George Gatewood

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.

The multichannel astrometric photometer and atmospheric limitations in the measurement of relative positions

The operational Multichannel Astrometric Photometer (MAP) now in use in the Allegheny Observatory astrometric program is the culmination of a decade of design and development effort. A detailed description of the system and its related software is followed by analysis of data acquired in four stellar regions. The study indicates an accuracy (in the sense of conformity to the best model), per night, for stars of the eighth magnitude or brighter, of 0.003 arcsec or better. These data points each have approximately twice the precision of the annual normal points obtained in our photographic program. Accuracy is shown to depend on: (1) the photon-count rate of the target star (it follows that the number of photons from the reference frame is also in important factor), (2) the duration of the observation, (3) the angular size of the reference frame, and (4) the quality of the astronomical seeing. Since (4) and, to a lesser extent, (1) involve the atmospheric characteristics at the time of observation, the probable performance at more favorable sites is discussed briefly.

The Pleiades, Map-based Trigonometric Parallaxes of Open Clusters. V.

The Multichannel Astrometric Photometer and Thaw Refractor (Thaw/MAP) of the University of Pittsburghs Allegheny Observatory have been used to determine the trigonometric parallax of the Pleiades star cluster. The parallax determined, 000764 with a standard error of ±000043 (corresponding to a distance modulus of 5.59 ± 0.12 mag) places the cluster significantly 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 fitting, 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 configuration. It thus supersedes our first determination of the parallax of the Pleiades cluster. A third Pleiades field is being selected for future measurement of the clusters trigonometric parallax, and assistance with the luminosity classification of reference stars is sought.

ALLEGHENY OBSERVATORY PARALLAXES FOR LATE M DWARFS AND WHITE DWARFS

Results of astrometric studies with the 76 cm Thaw red-light-refractor and the Multifiltered Astrometric Camera are presented for 21 stars. The parallax estimates range from 259.3 mas for Teegardens Star to 10.3 mas for the high-velocity star LSPM J2321+4704. This paper includes parallaxes for late M dwarfs, white dwarfs, and a high-velocity star. We also report the apparent binary motion of LSPM J0607+7372.

ASTROMETRIC STUDIES OF ALDEBARAN, ARCTURUS, VEGA, THE HYADES, AND OTHER REGIONS

The results of astrometric studies in the regions of Groombridge 34A, the Hyades, Aldebaran, Ross 47, BD+5° 1668, 81 Cancri, BD+15° 2620, Arcturus, Vega, and Ross 248 are presented. Estimates of the absolute parallax of each star are presented and a mass estimate is present for 81 Cancri. Comments include the discussion of the apparent motions of a few previously suggested planetary systems.

An Astrometric Study of Procyon

We report on the analysis of observations of Procyon obtained with the Multichannel Astrometric Photometer during the years 1986-2004. While the results significantly increase the precision of the estimated characteristics of the components of the system, they are generally in good agreement with the most recent studies. The F5 IV primary is found to have a mass of 1.43 ± 0.034 M⊙, while the white dwarf companions mass is 0.58 ± 0.014 M⊙. The distance modulus is 2.277 ± 0.005 mag, yielding an absolute visual magnitude for the combined light of the system of 2.640 ± 0.006 and an absolute visual magnitude of the secondary of 13.20 ± 0.05. The residuals of the fit of the positions of the photocenter to the excepted orbit support the parameters of that orbit, showing only a slight deviation in right ascension. The most significant improvements in our knowledge of the binary system can be made with further measurements of the separation of the components. The masses found here tend to reaffirm the recent models of the nature of the primary and secondary stars.

AN UPPER LIMIT TO THE MASS OF THE RADIAL VELOCITY COMPANION TO r 1 CANCRI

Doppler spectroscopy of r 1 Cnc has detected evidence of a companion with an orbital period of 14.65 days and a minimum mass of 0.88 Jupiter masses. Astrometric observations performed with the Hubble Space Telescope Fine Guidance Sensor 1r using a novel observing technique have placed an upper limit on the astrometric reflex motion of r 1 Cnc in a time period of only 1 month. These observations detected no reflex motion induced by the 14.65 day period radial velocity companion, allowing us to place a 3 j upper limit of ∼0.3 mas on the semimajor axis of this motion, ruling out the preliminary Hipparcos value of 1.15 mas. The corresponding upper limit on the true mass of the companion is ∼30 MJ, confirming that it is a substellar object. Subject headings: astrometry — planetary systems — stars: low-mass, brown dwarfs

Map-based trigonometric parallaxes of open clusters. II - The Hyades: 51 Tauri

The trigonometric parallax, mass, and mass ratio of 51 Tauri, a binary member of the Hyades star cluster, are determined. The parallax solution, 19.4 mas with a standard error of 1.1 mas, is slightly greater than its generally accepted value and yields a cluster distance of 4.30 +/-2.0 pc or a distance modulus equivalent of 3.16 +/- 0.10 mag, in excellent agreement with recent photographic parallax results. The weighted mean of all trigonometric parallax determinations of the cluster is now 22.9 +/-0.6 mas or a modulus of 3.20 +/- 0.06 mag. The total mass of the 51 Tauri system was found to be 2.56 +/- 0.44 solar masses. Adopting a magnitude difference in line with the spectral types of the components yields individual masses of 1.61 +/- 0.28 and 0.95 +/- 0.18 solar masses. This result places the mass of both the A8 V primary and the G0 secondary stars within the errors of the measurement of the mass-luminosity relation established for solar-neighborhood main-sequence stars.

AN ASTROMETRIC STUDY OF THE MIRA VARIABLE R LEONIS AND THE OCCULTATION BINARY 19 LEONIS

The Multichannel Astrometric Photometer (MAP) has been used to determine parallaxes of stars in the region centered on the Mira variable R Leonis. R Leonis, with a parallax of 8.3 ± 1.0 mas (120 ± 15 parsecs), is significantly closer than most recent estimates. At visual wavelengths, its mean physical diameter is 612 ± 96 times that of the sun. The photocentric motion of R Leonis is not suggestive of either optical or orbital companions, excluding possible secondary components over a wide range or magnitudes as well as orbital periods of up to approximately one decade. Nineteen Leonis, an occultation-speckle binary with a well-determined orbit and a reference star in the R Leonis field, has a distance of 92.6 ± 12 parsecs. The 15.2 year period binary exhibits a slight pertubation during the 4 years of MAP observations. The similar primary and secondary of this A7 Vn system are 2.41 ± 0.96 and 2.31 ± 0.92 solar masses respectively.

A Precise Orbit Determination of χ1 Orionis from Astrometric and Radial Velocity Data

ABSTRACT We have derived an accurate orbit of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape