Search

Previous analyses of various standard candles observed by the Gaia satellite have reported statistically significant systematics in the parallaxes that have improved from $\sim$250 $\mu$as in the first data release (DR1) to 50--80 $\mu$as in the second data release (DR2). Here we examine the parallaxes newly reported in the Gaia early third data release (EDR3) using the same sample of benchmark eclipsing binaries (EBs) we used to assess the DR1 and DR2 parallaxes. We find a mean offset of $-37\pm20$ $\mu$as (Gaia$-$EB), which decreases to $-15\pm18$ $\mu$as after applying the corrections recommended by the Gaia Mission team; global systematics in the Gaia parallaxes have clearly improved and are no longer statistically significant for the EB sample, which spans $5\lesssim{G}\lesssim12$ in brightness and 0.03--3 kpc in distance. We also find that the RUWE goodness-of-fit statistic reported in Gaia EDR3 is highly sensitive to unresolved companions (tertiaries in the case of our EB sample) as well as to photocenter motion of the binaries themselves. RUWE is nearly perfectly correlated ($r^2=0.82$) with photocenter motions down to $\lesssim$0.1 mas, and surprisingly this correlation exists entirely within the nominal "good" RUWE range of 1.0--1.4. This suggests that RUWE values even slightly greater than 1.0 may signify unresolved binaries in Gaia, and that the RUWE value can serve as a quantitative predictor of the photocenter motion., Comment: accepted for publication in ApJ Letters

Stellar distances constitute a foundational pillar of astrophysics. The publication of 1.47 billion stellar parallaxes from Gaia is a major contribution to this. Yet despite Gaia's precision, the majority of these stars are so distant or faint that their fractional parallax uncertainties are large, thereby precluding a simple inversion of parallax to provide a distance. Here we take a probabilistic approach to estimating stellar distances that uses a prior constructed from a three-dimensional model of our Galaxy. This model includes interstellar extinction and Gaia's variable magnitude limit. We infer two types of distance. The first, geometric, uses the parallax together with a direction-dependent prior on distance. The second, photogeometric, additionally uses the colour and apparent magnitude of a star, by exploiting the fact that stars of a given colour have a restricted range of probable absolute magnitudes (plus extinction). Tests on simulated data and external validations show that the photogeometric estimates generally have higher accuracy and precision for stars with poor parallaxes. We provide a catalogue of 1.47 billion geometric and 1.35 billion photogeometric distances together with asymmetric uncertainty measures. Our estimates are quantiles of a posterior probability distribution, so they transform invariably and can therefore also be used directly in the distance modulus (5log10(r)-5). The catalogue may be downloaded or queried using ADQL at various sites (see http://www.mpia.de/homes/calj/gedr3_distances.html) where it can also be cross-matched with the Gaia catalogue., Comment: Accepted to The Astronomical Journal (minor changes following referee report; catalogue unchanged). 28 pages, 26 figures. Access catalogue via http://www.mpia.de/homes/calj/gedr3_distances.html

The velocity field of main-sequence stars and red giants from the TGAS catalogue with heliocentric distances up to 1.5 kpc has been analyzed for various spectral types. To estimate the influence of a low accuracy of stellar parallax measurements on the results of a kinematic analysis of distant stars, first we have studied in detail how the kinematic parameters derived with 1/π distances are shifted when these distances are replaced by three other versions of distances from Astraatmadja et al. (2016b). We have obtained detailed tables in which the ranges of these shifts in the Ogorodnikov–Milne and Bottlinger model parameters are shown for the stars of each spectral type. We have the smallest shifts in the case of determining the Oort coefficients A and B, for which there are 10% shifts only for main-sequence stars of spectral type B. In the remaining cases, these shifts are 0–3%. For the remaining parameters the shifts do not exceed 30%. Thus, we have shown that using the 1/π distance scale in estimating the Ogorodnikov–Milne and Bottlingermodel parameters (except for the parameter Ω″0) yields reliable results even when using parallaxes with large relative errors (up to 60%). To study Parenago’s discontinuity, we have investigated the dependence of the Ogorodnikov–Milne and Bottlinger model parameters on color for 1 260 071 mainsequence stars and 534 387 red giants. As far as we know, such a data set is used for the first time to investigate Parenago’s discontinuity. The main result is the detection of maximum points at B − V = 0.75 after which the solar velocity component V and the Oort coefficient B decrease when moving from blue stars to red ones. This fact is a new feature of Parenago’s discontinuity, because the component V does not change in the classical case at B − V >0.6. We have made an attempt to represent the well-known Parenago’s discontinuity as a special case of the more complex effect of a gradual change in a number of kinematic parameters as the mean age and composition of the group of stars under study changes.

Using offset-corrected Gaia-EDR3 parallax measurements and spectrophotometric methods, we have determined distances for 69 massive stars in the Carina OB1 association and associated clusters: Trumpler 16 (21 stars), Trumpler 14 (20 stars), Trumpler 15 (3 stars), Bochum 11 (5 stars), and South Pillars region (20 stars). Past distance estimates to the Carina Nebula range from 2.2 to 3.6 kpc, with uncertainties arising from photometry and anomalous dust extinction. The EDR3 parallax solutions show considerable improvement over DR2, with typical errors $\sigma_{\varpi}/\varpi \approx$~3-5%. The O-type stars in the Great Carina Nebula lie at essentially the same distance ($2.35\pm0.08$ kpc), quoting mean and rms variance. The clusters have distances of $2.32\pm0.12$ kpc (Tr 16), $2.37\pm0.15$ kpc (Tr 14), $2.36\pm0.09$ kpc (Tr 15), and $2.33\pm0.12$ kpc (Bochum 11) in good agreement with the $\eta$ Car distance of around 2.3 kpc. O-star proper motions suggest internal (2D) velocity dispersions $\sim4$ km/s for Tr 14 and Tr 16. Reliable distances allow estimates of cluster sizes, stellar dynamics, luminosities, and fluxes of photoionizing radiation incident on photodissociation regions in the region. We estimate that Tr 14 and Tr 16 have half-mass radii $r_h = 1.5-1.8$ pc, stellar crossing times $t_{\rm cr} = r_h/v_m \approx 0.7-0.8$ Myr, and two-body relaxation times $t_{rh} \approx 40-80$ Myr. The underlying velocity dispersion for Tr 14, if a bound cluster, would be $v_m \approx 2.1^{+0.7}_{-0.4}$ km/s for $N = 7600^{+5800}_{-2600}$ stars. With the higher dispersions of the O-stars, mass segregation might occur slowly, on times scales of 3-6~Myr.

Carbon-deficient red giants (CDRGs) are a rare class of peculiar red giants, also called "weak G-band" or "weak-CH" stars. Their atmospheric compositions show depleted carbon, a low 12C/13C isotopic ratio, and an overabundance of nitrogen, indicating that the material at the surface has undergone CN-cycle hydrogen-burning. I present Stromgren uvby photometry of nearly all known CDRGs. Barium stars, having an enhanced carbon abundance, exhibit the "Bond-Neff effect"--a broad depression in their energy distributions at ~4000 A, recently confirmed to be due to the CH molecule. This gives Ba II stars unusually low Stromgren c1 photometric indices. I show that CDRGs, lacking CH absorption, exhibit an "anti-Bond-Neff effect": higher c1 indices than normal red giants. Using precise parallaxes from Gaia DR2, I plot CDRGs in the color-magnitude diagram (CMD) and compare them with theoretical evolution tracks. Most CDRGs lie in a fairly tight clump in the CMD, indicating initial masses in the range ~2 to 3.5 Msun, if they have evolved as single stars. It is unclear whether they are stars that have just reached the base of the red-giant branch and the first dredge-up of CN-processed material, or are more highly evolved helium-burning stars in the red-giant clump. About 10% of CDRGs have higher masses of ~4 to 4.5 Msun, and exhibit unusually high rotational velocities. I show that CDRGs lie at systematically larger distances from the Galactic plane than normal giants, possibly indicating a role of binary mass-transfer and mergers. CDRGs continue to present a major puzzle for our understanding of stellar evolution., Comment: Accepted by Astrophysical Journal

For application to surveys of interstellar matter and Galactic structure, we compute new spectrophotometric distances to 139 OB stars frequently used as background targets for UV spectroscopy. Many of these stars have updated spectral types and digital photometry with reddening corrections from the Galactic O-Star (GOS) spectroscopic survey. We compare our new photometric distances to values used in previous IUE and FUSE surveys and to parallax distances derived from Gaia-DR2, after applying a standard (0.03 mas) offset from the quasar celestial reference frame. We find substantial differences between photometric and parallax distances (at d > 1.5 kpc) with increasing dispersion when parallax errors exceed 8%. Differences from previous surveys arise from new GOS stellar classifications, especially luminosity classes, and from reddening corrections. We apply our methods to two OB associations. For Perseus OB1 (nine O-stars) we find mean distances of $2.47\pm0.57$ kpc (Gaia parallax) and $2.99\pm0.14$ kpc (photometric) using a standard grid of absolute magnitudes (Bowen et al. 2008). For 29 O-stars in Car OB1 associated with Trumpler-16, Trumpler-14, Trumpler-15, and Collinder-228 star clusters, we find $2.87\pm0.73$ kpc (Gaia parallax) and $2.60\pm0.28$ kpc (photometric). Using an alternative grid of O-star absolute magnitudes (Martins et al. 2005) shifts these photometric distances 7% closer. Improving the distances to OB-stars will require attention to spectral types, photometry, reddening, binarity, and the grid of absolute magnitudes. We anticipate that future measurements in Gaia-DR3 will improve the precision of distances to massive star-forming regions in the Milky Way., 29 pages, 5 tables, 5 figures, and lengthy appendix (minor changes, accepted to Astrophysical Journal)

The Oort constants describe the local variations of the stellar streaming field. Classically, they are determined from stellar proper motions. We discuss problems arising in this procedure. A large, hitherto overlooked, source of systematic error arises from longitudinal variations of the mean stellar parallax. Together with the solar reflex motion, these variations create contributions to the longitudinal proper motions mu_l*(l) that are indistinguishable from the Oort Constants at the 20% level. Fortunately, we can correct for this "mode mixing" using the latitudinal proper motions mu_b(l). We use ~10^6 stars from the ACT/Tycho-2 catalogs with proper motion errors of ~ 3 mas/yr. We find significant deviations from expectations based on a smooth axisymmetric equilibrium disk, in particular non-zero C for old red giant stars. We also find variations of the Oort constants with the asymmetric drift of the sub-sample. These correlations are different in nature than those expected for an axisymmetric Galaxy. The most reliable tracers for the ``true'' Oort constants are red giants, which are old enough to be in equilibrium and distant enough to be unaffected by possible local anomalies. For these stars we find, A ~ 16, B ~- 17, A - B ~ 33, and C ~- 10 km/s/kpc with internal errors of about 1-2 and external error of perhaps the same order. These values are consistent with our knowledge of the Milky Way (flat rotation curve and Omega=A - B ~ 28 +/- 2). Based on observations made with the ESA Hipparcos astrometry satellite. (Abridged), Submitted to ApJ

The inconsistency between the mean parallax of the Pleidaes open cluster from the Hipparcos catalog and that obtained from the stellar evolution theory and photometric measurements is probed by recomputing the Hipparcos data in a different way that reduces the propagation of the along-scan attitude errors. This is achieved by coupling observations of stars made nearly simultaneously in the two separate fields of view of the telescope. A direct calculation of astrometric quantities of 54 Pleiades members by the new method, based on the Intermediate Astrometry Data, provides a correction of -0.71 ± 0.14 mas to the weighted mean parallax of the cluster. The mean corrected parallax of the Pleiades is 7.75 ± 0.20 mas.

The Einstein timescale, which is the only information obtained from current microlensing experiments, results from a combination of the lens parameters that we want to determine. Of the methods for breaking the lens parameter degeneracy, the most promising and generally applicable method is to measure the lens parallax from simultaneous photometric observations of a lensing event from the ground and from a heliocentric satellite. However, the elegant idea of lens parallax measurement, which was proposed to resolve the lens parameter degeneracy, paradoxically suffers from another type of degeneracy, which arises because a given set of light curves can have two possible values of parallax (parallax degeneracy). In this paper we show that one can break the parallax degeneracy either from the simultaneous astrometric observations of an event by using a high-precision ground-based interferometer and a heliocentric satellite such as the Space Interferometry Mission or from a combination of the astrometric observations on one site (either the ground or the satellite) and the photometric observations on a different site. Since the proposed methods can measure both the lens parallax and proper motion at the same time, one can completely break the lens parameter degeneracy, and thus the physical parameters of individual lenses can be uniquely determined.

Original paper can be found at: http://www.astrosociety.org/pubs/cs/113-155.html--Copyright Astronomical Society of the Pacific

Comparisons show agreement at the 0.1-mag level between the calibration of the Cepheid period–luminosity (P–L) relation by Feast & Catchpole (FC) using the early release of Hipparcos data and four previous ground-based calibrations, three of which are either largely or totally independent of the distance to the Large Magellanic Cloud (LMC). Each of the comparisons has the sense that the FC calibration is brighter, but only at the level of ≲0.1 mag. In contrast, FC argue that their Hipparcos recalibration leads to a 0.2-mag revision in the distance to the LMC, and thereby to a 10 per cent decrease in the Hubble constant. We argue differently. The comparison of the Hipparcos recalibration with others should be made using only local Galactic Cepheids, not based on Cepheids in the LMC that require a set of precepts that are not germane to the direct Hipparcos recalibration. The comparison made here, using only Galactic Cepheids, gives a correction of ∼4 per cent or less to our value of H0 based on Type Ia supernovae, keeping all other factors and precepts the same. A second success of the Hipparcos mission is the calibration of the position of the main sequence in the Hertzsprung–Russell diagram as a function of metallicity using local subdwarfs. These data have been used by Reid and by Gratton et al. to obtain, similarly to FC, a brighter absolute magnitude of RR Lyrae stars by ∼0.3 mag from that often currently adopted. These new calibrations confirm the earlier brighter calibrations by Walker, by Sandage, and by Mazzitelli, D'Antona & Caloi, thereby reducing the ages of globular clusters by ∼30 per cent. This removes most of the cosmological time-scale problem if H0∼55 km s−1 Mpc−1. A similar conclusion, based on pulsation theory and MACHO data, has been reached by Alcock et al.

A quantitative study of the clustering properties of the cosmic web as a function of absolute magnitude and colour is presented using the SDSS Data Release 7 galaxy survey. Mark correlations are included in the analysis. We compare our results with mock galaxy samples obtained with four different semi-analytical models of galaxy formation imposed on the merger trees of the Millenium simulation. The clustering of both red and blue galaxies is studied separately., 8 pages, 5 figures, to be published in Baltic Astronomy

Now that the HIPPARCOS satellite safely revolves on his ‘degraded’ orbit - after the exploit of the ESA technicians to rescue it from the faulty ignition of its apogeum boost motor - and regularly provides the information needed for reaching the initial aim of the project, one may turn to examine the possible applications of the finalHipparcos Catalogue. In the present paper, we describe how double and multiple stars have been introduced in the Input Catalogue and what may be expected from their observation. We also try to evaluate how much the final HIPPARCOS data may influence future researches on these celestial bodies and what kind of ground-based observation programmes should be conducted in order to support the space results.

We present a measurement of the trigonometric parallax for a nearby very low mass star recently discovered by Delfosse et al. (2001). Using schmidt plates measured on the SuperCOSMOS plate scanning machine we nd a parallax of = 19237 mas, giving a distance of d =5 :2 +1:2 0:8 pc. Using this distance we nd the absolute magnitude and kinematics for the object to be consistent with an intermediate{age disk object of mass 0:08 M. This star is clearly nearby and is on these measurements between the 30th and 86th closest known stellar system to the Sun; it could be the closest M 9 dwarf to the Sun.

The distribution function of the bias in the absolute magnitude of a star, as determined from its trigonometric parallax, is calculated for the case where the error dispersion is imperfectly known. A general power law is assumed for the spatial distribution of stars. The distribution function is used to calculate most probable and mean bias corrections as well as confidence intervals for the correction, the latter providing an indication of the intrinsic accuracy of the Lutz-Kelker correction. The basic method may be used for bias correction in other situations also

New orbital elements are presented for the visual double stars W.D.S. 03537N5317, 11468N4839, and 14138N3100. The observations, residuals, ephemerides, dynamical parallaxes, and total masses of each system are also given.

One of the most important applications of the Astrometry is the study of the motions in our Galaxy. In this sense, an effort has been made to determine the kinematic parameters of the galactic rotation in the solar neighbourhood when the galactic velocity field is expanded until the second order terms of its Taylor development. For this purpose, it has been necessary to use a larger sample of stars and better distributed in galactic latitude than hitherto. As a first study, we present here the solution method applied and the results obtained when the hypothesis of cylindrical symmetry is made. The small correlations obtained between the second order partial derivatives indicate that some of these parameters can be solved at present. Furthermore, we have obtained a good concordance between our values and the hypothesis made by several previous authors concerning these terms: Trumpler and Weaver (1962), Clube (1974) and Brosche and Schwan (1981).

After a short summary of the results expected from the HIPPARCOS mission, the author gives several examples illustrating the needs that will arise afterwards in order to maintain the level of precision achieved and to extend it to many more objects. Three domains are particularly dealt with in this paper: 1) The maintenance of the HIPPARCOS reference system that will degrade at the rate of 2 mas per year. The instruments intended to contribute to this task should concentrate on observing a few stars as accurately as possible. The best candidates are astrolabes and optical interferometers. 2) The extension of the HIPPARCOS catalogue to more stars using astrographs, Schmidt telescopes, and photoelectric meridian circles. A particularly important objective would be to reduce the GSE catalogue to this system and determine its proper motions with second epoch plates. 3) The determination of new parallaxes and double star parameters in particular with CCD astrometry, astrometric photometry and various interferometric techniques.

Since 1983, a TI 800×800 CCD has been routinely used on the 1.5-m astrometric reflector during dark time to measure parallaxes of faint (V≥16.0) stars. The rest of the time remains devoted to the fine grain (Kodak type IIIa emulsions) photographic parallax program, which now uses magnitude compensating filters to extend the range of the program to V=0. Sufficient data are now available to evaluate the precisions of these programs. For CCD fields, the mean error for the relative parallax ranges from ±0".0025 for fields with relatively few observations to ±0".0005 for intensively observed fields such as vB10. The photographic program is now producing relative parallaxes with average mean errors of ±0".0026 compared to ±0".0038 previously obtained with coarse grain emulsions.

During the annular solar eclipse of 1987 September 23, Baily beads were observed at two locations on the southern and northern edges of the path. The observations were analysed to identify the features of the lunar profile which gave rise to the beads. They were then reduced to derive corrections to the ecliptic longitude and latitude of the Sun and the standard solar radius. The corrections obtained are: Δr=−0.11“±0.03” , Δ1=+0.57“±0.09” , Δb=+0.02“±0.02” , the errors are mean errors.

We investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry, e.g. in the context of exoplanet searches, and to quantify the expected effects in different regions of the HR-diagram. Stellar surface structures are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter, and closure phase. We use theoretical considerations supported by Monte Carlo simulations to derive statistical relations between the corresponding astrometric, photometric, and radial-velocity effects. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (, 12 pages, 4 figures, submitted to A&A

Initial orbit determination for distant objects can be made a simple linear problem with explicit solutions. This is similar to stellar parallax work, with very difficult observations but simple computations. A very simple procedure for computing ephemerides for distant objects is proposed. The optimum distribution of observations in time and regions is investigated. The problem of double solutions, linking, elongation, and latitude, and accuracy of elements are also discussed.

This paper presents results from speckle observations of binary stars carried out between 1987 and 1990 using a 212-cm telescope at the San Pedro Martir Observatory, Mexico.

Massive Compact Objects (Machos) are currently being discovered at substantially higher rates than would be expected from standard models of known stellar populations. To determine whether they are due to non-standard distri- butions of known populations or to a heretofore unknown (`dark') population, one must acquire more information about the individual events. Space-based parallaxes are potentially the best tool for extracting additional information. To leading order, parallax measurements by a single satellite result in a four- fold degeneracy: two possible values of $\tilde v$ and two possible signs for the component of motion perpendicular to the projected satellite-Earth vector. It had been believed that a second satellite would be required to break this degeneracy. I show that the velocity difference between the satellite and the Earth allows one to partially or totally break the degeneracy using a single satellite. For most Macho events it is possible to measure $\tilde v$ and $\tilde r_e$. For some it is also possible to measure $\Phi$. The proposed Space Infrared Telescope could measure $\sim 100$ parallaxes per year by applying $\sim 400\,$hr of telescope time., Comment: 12 pages, request PostScript file to gould@payne.mps.ohio-state.edu, OSU-TA-15/94

It is pointed out that a Cepheid period–luminosity relation with a zero-point from Hipparcos trigonometrical parallaxes and a consistent reddening system zero-point implies that some recent estimates of H0 based on the Cepheid scale should be increased by ∼8 per cent. This result avoids using the distance to the Large Magellanic Cloud (LMC) as an intermediary point but is not significantly different from the result obtained by Feast & Catchpole via the LMC. A number of other issues are discussed, including metallicity effects on Cepheid distances and reddenings, and the age of metal-poor globular clusters.

The proper motions of 512 stars reduced to the system of FK5 were used for the determination of the velocity components X,Y,Z of the solar motion, Dort's constants A,B, and corrections Δn, Δk to Newcomb's precession values. It was found that the transition from FK4 to FK5 gives negligible and unsignificant changes to all the determined quantities.

This is the fourth study in a series to determine the direct trigonometric parallaxes of four of the nearest open star clusters, the Hyades, the Pleiades, the Praesepe, and the nearby cluster in Coma (Gatewood et al. 1990; Gatewood et al. 1992); Gatewood & Kiewiet de Jonge 1994). The results for the open star cluster in Coma are compared with those of the other three clusters, and the members are found to be significantly subluminous. The trigonometric parallax of the cluster is estimated from that of three members studied with the Multichannel Astrometric Photometer (MAP) at the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of the cluster is +13.53 +/- 0.54 mass (0.00054 min), corresponding to a distance modulus of 4.34 +/- 0.09 mag. The U-B excess of the Coma cluster members may be used to adjust the observed absolute magnitudes and the B-V measurements as suggested by Sandage & Eggen (1959). The agreement obtained in this manner suggests that, like subdwarf stars, the stars of the Coma cluster appear subluminous because of line blanketing. One of the three members observed in this study was recognized as a member by its parallax and is the faintest known member of the cluster.

Recent-epochs positions and proper motions of nine dwarf carbon star candidates are presented along with finding charts for each object. Measurements are obtained from digitized Palomar Observatory Sky Survey (POSS) and Quik V plate archives at the Space Telescope Science Institute, and from recent CCD images.

Completion of long-term astrometric plate series yields revised results on the orbits and parallaxes. The subdwarf mu Cas seems to be overluminous; the distance of the supergiant epsilon Aur is near 600 pc. The unresolved companion in the red dwarf pair BD +6degree398 is of very low mass, and the companion of BD +67degree552 massive but probably underluminous.

Trigonometric parallaxes for stars in the regions of the binary stars Delta Equulei (HR 8123) and Chi(sup l) Orionis (HR 2047) are derived from data collected with the Multichannel Astrometric Photometer (MAP) and the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of all trigonometric studies of delta Equ is now +5.42 +/- 0.93 mas, corresponding to absolute magnitudes of 3.87 +/- 0.04 and 3.95 +/- 0.04 mag, respectively, for the primary and secondary. Using the Popper and Dworetsky orbit we find a photocentric semimajor axis of 2.9 +/- 0.8 mas and individual masses of 1.21 +/- 0.090 and 1.19 +/- 0.088 solar masses, respectively, for the primary and secondary components. The weighted mean trigonometric parallax of all studies of the binary star Chi(sup l) Ori is +111.0 +/- 0.92 mas, implying an absolute visual magnitude for the dominant GO V primary of 4.63 +/- 0.018 mag. The photocentric semimajor axis, derived from a fraction of the orbit, is 96.9 +/- 5.4 mas.

Trigonometric parallaxes for stars in the regions of the variable stars delta Cephei and EV Lacertae are derived from data collected with the Multichannel Astrometric Photometer (MAP) and the Thaw Refractor of the University of Pittsburgh's Allegheny Observatory. The weighted mean parallax of all trigonometric studies of delta Cephei is now + 0.0030 sec + or - 0.00093 sec, corresponding to a distance modulus of 7.61 + or - 0.67 mag. This indicates that this luminosity standard star is approximately one standard deviation more distance than has been generally accepted. The weighted mean trigonometric parallax of all studies of the variable star EV Lacertae (BD + 43 deg 4305) is + 0.1993 sec + or - 0.00093 sec, implying a distance modulus of - 1.498 + or - 0.0010 mag. The calculated absolute magnitude of this star is almost exactly that predicted by its (R-I)(sub Kron) magnitude and by the Gliese (R-I) main-sequence value for stars in the solar neighborhood. We also find a parallax of 0.0189 sec + or - 0.0008 sec for the FO IVn star, HR 8666 (BD + 43 sec 4300). The derived luminosity of this star is midway between that expected for luminosity class IV and V stars at the indicated temperature.

Trigonometric parallaxes, relative proper motions, and photometry are presented for 122 stars in 111 systems. Of these stars, 70 are brighter than V=10.0

First results are presented from a trigonometric parallax program being carried out with a CCD detector on the 60» telescope at Palomar Mountain. New parallaxes for six extremely late M-dwarfs (M bol >13) are presented, more than doubling the number of such stars with directly measured distances. Structure in the main sequences constructed with this new parallax data suggest that the faintest known stars may not be stably supported by nuclear burning. It is shown that trigonometric parallaxes with accuracies of ≤0.004″ can be obtained in a few years using standard CCD equipment on a common-user telescope

191 trigonometric parallaxes, 19 binary-star mass ratios, and three new unresolved red-dwarf binaries are listed and annotated

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.

This system consists of three red-dwarf components with masses from 0.4 to 0.1 solar masses. The orbits with periods of 16 and 323 years are rediscussed from astrometric plates to 1991.

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.

The orbital elements of the double stars WDS 00516N2238, WDS 17366N0722, WDS 17563N0259, and WDS 23020N4800 have been calculated for the first time. Dynamical parallaxes and masses have been determined for the last three.

Parallaxes and proper motions are presented for 19 stars in 15 star fields. Four of the stars were discovered in the course of the measures to have significantly large proper motions and the data for them are included in the tables. The other 15 are all stars considered to be subluminous. Photometric and metallicity data for most of them are presented along with those for the 15 subdwarfs covered in the two previous lists in this series. They appear not to conform to a straight linear relation between [Fe/H] determined from the measured ultraviolet excess and the absolute magnitude corrected to a B-V color of 0.6

The framework of General Relativity is applied to the problem of reduction of high-precision astrometric observations of the order of 1 μarcsec. Such precision is expected to be attained in the not so distant future by means of the space optical interferometers orbiting the Earth. Theoretical methods are described enabling one to construct astrometric reference systems involving the barycentric system, geocentric system, and satellite (observer) system. The relativistic transformations between the employed reference systems have been derived. The equations of geometric optics for the nonstationary gravitational field of the Solar system have been deduced

We have determined the orbit and parallax of the Procyon system (F5 IV-V primary and white dwarf secondary) using conventional absolute radial velocities, precise relative radial velocities determined with the hydrogen flouride (HF) absorption-cell technique, and astrometric and visual observations. Our least-squares analysis confirms and strengthens Strand's previous analysis based on more limited material. We derive an absolute parallax of piabs=0.2899 ± 0.0074 arcsec. Our parallax result is in excellent agreement with other parallax investigations based on independent material. If we adopt the latest USNO parallax of piabs=0.2864 ± 0.0023 arcsec, then from our orbital paramters we derive MA=1.751 ± 0.051 solar mass, and MB=0.622 ± 0.017 solar mass. For the derived mass of Procyon A (which is consistent with previous results), it has long been known that the luminosity predicted by stellar interior theory is approximately a factor of two larger than the luminosity derived from observations. The errors in this observed luminosity and the above formal standard errors (i.e., our best estimates of the statistical random errors) in our derived mass are too small to explain this discrepancy. If the observed separation of the binary components has been systematically overestimated by approximately 0.2 arcsec, then the derived mass would be reduced, thus resolving the discrepancy. However, the excellent consistency of the separations measured by Aitken and Barnard for a number of years at different sites argues against this interpretation. The stellar interior theory of Procyon A may need substantial revision.

Eta Virginis is a bright (V = 3.89) triple system of composite spectral type A2 IV that has been observed for over a dozen years with both spectroscopy and speckle interferometry. Analysis of the speckle observations results in a long period of 13.1 yr. This period is also detected in residuals from the spectroscopic observations of the 71.7919 day short-period orbit. Elements of the long-period orbit were determined separately using the observations of both techniques. The more accurate elements from the speckle solution have been assumed in a simultaneous spectroscopic determination of the short- and long-period orbital elements. The magnitude difference of the speckle components suggests that lines of the third star should be visible in the spectrum.

Broadband BVRI photometry on the Cousins system is presented for 127 stars with annual proper motions greater than 1/2 arcsec. The sample includes three spectroscopically confirmed degenerate stars, and a considerable number of cool subdwarfs, several of which are discussed. Two stars with peculiar colors are identified, and the significance of certain of the Luyten color classes is briefly considered

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.