent magnitudes are

4.2. Photometry

accurate at the 0.04 to 0.08 mag level, while other-

wise errors up to 0.2 mag in V will remain.

We obtained very reliable V and I magnitudes for 367

double stars of our sample so far. For 19 systems the

di erence in one of the comp onents was clearly to o

large > 0:3 mag to b e included in a detailed com-

parison 6 p er cent. For the remaining sample the

ACKNOWLEDGMENTS

comp onent information V magnitude of comp onent

A and B separately was compared to the Hipparcos

magnitudes.

We thank ESO for granting a Key Programme for

this work and the Hipparcos Science Team and

A direct comparison cannot b e done: at least some 448

Magnitude differences without colour correction (component A) Magnitude differences without colour correction (component B) 0.3 0.3 0.2 0.2 CCD CCD

A 0.1 B 0.1 0.0 0.0 Hip − V Hip − V A −0.1 B −0.1 −0.2 −0.2 Difference V −0.3 Difference V −0.3 −0.4 −0.4 −0.5 0.0 0.5 1.0 1.5 2.0 −0.5 0.0 0.5 1.0 1.5 2.0

V−I A V−I B

Magnitude differences with colour correction (component A) Magnitude differences with colour correction (component B) 0.3 0.3 0.2 0.2 CCD CCD

A 0.1 B 0.1 0.0 0.0 Hip − V Hip − V A −0.1 B −0.1 −0.2 −0.2

Difference V −0.3 Difference V −0.3 −0.4 −0.4 −0.5 0.0 0.5 1.0 1.5 2.0 −0.5 0.0 0.5 1.0 1.5 2.0

V−I A V−I B

Distribution of differences (component A) Distribution of differences (component B) 50 50

40 40

30 30

20 20

10 10

0 0 −0.3 −0.2 −0.1 0.0 0.1 0.2 0.3 −0.3 −0.2 −0.1 0.0 0.1 0.2 0.3

VAHip − VACCD VBHip − VBCCD

Figure4. The di erences between the Hp and the CCD V Figure5. The di erences between the Hp and the CCD V

magnitude a, between the from Hp derived V magnitude magnitude a, between the from Hp derived V magnitude

and the CCD V magnitude of the A components b and and the CCD V magnitude of the B components b and

their distribution c. their distribution c.

Workshop Visual Double Stars: Formation, Dy-

J.L. Falin for the preliminary data on double stars

namics and Evolutionary Tracks, eds. J.A. Do-

for which CCD results were previously transmitted

cob o, A. Elip e, H. McAlister, ASSL series, in press

to the Reduction Teams. This research has made use

of the Besancon Double and Multiple Star data base,

Landolt A.U., 1983, AJ, 88, 439

op erated at Besancon France, the Lausanne Photo-

Menzies, J. 1989, SAAO Circ., 13, 1-13.

metric data base, op erated at Geneva Switzerland

and the Simbad data base, op erated at CDS, Stras-

Mermillio d, J.C., Hauck, B. and Mermillio d, M.,

b ourg France. We also acknowledge nancial aid

1996, `General Catalogue of Photometric Data'.

from the French Minist ere de la Recherche et de la

[http://obswww.unige.ch/gcpd/gcpd.h tml]

Technologie at the start of this pro ject in 1992 and

Oblak, E., Argue, A.N., Brosche, P., et al., 1992a,

of the Research Programme of the Fund for Scienti c

IAU Coll. 135, ASP Conference Series 32, eds.

Research F.W.O., Flanders Belgium.

H.A. McAlister, W.I. Hartkopf, 454-456.

Oblak, E., Argue, A.N., Brosche, P., et al., 1992b,

ESO Messenger, 69,14

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Brosche, P., Sinachop oulos D., 1989, Bull. Inf. CDS,

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