643



THE DISTANCE OF THE AND NEARBY CLUSTERS

1 2 2 2 2

J.-C. Mermillio d ,C.Turon , N. Robichon , F. Arenou , Y. Lebreton

1

Institut d'Astronomie, Universit e de Lausanne, Switzerland

2

DASGAL/URA CNRS 335, Observatoire de Paris, 92 195 Meudon Cedex, France

ABSTRACT to account for the diference b etween the

Hyades and Pleiades clusters. This correction is

based on relations b etween metallicity estimators like

 U B orm and absolute magnitude di erence

We have determined the distance to nearby op en

1

M . Such a distance mo dulus correction should b e

clusters from the means of Hipparcos parallaxes re-

V

even larger b etween the and Coma Berenices

computed from the intermediate data. The rst re-

clusters, b ecause the di erence in chemical comp osi-

sult worth noticing is that Praesep e, Coma Ber,

tion is larger. Direct distances obtained by Hipparcos

Persei and Blanco 1 de ne the same main sequence

are therefore crucial to x the distance of the nearby

in spite of their di erences in metallicity. The second

clusters indep endently of any a priori knowledge of

and more surprising result is that the Pleiades, and

their prop erties, age, metallicity and C, N, O abun-

probably IC 2391 and 2602, de ne a sequence ab out

dances.

0.5 mag fainter than the previous one. The mean

parallax of the Pleiades is 8.60 mas, corresp onding to

116 p c. There is no way to reconcile the trigonomet-

The results obtained from the analysis of Hipparcos

ric and photometric distances of the Pleiades. These

parallaxes do not corresp ond to any exp ectation in

results contradict the commonly accepted interpre-

this resp ect: the relative p ositions of the sequences

tation of the metallicity e ects. The results are the

of each cluster in the colour-magnitude diagram are

same for the UBV, uvby and Geneva photometric sys-

not simply correlated with metallicityorany known

tems. The parameter resp onsible for these di erences

parameter. Accumulated evidences do not p ermit to

has not yet b een identi ed, although they could be

cast doubts on Hipparcos data, but show that the

accounted for by a signi cant di erence in the helium

standard analysis metho ds, mainly fo cused on the

abundance. However there is presently no indication

e ect of metallicity alone, were to o naive.

ofamuch higher helium abundance in nearbyyoung

clusters or asso ciations. The present result could b e

linked to the so-called Hyades anomaly.

2. MEAN PARALLAX DETERMINATION

Key words: op en cluster; Hipparcos; distance; chem-

Although the Hipparcos parallaxes are probably un-

ical comp osition.

biased, the op eration mo de of the satellite induces

correlations between the abscissae obtained on a

given `reference great circle'. This e ect may b e sig-

1. INTRODUCTION

ni cant for within a few degrees of the sky, such

as clusters Arenou et al. 1997a. As a consequence,

the straight average of the parallaxes of the cluster

stars is not an optimal estimate of the mean clus-

The direct determination of the distance to nearby

ter parallax, and moreover the standard error on this

op en clusters is of paramount imp ortance b ecause

estimate would b e clearly underestimated.

op en clusters are cornerstones in metallicity deter-

mination and distance-scale calibration, while their

ages are imp ortant to understand the characteristics

The prop er way to obtain an optimal mean parallax is

of di erent p opulations and their evolution. Conse-

to use the Hipparcos intermediate astrometric data.

quently op en clusters have b een widely used to de-

The correlation between the abscissae of two stars

ne the Zero Age Main Sequence ZAMS, lo ci of

on each reference great circle has b een calibrated as

constant ages empirical iso chrones and of constant

a function of the abscissae di erence allowing to ob-

chemical comp ositions in photometric diagrams.

tain the full covariance matrix between all the ob-

servations for the given cluster. These observations

So far, only indirect reasoning has b een used to cor-

together with their covariance matrix were then pro-

rect the distances obtained by the main sequence t-

cessed with a least-squares programme where either

ting pro cedure for metallicity e ects. Turner 1979

the mean parallax or prop er motion or b oth have

estimated a distance mo dulus correction of 0.15 mag

b een considered as the same unknowns for the clus-

ter, the other astrometric parameters of the stars re-



Based on data from the ESA Hipparcos satellite.

maining determined individually.

644

van Leeuwen & Hansel Ruiz 1997 have indep en- c parameter: d = c +0:37 in the sp ectral range

1 1

dently investigated the distance of the Pleiades and covered by the Hyades A0 - G8. The signi cation

incorp orated more precise ground based prop er mo- of  varies along the main sequence. In the domain

tions determined from astrometric plates sp ending a of solar-typ e stars, it is sensitive to chemical com-

large time interval. They basically obtained the same p osition. The e ect of the di erent chemical com-

value of the mean parallax. p osition b etween the Hyades and Coma Berenices is

clearly apparent in Figure 1. This di erence has also

b een well do cumented in the uvby system by Craw-

ford & Stromgren 1966 who analysed in detail the

3. CLUSTER DISTANCES

various diagrams built on the uvby data. The same

plane Figure 2 for Praesep e  lled squares and the

Pleiades op en squares shows ab out the same dif-

The cluster stars chosen for the Hipparcos mission

ference b etween the two sequences. The Hyades and

were selected according to the available memb ership

Praesep e share the same sequence in this plane and

criteria prop er motion, radial velo city and photom-

the other nearby clusters discussed in this study fol-

etry. Known double stars were rejected from the

low the same trend as the Pleiades or Coma Berenices

sample. Hipparcos parallaxes and prop er motions

clusters.

con rm the memb ership of the stars included in the

initial sample.

The cluster mean parameters derived from the

present analysis are displayed in Table 1. It gives

the cluster designation, numb er of memb er stars used

in the solution, mean parallax from Hipparcos,  on

this parallax, the inferred distance in [p c], the e ect

on the distance of a change of 1  on the mean par-

allax + 1  , { 1  , the distance mo dulus and

 

again the e ect in magnitude of a change of 1  on

the mean parallax + 1  , {1  , the next three

 

columns give the distance mo duli, EB-V, [Fe/H]

from Lynga's 1987 and ages from the recent liter-

ature. Part of the di erence with Lynga's mo duli is

due to the change of the Hyades distance m M

= 3.33 determined by Perryman et al. 1997, as

presented by Brown et al. 1997 at this conference

Lynga still used 3.01. But, even with this value of

the Hyades distance, the distances of the three `p ecu-

liar' clusters Pleiades, IC 2391 and IC 2602 based

on Hipparcos parallaxes are smaller than those de-

duced from Lynga's compilation. The distance for

Figure 1. Comparison of the sequences of the Hyades

Persei is left unchanged.

 l led squares and Coma Ber open squares clusters.

The di erence is interpreted in terms of chemical compo-

The [Fe/H] values are from Cayrel de Strob el 1990

sition:  increases with increasing metal content.

for Coma Ber, Pleiades, Praesep e and Per, from

Edvardsson et al. 1995 for Blanco 1 and from

Lynga 1987 for IC 2391 and IC 2602. No recent

determination of the iron abundance in IC 2391 and

IC 2602 has b een found in the literature, nor in the

new catalogue of Th evenin 1997.

4. NEW WORKING PARADIGM

It has b een known for a long time, and con rmed

by the results presented at this conference by Giusa

Cayrel Cayrel de Strob el et al. 1997, that the

Hyades and Coma Ber clusters have di erent metal-

licities. This is displayed in Figure 1 which depicts

a p ortion of the d,  plane from the Geneva pho-

tometric system with the sequences of Hyades  lled

squares and Coma Berenices op en squares. d and

 are two reddening-free parameters de ned by the

linear combination of colour indices:

d = U B  1:430 B B 

1 1 2

Figure 2. Comparison of the sequences of the Praesepe

 = U B  0:832 B G

2 2

 l led squares and Pleiades open squares open clus-

ters. The di erence is interpreted in terms of chemical

Golay 1973, 1980. d is a measure of the Balmer dis-

composition:  increases with increasing metal content.

continuity. It has a go o d correlation with Stromgren

645

Table 1. New cluster distances.

Cluster N   d { + m{M { + m{M EB{V [Fe/H] Age Ref



[mas] [mas] [p c] [p c] [p c] [mag] [mag] [mag] [mag] [mag] [dex] [Myr]

Lynga

Coma Ber 26 11.34 0.22 88.2 1.7 1.7 4.73 0.04 0.04 4.49 0.00 {0.065 455 1

Pleiades 51 8.60 0.24 116.3 3.2 3.3 5.33 0.06 0.06 5.61 0.04 +0.026 100 2

IC2602 13 6.81 0.22 146.8 4.6 4.9 5.83 0.07 0.07 6.02 0.04 {0.200 30 3

IC2391 7 6.78 0.25 147.5 5.2 5.6 5.84 0.08 0.08 5.96 0.01 {0.040 30 3

Praesep e 24 5.65 0.31 177.0 9.2 10.3 6.24 0.12 0.12 5.99 0.00 +0.095 830 1

Per 32 5.43 0.22 184.2 7.2 7.8 6.33 0.09 0.09 6.36 0.09 +0.061 50 2

Blanco 1 11 3.96 0.48 252.5 27.3 34.8 7.01 0.25 0.28 6.97 0.02 +0.230 100 4

References for the ages: 1 Lynga 1987, 2 Meynet et al. 1993, 3 Stau er et al. 1997, 4 this work.

parameter  or the iron abundance [Fe/H], do not In view of the di erences noticed in the d,  planes,

explain the observed relative p ositions of the main se- it is interesting to lo ok at the relative p ositions of

quences of the nearby op en clusters. Cayrel de Stro- the main sequences in the colour-magnitude diagrams

bel et al. 1997 have shown, from high-resolution M ; B V   in the Geneva photometry. This

V 2 1 o

sp ectra, that the of these clusters were system has b een chosen b ecause it provides homo-

not so di erent from that of the Sun, with the excep- geneous magnitudes and colours from one source

tion of the Hyades cluster. Rufener 1989, which is not the case for the other

systems. We nd the following situation:

The estimate of chemical comp osition is a dicult

question and it is now evident that the photometric

 Coma Berenices sequence well matches that of

and sp ectroscopic approaches may pro duce di ering

Praesep e or Hyades in spite of the mentioned

results, which is not surprising b ecause the measured

di erence in chemical comp osition Figure 3.

features are di erent. For example, from a photomet-

The same is true for the Persei and Blanco

ric p oint of view the Hyades and Praesep e are very

1 clusters;

similar, while in a recent analysis of high disp ersion

sp ectra, Praesep e app ears to be more solar in iron

 the Pleiades de nes a main sequence di erent

abundance Cayrel de Strob el et al. 1997. Blanco 1

from that of Praesep e or Coma Ber Figure 4.

o ers a second example: Eggen 1970 claimed that

This di erence is large ab out 0.5 mag and very

this cluster shows a mo derate metal de ciency, while

surprising due to the similarity of the b ehaviour

avalue of [Fe/H] = +0.23 was obtained by Edvards-

of the Pleiades and Coma Berenices clusters in

son et al. 1995 from a high disp ersion sp ectroscopic

the d,  diagram. The two young southern

analysis, which makes Blanco 1 even more metal rich

op en clusters IC 2391 and IC 2602 seem to follow

than the Hyades. This is at o dds with the observed

the same trend as the Pleiades. Results from a

U B  excess: the metal de ciency is con rmed by

p oster displayed at this conference by Robichon

the b ehaviour of Blanco 1 in the photometric dia-

et al. 1997 show that the sequences of other

grams of the uvby and Geneva systems.

clusters, and esp ecially that of NGC 2516, are

also lo cated well b elow the Hyades-Praesep e se-

The results shown by Figures 3 and 4 for the Geneva quence;

photometric system are con rmed in the UBV and

 the di erence is also observed between the

uvby systems. The same clusters Coma Berenices,

Pleiades and Persei clusters which are often

Persei, Blanco 1 match the sequence of Praesep e,

asso ciated as young clusters to compare their

while the di erence of the Pleiades is also quite strik-

prop erties to those of the older pair formed by

ing. The main di erence between the Pleiades and

the Hyades and Praesep e.

the other clusters is really on the magnitude: the

diagrams M , d Figure 5 and M ,  Fig-

V V

ure 6 for Praesep e  lled squares and the Pleiades

The analysis of the three clusters, Praesep e, Coma

op en squares clearly show that all the sequences

Berenices and Pleiades is sucient to describ e the

are displaced vertically. Evolutionary e ects should

whole problem: Praesep e and Coma Berenices are

b e taken into account in the appreciation of the upp er

di erent in the d,  diagram but de ne the same

main sequence relative p ositions. Similar conclusions

zero age main sequence, while the Pleiades and Coma

are obtained by examination of the M ;  diagram

V

Berenices clusters are very similar in the d,  dia-

Figure 7.

gram, but strongly di er in the colour-magnitude di-

agram. The di erence b etween the p osition of Prae-

The comparison of the cluster pairs formed by

sep e and the Pleiades 0.5 mag is larger than de-

Persei and Coma Ber Figure 8, Persei and the

termined on the basis of the di erence in chemical

Pleiades Figure 9 show that Per is very similar

comp osition alone. Turner 1979 obtained a ZAMS

to Coma Ber but di ers from the Pleiades.

correction of 0.15 mag from the observed U B  ex-

cess of 0.03 mag. Furthermore, such an e ect is not

Another presentation at this conference de Zeeuw observed b etween Praesep e and Coma Ber, although

et al. 1997 demonstrated that the distances to the the di erence in chemical comp osition is commonly

asso ciations de ning the Gould Belt are reduced by considered to be larger. Therefore, it can safely be

ab out 10 p er cent. Therefore the e ect found for the argued that, contrary to the exp ectation, the di er-

Pleiades, IC 2391 and 2602 is not a unique example. ences in chemical comp osition, as measured by the

646

Figure3. Comparison of the sequences of Praesepe  l led squares and Coma Ber open squares clusters. The distance

is derived directly from Hipparcos paral laxes. The agreement of the sequences is very good in the unevolvedpart, in the

range 0.2

2 1

The main factor could be the helium abundance. If the photometric distances to these asso ciations are

Mo dels computed by Y. Lebreton Figure 10 show primarily based on that to the Pleiades, the smaller

that the large di erence could b e explained by an en- distance is then exp ected. If their distances are more

richment in helium for the Pleiades. The existence of indep endent of that of the Pleiades, it o ers another

a larger helium abundance in the Gould Belt asso cia- example of the same di erence. In b oth cases, it adds

tions could nd some supp ort in the observations by credibility to the Pleiades' new parallax.

Nissen 1976 and Peterson & Shipman 1973 who

found that helium abundance in Sco-Cen, Lac OB1

and Ori OB1 is larger than that in NGC 2264, h &

5. WHAT IS THE GOVERNING FACTOR?

Persei or Cep OB3. The values found in the liter-

ature for the Pleiades Klo chkova& Panchuk 1986

is probably not reliable b ecause of the large di er-

ences between these values and those published by

The colour indices used as temp erature indicator,

Wol & Hasley 1985 for clusters in common. How-

namely B V , B V and b y are not very sensi-

2 1

ever the change in abundance needed to repro duce

tive to small di erences in metallicity around the so-

the 0.5 mag e ect observed in the colour-magnitude

lar value due to comp ensation of blanketing b etween

diagram is to o large to b e acceptable.

the lters B and V on the one hand and b and y

2 1

on the other. Therefore the use of a single zero main

sequence is probably appropriate for many op en clus-

ters. However the is sensitive,

Abundances of C, N, O elements could also play at least in the case of the Pleiades and of the Gould

a role. Di erences have b een found by Alexander Belt asso ciations, to an unknown factor which makes

1986 in the C molecule measured in the Hyades them app ear fainter for their distance. Comparisons

2

and Coma Berenices. However, the problem is more of the apparent V magnitudes of Pleiades stars be-

complex, b ecause in the case of the Hyades and Coma tween the three photometric systems Geneva, UBV

Berenices clusters we have to understand why they and uvby show a rather good agreement within 

share the same main sequence. Therefore any di er- 0.02 mag and, as a systematic error of 0.5 mag in the

ence in element- or molecule abundances worsens the zero p oint is excluded, there is no reason to susp ect

problem instead of solving it. the measured magnitudes.

647

Figure4. Comparison of the sequences of the Praesepe  l led squares and Pleiades open squares clusters. For B V >

2 1

0.20, the di erenceisroughly constant and reaches 0.5 mag. The scatter observedinboth sequences is mainly due to the

presence of binary stars Mermil liod et al. 1992, 1994.

sistency in connection with the determination of its 6. THE HYADES ANOMALY

distance'. The resulting distance mo duli are rather

di erent from those obtained by standard main se-

The results from the analysis of Hipparcos data and

quence tting pro cedure. If the distance mo duli of

the problem raised by the distance to the Pleiades are

his Table 6 are adjusted to the Hipparcos Hyades

probably related to the so-called Hyades anomaly de-

{ Praesep e distances, that of the Pleiades b ecomes

scrib ed by Crawford & Stromgren 1966. In the c ,

close to the direct Hipparcos determination. This

1

b y  plane, the sequence of unevolved Hyades stars

correction is admittedly crude and it would b e neces-

deviates from the ZAMS sequence of eld stars with

sary to redo the calibration with the latest values, but

the same overall metal abundance as the Hyades. Af-

at least the order of magnitude of the e ect has the

ter a thorough discussion of the anomaly, Stromgren

right size. However the corrected mo duli for Persei

et al. 1982 have suggested that it is due to a he-

and Coma Berenices are in bad agreement with those

lium abundance di erence. However, a combination

deduced from Hipparcos parallaxes.

of high resolution sp ectroscopybyCayrel de Strob el

et al. 1997 and new mo dels adapted to the Hyades

chemical comp osition discussed at this conference by

Furthermore, in Nissen's 1988 Figure 9, the clusters

Lebreton et al. 1997 shows that the helium abun-

that de ne the same zero age main sequence Hyades,

dance of the Hyades is Y = 0.26  0.02, and close

Praesep e, Coma Ber and Persei are on the same

to the solar one. This con rms earlier conclusion

line de ned by c = 0.25 [Fe/H]. The Pleiades and

o

reached by Dobson 1990.

NGC 2516 are clearly outside. Both clusters are well

b elow the Hyades main sequence as shown in this

Alexander 1986 has shown that four parameters are

contribution for the Pleiades and by Robichon et al.

needed to understand the colours of G dwarfs. Fur-

1997 for NGC 2516.

ther insight has b een provided by Nissen 1988 who

observed unevolved F-typ e stars in 7 op en clusters.

He estimated photometric values of [Fe/H] from m

o

Recently, Joner & Taylor 1995 have obtained new and the value of c for each cluster. He corrected the

o

uvby data and found that Praesep e do es not share absolute magnitude M by the factor fc ,f b eing

V o

the Hyades anomaly and is undistinguishable from de ned by Crawford 1975, 1979. As stated by Nis-

Coma Berenices in that regard. sen, `the c anomaly of a cluster leads to an incon- o

648

Figure 5. Comparison of the sequences of the Praesepe Figure 6. Comparison of the sequences of the Praesepe

 l led squares and Pleiades open squares clusters. This  l led squares and Pleiades open squares clusters. The

plane is equivalent to M ;c . al l sequence of the Pleiades is displaced vertical ly.

V 1

will b e however dicult to prove the validity of this

7. DISCUSSION

assumption b ecause no direct estimates of other dis-

tances can be obtained. Furthermore it cannot be

excluded that similar anomalies also exist elsewhere

In the present situation, no simple correlation can

in the solar vicinity.

be deduced from the available data. When clusters

are taken by pairs of similar or di erent chemical

The rst question is thus to determine if the Pleiades

comp osition, all situations arise:

anomaly is an isolated case or a frequent one. The

answer is obviously not straightforward. The second

 clusters with similar b ehaviour in the d, 

question is whether there could be a 1.5 mas error

plane either de ne the same main sequence in

in the parallax of the Pleiades: although errors in

the colour-magnitude diagram, as shown by the

Hipparcos data cannot b e precluded, various checks

pair Coma Ber { Persei, or de ne sequences

have b een done ab out the parallax zero-p oint and

that di er by 0.5 mag on the absolute magnitude

the systematic error was found to be negligible on

axis, as shown by the pairs Persei { Pleiades

the average Arenou et al. 1997b.

or Coma Ber { Pleiades;

In another domain, the low detection rates of X-ray

 conversely, clusters with di erent b ehaviour in

sources in Praesep e in comparison to the rates ob-

the d,  plane, like Praesep e and Coma Ber, or

served in the Hyades Randich & Schmitt 1995 is

Praesep e and Blanco 1, de ne closely the same

another fact revealed by space observations of nearby

main sequence.

clusters that challenges the present understanding of

our visible Universe.

Wehave so far relied mainly on one parameter, the

metallicity, as measured by the ratio [Fe/H] or sev-

eral photometric parameters, and assumed that the

8. CONCLUSION

abundances of H and He were more or less similar.

Now, we p erhaps need to determine not only [Fe/H]

but also the helium abundance. Even if the varia-

The analysis of Hipparcos parallaxes for nearby clus-

tions of these abundances are not correlated, it could

ters have pro duced a complex result whichchallenges

happ en that their e ect sometimes cancels, namely

the common ideas and practice. On the one hand,

if Y /Z  5. This would explain why no di er-

Praesep e, Coma Berenices, Persei and Blanco 1

ence is observed b etween Praesep e and Coma Ber or

de ne a common main sequence, and, on the other

Blanco 1.

hand, the Pleiades, together with IC 2391 and IC

2602, de nes another sequence ab out 0.5 mag fainter.

The Pleiades anomaly could be a p eculiarity of the

No factor explaining this situation has yet b een

Lo cal Asso ciation describ ed by Eggen 1983a, 1983b

found. Metallicity e ects alone can b e probably ruled

in which he included the Pleiades, IC 2602 and NGC

out. Di erences in the abundances of helium and/or

2516, three clusters a ected by the Pleiades syn-

C, N, O elements could explain the observations.

drome, and of the Gould Belt. If the link between

However the e ect is not of the order of a few hun-

the observed di erences and the memb ership to the

dredths of magnitude: it reaches half a magnitude.

Lo cal Asso ciation or the Gould Belt is correct and

The displacement of the Pleiades sequence is not due explains the e ect shown by Hipparcos data, thus

to change in colour indices or parameters b ecause the one could hop e that the distances determined by the

global e ect is the same in several diagrams involving ZAMS tting metho d for other clusters is safe. It

649

Figure 8. Comparison of the sequences of Per  l led

squares and Coma Ber open squares clusters. This di-

Figure 7. Comparison of the sequences of the Praesepe

agram o ers an example of a good agreement of the main

 l led squares and Pleiades open squares clusters. The

sequences of both clusters.

same di erence in the referencesequences is also appar-

ent.

various parameters B V , d, ,  which have

2 1

di erent sensitivities to metallicity e ects. Most of

the e ect is observed on the V magnitude. At a given

B V value, a Pleiades app ears to b e 0.5 mag

fainter than a Coma Berenices or Praesep e star.

This result questions the simple main-sequence t-

ting metho d used so far to determine the distance of

op en clusters, and the predominent role of metallicity

to explain the relative p osition of the ZAMS of dif-

ferent clusters. In addition to the metal and helium

abundances, another parameter app ears necessary to

explain the b ehaviour of the Pleiades with resp ect

to the other nearby clusters. Alexander 1986 and

Nissen 1988 have already suggested that a fourth

parameter was needed to describ e solar-typ e stars.

In addition to the dramatic di erence between the

Pleiades and Praesep e, one should also remember

Figure 9. Comparison of the sequences of the Per

the similarity of Coma Berenices and Praesep e, al-

 l led squares and Pleiades open squares clusters. The

though the Coma Ber cluster was at the origin of

Pleiades arelocatedbelow the sequenceof Per.

the Hyades anomaly, and consequently the di erence

between the Pleiades and Coma Berenices, although

they have a similar value of c , according to Nissen

o

1988. No simple correlation is found when compar-

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It is also interesting to consider the asso ciations

Volume 3, Chapter 17

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Arenou, F., Mignard, F., Palasi, J. 1997b, The Hip-

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