Van Vleck Observatory Research Articles

The Variability and Rotation of Pre-Main-Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?

A variability study of the young cluster IC 348 at Van Vleck Observatory has been extended to a total of 7 yr. Twelve new periodic stars have been found in the last 2 yr, bringing the total discovered by this program to 40. In addition, we confirm 16 of the periods reported by others and resolve some discrepancies. The total number of known rotation periods in the cluster from all studies has now reached 70. This is sufficient to demonstrate that the parent population of K5-M2 stars is rotationally indistinguishable from that in the Orion Nebula Cluster, even though their radii are 20% smaller and they would be expected to spin about twice as fast if angular momentum were conserved. The median radius and therefore the inferred age of the IC 348 stars actually closely matches that of NGC 2264, but the stars spin significantly more slowly. This suggests that another factor besides mass and age plays a role in establishing the rotation properties within a cluster, and we suggest that it is environment. If disk locking were to persist for longer times in less harsh environments because the disks themselves persist for longer times, it could explain the generally slower rotation rates observed for stars in this cluster, whose earliest type star is of class B5. We have also obtained radial velocities, the first for pre-main-sequence stars in IC 348, and v sin i measurements for 30 cluster stars to assist in the study of rotation and as an independent check on stellar radii. Several unusual variable stars are discussed; in some or all cases their behavior may be linked to occultations by circumstellar material. A strong correlation exists between the range of photometric variability and the slope of the spectral energy distribution in the infrared. Nineteen of the 21 stars with I ranges exceeding 0.4 mag show infrared evidence for circumstellar disks.

The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004

We present results from the most recent (2002–2004) observing campaigns of the eclipsing system KH 15D, in addition to rereduced data obtained at Van Vleck Observatory (VVO) between 1995 and 2000. Phasing 9 yr of photometric data shows substantial evolution in the width and depth of the eclipses. The most recent data indicate that the eclipses are now approximately 24 days in length, or half the orbital period. These results are interpreted and discussed in the context of the recent models for this system put forward by Winn and coworkers and Chiang & Murray-Clay. A periodogram of the entire data set yields a highly significant peak at 48.37 ± 0.01 days, which is in accord with the spectroscopic period of 48.38 ± 0.01 days determined by Johnson and coworkers. Another significant peak, at 9.6 days, was found in the periodogram of the out-of-eclipse data at two different epochs. We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion. If so, application of Hut's theory implies that the eccentricity of the orbit is e = 0.65 ± 0.01. Analysis of the Ultraviolet and Visual Echelle Spectrograph VLT spectra obtained by Hamilton and coworkers shows that the v sin i of the visible star in this system is 6.9 ± 0.3 km s-1. Using this value of v sin i and the measured rotation period of the star, we calculate the lower limit on the radius to be R = (1.3 ± 0.1) R⊙, which concurs with the value obtained by Hamilton and coworkers from its luminosity and effective temperature. Here we assume that i = 90°, since it is likely that the spin and orbital angular momenta vectors are nearly aligned. One unusually bright data point obtained in the 1995/1996 observing season at VVO is interpreted as the point in time when the currently hidden star B made its last appearance. Based on this datum, we show that star B is 0.46 ± 0.03 mag brighter than the currently visible star A, which is entirely consistent with the historical light curve. Finally, well-sampled VJ and IJ data obtained at the Cerro Tololo Inter-American Observatory Yale 1 m telescope during 2001/2002 show an entirely new feature: the system becomes bluer by a small but significant amount in very steady fashion as it enters eclipse and shows an analogous reddening as it emerges from eclipse. This suggests an extended zone of hot gas located close to but above the photosphere of the currently visible star. The persistence of the bluing of the light curve shows that its length scale is comparable to a stellar radius.

A Multiyear Photometric Study of IC 348

The extremely young cluster IC 348 has been monitored photometrically over 5 observing seasons from Dec 1998 to March 2003 in Cousins I with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16 irregular variables have been identified. Among the brighter stars, 14 of the 16 known K or M-type WTTS were found to be periodic variables, while all 5 of the known CTTS were found to be irregular variables. In the full sample, which includes 150 stars with I mag as faint as 18, we find that 40% of the 63 WTTS are detected as variables, nearly all of them periodic, while 55% of the 20 CTTS are also detected as variable, with none of them periodic. Our study suggests that 80-90% of all WTTS in young clusters will be detected as periodic variables given sufficiently precise and extended monitoring, whereas CTTS will reveal themselves primarily or solely as irregular variables. This has clear consequences for PMS rotational studies based on photometric periods. We examine the stability of the periodic light curves from season to season. All periodic stars show modulations of their amplitude, mean brightness and light curve shape on time scales of less than 1 yr, presumably due to changes in spot configurations and/or physical characteristics. In no case, however, can we find definitive evidence of a change in period, indicating that differential rotation is probably much less on WTTS than it is on the Sun. Among the non-periodic stars, we report the detection of two possible UXors as well as a pre-main sequence star, HMW 15, which apparently undergoes an eclipse with a duration exceeding three years.

Periodic Variability in the Pre-Main-Sequence Object CB 34V

A photometric study of the G-type pre–main-sequence star CB 34V was undertaken at Van Vleck Observatory during the 2001–2002 observing season. The star remained in its bright state throughout the 5 month monitoring period, showing no evidence of the large amplitude variability that originally drew attention to it. However, it did exhibit periodic variability with an amplitude of ~0.6 mag. A highly significant peak is present in the periodogram at 1.186 days, but analysis of the light curve suggests that the actual rotation period of the star is twice that—2.372 days. Apparently, the star had similar spots on opposite hemispheres during this time and we view it close to equator-on. This circumstance allows us to place additional constraints on the radius and luminosity of the star. The new data argue against its being an FU Orionis star and in favor of its being either a UX Orionis star or an object (similar to KH 15D) that is eclipsed episodically by circumstellar matter.

The Rotation and Variability of T Tauri Stars: Results of Two Decades of Monitoring at Van Vleck Observatory

AbstractA brief history of photometric monitoring of T Tauri stars with a 0.6 m telescope at Wesleyan University is given. This is followed by discussion of three recent results: 1) the mass dependence of stellar rotation rates in the Orion Nebula Cluster, 2) a check on stellar radii estimates of pre-main sequence stars, and 3) the nature of a very peculiar object in NGC 2264.

Bridges and Outdoor Lighting

Northeast Utilities, the electric power distributor for Connecticut, has devised a plan to light the Arrigoni Bridge across the Connecticut River at Middletown. This paper descibes the plan and its impact on the surrounding region. The floodlights will affect migratory birds, and teaching and research at the nearby Van Vleck Observatory on Wesleyan Campus. Examples of bridges at other sites are also discussed, including the suspension spans of New York and San Francisco, that bear only tracer lights, or no lights at all.

A Variability Study of Pre–Main-Sequence Stars in the Extremely Young Cluster IC 348

The extremely young cluster IC 348 has been monitored in the Cousins I band with a 0.6 m telescope at Wesleyan's Van Vleck Observatory. Photometry of 150 stars was obtained on 76 images taken on 27 separate nights during the period December, 1998, through March, 1999. As expected, spectral characteristics largely determine the nature of a star's variability in this cluster. None of the stars with H-alpha in absorption were found to be variables. On the other hand, all 16 stars identified as CTTS by their H-alpha emission equivalent widths and the majority of the 49 WTTS in the part of the cluster we monitored showed evidence of variability. Nineteen stars were found to be periodic, with periods ranging from 2.24 to 16.2 days and masses ranging from 0.35 to 1.1 M-sun. Seventeen of these are WTTS and the other 2 are of unknown spectral class. The period distribution is remarkably similar to what is found in the Orion Nebula Cluster for stars in the same mass range. Namely, it is bimodal with peaks at 2-3 days and 7-8 days, although there are not enough periods known to define these features significantly by the IC 348 data alone. The three fastest rotators are also the three most massive stars in the periodic sample.

The challenges and frustrations of a veteran astronomical optician: Robert Lundin, 1880-1962

Robert Lundin, apprenticed in nineteenth century optical craftsmanship but employed in twenty century fabrication and engineering, suffered many frustrations during a nonetheless productive career. Son of Carl A R Lundin, a senior optician at the famous American firm of Alvan Clark & Sons, Robert grew up building telescopes. As a teenager, he assisted with projects including the 1-m [40-inch] objective for Yerkes Observatory. After his father's death in 1915, he became manager of the Clark Corporation and was responsible for many smaller, successful refractors and reflectors. Lundin also completed major projects, including a highly praised 50.8-cm achromat for Van Vleck Observatory, as well as a successful 33-cm astrograph used at Lowell to discover Pluto. In 1929, a dispute with the owners of the Clark Corporation led to Lundin's resignation and his creation of a new business, "C. A. Robert Lundin and Associates." This short-lived firm built several observatory refractors, including a 26.7 cm for E W Rice, the retired chairman of General Electric. But none was entirely successful, and the Great Depression finished off the company. In 1933, Lundin took a job as head of Warner & Swasey's new optical shop, only to experience his greatest disasters. The 2.08-m [82-inch] reflector for McDonald Observatory was delayed for years until astronomers uncovered an error in Lundin's procedure for testing the primary mirror. A 38.1-cm photographic lens for the Naval Observatory was a complete failure. Under pressure to complete a 61-cm Schmidt camera, Lundin seems to have attempted to deceive visiting astronomers. After retirement in the mid 1940s, Lundin moved to Austin, Texas, the home of his daughter, where he died. His difficulties should not obscure his success with many instruments that continue to serve as important research and education tools.

Additional Periodic Variables in NGC 2264

We present results from a second season of monitoring fields in the young cluster NGC 2264 at Van Vleck Observatory. In Paper I of this series, we reported the discovery of nine periodic variables—all interpreted as spotted pre–main-sequence stars. That has now been increased to 31, one of which is quite unusual. The preliminary result reported in Paper I—that the frequency distribution of rotation periods in NGC 2264 is significantly different from what is seen in the Orion Nebula cluster (ONC)—is confirmed. In particular, the distribution peaks at a period of about 4 days, precisely where there is a gap in the ONC distribution. There is also a good number of very rapidly rotating stars and a long tail of slow rotators. The distribution can be understood, at least qualitatively, in terms of a disk-locking model of rotational evolution. In this interpretation, the majority of stars in NGC 2264 must have unlocked from their disks by an age of about 1 million years, and spun up roughly in accordance with conservation of angular momentum. A minority (about 20%) are conserving angular velocity instead (i.e., locked to their disks). The unusual star, 15D, has an apparent period of about 48 days and a light curve indicative of an eclipse by a nonstellar object. Its amplitude exceeds 3 mag. Our fragmentary data suggest a complex structure for the eclipsing body. It could be a feature (protoplanet?) in this star's circumstellar disk and, as such, deserves observational attention.

Rotation Periods and Variability of Stars in the Trapezium Cluster

view Abstract Citations (31) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rotation Periods and Variability of Stars in the Trapezium Cluster Eaton, N. L. ; Herbst, W. ; Hillenbrand, L. A. Abstract Results from a fourth season (1993/94) of monitoring the Trapezium cluster in the Cousins I band at Van Vleck Observatory are reported. Data were obtained on 28 nights with multiple exposures on most. A periodogram technique described by Horne & Baliunas [ApJ, 302, 757 (1986)] based on Scargle's method was used to search for periods. False alarm probabilities must be carefully assessed when the sampling intervals are distributed in the manner of this study. Multiple observations per night allow us to effectively resolve problems with aliasing. Rotation periods are now known for 19 cluster members; ten are confirmations of periods reported by Mandel & Herbst [ApJ, 383, L75 (1991)] and/or Attridge & Herbst [ApJ, 398, L61 (1992)] and nine are newly discovered. 17 of these are likely Type I (cool spot) variables and two are of Type IIp (hot spot). Periods range from 1.68 to 34.5 d and have a bimodal distribution similar to that found for the Orion Nebula cluster as whole. In this limited, but homogeneous sample, we find no trends in angular velocity with stellar mass, stellar age, or presence on an inner disk, as inferred from H - K excess. The existence of a bimodal period distribution is, however, strong evidence in favor of disk-regulated angular momentum evolution. Irregular variables are also common in the Trapezium cluster and we identify 14 that have ranges exceeding 0.5 mag. Most of them exhibit light curves typical of classical T Tauri stars (Type II), but one has a much slower (perhaps FUor-like?) behavior, and three showed eclipse-like drops in brightness. They could be eclipsing binaries but no periods were found, so they are more likely to be Type III irregular variables (UXors). VIJHK photometry demonstrates that the large amplitude variables (LAVs), as a group, have substantial IR excesses, indicative of accretion disks, signifying that they are indeed CTTS. None of these stars are brighter than V = 14, in accord with the failure of spectroscopic surveys to detect CTTS in the Trapezium cluster, heretofore. Our data indicate that both CTTS and WTTS are present in the Trapezium cluster, but that the brightest WTTS are systematically brighter than the brightest CTTS by about 1.6 mag in V and 1.8 mag in I. A simple interpretation of this fact is that the CTTS (at least the LAVs) are of lower mass and cooler than the WTTS in this cluster. Publication: The Astronomical Journal Pub Date: October 1995 DOI: 10.1086/117646 Bibcode: 1995AJ....110.1735E Keywords: STARS: ROTATION; STARS: OSCILLATIONS; STARS: EARLY-TYPE full text sources ADS | data products SIMBAD (38)

The Feasibility of a CCD for an Astrometric Refractor

The 0.5m refracting telescope of the Van Vleck Observatory has been active in the determination of trigonometric parallaxes since its first observations in 1922. Its lenses were ground by C.A.R. Lundin of the Alvan Clark Co. for photographic use. Coma was minimized across the field and vignetting was also kept to a minimum. Partly as a consequence the focal curve is very steep in the blue and green regions of the spectrum, as is shown in Fig. 1. A Wratten No. 12 minus blue filter is used to filter out all wavelengths to the blue of about 5200 å. The region between 5200 å and 6000 å is very flat with the focal plane varying over a range of about one millimeter. Towards the red region it steepens, although not enough to impair images on photographic plates of emulsion types 103a-D and IIIa-F, the two in widespread use in recent years.

CCD photometry of three Type IA supernovae - V, R, and I light curves

view Abstract Citations (59) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CCD Photometry of Three Types 1a Supernovae: V,R, and I Light Curves Ford, Charles H. ; Herbst, William ; Richmond, Michael W. ; Baker, Michael L. ; Filippenko, Alexei V. ; Treffers, Richard R. ; Paik, Young ; Benson, Priscilla J. Abstract Cousins system VRI photometry has been obtained for the Type Ia supernova 1992G with CCDs attached to small telescopes at Leuschner Observatory (LO), Van Vleck Observatory (VVO), and Whitin Observatory (WO). Similar equipment, observing and reduction procedures have been adopted at all three sites and there is generally good agreement among the data sets. Two other Type Ia supernovae, 1991M and 1991T, were observed only at VVO. The shapes of the light curves of all three supernovae, including the spectroscopically peculiar SN 1991, are quite similar, and the combined light curve in V matches Leibundgut's [Ph.D. thesis, University of Basel (1988)] template reasonably well. Since very little R and I photometry of supernovae has been done previously, our data provide the first real definition of SN Ia templates in those bands. The I template is of particular interest since it shows a secondary maximum 23+/-1 days after the primary peak, similar to what is seen in normal Type Ia supernovae at J, H, and K wavelengths. The behavior in R is intermediate between V and I. The existence of substantial structure in the R and I light curves of normal Type Ia supernovae (and its absence in the peculiar Type Ia SN 1991bg) enhances their utility as cosmological probes and provides an observational challenge to models of the event. The observed colors of SN 1991M and SN 1991T as a function of time are quite similar, indicating that they suffer about the same amount of reddening. SN 1992G is clearly more heavily reddened than these two, by about 0.1 mag in R -I. The intrinsic colors of these Type Ia supernovae at V_max_ are V-R =0.0 and R -I= -0.3, with an uncertainty of around 0.05 mag, if the reddening of SN 1991T is E(B-V) = 0.13. If Type Ia supernovae are assumed to be good standard candles, then NGC 4527, the parent galaxy of SN 1991T, must have a peculiar velocity of 1200+/-150 km s^-1^ relative to the Hubble flow and directed away from the Galaxy. If the peculiar velocity of NGC 4527 is less than this, then SN 1991T must be an overluminous SN Ia. Tully & Shaya (1984) regard NGC 4527 as a near-side member of the Virgo cluster falling in at approximately 1000 km s^-1^. Our data alone yield a Hubble constant of 75+/-15 km s^-1^ Mpc^-1^ if M_V_ at maximum light is -18.7 mag, and 50+/-10 km s^-1^ Mpc^-1^ if it is -19.6 mag. Publication: The Astronomical Journal Pub Date: September 1993 DOI: 10.1086/116708 Bibcode: 1993AJ....106.1101F Keywords: Astronomical Photometry; Charge Coupled Devices; Light Curve; Supernovae; Astronomical Observatories; Interstellar Extinction; Peculiar Stars; Stellar Color; Stellar Magnitude; Astrophysics; SUPERNOVAE: INDIVIDUAL: 1992G; SUPERNOVAE: INDIVIDUAL: 1992M; SUPERNOVAE: INDIVIDUAL: 1992T full text sources ADS | data products SIMBAD (10) NED (6)

The Measure of Light Pollution and the Populations of Cities

ABSTRACTA program to measure the night sky brightness has been in progress for some years in order to calibrate the extent of the night sky brightness surrounding the Van Vleck Observatory. Both the central intensity and the areal extent of the brightest sky caused by campus and city were repeatedly measured in order to gauge the extent of the problem. For this purpose, portable visual photometers were designed which have remained stable and usable for nearly a decade. They are now useful for the measurement of the effects of increases in urban growth and of the more flexible attitude of the campus administration toward excess campus lighting. The inability to define city populations which realistically model and predict the measured sky brightness is the largest source of uncertainty. The observatory is in the Northeastern Corridor where the observed brightness is the sum of the illumination from a number of overlapping city sources. Present light pollution studies have not correctly defined the model for the population of an urban area. The two paradigmatic definitions now available for use are shown to be flawed for light pollution comparisons. An algorithm, unlike any in present use, must be sought which distinguishes between individual core cities within metropolitan areas.

TiO photometry of six T Tauri stars

Wing system and UBVRI photometry of six T Tauri stars: RY Tau, T Tau, V410 Tau, SU Aur, RW Aur, and CO Ori, was carried out with the Perkin telescope at Van Vleck Observatory during the 1988/1989 Milky Way season. A TiO sensitive index was formed from two of the Wing system filters. It is found that the weak T Tauri star V410 Tau shows enhanced TiO absorption when fainter, in rough quantitative agreement with cool spot models. The large amplitude irregular variables RY Tau, RW Aur, and CO Ori do not show such enhancements. SU Aur and T Tau did not vary sufficiently for us to determine their TiO behavior as a function of brightness. The data demonstrate that one cannot extend the simple cool spot model, which explains periodic variability in some T Tauri stars, to the irregular variations more typical of classical T Tauri stars such as RY Tau and CO Ori. 16 refs.

The variability of T Tauri stars

For many years we have monitored the variability of some bright T Tauri stars with the 0.6 m “Perkin telescope” at Van Vleck Observatory. Collaborators in this endeavor have included several astronomers at the Crimea Astrophysical Observatory, at Capidomonte Observatory, at the U.S. Naval Observatory, and even some sophisticated amateurs.

Discussion of trignometric parallaxes determined along right ascension and declination

We discuss the data of trignometric parallaxes determined along α and δ published by the US Naval Observatory and Van Vleck Observatory. We found that the absolute difference δπ = π x − π y shows a very similar periodic variation with α for both observations. This variation is shown to be due to errors in π y and to be related to the current practice of maximising the parallax factor for parallax determination in right ascension. Therefore, if possible, we should increase observations where the parallax factor in declination is maximised.

Phase-dependent variations of H-alpha equivalent width in VW CEP

view Abstract Citations (2) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Phase-Dependent Variations of Hα Equivalent Width in VW CEP Herbst, William ; Bishel, Lynn E. Abstract Hα photometry of the W UMa type eclipsing binary VW Cep has been obtained on four nights in 1985 at Van Vleck Observatory. A phase-dependent variation of Hα equivalent width (EW) has been discovered. The absorption EW decreases by a small (0.1 - 0.2 Å) but significant amount during both eclipses. No significant difference in the amplitude of the effect is seen between primary and secondary eclipse. The most likely interpretation is that the photospheric absorption-line EW decreases during eclipse. It is also possible that an emission component to the line is seen more prominently during both eclipses, resulting in a net decrease in the absorption EW. No evidence for plage-like concentrations of Hα flux on either component of the binary is found. Publication: The Astronomical Journal Pub Date: October 1987 DOI: 10.1086/114539 Bibcode: 1987AJ.....94.1051H Keywords: Astronomical Photometry; Eclipsing Binary Stars; H Alpha Line; Spectral Line Width; Absorption Spectra; Photosphere; Stellar Oscillations; Astrophysics; STARS: BINARIES; STARS: ECLIPSING BINARIES full text sources ADS | data products SIMBAD (2)

H-alpha photometry of dwarf K and M stars

view Abstract Citations (37) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hα Photometry of Dwarf K and M Stars Herbst, William ; Layden, Andrew C. Abstract Hα photometry of 188 dM and dK stars has been obtained with the Perkin telescope at Van Vleck Observatory. The photometric index, N-W, has been calibrated using 56 stars with equivalent widths determined spectroscopically by Stauffer and Hartmann (1986). An external standard error of 0.08 Å is found for the equivalent width (EW) measurements of non-emission stars; it may be higher for those with emission lines. The distribution of stars in the N-W, R-I plane is discussed. Most lie along a single "main sequence" of absorption which slopes up towards 0 EW with increasing R-I. Eighteen dKe or dMe stars are found, all clearly separated from the absorption sequence. The absorption EWs are large compared with expectation based on photospheric models, indicating either that the models are in error or that chromospheres are present in essentially all late-type dwarfs. Three stars (Gl 182, Gl 873AB, and Gl 890) were found to vary significantly in N-W. All are emission stars, and the variability can probably be attributed to flares. Publication: The Astronomical Journal Pub Date: July 1987 DOI: 10.1086/114457 Bibcode: 1987AJ.....94..150H Keywords: Dwarf Stars; H Alpha Line; K Stars; M Stars; Stellar Spectrophotometry; Variable Stars; Emission Spectra; Hertzsprung-Russell Diagram; Main Sequence Stars; Stellar Magnitude; Astrophysics; STARS: ABUNDANCES; STARS: SPECTRAL CLASSIFICATION full text sources ADS | data products SIMBAD (171)

T TAURI VARIABLES.

ABSTRACT A review is given of the variability characteristics of T Tauri stars monitored at Van Vleck Observatory. A list of T Tauri stars with known rotation periods is compiled and used to compare limits on the radii, derived from v sin i measures, with expectation based on the luminosities and effective temperatures of the stars. The large-amplitude quasi-perodic or aperiodic component of the light variation, which is more typical of the T Tauri phenomenon than periodic behavior, is discussed. Correlations between brightness and color and brightness and spectral change are reviewed. A typical time scale of 30 to 60 days is suggested for the phenomenon in the Van Vleck Observatory sample. A plausible interpretation of the variability is given, which involves the growth and decay of active regions on the photosphere of the star.

Variations in External Parallax Errors

The present astrometric program of the Van Vleck Observatory began in 1967. Since then it has emphasized parallaxes and proper motions of stars of the lower main sequence which are not identified on the basis of proper motion and are therefore not biased towards high space velocity. Later an analysis was made from parallax and proper motion data from 70 stars in the spectral range dK3-M2 (Upgren 1973) which found the average external mean error in parallax to be 8.0 ± 1.7 mas (milliarcseconds). This external error was found to vary little from one parallax to another and to have no correlation with the formal internal errors for the individual parallax determinations. Hanson and Lutz (1983) confirm this result using the parallaxes of 14 members of the Hyades cluster also determined at Van Vleck. They find a mean external parallax error of 9.4 ± 1.8 mas and suggest that the proximity of the two determinations may be evidence that Van Vleck parallaxes may be characterized by a single external error. Their slightly larger figure may or may not reflect the relatively fewer plates and epochs of observation from which the Hyades parallaxes were determined.