SKY-MAP.ORG
 Home Getting Started To Survive in the Universe News@Sky Astro Photo The Collection Forum Blog New! FAQ Press Login

# α Vol

Contents

### Images

DSS Images   Other Images

### Related articles

 Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single starsAims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream. Decay of Planetary Debris DisksWe report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation. Statistical Constraints for Astrometric Binaries with Nonlinear MotionUseful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr). Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin iThis work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897 Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphereWithin the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105 Ages of A-Type Vega-like Stars from uvbyβ PhotometryWe have estimated the ages of a sample of A-type Vega-like stars byusing Strömgren uvbyβ photometric data and theoreticalevolutionary tracks. We find that 13% of these A stars have beenreported as Vega-like stars in the literature and that the ages of thissubset run the gamut from very young (50 Myr) to old (1 Gyr), with noobvious age difference compared to those of field A stars. We clearlyshow that the fractional IR luminosity decreases with the ages ofVega-like stars. Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statisticsThe Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521 The proper motions of fundamental stars. I. 1535 stars from the Basic FK5A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222 The Disk of β Pictoris in the Light of Polarimetric DataWe model the linear polarization of the radiation of β Picscattered by dust particles in the circumstellar disk. The observedspatial distribution and the wavelength dependence of the polarizationtogether with the colors of the β Pic disk require that particlesin a wide size range be present in the disk, with the grains smallerthan a few microns in size being somewhat depleted but still ofimportance for the polarization and colors. The inferred sizedistribution is consistent with the production and loss mechanisms: thesources-presumably collisions and evaporation of largebodies-continuously produce dust with a power-law size distribution withthe exponent ~3.5 over a broad range of sizes, but the particles smallerthan a few microns are blown away by the radiation pressure, whichshortens the time they spend in the disk and decreases their numberdensities. Compact (or slightly porous) silicates are found to givebetter agreement with the observations, although other materials arestill not ruled out and a high fluffiness of the large particles ispossible. The observed asymmetry in the polarization of two wings can beexplained if more small grains (by 20%-30%) are present on the northeastside of the disk. We show that such an asymmetry in the sizedistributions in two wings might be caused by an influence of theinterstellar medium; a required amount of small grains could be producedby destructive collisions of interstellar grains with the circumstellardust particles. Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutionsThe FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included). Radial velocities of HIPPARCOS southern B8-F2 type starsRadial velocities have been determined for a sample of B8-F2 type starsobserved by the Hipparcos satellite. Observations were obtained withinthe framework of an ESO key-program. Radial velocities have beenmeasured using a cross-correlation method, the templates being a grid ofsynthetic spectra. The obtained precision depends on effectivetemperature and projected rotational velocity of the star as well as ona possible asymmetry of the correlation peak generally due to secondarycomponents. New spectroscopic binaries have been detected from theseasymmetries and the variability of the measured radial velocity.Simulations of binary and triple systems have been performed. Forbinaries our results have been compared with Hipparcos binary data.Adding the variable radial velocities, the minimum binary fraction hasbeen found 60% for physical systems. Radial velocities have beendetermined for 581 B8-F2 stars, 159 being new. Taking into accountpublished radial velocities, 39% south A-type stars with V magnitudelower than 7.5 have a radial velocity. Based on observations obtained atthe European Southern Observatory (ESO, La Silla, Chile) and on datafrom the ESA Hipparcos astrometry satellite.}\fnmsep \thanks{Tables 7, 8and 9 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html A search for circumstellar gas around normal A stars and Lambda Bootis starsWe have searched for interstellar or circumstellar absorption lines inthe center of Ca II K towards bright A-type stars that are mostly within80 pc of the Sun. Narrow absorption features are found in about 30 % ofthe 28 normal main-sequence A stars and 18 metal-deficient lambda Bootisstars studied. We have determined surface gravities and projectedrotational velocities. Most of the stars with detectable Ca K featureshave comparatively low gravities and high projected rotationalvelocities. This correlation with stellar properties implies that mostof the narrow absorption features are of circumstellar rather thaninterstellar origin. The preference of low gravity and rapid rotationfurthermore suggests that most of the gas shells around A stars developin the pre-main-sequence phase of evolution, and disappear largelybefore the star arrives at the ZAMS. Among the normal A stars studied,about 50 % are known to have dust disks. Unlike A stars withcircumstellar gas, these dusty stars do not prefer low log g and high vsin i. This results in an apparent lack of correlation between gas anddust, and indicates that normal A stars with gas shells and those withdust disks are not in the same evolutionary stage. We conjecture thatdust disks tend to develop after most of the gas has disappeared. Basedon observations collected at the European Southern Observatory, LaSilla, Chile Dusty and dust-free A starsWe present preliminary results of our search for circumstellarabsorption features in the Ca K lines based on high S/N observationsobtained with the ESO CAT/CES system. The Sirius Supercluster and Missing Mass near the SunThe Hipparcos results confirm some 50 members of the Sirius superclusterin the Bright Star Catalogue. The resulting, well-definedcolor-luminosity array indicates an age of 4 x 10^8 yr from conventional(no overshoot) models. A comparison of the luminosity function obtainedfrom members in the Bright Star Catalogue, unbiased as to proper motion,and in the Catalogue of Nearby Stars (~27 pc), which are strongly biasedtoward large proper motions, reveals that either most of the smallproper-motion stars near the Sun remain to be identified or theluminosity function of the supercluster is drastically different fromthat of the field stars. A search for low-mass members in a 6 deg x 6deg field in the center of the Ursa Major cluster yielded 10 possiblemembers that lie on the white dwarf sequence, 5 mag below the mainsequence. Luyten (field LP 131) found 368 stars in this field withproper motion exceeding 0.08", and only 19 of these have a positionangle of their proper motion between 0 deg and 98 deg (i.e., thequadrant containing UMa cluster members). The assumption of clustermembership yields a parallax very close to that obtained for brightmembers. Aside from white dwarfs, the possibilities are discussed thatthese stars are (1) brown dwarfs, (2) members of a cluster in the lineof sight to UMa but 100 pc more distant, or (3) reflecting a chain ofvery unlikely coincidences. Effective temperatures of AP starsA new method of determination of the effective temperatures of Ap starsis proposed. The method is based on the fact that the slopes of theenergy distribution in the Balmer continuum near the Balmer jump fornormal" main sequence stars and chemically peculiar stars with thesame Teff are identical. The effective temperaturecalibration is based on a sample of main sequence stars with well knowntemperatures (\cite[Sokolov 1995]{sokolov}). It is shown that theeffective temperatures of Ap stars are derived by this method in goodagreement with those derived by the infrared flux method and by themethod of \cite[Stepien & Dominiczak (1989)]{stepien}. On the otherhand, the comparison of obtained Teff with Teffderived from the color index (B2-G) of Geneva photometry shows a largescatter of the points, nevertheless there are no systematicaldifferences between two sets of the data. Comparison of the ACRS and PPM Catalogs with the FK5 in the Southern HemisphereA comparison of the Astrographical Catalog of Reference Stars (ACRS) andthe Positions and Proper Motions Catalog (PPM) with the FK5 in thesouthern hemisphere, is presented. To this aim, the positions of FK5stars uniformly spread over the celestial sphere were astrographicallytaken. These positions were reduced in the ACRS and PPM systems and thencompared with those from the FK5 for the epoch of observation. The (FK5- ACRS) and (FK5 - PPM) systematic differences thus obtained, in thedeclination range from -30o down to the South Pole, for themean epoch of 1994.50, are shown. The Angular Momentum of Main Sequence Stars and Its Relation to Stellar ActivityRotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars) Dust around Main-Sequence Stars: Nature or Nurture by the Interstellar Medium?Dust from the interstellar medium (ISM) can collide with and destroyparticles in the circumstellar dust disks around main-sequence stars(Vega/ beta Pic stars). Two current theories tying the occurrence of theVega/ beta Pic phenomenon to the erosive influence of the ISM arecritically reconsidered here. Using the local standard of rest frame, wefind little evidence for a correlated motion (streaming) of prominentdisk systems, which one theory suggests would result from a passageabout 107 yr ago of these stars, but not the control A-type stars,through the nearby Lupus-Centaurus interstellar cloud complex. Moreover,the prototype system of beta Pic could not have retained dust producedin such a passage for much longer than 104 yr. We show theoreticallythat the ISM sandblasting of disks has minor importance for thestructure and evolution of circumstellar disks, except perhaps in theiroutskirts (usually >400 AU from the stars), where under favorableconditions it may cause asymmetries in observed brightness and color.The ISM neither produces the disks (as in one theory) nor depletes andeliminates them with time (as in another theory), because typical ISMgrains are subject to strong radiative repulsion from A- and F-typedwarfs (a few to 100 times stronger than gravity). Atypically large ISMgrains are not repelled strongly, but are unimportant on account oftheir small number density. Dust production and destruction in betaPic-type disks results mainly from their collisional nature enhanced bythe radiatively produced eccentricities of particle orbits, rather thanfrom nurture in a hostile ISM. The residence times of the few-microndust grains predominant in the densest part of the beta Pic disk is only104 yr, or a few dozen orbital periods. Submicronic debris is blown outas beta meteoroids, carrying away from this system an equivalent of thesolar system's total mass in solids (~120 Earth masses) in only ~65 Myr.This rate of collisional erosion exceeds almost 108 times that of thezodiacal light disk of our own system. A massive and relatively young(<~102 Myr) planetesimal disk appears to surround beta Pic, destinedto decline in dust density over time comparable to its age. Other dustdisks, like those around Fomalhaut and Vega, contain much less dust andmay be much older than the beta Pic disk, but like the beta Pic diskthey are also derived from and replenished many times during theirlifetimes by unseen parent bodies. The Pulkovo Spectrophotometric Catalog of Bright Stars in the Range from 320 TO 1080 NMA spectrophotometric catalog is presented, combining results of numerousobservations made by Pulkovo astronomers at different observing sites.The catalog consists of three parts: the first contains the data for 602stars in the spectral range of 320--735 nm with a resolution of 5 nm,the second one contains 285 stars in the spectral range of 500--1080 nmwith a resolution of 10 nm and the third one contains 278 stars combinedfrom the preceding catalogs in the spectral range of 320--1080 nm with aresolution of 10 nm. The data are presented in absolute energy unitsW/m(2) m, with a step of 2.5 nm and with an accuracy not lower than1.5--2.0%. Infrared excesses in A-type starsNot Available Star Streams and Galactic StructureAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.1595E&db_key=AST A microwave survey of southern early-type starsA multi-epoch survey with the Parkes telescope of a completedistance-limited sample of 57 stars earlier than F6 has detectedpossible 8.4-GHz emission from 16 stars. Single-epoch partial synthesisobservations with the Australia Telescope Compact Array (ATCA) at 4.8GHz on 27 stars from the same sample (including the possible Parkesdetections) found no emission at the stellar positions above a fluxdensity limit of 1.2-1.9 mJy, but the maps show that the Parkesdetections are not merely the results of confusion of sources within theParkes beam. Three early F stars with UV and/or X-ray emission wereobserved simultaneously at 4.8 and 8.4 GHz in 12-h syntheses with the6-element ATCA. Two of these stars were from the above sample and thethird was the supergiant Alpha Carinae. We detected only alphaCar withflux densities of 300+/-65 and 140+/-65 muJy at 4.8 and 8.6 GHz(S~nu^-1.3+/-1.3). We discuss the legitimacy of the Parkes 3-6sigmadetections and show that, although none has been detected by synthesisobservations, there is no compelling reason for rejecting them on theinternal evidence. The power emitted by the supergiant alphaCar issimilar to that of the 16 possible Parkes detections, although itsactivity index is orders of magnitude lower. We show that this emissioncannot be thermal bremsstrahlung from the 10^7.2-K corona of the starbut is probably synchrotron emission from a magnetically maintainedcorona. Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with. Optical Polarization of 1000 Stars Within 50-PARSECS from the SunAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..101..551L&db_key=AST Newly identified main-sequence A stars with circumstellar dustThe IRAS Faint Source Survey data base and the ADDSCAN/SCANPI softwareare used to search for systems with circumstellar dust in two samples:all of the 62 A stars in Woolley's catalog, which lie near the mainsequence and are typically within 25 pc of the sun, and all A stars inthe Bright Star Catalogue with mV in the range 4-5 and v sin i not lessthan 100 km/s. In the first sample, 11 nearby A stars having (12)-(25)and (25)-(60) colors consistent with circumstellar dust are found. Theanalysis of the second sample demonstrates that the use of the FSS database increases the likelihood of identifying A stars with circumstellardust fainter than mV = 4. Three more new dusty systems are found in thissample. Beta Pictoris (A5 IV-V) is the only star within 25 pc of the sunthat displays IR colors indicative of both warm and cool dust. It isinferred from the absence of cool dust in the vast majority of thesystems that the dust disks of these stars are much less extended thanthat seen around Beta Pictoris. Velocity streaming of IRAS main-sequence disk stars and the episodic enhancement of particulate disks by interstellar cloudsAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1992ApJ...388..190W&db_key=AST The Sirius supercluster in the FK5Fifty-five stars in the FK5 or with PPM(H) quality proper motions arediscussed as members of the Sirius supercluster. The convergent point ofthe supercluster motion is (A, D) = (20.55h -38.1 deg) and the spacevelocity is V(T) = 18.6 + 0.005X km/s, where X is the radial distance ofthe members from the sun. The member stars indicate (Fe/H) Hyades -(Fe/H) Sirius = 0.18 dex from both photometric and spectroscopicevidence. The bulk of the supercluster members fall into two age groups,6.3 x 10 exp 8 and 10 exp 9 yr on the basis of models with convectiveovershoot. The available observations of chromospheric activity in Mg II(h and k) are quantized into three well-defined groups for both Siriussupercluster and Hyades cluster dwarfs. For 20 supercluster members withcluster parallax greater than 0.03 arcsec the ratio ofpi(cluster)/pi(trigonometric) = 0.95 +/- 0.20(sigma). An optical search for Beta Pictoris-like disks around nearby starsA coronagraphic survey of more than one hundred stars has been carriedout in a search at optical wavelengths for circumstellar materialsimilar to that found in the Beta Pictoris disk. The survey stars wereprimarily dwarfs in the spectral range A to K and most were closer than100 pc. No evidence of circumstellar material was found around any ofthe stars, suggesting that Beta Pictoris is an abnormal object,surrounded by an unusually large amount of optically scatteringmaterial. Second Rio Grande catalogue (RG2).Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990RMxAA..21..286H&db_key=AST Physical data of the fundamental stars.Not Available
Submit a new article