Small Proper Motion Research Articles

SN 2001em: No Jet-driven Gamma-Ray Burst Event

We report on our second-epoch VLBI and VLA observations of the Type Ib/c supernova 2001em, five years after the explosion. It was suggested that SN 2001em might be a jet-driven gamma ray burst (GRB), with the jet oriented near the plane of the sky, which would entail relativistic expansion or motion. Our VLBI image shows that SN 2001em is still unresolved five years after the explosion. For a distance of 83 Mpc (H_0 = 70 km/s/Mpc), the nominal expansion velocity is 5800 +/- 10,000 km/s, and the proper motion is 33,000 +/- 34,000 km/s. Our values are inconsistent with either relativistic expansion or motion, but are consistent with the non-relativistic expansion speeds and small proper motions seen in other supernovae. In particular these values are consistent with radio emission from SN 2001em being due to normal, non-relativistic supernova ejecta interacting with the circumstellar medium. Our VLA observations show a power-law decay in flux density since the time of the peak in the 8.4 GHz radio lightcurve in ~2003.

Chemical Composition and Differential Time-Series CCD Photometry of V2314 Ophiuchi: A New λ Bootis-Type Star

We investigated V2314 Oph using high-resolution (R = 45,000) spectral observations. The atmospheric parameters were determined using iron-line abundance analysis. We derived a mean effective temperature of Teff = 7510 K, mean surface gravity of log g = 3.30, metallicity of [Fe/H] = -0.59, and rotation velocity of v sin i = 38 km s-1. The abundances of 19 chemical elements were determined. The abundance pattern of V2314 Oph was found to be similar to that of λ Bootis-type stars. V2314 Oph is also a variable star, as are a majority of λ Bootis stars. A frequency analysis of 10 selected nights of precise differential photometry has preliminarily revealed four pulsational modes. The multiperiodic solution of the light curve is probably more similar to a low-amplitude Population I δ Scuti variable pattern, as is apparent for many λ Bootis stars. For asteroseismic investigations of the star it is necessary to obtain more prolonged, continuous time-series observations. These features, as well as the small proper motion and relatively large rotational velocity of V2314 Oph, specify the star as not likely to be a field SX Phoenicis Population II variable, as was suggested previously.

Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J17072343-0558249AB

We present Infrared Telescope Facility SpeX observations of the M/L binary system 2MASS J17072343-0558249. SpeX imaging resolves the system into a 101 ± 017 visual binary in which both components have red near-infrared colors. Resolved low-resolution (R ~ 150) 0.8-2.5 μm spectroscopy reveals strong H2O, CO, and FeH bands and alkali lines in the spectra of both components, characteristic of late-type M and L dwarfs. A comparison to a sample of late-type field dwarf spectra indicates spectral types M9 and L3. Despite the small proper motion of the system (0100 ± 0009 yr-1), imaging observations over 2.5 yr provide strong evidence that the two components share common proper motion. Physical association is also likely due to the small spatial volume occupied by the two components (based on spectrophotometric distance estimates of 15 ± 1 pc) as compared to the relatively low spatial density of low-mass field stars. The projected separation of the system is 15 ± 3 AU, similar to other late-type M and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of the binary components to be 0.072-0.083 and 0.064-0.077 M⊙, with an orbital period of roughly 150-300 yr. While this is nominally too long a baseline for astrometric mass measurements, the proximity and relatively wide angular separation of the 2MASS J1707-0558AB pair make it an ideal system for studying the M dwarf/L dwarf transition at a fixed age and metallicity.

Formation and progenitor of PSRJ0737−3039: New constraints on the supernova explosion forming pulsar B

We investigate the formation of the double pulsar PSR $J0737\mathrm{\text{\ensuremath{-}}}3039$ and examine its most likely progenitors, taking into account the most recent and all currently available observational constraints. We show that the most likely kick velocity and progenitor parameters depend strongly on the consideration of the full five-dimensional probability distribution function for the magnitude and direction of the kick velocity imparted to pulsar B at birth, the mass of pulsar B's presupernova helium star progenitor, and the presupernova orbital separation rather than marginalized one- or two-dimensional distributions for the kick velocity and progenitor mass. The priors that enter the analysis are the age of the system, the minimum helium star mass required to form a neutron star, the transverse systemic velocity, and the treatment of the unknown radial velocity. Since the latter cannot be measured observationally, we adopt a statistical approach and use theoretical radial-velocity distributions obtained from population synthesis calculations for coalescing double neutron stars. We find that when the minimum presupernova helium star mass required for neutron star formation is assumed to be $2.1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, the most likely kick velocity ranges from $70\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ to $180\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. When, on the other hand, masses lower than $2.1\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ are allowed as neutron star progenitors, the most likely kick velocity can reach very low values (as low as a few $\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$), although the majority of the models still yield most likely kick velocities of $50\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ to $170\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Hence, we agree with Piran and Shaviv [T. Piran and N. J. Shaviv, Phys. Rev. Lett. 94, 051102 (2005).] that the observed system properties, including the low transverse systemic velocity, can indeed be compatible with low progenitor masses and low kick velocities. Equally important though, we show that this is not the only likely formation path of pulsar B, due to the role of different prior assumptions that are necessary in the analysis. Moreover, in contrast to earlier claims in the literature, we show that the proximity of the double pulsar to the Galactic plane and the small proper motion do not pose stringent constraints on the kick velocity and progenitor mass of pulsar B at all. Instead, the constraints imposed by the current orbital semimajor axis and eccentricity and the orbital dynamics of asymmetric supernova explosions turn out to be much more restrictive. We conclude that without further knowledge of the priors, the currently available observational constraints cannot be used to unambiguously favor a specific core-collapse and neutron star-formation mechanism. Both electron capture and neutrino-driven supernovae therefore remain viable formation mechanisms for pulsar B.

Statistical Constraints for Astrometric Binaries with Nonlinear Motion

Useful constraints on the orbits and mass ratios of astrometric binaries in the Hipparcos catalog are derived from the measured proper motion differences of Hipparcos and Tycho-2 (??), accelerations of proper motions (), and second derivatives of proper motions (). It is shown how, in some cases, statistical bounds can be estimated for the masses of the secondary components. Two catalogs of astrometric binaries are generated, one of binaries with significant proper motion differences and the other of binaries with significant accelerations of their proper motions. Mathematical relations between the astrometric observables ??, , and and the orbital elements are derived in the appendices. We find a remarkable difference between the distribution of spectral types of stars with large accelerations but small proper motion differences and that of stars with large proper motion differences but insignificant accelerations. The spectral type distribution for the former sample of binaries is the same as the general distribution of all stars in the Hipparcos catalog, whereas the latter sample is clearly dominated by solar-type stars, with an obvious dearth of blue stars. We point out that the latter set includes mostly binaries with long periods (longer than about 6 yr).

Astrometric proof of companionship for the L dwarf companion candidate GJ1048B

Gizis et al. (2001, AJ, 121, 2185) reported a companion candidate of spectral type L1 near the K2 dwarf GJ1048 using the Two Micron All-Sky Survey (2MASS). At that time it was not possible to verify companionship astrometrically using the 2MASS data alone due to the small proper motion of GJ1048. We now show that both objects share the same proper motion by using data from the UK Schmidt Telescope Near-infrared (IVN) Southern Survey as the first epoch and data from 2MASS as the second epoch. Our technique of subtracting the PSF of the primary from the SuperCOSMOS I scans of the Southern Survey enables the astrometry of the companion candidate to be measured directly.

Water Masers as Tracers of Protostellar Disks and Outflows in the Intermediate‐Mass Star‐forming Region NGC 2071

We have mapped the water maser emission associated with the infrared centers IRS 1 and IRS 3 of the NGC 2071IR star-forming region at four epochs over ~4 months with the Very Long Baseline Array. We detected 269 maser features with ~1 km s-1 line widths and measured 30 proper motions. In each infrared center, the water maser emission appears to trace parts of a protostellar disk and collimated outflow. The disk components are ~9 and ~17 AU long in IRS 3 and IRS 1, respectively, and ~2 AU wide. They are identified as disks by their compact size, elongation parallel to the direction of known IR polarization, central location in the maser maps, small internal proper motions, and proximity to λ1.3 cm continuum emission. The outflows have axes perpendicular to the disks and exhibit proper motions of up to ~42 km s-1. They are outlined by maser emission up to ~260 AU from the protostars. The IRS 3 outflow appears to be conical on one side, while the IRS 1 outflow comprises a narrowly collimated bipolar flow surrounded by outward-facing, funnel-shaped cavities. The detection of water maser emission tracing such compact disk components and specifically conical or funnel-shaped structures is unusual. The fact that the distributions are similar in IRS 3 and IRS 1 may indicate that the two infrared centers are roughly coeval. NGC 2071IR provides a rare opportunity to resolve the structures and dynamics of disks and outflows together and to do so for two protostars that are only ~2000 AU apart (in projection) in a deeply embedded star-forming region of intermediate luminosity.

A New Very Cool White Dwarf Discovered by the Sloan Digital Sky Survey

Early data taken during commissioning of the Sloan Digital Sky Survey (SDSS) have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision-induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.

High-resolution near-infrared observations of Herbig-Haro flows -- I. H2 imaging and proper motions

We present results of an H2 proper motion study of three Herbig-Haro flows: HH 7-11, 25-26 and 33/40. These are the first proper motion measurements for these objects in the near-infrared, and are complementary to a velocity-resolved, echelle spectroscopy study of the H2 line profile from these objects, presented in a companion paper (Paper II, this issue). The results presented here cover a 4-5 year time-span. The HH 7-11 outflow components have high proper motions, ranging between -200 and 450 km s-1. The directions of their propagation are remarkably uniform, leading directly away from the outflow source. This proper motion pattern is in accordance with recent numerical simulations by Volker et al., and leads us to suggest that the HH 7-11 chain is produced by a (pulsed) heavy jet, of which HH 7 is the working surface, driving into a low-density but clumpy medium. The HH 25-26 system is quite intriguing: the shock components furthest from the source have high proper motions (?200km s-1), while those closest to the source have negligibly small proper motions (?70 km s-1). This system is interpreted in terms of working surface bow shocks internal to the jet, and a turbulent mixing layer between the flow and the ambient medium. Indeed, HH 26A and 25C may represent the jet being deflected off nearby dense material. The HH 33/40 flow does not show any significant amount of proper motion, with an upper limit of ?40-70 km s-1. Situated at the terminus of the giant 'parsec-scale' HH 34 outflow, HH 33 is probably the working surface of the outflow while HH 40 must be material caught up in the general flow of the system. Finally, we show that the limb-brightened leading edges of the bow shocks in HH 7 and 33 are resolved. If the widths of these leading edges represent cooling lengths, then the shocks are probably C-type, as opposed to J-type, bow shocks driving through pre-shock material with <1-mG magnetic fields.

HDE 233517: Lithium and Excess Infrared Emission in Giant Stars

Recent studies have identified a small class of moderately rapidly rotating, chromospherically active, single giants, some of which are lithium rich. We present evidence suggesting the peculiar K-type star HDE 233517 is one such object. Previously, HDE 233517 has been suggested to be a young star, consistent with its large far-infrared excess and our log e(Li) ~ 3.3. However, our high-resolution spectroscopic observations show it is likely a single, post-main-sequence K2 giant with v sin i = 15 km s-1 and modest Ca II H and K emission. The giant status of HDE 233517 is determined directly from luminosity-sensitive line ratios and a lack of significant line wings, and is further supported by a large radial velocity (46.5 km s-1), small proper motion, and the presence of interstellar absorption features. Interpretation of the data in the context of a recent mass outflow model for giant stars proposed by de la Reza and coworkers indicates that HDE 233517 has the largest mass-loss rate, ~3 × 10-7 M☉ yr-1, of any known luminosity class III giant. We suggest that the processes causing rapid rotation, large lithium abundance, and infrared excess are triggered at the base of the giant branch when the convection zone reaches the rapidly rotating core of low-mass stars.

Earth-crossing asteroids: The last days before earth impact

view Abstract Citations (10) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Earth-Crossing Asteroids: The Last Days Before Earth Impact Hills, Jack G. ; Leonard, Peter J. T. Abstract We have determined the observational and dynamical properties of Earth-crossing asteriods in the final weeks before their impacts with Earth. An asteroid that is within a few weeks of either Earth impact or of a very close approach to Earth has a large parallax and a small proper motion. The proper motion of such an asteroid has a maximum value of only a few minutes of arc per day, and it vanishes for some approach directions. An asteroid that is 10 days from impact has a parallax of several minutes of arc when viewed from two positions on Earth having a projected separation of 1 R (solar halo). Wide-field cameras can easily detect this parallax. Impacting asteroids greater than 100 m in diameter, which is near the minimum size that produces significant local damage, appear as stars of at least visual magnitude 18 during their final 10 days to impact unless they approach Earth from the vicinity of the Sun in the sky. They are detectable with relatively small telescopes equipped with CCDs. A typical proposed station for detecting them may have 20 telescopes with apertures between 5 and 16 in. and 1 telescope with an aperture of 36 in. (0.9 m). The largest telescope would only be used to search for objects near the sun (near dusk and dawn), and it would be available during the remainder of the night to improve the orbits and physical characterization of objects that may pass near or hit Earth. While the apertures of the required telescopes are relatively modest, such a systematic search for Earth-approaching asteroids will be a computer intensive task and it will require the correlation of the positions of anomalous objects between two or more sites in near real time to determine whether or not they have a measurable parallax. Thermal infrared emission may provide a complementary way of detecting impacting asteroids especially if they approach Earth from near the direction of the Sun. Publication: The Astronomical Journal Pub Date: January 1995 DOI: 10.1086/117285 Bibcode: 1995AJ....109..401H Keywords: Asteroids; Impact Prediction; Parallax; Radial Velocity; Solar Orbits; Charge Coupled Devices; Monte Carlo Method; Telescopes; Astronomy; METEOROIDS full text sources ADS |

Fundamental astrometry and millisecond pulsars

view Abstract Citations (76) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Fundamental Astrometry and Millisecond Pulsars Rawley, L. A. ; Taylor, J. H. ; Davis, M. M. Abstract Using a new data acquisition system designed specifically for timing millisecond pulsars, the authors have obtained 2 yr of data on PSRs 1937+21 and 1953+29 and 1 yr on PSR 1855+09. For PSR 1937+21, pulse arrival times have been combined with previously published data for two additional years in an attempt to detect annual parallax and proper motion. The authors obtain a limit π < 0.4 mas for the parallax and, in the nearly inertial reference frame of the PEP740R solar system ephemeris, a surprisingly small proper motion in galactic longitude, μl = -0.6±0.3 mas yr-1. Interstellar scintillation parameters derived from the same set of observations are consistent with a pulsar moving with transvese velocity less than 40 km s-1 relative to interstellar gas along the line of sight. For the other two millisecond pulsars, PSRs 1855+09 and 1953+29, the new timing data yield improved estimates of celestial coordinates and rotation and orbital parameters. Publication: The Astrophysical Journal Pub Date: March 1988 DOI: 10.1086/166153 Bibcode: 1988ApJ...326..947R Keywords: Astrometry; Interstellar Matter; Magnetic Stars; Pulsars; Pulse Duration; X Ray Binaries; Data Acquisition; Galactic Mass; Neutron Stars; Stellar Rotation; White Dwarf Stars; Astronomy; ASTROMETRY; INTERSTELLAR: MATTER; PULSARS; STARS: BINARIES; STARS: MAGNETIC full text sources ADS | data products SIMBAD (3)

Dwarf K and M stars of small proper motion found in a large spectroscopic survey

Dwarf K and M stars of small proper motion found in a large spectroscopic survey

Photometric parallaxes for selected stars of color class M from the NLTT catalog

view Abstract Citations (52) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometric parallaxes for selected stars of color class M from the NLTT catalog. Weis, E. W. Abstract VRI photometry is presented for a sample of 413 stars selected from among the stars of color class m in the NLTT catalog for which no trigonometric parallaxes have been measured and which are not known to be on a trigonometric parallax program. Additional VRI photometry of stars with good trigonometric parallaxes provides the basis for the calibration of color/absolute-magnitude relations used to compute photometric parallaxes for the program stars, about 90 of which are found to have photometric parallax greater than or equal to 0.040 arscec. Attention is called to a few apparently subluminous stars of possible interest. On the basis of the present sample, it appears that the catalog color index m(pg)-m(R) will not be useful in identifying nearby stars from among the thousands of color-class-m stars with small proper motion in the NLTT catalog. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1984 DOI: 10.1086/190956 Bibcode: 1984ApJS...55..289W Keywords: Astronomical Catalogs; Stellar Parallax; Stellar Spectrophotometry; Calibrating; Dwarf Stars; Northern Sky; Stellar Color; Stellar Luminosity; Astronomy full text sources ADS | data products SIMBAD (494) GCPD (1) ARI (1) Related Materials (3) Part 2: 1986AJ.....91..626W Part 3: 1987AJ.....93..451W Part 4: 1988AJ.....96.1710W

Observations of H-alpha-emission stars in the young cluster NGC 2264

view Abstract Citations (12) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Halpha -emission stars in the young cluster NGC 2264. Rydgren, A. E. Abstract UBVRI photometry is given for a sample of 25 late-type H-alpha-emission stars in the young cluster NGC 2264. The stars are in the V magnitude range from 12 to 16. Some but not all appear to be T Tauri stars. The color-color diagrams support the view that the deviations from normal photospheric colors (due to 'spectral veiling' and line emission) decrease with increasing wavelength between the U and I filters. In the (V, V - R) diagram, the H-alpha-emission stars lie in a well-defined pre-main-sequence band. Within this sample, there is a trend toward stronger line emission and spectral veiling with later spectral type. All of the likely legitimate T Tauri stars have inferred spectral types later than about K3. The question of cluster membership for stars in the cluster field with very small proper motions is considered. Publication: The Astronomical Journal Pub Date: January 1979 DOI: 10.1086/112393 Bibcode: 1979AJ.....84...90R Keywords: Emission Spectra; H Alpha Line; Late Stars; Stellar Spectrophotometry; Ubv Spectra; Color-Color Diagram; Line Spectra; Main Sequence Stars; Photoelectric Emission; Star Clusters; Astrophysics; Late-Type Stars:Open Clusters; Open Clusters:T Tauri Stars full text sources ADS | data products SIMBAD (1)

AN EMISSION-LINE PROPER-MOTION STAR, GD 552.

ABSTRACT An emission-line blue star of small proper motion, GD 552, has been found among the suspected white dwarfs of the Lowell lists. It has a nearly flat continuum and strong Balmer emission lines, about 2000 km/s in width. Lowell photographs do not show any large variability, yet its spectrum resembles that of an old nova or U Gem star. It is interpreted as a white dwarf with a weak accretion disk, each contributing about one-half the light. Its absolute magnitude is about + 13, considerably fainter than the normally accepted value for U Gem stars.

Toward the resolution of the local missing mass problem

view Abstract Citations (10) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Toward the resolution of the local missing mass problem Krisciunas, K. Abstract It is shown that if we know the velocity dispersions of (1) a total population of stars, (2) the observed biased sample (detected on the basis of proper motion data), and (3) the unobserved group (to be found in objective-prism surveys), we can infer the bias in the number density of the stars. The known mass density due to stars can then be corrected for undetected faint stars with small proper motions, resulting in a more accurate estimate of the directly observed local mass density. Using this method it is shown that approximately 0.02 M0 can be accounted for in the form of un- detected M3-M8 dwarfs. The total local mass density, due to stars and interstellar matter, is then 0.09 M0 . This is still less than the most likely value of the Oort limit (0.14 M0 ), but it must be kept in mind that the Oort limit is observationally derived and not accurately known. Publication: The Astronomical Journal Pub Date: March 1977 DOI: 10.1086/112030 Bibcode: 1977AJ.....82..195K full text sources ADS | Related Materials (1) Erratum: 1977AJ.....82..662K

Determination of an Extragalactic Radio Source Position Catalogue Suitable for Use in Monitoring Geophysical Phenomena

The use of radio interferometric techniques to monitor geophysical phenomena such as UTI-UTC variations, polar motion, and tectonic plate motion requires that there be available a fairly uniform distribution of extragalactic radio sources whose relative positions are known to better than 0″.01. These sources must also have very small proper motion, and not be subject to asymmetric changes in radio structure or to very large variations in intensity. In addition, to facilitate tying this frame of reference to the optical star frames, it is desirable that many of these radio objects have optical counterparts. This paper describes our progress in obtaining such a frame and sets forth our desiderata for further work by radio and optical astronomers.

On the Nature of Faint Blue Objects in High Galactic Latitudes. I. Photometry, Proper Motions, and Spectra in PHL Field 1:36+6° and Richter Field M3, II

view Abstract Citations (85) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Nature of Faint Blue Objects in High Galactic Latitudes. I. Photometry, Proper Motions, and Spectra in PHL Field 1:36+6° and Richter Field M3, II Sandage, Allan ; Luyten, Willem J. Abstract Astrometric, photometric, and spectrographic data are given for a representative sample of objects in a single field of 40 sq deg of the PHL catalogue. F and G subdwarfs, white dwarfs, and "radio-quiet" quasi-stellar galaxies (QSG) are the most frequent types of objects, although a few horizontal-branch stars, U Gem variables, and RR Lyrae stars are also present. Ten positive QSG identifications have been made from spectra of candidate objects in a list compiled from the bluest objects of small proper motion. The redshifts, Ax/x0, of PHL 1127,1194, and 3424 are 1.990, 0 298, and 1.847, respectively. Spectra of PHL 1027,1070,1072,1186, 1222, 1226, and 3375 prove these to be definite QSG also, but with undetermined redshifts. PHL 1092 is a possible QSG. A firm lower limit for the surface density of QSG in this field is S = 0 25 QSG/sq deg to B = 18 mag, based on ten QSG in 40 sq deg, but the true density is undoubtedly larger, since not all objects in the field have been spectrographically sampled An upper limit is S = 0 5 QSG/sq deg to B = 18 mag A small photometric sample of the Richter-Sahakjan list gives S between 1 QSG/sq deg and 3 QSG/sq deg to an optical limit of 19.7 mag These values far exceed S = 0004 QSS/sq deg for radio quasi-stellar sources in the 3C R catalogue. It is shown that very strong observational selection operates against finding QSGs of small absolute radio power when optical identifications are made from existing radio catalogues (Fig 4). The number of QSGs may exceed 100,000 over the entire sky to an optical limit of B = 19.7 mag. Publication: The Astrophysical Journal Pub Date: June 1967 DOI: 10.1086/149200 Bibcode: 1967ApJ...148..767S full text sources ADS | data products SIMBAD (69) NED (8)

The H-R diagram and space distribution of the members of the Ursa Major cluster.

Introduction. In two papers on the Ursa Major cluster, published in 1939, Smart examined the claim to cluster membership of 136 stars. He concluded that 42 of these stars are definitely members of the cluster while the status of 52 was doubtful. The purpose of this investigation has been to obtain spectroscopic parallaxes of these stars and, combining these with the remaining data in Smart's papers, to reexamine the question of the membership of the cluster and to give a revised H-R diagram. A number of stars with small proper motions, for which the data are inconclusive, have been excluded by the limits on the position angle residual of 100 for definite members and 200 for probable members. The more accurate spectroscopic parallaxes have also excluded a few stars; the number has been reduced to 32 definite members and 12 probable members. There are probably also several members south of ~3o0 which have not been considered in this investigation. Space distribution. The cluster has a fairly compact nucleus containing I I stars within nine parsecs. The remaining stars are scattered up to a distance of 135 parsecs from this nucleus. The Coma and the Hyades clusters are included in the volume of space occupied by the Ursa Major cluster; Praesepe and the Pleiades are almost within it. In contrast to the Ursa Major cluster, the Coma cluster contains about 30 stars of similar absolute magnitude, Praesepe and the Hyades, about 75, and the Pleiades, several hundred members, each in a region less than eight parsecs across. H-i? Diagram. Nine of the nucleus stars give a smooth main sequence; the others include a metallic line star, a peculiar A star, and probably a Go subdwarf. The latter was discovered in an attempt to find the fainter members of the cluster in the vicinity of the nucleus from their proper motion, and has no radial velocity. The outlying members are also primarily main sequence stars with no stars in the cluster earlier than Ao. The cluster also probably contains an Mo giant, six late G giants, two early F giants, and four peculiar stars. Dynamical considerations. If the distribution of velocities near the sun were actually random, 575 of the stars in the Bright Star Catalogue within a hundred parsecs would have speeds within 4 km/sec of the cluster speed. However, only four of these would be moving within 100 of the direction of the cluster motion. A calculation of the effect of stellar encounters shows that a cluster star will probably change its kinetic energy with respect to the nucleus of the cluster by less than 20 per cent in 3 X IO~ years. This means that the life time of the cluster is of the same order as the galactic time scale. Yerkes Observatory, Williams Bay, Wis.