THE ASTROPHYSICAL JOURNAL, 533:938È943, 2000 April 20 ( The American Astronomical Society. All rights reserved. Printed in U.S.A.

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1 THE ASTROPHYSICAL JOURNAL, 533:938È943, 2000 April 20 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. THE PLEIADES, MAP-BASED TRIGONOMETRIC PARALLAXES OF OPEN CLUSTERS. V. GEORGE GATEWOOD AND JOOST KIEWIET DE JONGE University of Pittsburgh, Allegheny Observatory, Observatory Station, Pittsburgh, PA AND INWOO HAN Korea Astronomy Observatory and Bohyunsan Optical Astronomy Observatory, Jacheon Post Office, Young-Cheon, Kyung-Book, , Korea Received 1999 March 8; accepted 1999 November 30 ABSTRACT The Multichannel Astrometric Photometer and Thaw Refractor (Thaw/MAP) of the University of PittsburghÏs Allegheny Observatory have been used to determine the trigonometric parallax of the Pleiades star cluster. The parallax determined, 0A with a standard error of ^0A (corresponding to a distance modulus of 5.59 ^ 0.12 mag) places the cluster signiðcantly further away than indicated by the mean parallax of cluster members drawn from the Hipparcos catalog. The distance derived here is in general agreement with values based on main-sequence Ðtting, indicating that cluster members are not subluminous as suggested by the Hipparcos-based results. The current study combines the data from our initial study of this cluster with new observations of that region and of a second Pleiades region in an overlapping conðguration. It thus supersedes our Ðrst determination of the parallax of the Pleiades cluster. A third Pleiades Ðeld is being selected for future measurement of the clusterïs trigonometric parallax, and assistance with the luminosity classiðcation of reference stars is sought. Subject headings: astrometry È Hertzsprung-Russell diagram È open clusters and associations: individual (Pleiades) È stars: distances È stars: evolution 1. BACKGROUND With the advent of electronic astrometric detectors and, more recently, the completion of the Hipparcos mission (ESA 1997), it is now possible to compare the absolute luminosities of the main-sequence stars of several of the nearest open star clusters directly (e.g., Eggen 1998; Pinsonneault et al. 1998). The results have not been altogether satisfactory. Eggen concurs with Pinsonneault et al. that the Hipparcos measure of the parallax of the Pleiades cluster cannot be reconciled with the Hyades main sequence. Pinsonneault et al. Ðnd that the Hipparcos-based cluster parallaxes of both the Pleiades and Coma clusters di er from the mainsequenceèðtting (MSF) distance by more than 3 standard errors. Interestingly, the straight mean of the Hipparcosbased parallax for the Pleiades and that of all previous ground-based parallax studies 6.9 ^ 0.7 (van Altena et al. 1995) agrees very well with the distance derived by MSF. As detailed in a series of papers from Lindegren (1988) through Robichon et al. (1999), derivation of the Hipparcos mean value of the parallax of a cluster is not straightforward (see also van Leeuwen & Hansen Ruiz 1997). Published estimates of the Hipparcos-based mean parallax of the Pleiades range from 8.72 mas for 19 bright members (Eggen 1998) to 8.45 ^ 0.25 mas (van Leeuwen 1999). The variation results from the fact that the brightest cluster members, which are conðned to the center of the cluster, have a noticeably higher average parallax than the majority of the cluster members listed in the Hipparcos catalog. Selecting from the 54 members chosen by Robichon et al. (1999) for their study of the cluster parallax, we Ðnd that the 10 stars brighter than sixth visual magnitude have a mean parallax of 8.94 ^ 0.14 mas, while the 18 stars fainter than eighth visual magnitude have a mean parallax of 7.91 ^ 0.68 mas. That the e ect is position-related, not magnitude-related, is indicated by the fact that even the fainter stars show a higher mean parallax near the center of the cluster. We note that the 30 stars in this set more than 1.5 from Alcyone have an average parallax of 8.21 ^ 0.41 mas, a value that is within the standard error of its di erence from the value we Ðnd from our own parallax measurements. That the measured parallax values of Pleiades member stars within 1.5 of Alcyone are correlated is evident from the fact that their dispersion is less than that suggested by their cataloged standard errors. This is also very evident in the parallax of the 10 stars brighter than sixth magnitude, noted above. Those outside that distance, on the other hand, have a larger parallax dispersion than suggested by their cataloged standard errors. One possible reason for the latter is that the standard errors listed in the Hipparcos catalog have been underestimated. Narayanan & Gould (1999) have suggested a more elegant technique for the estimation of the precision Hipparcos parallax estimates using measurements of the Hyades. Several techniques are used to investigate the systematic and accidental errors of a parallax catalog. One of the most obvious is to compare the parallax measurements with those found in other catalogs of similar precision. There have now been two small studies of this kind (Gatewood, de Jonge, & Persingeret 1998; Harris, Dahn, & Monet 1997). Reassuringly, neither study Ðnds a signiðcant systematic di erence; however, both indicate that the errors of the di erences are larger than expected and suggest that the Hipparcos errors are probably approximately 1.5 times that stated in the catalog. During the late 1980s, we instituted a Multichannel Astrometric Photometer and Thaw Refractor (Thaw/MAP) (Gatewood 1987) observing e ort at the Allegheny Observatory to measure the distances of four of the nearest open star clusters. Earlier publications in this program involve the Coma (Gatewood 1995), Hyades (Gatewood et al. 1992), 938

2 PLEIADES, MAP-BASED PARALLAXES OF OPEN CLUSTERS. V. 939 Pleiades (Gatewood et al. 1990, superceded by this paper), and Praesepe (Gatewood & de Jonge 1994) clusters. In a recent comparison of the parallaxes common to the Hipparcos and Allegheny Observatory catalogs (Gatewood et al. 1998), we found no signiðcant systematic di erences between the two parallax systems. We did, however, note meaningful di erences between some individual star parallaxes. A case in point involves the stars in the Pleiades. 2. DATA AND DATA REDUCTIONS The instrumentation and reduction procedures utilized here have been described extensively (Gatewood 1987). The transformation of the phase data produced by the MAP to the star constants listed below utilizes the central overlap technique. The algorithm by which the absolute parallaxes are determined includes the estimation of the intrinsic luminosities of the reference stars. Much of the information for the latter is obtained from a parallel series of reports detailing intermediate-band photometry results for Thaw/MAP reference stars (Castelaz & Persinger 1989; Persinger & Castelaz 1990; Castelaz et al. 1991; Persinger & Castelaz 1999, private communication). Both of the Thaw/MAP regions chosen for this study are near the apparent center of the cluster and partly overlap. Four stars are common to the two regions allowing the observations of these stars to be used to enforce overlap constraints (e.g., Eichhorn 1974 p. 78, 1998). To estimate the clusterïs distance, nine stars were chosen from bright background objects with proper motions signiðcantly di erent from that of the cluster (Hertzsprung et al. 1947). Preference was given to objects that appeared, from a comparison of apparent magnitudes derived from plates taken with the Thaw 0.76 m red and blue objective lenses, to lie o of the clusterïs main sequence. Three years after the initial study of this region, Gatewood, de Jonge, & Stephenson (1993) improved the reduction process. At the high-precision level of the Thaw/ MAP system, stellar parallax causes signiðcant residuals in the positions of the reference frame. Thus, we improved the process by including the best estimate of the spectroscopic parallax of each reference star starting with the Ðrst iteration in the reduction algorithm. Parallax terms remain, in all following iterations, among the unknowns that model each starïs position and motion. Like the catalog positions, the estimates of the parallax of the reference stars are subject to veriðcation and possible adjustment. Di erences between the predicted and observed parallax can lead to a reevaluation of the spectroscopic parallax of a reference star, as was the case for stars AO 828 and AO 1299 in the present study. Since they are target stars, the initial pass does not use cluster members for reference. Thus, there is no initially assumed value for the parallax of the cluster. AO 828 was classiðed as a K2 III star by Persinger & Castelaz (1990) with a corresponding spectroscopic parallax of 3.0 mas and a visual absorption of 0.3 mag. The preliminary reductions suggested that the parallax was near 4.5 mas. Reevaluation of the 10 band photometry did not change the luminosity classiðcation but was suggestive of a slightly hotter star with greater absorption, a K1 III type with a visual absorption of 0.45 mag and a parallax of 4.2 mas. With regard to star AO 1058, photometric data published by Castelaz et al. (1991) are confusing. Listed as a G0 I star, it was nevertheless assigned an absolute magnitude of 6.1 and a parallax of 19 mas. Thus, we did not use this star in the initial reduction of the Thaw/MAP data. The preliminary parallax was calculated at approximately 5 mas. Binnendijk (1946) classiðed this star as an A3. Assuming that it is a main-sequence star and adopting the photometry listed by Castelaz et al. (1991), we now Ðnd a visual absorption of 0.75 magnitudes and an estimated parallax 3.2 mas. Three stars had not been classiðed in the Castelaz & Persinger series, namely AO numbers 1298, 1299, and AO 1300 is Hipparcos Binnendijk classiðed this star as type K5. The Hipparcos parallax, which is consistent with the luminosity of a K5 III star, was adopted. With reference to star AO 1299, we based our temperature classi- Ðcation on its Hb photometry, its HD temperature classi- Ðcation, and the B, V, R, and I photometry of Persinger & Castelaz (1999, private communication). Assuming a mainsequence luminosity, we Ðnd a visual absorption of 0.59 mag and a parallax of 4.3 mas. Finally, we consider the luminosity classiðcation of star AO 1298 to be the most problematical in this study. Fortunately, it is not a highweight reference star. Binnendijk lists its temperature class as F2. If it is a main-sequence star, its parallax is approximately 7 mas, but the Ðrst reduction iteration pointed to a parallax of approximately 1 mas. The star is well placed in the Ðeld and not very faint, so we adopted the parallax the star would have as an F2 III. Using the Persinger & Castelaz (1999, private communication) photometry, we Ðnd a visual absorption of 0.34 mag and an estimated parallax of 2.1 mas. Since the preliminary parallax was our only justiðcation for changing the initially assumed main-sequence luminosity of star AO 1299 to that of a giant, we considered dropping the reference star. Removing it from the calculations in Table 1 would not change the derived cluster parallax and would only increase its error by 0.03 mas, so we retained this star in the solution. An unweighted estimate of the adjustment to absolute parallax is used during the computation of the individual sets of Ðeld variates. Thus, the adjustment of the parallaxes to absolute can still be improved (e.g., Stein 1991). Listed in Table 1 are the Allegheny Observatory (AO) catalog star number and the adopted spectral classiðcation-luminosity type of the noncluster members for which trigonometric and photometric studies were meaningful. Most of the tabulated spectral classiðcations come from the multiband photometry of Persinger & Castelaz (1990) and Castelaz et al. (1991). Next we list the visual absorption found by Ðtting the photometry to the intrinsic colors published by Johnson (1966) and Bessell & Brett (1988). If not mentioned above, the photometric parallaxes, listed in the next column of Table 1, are from Persinger & Castelaz. The estimated standard error of the photometric parallax is 35% of the parallax (see Castelaz et al. 1991). It includes an allowance for the spread of luminosities within a luminosity class. The provisional absolute parallax and its calculated standard error, listed in columns (6) and (7), are the result of the astrometric reduction before the weighted adjustment. Since the calculated standard error, in column (7), does not yet contain an allowance for the standard error of the adjustment being calculated in Table 1, it is somewhat smaller than the standard error listed in Table 2. Column (8), the weighted adjustment, is calculated by multiplying the di erence between the provisional parallax and the spectroscopic parallax by the weight, listed in the following column. The weights are the inverse of the sum of the variances of the

3 TABLE 1 ADJUSTMENT TO WEIGHTED ABSOLUTE PARALLAX IN THE COMBINED PLEIADES REGIONS Spectral- AO A V Spectral Parallax Estimated S.E. Provisional Parallax Calibrated S.E. Weighted Adjustement Observed Parallax Observed Number Spectral Class (mag) (mas) (mas) (mas) (mas) (mas) Weight (mas) (mas) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) K1 III [ [ K1 III ] K1 III [ [ M1 III ] G0 V ] A3 V [ [ F2 III ] A2 V [ [ a... K5 III [ [0.14 NOTE.ÈWeighted adjustment to mean \]0.09 mas. Standard error of weighted adjustment to mean \ 0.31 mas. As in the Castelaz & Persinger (see text), photometric calibrations A \ 3.10E(B [ V ). a Hipparcos parallax used (see text). V

4 PLEIADES, MAP-BASED PARALLAXES OF OPEN CLUSTERS. V. 941 TABLE 2 STAR PARAMETERS IN THE REGION OF THE PLEIADES R.A. PM Decl. PM AO V Parallax (2000) (R.A.) (2000) (Decl.) W Numbera Deviceb (mag) B[V (mas) (h m s) (s) (arcsec) (Int.) [ [ [ [ [ * [ [ [ [ [ [ * [ * [ * [ * [ [ [ [ [ * [ * [ [ [ [ [ * NOTE.ÈThe parallax standard errors include the uncertainty of the adjustment to absolute. An asterisk (*) denotes that the star is a Pleiades cluster member. Except for the parallaxes, the standard errors (second row of each entry, for example those of the positions) are strictly internal and do not allow for the zeropoint errors of the reference system. The precession for ]50 yr at the target object is minutes of time in R. A. and 9@.22 in decl. a AO numbers are part of a continuing series beginning with the use of the central overlap technique. The numbers are assigned with their Ðrst publication and all but 1297È1300 have appeared in previous publications. b The 2 ÏÏ denotes that the device used to gather the astrometric data was the Thaw/MAP. provisional parallax and the estimated spectroscopic parallax expressed in thousands of an arcsecond. The adjustment to a weighted mean, ]0.09 mas, and its standard error, ^0.31 mas, are listed at the bottom of the Table. The adjustment is based upon the luminosity classiðcations adopted in Table 1, the absolute magnitudes given by Allen (1976 p. 200), and the estimated individual interstellar absorption corrections. The adjustment is applied throughout Table 2 and elsewhere in this paper. The study weights, positions, and motions at the epoch and equinox of J2000 of the stars under study are listed in the last Ðve columns of Table 2 above their corresponding standard errors. The positions and motions are on the system of the ICRS as realized in the optical part of the spectrum by the ACT Reference catalog (Urban, Corbin, & Wyco 1997) and the Hipparcos catalog (ESA 1997). The standard errors are given in units of the last shown digit of the parameter to which they pertain and are strictly internal at J2000. We note that they do not include an allowance for the zero point, scale, orientation, or proper-motion uncertainties of the reference system. If desired, the internal standard errors of the parallaxes can be recovered by taking the square root of divided by the weight listed in the last column of Table 2. Easy cross identiðcation of these stars is a orded by Table 3, which lists the AO, Hertzsprung II, Bonner Durchmusterung (Argelander 1862), and Hipparcos numbers for the stars in this study. 3. DISCUSSION The last column of Table 1, the unweighted residuals of the adjustment, provides a quick visual inspection of the results. The values listed in column (8) may be used to obtain a feeling for the reliability of the adjustment to absolute and its standard error. For example, one may test the

5 942 GATEWOOD, DE JONGE, & HAN Vol. 533 TABLE 3 NUMBER CROSS INDEX AO Hertzsprung BD Hipparcos e ect of removing any reference star from the reduction. Nine values are obtained by removing a di erent reference star each time and calculating the new weighted adjustment to absolute. These estimates have a range of [0.20 to ]0.23 mas. The standard deviation of these values is ^0.12 mas. If one eliminates either the two most positive or the two most negative weighted adjustments the resultant weighted mean adjustments are ]0.32 and [0.26 mas, respectively. All but one of these values are within the estimated standard error of the adjustment calculated in Table 1, a value that is itself in line with that expected of nine stars with the indicated weights. Since the weighted mean parallax of the reference system is 1.95 mas, the e ective distance of the reference system is approximately 4 times that of the cluster. Thus, errors in the assumed absolute magnitudes of the luminosity classes represented in Table 1, here taken from Allen (1976 p. 200), are reduced by a factor of 4 in their e ect on the distance modulus derived for the cluster. To give a correct estimate of the reliability of the parallax of each individual star, the standard error of the mean of the residuals (observed minus spectroscopic) of the reference star parallaxes has been added, in quadrature, to the internal standard error of each stellar parallax. Thus, the standard error of the parallax listed in Table 2 di ers from the internal standard error. To obtain the correct weighted parallax of the cluster, weights derived from the internal standard errors must be used to Ðnd the mean, 7.64 mas, and its internal standard error, ^0.30 mas. The standard error of this weighted mean is then added in quadrature to the standard error of the mean of the residuals of the reference star parallaxes, ^0.31 mas, to obtain the standard error of the cluster parallax, ^0.43 mas. Naively computing the mean from Table 2 would yield an ingenuously low estimate of the error. Herein lies a danger of what could become a common practice. Until the external errors of the Hipparcos catalog are better understood, we should be cautious in our calculation of the weight of any parallax derived by computing a simple weighted mean (Gatewood et al. 1998). The Hipparcos program overcame extraordinary mission difficulties and exceeded its design goals by a factor of 2. This exceptional success does not mean, however, that the product is without statistical limitations. The mean cluster parallax derived in the present study di ers from the value found in our 1990 study by 1.1 times that standard error of the di erence. We believe that the change has its origins in the increased sophistication of the reduction algorithm, the increased number of reference stars, minor improvements in the hardware, and the new data. Improvements in the algorithm are reñected in the relative weights of the two studies. While this study includes approximately twice as much data, including an additional year of observations of the original Ðeld and twice as many reference stars, its statistical weight is 3.5 times that of the original study. Part of the improved precision stems from the early introduction of the estimated parallaxes into the iterative reductions. We also noticed a signiðcant improvement in the stability of the results with the introduction of overlap conditions for the stars common to the two Thaw/ MAP regions in this study. As noted in the cross index, Table 3, four stars in the present study are listed in the Hipparcos catalog. A comparison indicates that the di erences in the measured parallaxes of the last three stars, two of which are cluster members, are all well within their standard errors. For these stars the mean di erence is [ 0.2 ^ 0.7 mas. The Ðrst star is another cluster member, however, the di erence in the measured parallax is ]1.81 ^ 1.10 mas. All of the stars in our two regions fell within the 1.5 degree inner circle discussed above. 4. CONCLUSION The weighted mean parallax, calculated as indicated above, of seven members of the Pleiades open star cluster is 7.64 ^ 0.43 mas. There is no evidence that the depth of the cluster is a signiðcant factor in the calculated dispersion. This parallax is equivalent to a distance modulus of 5.59 ^ 0.12 mag, in good agreement with early photometric estimates (e.g., Johnson 1958) and more sophisticated mainsequenceèðtting techniques (e.g., Pinsonneault et al. 1998). The di erence in the most recent Hipparcos-based parallax (van Leeuwen 1999) reduces the di erence with that found here to 0.81 mas. The standard error of this di erence is ^0.50 mas, indicating a greater than 90% likelihood that the di erence is signiðcant and likely systematic. Yet Gatewood et al have indicated that they found no systematic di erences related to magnitude, color, or global position. Pinsonneault et al. (1998) suggest a more narrow angle e ect in right ascension, one which would not have been evident in the Gatewood et al comparison of the Hipparcos and Allegheny Observatory Parallax catalogs. This assertion led us to the idea of using the more widely separated members to estimate the Hipparcos parallax noted in 1. Clearly, the Hipparcos parallax estimates of members of the Pleiades are correlated by position and, thus, by magnitude. Perhaps it is the use of image dissector tube technology that relates one to the other, but with variations in the mean parallax of di erent sets of Hipparcos measurements ranging by more than 1.0 mas and no obvious way to establish a zero point in this range, it is difficult to establish a meaningful mean parallax to a much greater precision. The results of Pinsonneault et al., Eggen, and those given here indicate that further improvement in our knowledge of the distances to the nearby star clusters and thus to some extent our knowledge of the cosmic distance scale is dependent upon further measurements of the parallaxes of cluster

6 No. 2, 2000 PLEIADES, MAP-BASED PARALLAXES OF OPEN CLUSTERS. V. 943 members. Without a current replacement for the Hipparcos program, this e ort must fall on ground-based programs. The Thaw/MAP e ort is the only milliarcsecond-parallax program currently working on cluster parallaxes. The greatest weakness in the narrow Ðeld studies conducted with the Thaw/MAP is the adjustment to absolute (see Table 1). Since the standard error of the parallax of a reference star is approximately 0.35 times the parallax, the most direct way to reduce the uncertainty in this value is to use more distant reference stars. Thus, until a new Hipparcoslike catalog is published, the key to improving the parallaxes of the nearby open clusters is through a photometric and spectroscopic selection of distant reference stars. The selection of 10 reference stars with parallaxes of 1.0 mas would reduce the error of the adjustment to absolute to 0.1 mas. To encourage others to assist in this e ort, the coordinates of prospective reference stars are being posted on our web site. With the recent installation of two additional high quantum efficiency channels on the Thaw/MAP, we can now observe up to 10 reference stars and four cluster members in each study. With careful selection of the reference stars, a concentrated Thaw/MAP program, involving several regions in each cluster, should be able to reduce the parallax error of each of the four clusters to approximately ^0.2 mas. This e ort has received support from the National Science Foundation through grant AST and the National Aeronautics and Space Administration through grant NAG 253. Additional support has been received from the University of Pittsburgh and the Allegheny Observatory Endowment Fund. Obviously, no e ort of this size is accomplished by a few people. The entire sta of the Allegheny Observatory contributed to this paper, and to them the authors owe their deepest thanks. Some of the references used in this study were retrieved through SIMBAD, the database of the Strasbourg, France, Astronomical Data Center. Allen, C. W. 1976, Astrophysical Quantities (London: Athlone) Argelander, F. W. A. 1862, Bonner Sternverzeichniss, Astron. Beob. Ko nigl Sternwarte (Bonn: Friedrich-Wilhelms Univ.) Bessell, M. S., & Brett, J. M. 1988, PASP, 100, 1134 Binnendijk, L. 1946, Annal. Sterrew. Leiden XIX, Tweede Stuk, 5 Castelaz, M. W., & Persinger, T. 1989, AJ, 98, 1768 Castelaz, M. W., Persinger, T., Stein, J. W., Prosser, J., & Powell, H. D. 1991, AJ, 102, 2103 Eggen, O. J. 1998, AJ, 116, 1810 Eichhorn, H. 1974, Astronomy of Star Positions (New York: Frederick Ungar) ÈÈÈ. 1998, Dynamical Astronomy, unpublished ESA 1997, The Hipparcos and Tycho Catalogs (ESA SP-1200, Noordwijk: ESA) Gatewood, G. 1987, AJ, 94, 213 ÈÈÈ. 1995, ApJ, 445, 712 Gatewood, G., Castelaz, M., Han, I., Persinger, T., Stein, J., Stephenson, B., & Tangren, W. 1990, ApJ, 364, 114 Gatewood, G., Castelaz, M., de Jonge, J. K., Persinger, T., Stein, J., & Stephenson, B. 1992, ApJ, 392, 710 Gatewood, G., & de Jonge, J. K. 1994, AJ, 105, 1179 Gatewood, G., de Jonge, J. K., & Persinger, T. 1998, AJ, 116, 1501 Gatewood, G., de Jonge, J. K., & Stephenson, B. 1993, AJ, 105, 1179 REFERENCES Harris, H. C., Dahn, C. C., & Monet, D. G. 1997, in Hipparcos Venice 1997, ed. B. Battrick & M. A. C. Perryman (Paris: ESA), 689 Hertzsprung, E. 1947, Ann. Leiden Obs., 19, 1 Johnson, H. L. 1958, ApJ, 128, 121 ÈÈÈ. 1966, ARA&A, 4, 193 Lindegren, L. 1988, in ScientiÐc Aspects of the Input Catalogue Preparation II, ed. J. Torra & C. Turon (ESA SP-1111; Paris: ESA), 179 Narayanan, V. K., & Gould, A. 1999, ApJ, 515, 256 Persinger, T. & Castelaz, M. W. 1990, AJ, 100, 1621 Pinsonneault, M. H., Stau er, J., Soderblom, D. R., King, J. R., & Hanson, R. B. 1998, ApJ, 504, 170 Robichon, N., Arenou, F., Turon, C., & Mermilliod, J. C. 1999, A&A, 345, 471 Stein, J. W. 1991, ApJ, 377, 669 Urban, S. E., Corbin, T. E., & Wyco, G. L. 1997, The ACT Reference Catalog (Washington, DC: US Naval Obs.) (CD ROM) van Altena, W. F., Lee, J. T., & Hoffleit, D. E. 1995, The General Catalogue of Trigonometric Stellar Parallaxes (4th ed.; New Haven: Yale Univ. Obs.) van Leeuwen, F. 1999, A&A, 341, L71 van Leeuwen, F., & Hansen Ruiz, C. S. 1997, in Hipparcos Venice 1997, ed. B. Battrick & M. A. C. Perryman (Paris: ESA), 689