Joost Kiewiet de Jonge

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.

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.

Multichannel astrometric photometer and photographic astrometric studies in the regions of Lalande 21185, BD 56 deg 2966, and HR 4784

The parallaxes of stars determined from the multichannel astrometric photometer (MAP) in the regions of Lalande 21185, BD 56 deg 2966, and from photographic plates in the region of BD 56 deg 2966 and the region of HR 4783 and HR 4784 are reported. The parallax determined for Lalande 21185, 395.1 +/-1.0 mas yields what is believed to be the most precise distance modulus (-2.984 +/-0.0055) known for a star other than the sun. The absolute magnitude of Lalande 21185 is found to be 10.474 +/-0.008. No significant evidence of unseen companions is detected. The photographic study of BD 56 deg 2966 indicates that the long-term (65 yr) motion of the star is linear to better than +/-16 mas, while the MAP study indicates that the motion is linear to better than +/-2.5 mas over a period of some 5 yr. The photographically determined parallaxes are compared with those observed with the MAP. The photographically calculated standard errors are found to be in agreement with the standard error of the differences.

Astrometric studies in the region of Algol

The distance and masses of the components of the triple star Algol (Beta Persei) are derived from photographic and electronic data collected with the two 0.76 m objectives of the Thaw Refractor at the University of Pittsburghs Allegheny Observatory and with the 0.61 m visual refractor at Swarthmore Colleges Sproul Observatory. These datasets constitute the major narrow-field astrometric collections of this star. The newly derived weighted mean trigonometric parallax of Algol is now +0.0343 +/- 0.00085 corresponding to a distance modulus of 2.32 +/- 0.054 mag. The total mass of the Algol system is estimated at 6.05 +/- 0.45 solar mass. The photocentric semimajor axis of the AB/C orbit is determined in the blue, yellow, and red bandpasses of the three independent astrometric systems. The total mass for the A and B component stars is found to be 4.63 +/- 0.35 solar mass and the mass of the C component is determined to be 1.42 +/- 0.13 solar mass. Residuals to this solution showed no meaningful correlation to the orbital motion of A/B. Thus it is probable that the photocenter is located near the center of mass of the A/B subsystem. There is no evidence of a previously suggested 32 yr orbital motion and no significant acceleration is evident in the 60 yr photographic series. It is therefore unlikely that the system has a fourth stellar mass component. It is noted that the visual band interferometric studies of the Algol system have the AB and C components interchanged.

Correlation of the Hipparcos and Allegheny Observatory Parallax Catalogs

No significant difference is found between the systems of the Hipparcos and Allegheny Observatory MAP parallax catalogs. The correlation of the parallaxes of 63 stars common to both programs is 0.9995 ± 0.0001, with an average standard deviation of the difference of 00023. While there is no indication of systematic difference in the two programs, our study suggests that the formal errors in one or both catalogs are somewhat underestimated.

Hipparcos and MAP Studies of the Triple Star π Cephei

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.

Multichannel astrometric photometer parallax studies in the regions of Groombridge 1618, Zeta Bootis, and Sigma Draconis

Improved trigonometric parallaxes are reported for stars in the regions of Groombridge 1618, Zeta Bootis, and Sigma Draconis which differ from generally accepted parallaxes by two to five standard deviations. The weighted mean parallax of Groombridge 1618 becomes 0.2079 +/- 0.0013 arcsec, indicating a distance modulus of -1.59 +/- 0.014 and yielding an absolute magnitude of this K7 V star of 8.20 +/- 0.017. The parallax determined for the visual binary Zeta Bootis is 0.0202 +/- 0.0010 arcsec and implies an average absolute visual magnitude of 1.067 +/- 0.12 for the spectroscopically similar components. The weighted mean parallax of Sigma Draconis becomes 0.1747 +/- 0.0010 arcsec, indicating a distance modulus of -1.210 +/- 0.012 and yielding an absolute visual magnitude of 5.90 +/- 0.012 for this K0 V star. The absolute visual magnitudes of the high velocity star Roman 336 and of the subdwarf star AO 1187 are found to be + 4.0 +/- 0.13 and + 5.9 +/- 0.17, respectively.

A study of Upgren 1

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.

A New Astrometric System

The 30-inch Thaw Refractor of the University of Pittsburgh’s Allegheny Observatory has been completely rebuilt. Changes include a new objective lens, a new detector system, and computer control and data acquisition. With an observational accuracy exceeding one-thousandth of an arc second, the new system will greatly expand the domain of astrometric research.

A new objective for the Allegheny Observatory 30-inch refractor

As a follow-up to the development of astrometric detectors of sufficient accuracy to detect extrasolar planetary systems, the Allegheny Observatory is modernizing their 30-inch Thaw Refractor. Changes include a new objective lens and the transition of the observational bandpass from blue light to red. The new system will have sufficient accuracy to detect jovian planets orbiting any of several dozen of the Sun’s nearest stellar neighbors.