Diane B. Paulson

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.

Exploring the Frequency of Close-in Jovian Planets around M Dwarfs*

We discuss our high-precision radial velocity results of a sample of 90 M dwarfs observed with the Hobby-Eberly Telescope and the Harlan J. Smith 2.7 m Telescope at McDonald Observatory, as well as the ESO VLT and the Keck I telescopes, within the context of the overall frequency of Jupiter-mass planetary companions to main-sequence stars. None of the stars in our sample show variability indicative of a giant planet in a short-period orbit, with a ≤ 1 AU. We estimate an upper limit of the frequency f of close-in Jovian planets around M dwarfs as 3.8MJ and a ≤ 0.7 AU. For eccentric orbits (e = 0.6) the survey completeness is 95% for all planets with m sin i > 3.5MJ and a ≤ 0.7 AU. Our results point toward a generally lower frequency of close-in Jovian planets for M dwarfs as compared to FGK-type stars. This is an important piece of information for our understanding of the process of planet formation as a function of stellar mass.

Searching for Planets in the Hyades. IV. Differential Abundance Analysis of Hyades Dwarfs

We present a differential abundance analysis of Hyades F–K dwarfs in search for evidence of stellar enrichment from accreted hydrogen-deficient disk material. Metallicities and relative abundance ratios of several species have been determined. We derive a cluster mean [Fe/H] = 0.13 ± 0.01. Two stars show abundances ~0.2 dex larger than the cluster mean. In addition, one star, which was added by a recent study as a cluster member, shows significantly lower abundances than the cluster mean. These three stars have questionable membership characteristics. The remaining stars in the survey have an rms of 0.04 dex in the differential [Fe/H] values. The Hyades cluster members have apparently not been significantly chemically enriched. The abundance ratios of Si, Ti, Na, Mg, Ca, and Zn with respect to Fe are in their solar proportions.

The Extrasolar Planet ϵ Eridani b: Orbit and Mass*

Hubble Space Telescope (HST) observations of the nearby (3.22 pc) K2 V star Eridani have been combined with ground-based astrometric and radial velocity data to determine the mass of its known companion. We model the astrometric and radial velocity measurements simultaneously to obtain the parallax, proper motion, perturbation period, perturbation inclination, and perturbation size. Because of the long period of the companion, Eri b, we extend our astrometric coverage to a total of 14.94 yr (including the 3 yr span of the HST data) by including lower precision ground-based astrometry from the Allegheny Multichannel Astrometric Photometer. Radial velocities now span 1980.8-2006.3. We obtain a perturbation period, P = 6.85 ± 0.03 yr, semimajor axis α = 1.88 ± 0.20 mas, and inclination i = 301 ± 38. This inclination is consistent with a previously measured dust disk inclination, suggesting coplanarity. Assuming a primary mass M* = 0.83 M⊙, we obtain a companion mass M = 1.55MJ ± 0.24MJ. Given the relatively young age of Eri (~800 Myr), this accurate exoplanet mass and orbit can usefully inform future direct-imaging attempts. We predict the next periastron at 2007.3 with a total separation ρ = 03 at position angle P.A. = -27°. Orbit orientation and geometry dictate that Eri b will appear brightest in reflected light very nearly at periastron. Radial velocities spanning over 25 yr indicate an acceleration consistent with a Jupiter-mass object with a period in excess of 50 yr, possibly responsible for one feature of the dust morphology, the inner cavity.

Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum

Aims. Our aim is to confirm the nature of the long period radial velocity measurements for β Gem first found by Hatzes & Cochran (1993). Methods. We present precise stellar radial velocity measurements for the K giant star β Gem spanning over 25 years. An examination of the Ca II K emission, spectral line shapes from high resolution data (

Chemical Homogeneity in the Hyades

R = 210\,000

SEARCHING FOR PLANETS IN THE HYADES. V. LIMITS ON PLANET DETECTION IN THE PRESENCE OF STELLAR ACTIVITY

), and Hipparcos photometry was also made to discern the true nature of the long period radial velocity variations. Results. The radial velocity data show that the long period, low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. Furthermore, the Ca II K emission, spectral line bisectors, and Hipparcos photometry show no significant variations of these quantities with the radial velocity period. An orbital solution assuming a stellar mass of 1.7

Searching for Planets in the Hyades. II. Some Implications of Stellar Magnetic Activity

M_\odot

MAGNESIUM ISOTOPE RATIOS IN HYADES STARS

yields a period,

Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M4 and M5

P = 589.6