Robert B. Hanson

The Problem of Hipparcos Distances to Open Clusters. I. Constraints from Multicolor Main-Sequence Fitting

Parallax data from the Hipparcos mission allow the direct distance to open clusters to be compared with the distance inferred from main-sequence (MS) fitting. There are surprising differences between the two distance measurements, indicating either the need for changes in the cluster compositions or reddening, underlying problems with the technique of MS fitting, or systematic errors in the Hipparcos parallaxes at the 1 mas level. We examine the different possibilities, focusing on MS fitting in both metallicity-sensitive B-V and metallicity-insensitive V-I for five well-studied systems (the Hyades, Pleiades, α Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and α Per are within 1 σ of the MS-fitting distance in B-V and V-I, while the Hipparcos distances to Coma Ber and the Pleiades are in disagreement with the MS-fitting distance at more than the 3 σ level. There are two Hipparcos measurements of the distance to Praesepe; one is in good agreement with the MS-fitting distance and the other disagrees at the 2 σ level. The distance estimates from the different colors are in conflict with one another for Coma but in agreement for the Pleiades. Changes in the relative cluster metal abundances, age related effects, helium, and reddening are shown to be unlikely to explain the puzzling behavior of the Pleiades. We present evidence for spatially dependent systematic errors at the 1 mas level in the parallaxes of Pleiades stars. The implications of this result are discussed.

The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones

We continue our series of papers on open cluster distances with a critical assessment of the accuracy of main-sequence fitting using isochrones that employ empirical corrections to the color-temperature relations. We use four nearby open clusters with multicolor photometry and accurate metallicities and present a new metallicity for Praesepe ([Fe/H]=+0.11±0.03) from high-resolution spectra. The internal precision of distance estimates is about a factor of 5 better than the case without the color calibrations. After taking into account all major systematic errors, we obtain distances accurate to about 2%–3% when there exists a good metallicity estimate. Metallicities accurate to better than 0.1 dex may be obtained from BVICKs photometry alone. We also derive a helium abundance for the Pleiades of Y=0.279±0.015, which is equal within the errors to the Suns initial helium abundance and that of the Hyades. Our best estimates of distances are (m-M)0=6.33±0.04,8.03±0.04, and 9.61±0.03 to Praesepe, NGC 2516, and M67, respectively. Our Pleiades distance at the spectroscopic metallicity,(m-M)0=5.66±0.01(internal)±0.05(systematic), is in excellent agreement with several geometric distance measurements. We have made calibrated isochrones for -0.3≤[Fe/H]≤+0.2 available online.

Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample

We present a revised catalog of 2106 Galactic stars, selected without kinematic bias and with available radial velocities, distance estimates, and metal abundances in the range -4.0 ≤ [Fe/H] ≤ 0.0. This update of the 1995 Beers & Sommer-Larsen catalog includes newly derived homogeneous photometric distance estimates, revised radial velocities for a number of stars with recently obtained high-resolution spectra, and refined metallicities for stars originally identified in the HK objective-prism survey (which account for nearly half of the catalog) based on a recent recalibration. A subset of 1258 stars in this catalog have available proper motions based on measurements obtained with the Hipparcos astrometry satellite or taken from the updated Astrographic Catalogue (second epoch positions from either the Hubble Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan Southern Proper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog. Our present catalog includes 388 RR Lyrae variables (182 of which are newly added), 38 variables of other types, and 1680 nonvariables, with distances in the range 0.1 to 40 kpc.

The Distances to Open Clusters as Derived from Main-Sequence Fitting. II. Construction of Empirically Calibrated Isochrones*

In the first of a new series of papers on open cluster distances, we use updated stellar evolution models to construct an isochrone appropriate for the Hyades, and compare it with the Hyades eclipsing binary system vB 22. We find that the absolute and relative luminosities of the two stars are in good agreement with the model, but the radii do not match the values inferred from eclipse data. We present evidence that there is a consistency problem with the flux ratios and the inferred radii, and discuss possible theoretical effects that could be responsible for the mismatch in the radii. We derive a helium abundance for the Hyades of Y = 0.271 ± 0.006, which is equal within the errors to the Sun’s initial helium abundance even though the Hyades is considerably more metal-rich. Subject headings: binaries: (eclipsing), stars: distances, stars: abundancesWe continue our series of papers on open cluster distances by comparing multicolor photometry of single stars in the Hyades with theoretical isochrones constructed with various color-temperature relations. After verifying that the isochrone effective temperatures agree well with spectroscopically determined values, we argue that mismatches between the photometry and the theoretical colors likely arise from systematic errors in the color-temperature relations. We then describe a method for empirically correcting the isochrones to match the photometry and discuss the dependence of the isochrone luminosity on metallicity.

On the Use of [Na/Fe] and [α/Fe] Ratios and Hipparcos-based (U, V, W) Velocities as Age Indicators among Low-Metallicity Halo Field Giants

We have examined the [Na/Fe] and [Mg/Fe] ratios in a sample of 68 field halo giants with -3 [Fe/H] -1. We recalculated the Galactic (U, V, W) velocity components for these stars, using Hipparcos proper motions and a new Hipparcos-based distance scale. We used these data to see how the abundance ratios may relate to kinematical substructure in the Galactic halo. To isolate a set of true halo stars, we eliminated metal-weak thick-disk stars, about 10% of our sample. The field halo giants show the expected correlation of Na and Mg abundances, so we can use Na as a surrogate for Mg and the α-elements. The most metal-poor stars show a wider dispersion of [Na/Fe] ratios than do the less metal-poor stars; the difference is most striking for stars on retrograde galactic orbits. Some 20% of our retrograde giants and 13% of all our halo giants have [Na/Fe] ≤ -0.35 and may be significantly younger than the oldest halo objects. Halo giants considered young by this Na abundance criterion show a preference for retrograde orbits. Giants in some globular clusters (e.g., M13) do not exhibit the Mg versus Na correlation found among halo field giants. Instead, they have very large [Na/Fe] ratios and widely scattered [Mg/Fe] ratios, probably induced by deep mixing, which field halo giants apparently do not experience.

Space velocities of 14 globular clusters

Relative proper motions for 14 globular clusters are converted to absolute proper motions using a new procedure that uses results from the Lick program of proper motions relative to galaxies. These absolute proper motions are then used to derive space velocities for the clusters. The space velocities generally confirm the picture of disk and halo populations of globulars that had been deduced from radial velocity data alone, but there are a few surprises; most notably M4 and M28 are moderately metal-poor but in orbits confined to the disk. We also find a very high space velocity for M5

The Problem of HIPPARCOS Distances to Open Clusters. II. Constraints from Nearby Field Stars

This paper examines the discrepancy between distances to nearby open clusters as determined by parallaxes from Hipparcos compared to traditional main-sequence fitting. The biggest difference is seen for the Pleiades, and our hypothesis is that if the Hipparcos distance to the Pleiades is correct, then similar subluminous zero-age main-sequence (ZAMS) stars should exist elsewhere, including in the immediate solar neighborhood. We examine a color-magnitude diagram of very young and nearby solar-type stars and show that none of them lie below the traditional ZAMS, despite the fact that the Hipparcos Pleiades parallax would place its members 0.3 mag below that ZAMS. We also present analyses and observations of solar-type stars that do lie below the ZAMS, and we show that they are subluminous because of low metallicity and that they have the kinematics of old stars.

The Distances to Open Clusters from Main-Sequence Fitting. I. New Models and a Comparison with the Properties of the Hyades Eclipsing Binary VB 22

We continue our series of papers on open cluster distances by comparing multicolor photometry of single stars in the Hyades to theoretical isochrones constructed with various color-temperature relations. After verifying that the isochrone effective temperatures agree well with spectroscopically determined values, we argue that mismatches between the photometry and the theoretical colors likely arise from systematic errors in the color-temperature relations. We then describe a method for empirically correcting the isochrones to match the photometry, and discuss the dependence of the isochrone luminosity on metallicity. Subject headings: stars: distances, stars: abundances, stars: evolution, open clusters: individual (Hyades)In the first of a new series of papers on open cluster distances, we use updated stellar evolution models to construct an isochrone appropriate for the Hyades and compare it with the Hyades eclipsing binary system VB 22. We find that the absolute and relative luminosities of the two stars are in good agreement with the model but that the radii do not match the values inferred from eclipse data. We present evidence that there is a consistency problem with the flux ratios and the inferred radii and discuss possible theoretical effects that could be responsible for the mismatch in the radii. We derive a helium abundance for the Hyades of Y ¼ 0:271 � 0:006, which is equal within the errors to the Sun’s initial helium abundance, even though the Hyades is considerably more metal-rich. Subject headings: binaries: eclipsing — open clusters and associations: individual (Hyades) — stars: abundances — stars: distances

A Preliminary Discussion of the Kinematics of BHB and RR Lyrae Stars Near the North Galactic Pole

The radial velocity dispersion of 67 RR Lyrae variable and blue horizontal branch (BHB) stars that are more than 4 kpc above the galactic plane at the North Galactic Pole is 110 km/sec and shows no trend with Z (the height above the galactic plane). Nine stars with Z 4 kpc have a Galactic V motion that is < -200 km/sec and which is characteristic of the halo. Thus the stars that have a flatter distribution are really halo stars and not members of the metal-weak thick-disk.

The Distance to the Hyades Cluster Based on Hubble Space Telescope Fine Guidance Sensor Parallaxes

Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m - M = 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.