Inwoo Han

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.

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

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

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AN ASTROMETRIC STUDY OF THE LOW-MASS BINARY STAR ROSS 614

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Hipparcos and MAP Studies of the Triple Star π Cephei

\end{document} Orionis (HD 39587, HIP 27913, HR 2047) by combining precise astrometric and radial velocity data. The combined orbital analysis yields \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} \ne...

A STUDY OF THE ACCURACY OF NARROW FIELD ASTROMETRY USING STAR TRAILS TAKEN WITH THE CFHT

Long accepted as the quintessential low-mass star, the secondary of the nearby diminutive astrometric binary Ross 614 has attracted considerable astrophysical interest. Unfortunately, the orbital period of 16.6 yr exceeds the duration of the mission-limited studies of most space-borne or instrumental-proving observational programs. As with most such binaries, the only full-orbit studies are based on photographic materials. The last extended study of this system was based upon the plate collections of the McCormick and Sproul Observatories. The work reported here combines data from the Multichannel Astrometric Photometer, the Hipparcos Intermediate Astrometric Data, the previously unmeasured photographic plates of the Allegheny Observatory, published observations of the visual binary, and recently published radial velocities of the system. Together, these data span more than three orbits of the low-mass binary system. Limiting our analysis to the most recent observations of the binary, and five older observations that are in fair agreement with them, we find masses of 0.2228 ± 0.0055 and 0.1107 ± 0.0028 M⊙ for the primary and secondary, respectively, with the largest source of error being the visual separations of the system. We find a parallax of 244.07 ± 0.73 mas, a period of 16.595 ± 0.0077 yr, and an increased estimate of the semimajor axis of 1101.2 ± 8.2 mas. The latter led to a significant increase in the computed masses. All other aspects of the orbital elements and astrometry are in excellent agreement with those found in the independent study of the McCormick and Sproul plates. The importance of long-term astrometric coverage is pointed out by the fact that the orbital motion of the system only resulted in an acceleration during the compilation of the Hipparcos Catalogue. No orbital parameters or mass estimates can be discerned from these high-precision but short-term data.

A study of Upgren 1

We reexamine the visual and radial velocity data for the π Cephei triple star system and derive the masses of the three components. Our study is based upon the analysis of Hipparcos Intermediate Astrometric Data (IAD) measurements obtained with the red light Thaw refractors Multichannel Astrometric Photometer (Thaw/MAP) and positions obtained from photographic plates acquired with the Thaw refractors original photographic objective. The weighted mean parallax of the star system is now 13.8 ± 0.41 mas, corresponding to a distance modulus of 4.30 ± 0.065, yielding absolute visual magnitudes of 0.24 ± 0.065 and 2.50 ± 0.070 for the A and B components, respectively, and a total system mass of 8.81 ± 0.87 M☉. The 58 yr span of the plate collection makes possible the first detection of the photocentric motion caused by the 160 yr orbit of π Cep A/B yielding masses of 6.88 ± 0.69 M☉ and 1.93 ± 0.23 M☉, respectively, for the central spectroscopic giant binary star and the late A secondary component. Although of shorter time span, the higher precision of the IAD and Thaw/MAP data allow the first detection of the astrometric motion caused by the spectroscopic companion of the A component. A comparison of the astrometric, spectroscopic, and photometric studies of the π Cep suggests that the spectroscopic binary is composed of a pair of red giants with similar masses, Aa = 3.63 ± 0.53 M☉, Ab = 3.27 ± 0.48 M☉, and a red magnitude difference of approximately 1.7 mag. The importance of long-term astrometric coverage is pointed out by the fact that the motion of both the A/B system and Aa/Ab system were missed during the compilation of the Hipparcos catalog and are only revealed in those data after their detection in the MAP/Thaw measurements.

Preliminary Astrometric Masses for Proposed Extrasolar Planetary Companions

We extend the study of Han (1989a) to angles as small as 7 arcsec. To investigate the relation between the relative motion and the separation of two stars in astrometric observations, we analyzed star trail plates taken with the CFHT by Christian and Racine (1985). The result shows that on Mauna Kea the r.m.s. relative image motion for 1 sec of integration time is given by sigma = 0.054 arcsec (theta/10 arcminute)0.35, where theta is the separation between two stars. The standard theory on atmospheric turbulence predicts that sigma is proportional to the 1/3th power of theta. Our study shows that it is valid for the range of 7 arcsec < theta < 1100 arcsec. Han, using data taken with MAP (Multichannel Astrometric Photometer, Gatewood 1987) of Allegheny Observatory of the University of Pittsburgh, showed that in Pittsburgh sigma = 0.143 arcsec (theta/10arcminute)0.34. This implies that the accuracy of very narrow field astrometric observations at an excellent site may be more than 7 times higher than those of the currently most precise astrometric programs.

A Combined Hipparcos and Multichannel Astrometric Photometer Study of the Proposed Planetary System of ρ Coronae Borealis

Upgren and Rubin (1965) have suggested that a small group of bright F stars, in the direction of the galactic north pole, may be the remnant of an old cluster. The multichannel astrometric photometer (MAP) has been used to determine the parallaxes of six of the candidate stars with an average precision of 1.1 thousandths of an arcsecond (1.1 mas). The derived distances are, as suggested, similar, and an unusual space density of F stars seems indicated. However, the derived space velocities indicate that the proposed cluster is composed of members of two dynamically different groups.