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The Distance of the Pleiades and Nearby Clusters

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The Distance of the Pleiades and Nearby Clusters 643 THE DISTANCE OF THE PLEIADES 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 metallicity 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 Hyades 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 stars 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.
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