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Analysis of Seven Nearby Open Clusters Using Hipparcos Data

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Analysis of Seven Nearby Open Clusters Using Hipparcos Data 567 ANALYSIS OF SEVEN NEARBY OPEN CLUSTERS USING HIPPARCOS DATA 1 1 1 2 1 N. Robichon , F. Arenou ,C.Turon J.-C. Mermillio d , Y. Lebreton 1 DASGAL, URA CNRS 335, Observatoire de Paris, 92195 Meudon, France 2 Institut d'Astronomie, Universit e de Lausanne, Switzerland calculate the cluster mean distance with Hipparcos ABSTRACT parallaxes as well as the mean space velo city using Hipparcos data and ground based radial velo cities. From Hipparcos data in seven op en cluster elds Praesep e, IC 4756, NGC 2516, NGC 3532, NGC A eld star is considered as a memb er if its Hippar- 6475, NGC 6633 and Sto ck 2 we have computed cos parallax and prop er motion are consistent with the memb ership of stars. The mean cluster distances the mean cluster values at a 3 level. Stars for which have b een derived from intermediate Hipparcos data. the p osition in the observational colour-magnitude diagram was not in agreement with the cluster se- quence were also rejected. Hipparcos double stars Cluster sequences in the HR diagram have b een de- were rejected when their duplicity could bias the duced from their distances, the photometry coming mean prop er motion and parallax values, i.e. when from the `Base des Amas' Mermillio d 1988. Com- the eld H59 of the Hipparcos Catalogue was equal parison b etween the relative sequence p ositions shows to G, O, V or X see ESA 1997. that metallicity is probably not the only parameter which in uences the p osition of the ZAMS. The numb er of stars selected in each cluster is given in Table 1. It varies b etween 6 and 24. Key words: op en clusters; Hipparcos; Hertzsprung- Russell diagram; distances; chemical comp osition. 3. CLUSTER MEAN PARALLAXES 1. INTRODUCTION 3.1. Hipparcos Intermediate Data Op en clusters have b een used for a long time to cali- The mean cluster parallaxes cannot be computed brate the main sequences in the Hertzsprung-Russell without caution. As was predicted b efore the satel- diagram as a function of age and metallicity. They lite launch Lindegren 1988, the estimation of the also de ne one of the rst steps in the distance scaling mean parallax or prop er motion of a cluster observed of the Universe. The advent of the Hipparcos Cata- by Hipparcos must takeinto account the observation logue allows, for the rst time, to determine, without mo de of the satellite. This is due to the fact that stars anyphysical assumption, the lo cations of cluster se- within a small area in the sky have frequently b een quences in the HR diagram. observed in the same eld of view of the satellite. Consequently, one may exp ect correlations b etween The comparison of the cluster sequences presented measurements done on stars separated by a few de- here and in Mermillio d et al. 1997 leads to amazing grees, or with a separation b eing a multiple of the results. The p ositions of cluster sequences are not basic angle b etween the two elds of view. correlated with metallicity as one could have thought b efore. This means that, when averaging the parallaxes or prop er motions for these n stars, the improvement p factor do es not follow the exp ected 1= n law and will p 2. MEMBER SELECTION if is the mean not b e asymptotically b etter than p ositive correlation between data. In the case of 0:35 clusters, the improvement is ab out n Lindegren The selection of memb ers in an op en cluster is al- 1988. The straightaverage of individual parallaxes ways a critical issue. The selection was done with would not b e an optimal estimate of the mean cluster the assumption that all stars b elonging to the cluster parallax, and moreover its standard error would be have the same space velo city and that they lie in a seriously underestimated. 10 parsec radius sphere centred on the cluster centre. The selection is iterative but converges after 2 or 3 The prop er way to take these correlations into ac- iterations: with a set of well known memb ers, we count is to come back to the reference great circle 568 Table 1. New cluster distances. cluster N d { + m{M { + m{M EB V [Fe/H] age [mas] [mas] [p c] [p c] [p c] [mag] [mag] [mag] [mag] [mag] [dex] [Myr] Lynga Praesep e 24 5.65 0.31 177.0 9.2 10.3 6.24 0.12 0.12 5.99 0.00 0.07 830 IC 4756 9 3.46 0.30 289.0 23.1 27.4 7.30 0.18 0.20 8.58 0.20 0.04 830 1 NGC 6475 21 3.43 0.30 291.5 23.4 27.9 7.32 0.18 0.20 7.08 0.06 0.03 130 NGC 6633 6 3.43 0.53 291.5 39.0 53.3 7.32 0.31 0.36 8.01 0.17 -0.11 630 2 Sto ck02 8 3.16 0.47 316.5 41.0 55.3 7.50 0.30 0.35 8.62 0.38 -0.14 170 NGC 2516 15 2.87 0.20 348.4 22.7 26.1 7.71 0.15 0.16 8.49 0.13 -0.23 70 1 NGC 3532 7 2.40 0.40 416.7 59.5 83.3 8.10 0.33 0.40 8.53 0.04 -0.10 290 1 from Piatti et al. 1995 2 from Claria & Piatti 1996 variations of the normalised di erences = level, to calibrate the correlation b etween the refer- as a function of apparent magnitude V b ottom and ence great circle abscissae, so that the full covariance colour index B V top are represented in Fig- matrix b etween observations allows to nd the opti- ure 1. The indep endence between these normalised mal astrometric parameters. As part of the least- di erences and apparent magnitudes or colour indices squares pro cedure, the nal covariance matrix be- was not rejected byaPearson or Kendall statistical tween astrometric parameters is also found. The test. adopted metho d is similar to that of van Leeuwen & Evans 1997 with the exception that the cali- B - V bration of correlation co ecients has b een done on 0.0 0.5 1.0 1.5 2.0 h reference great circle. This has b een done us- eac 3 ulae by Lindegren 1988 to ing the theoretical form 2 which harmonics were added through the use of co- 1 π sine transform Press et al. 1992. σ )/ moy 0 π − ( or a given cluster, either the mean parallax or the F -1 Praesepe prop er motion or b oth may b e considered to b e the -2 IC 4756 unknowns for the cluster, the other astro- same NGC 6475 -33 NGC 6633 parameters of cluster stars remaining deter- metric Stock 02 NGC 2516 individually. In our case, only the parallax mined 2 NGC 3532 b een considered constant, the resulting values has 1 π σ b eing given Table 1. From these mean parallaxes )/ moy 0 , and asso ciated standard errors, the distance and π − ( -1 distance mo duli are also indicated, together with a 1 variation. In the right part of Table 1, the dis- -2 tance mo duli, colour excesses, and ages, quoted by -3 56789101112 a 1987, and metallicities from Lynga 1987, Lyng V Piatti et al. 1995 or Claria & Piatti 1996 are also indicated. Figure1. Normalised di erences of the 94 cluster mem- bers versus B-V top and V bottom. 3.2. Accuracy of the Results 4. CLUSTER COLOUR{MAGNITUDE The distances and the distance mo duli given in Ta- DIAGRAMS ble 1 deserve some more comments. Since the trans- formation from parallax to distance is not linear, a bias in the quoted distances could b e exp ected. How- In each cluster, Johnson BV photo electric photom- ever, the relative error = is small b etween 6 and etry compiled in the `Base des Amas' was used to 17 p er cent so the e ect is probably negligible see build well de ned cluster sequences in the observa- Brown et al. 1997. tional Hertzsprung-Russell diagram. For IC 4756 and Sto ck 2, not enough photo electric photometry When computing the mean cluster parallax, one im- was available to obtain a clean sequence. Redden- plicitly assumes that the disp ersion in individual par- ing of each cluster from Lynga 1987 and Hippar- allaxes is only due to the measurements errors. The cos mean cluster parallaxes were used to obtain the depth of the cluster should be taken into account; absolute magnitude M and the dereddened colour V however, except p erhaps for Praesep e, it is small indices B V . The higher part of the Figure 2 0 compared to the quoted error on distance and may shows the sup erp osition of the 5 cluster sequences in b e neglected. the M ; B V diagram. The lower part repro- V 0 duces the sequences of Praesep e and NGC 2516 with Finally one may ask whether there could b e a mag- the error bars on absolute magnitudes derived from nitude or colour e ect in the Hipparcos parallaxes Hipparcos data see Table 1. The cluster sequences which could bias the mean parallax estimation. The separate into two groups: Praesep e and NGC 6475 569 -2 -1 0 1 V 2 M 3 Praesepe 4 NGC 6475 NGC 3532 5 NGC 6633 6 NGC 2516 -2 -1 0 1 V 2 M 3 4 5 6 -0.25 0 0.25 0.5 0.75 1 1.25 (B-V)0 Figure2.
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