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Публикации по объекту
The 78th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1706 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.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.
| The veiling spectrum of DI Cephei and its relationship to emission line profiles
High spectral resolution variability studies of classical T Tauri stars(CTTS) are an essential instrument for probing the physical conditionsand dynamics of their atmospheres and immediate vicinity. The shapes ofthe excess continuum emission and of the line profiles, theirvariability and relationship are all crucial tools to achieve that goal.We use high spectral resolution optical data of the CTTS DI Cep tosuggest a new diagnostic tool to investigate the relationship betweenthe line emission/absorption and the excess continuum emission. Bycorrelating the veiling continuum to the line flux in discrete velocitybins across the emission line we obtain a correlation profile, fromwhich one can discriminate between parts of the line that relatedifferently to the veiling. An earlier report of an unexpected humparound 5300 Å in the continuum excess emission spectrum of acouple of CTTS is not explained by current models of those stars. Weidentified a similar feature in the veiling spectrum of DI Cep anddiscuss, in this context, the relevance of the broad photosphericabsorption features present in the spectra of late-type stars. RegardingDI Cep, we find that its radial velocity seems to be variable but nosignificant periodicity could be derived, possibly due to inadequatetime sampling. We argue that this CTTS is most probably observed nearlyequator on. Accretion flows could not be identified directly in theemission lines, but their presence is inferred from the analysis of theveiling spectrum, which yields typical projected accretion rates around2.5×10-7 M_ȯ yr-1.
| High-Resolution Spectroscopy of Ursa Major Moving Group Stars
We use new and extant literature spectroscopy to address abundances andmembership for UMa moving group stars. We first compare the UMa, Coma,and Hyades H-R diagrams via a homogeneous set of isochrones and findthat these three aggregates are essentially coeval; this (near)coevality can explain the indistinguishable distributions of UMa andHyades dwarfs in the chromospheric emission versus color plane. Ourspectroscopy of cool UMa dwarfs reveals striking abundanceanomalies-trends with Teff, ionization state, and excitationpotential-like those recently seen in young, cool M34, Pleaides, andHyades dwarfs. In particular, the trend of rising λ7774-based O Iabundance with declining Teff is markedly subdued in UMacompared to the Pleiades, suggesting a dependence on age or metallicity.Recent photometric metallicity estimates for several UMa dwarfs aremarkedly low compared to the group's canonical metallicity, and similardeviants are seen among cool Hyads as well. Our spectroscopy does notconfirm these curious photometric estimates, which seem to be calledinto question for cool dwarfs. Despite disparate sources of Li data, ourhomogeneous analysis indicates that UMa members evince remarkably smallscatter in the Li-Teff plane for Teff>=5200 K.Significant star-to-star scatter suggested by previous studies is seenfor cooler stars. Comparison with the consistently determined HyadesLi-Teff trend reveals differences that are qualitativelyconsistent with this cluster's larger [Fe/H] (and perhaps slightlygreater age). However, quantitative comparison with standard stellarmodels indicates the differences are smaller than expected, suggestingthe action of a fourth parameter beyond age, mass, and [Fe/H]controlling Li depletion. The UMa-Coma cool star Li abundances may showa slight 0.2 dex difference; however, this may be mass-independent andthus more consistent with a modest initial Li abundance difference.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.Based on observations obtained at Kitt Peak National Observatory, adivision of the National Optical Astronomy Observatories, which isoperated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation.
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| Stellar encounters with the solar system
We continue our search, based on Hipparcos data, for stars which haveencountered or will encounter the solar system(García-Sánchez et al. \cite{Garcia}). Hipparcos parallaxand proper motion data are combined with ground-based radial velocitymeasurements to obtain the trajectories of stars relative to the solarsystem. We have integrated all trajectories using three different modelsof the galactic potential: a local potential model, a global potentialmodel, and a perturbative potential model. The agreement between themodels is generally very good. The time period over which our search forclose passages is valid is about +/-10 Myr. Based on the Hipparcos data,we find a frequency of stellar encounters within one parsec of the Sunof 2.3 +/- 0.2 per Myr. However, we also find that the Hipparcos data isobservationally incomplete. By comparing the Hipparcos observations withthe stellar luminosity function for star systems within 50 pc of theSun, we estimate that only about one-fifth of the stars or star systemswere detected by Hipparcos. Correcting for this incompleteness, weobtain a value of 11.7 +/- 1.3 stellar encounters per Myr within one pcof the Sun. We examine the ability of two future missions, FAME andGAIA, to extend the search for past and future stellar encounters withthe Sun.
| Detection of moving clusters by a method of cinematic pairs.
Not Available
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Stellar Encounters with the Oort Cloud Based on HIPPARCOS Data
We have combined Hipparcos proper-motion and parallax data for nearbystars with ground-based radial velocity measurements to find stars thatmay have passed (or will pass) close enough to the Sun to perturb theOort cloud. Close stellar encounters could deflect large numbers ofcomets into the inner solar system, which would increase the impacthazard at Earth. We find that the rate of close approaches by starsystems (single or multiple stars) within a distance D (in parsecs) fromthe Sun is given by N= 3.5D^2.12 Myr^-1, less than the number predictedby a simple stellar dynamics model. However, this value is clearly alower limit because of observational incompleteness in the Hipparcosdata set. One star, Gliese 710, is estimated to have a closest approachof less than 0.4 pc 1.4 Myr in the future, and several stars come within1 pc during a +/-10 Myr interval. We have performed dynamicalsimulations that show that none of the passing stars perturb the Oortcloud sufficiently to create a substantial increase in the long-periodcomet flux at Earth's orbit.
| Orbital Solutions for Three RS CVn Systems: WY Cnc, SV Cam and XY UMa
Near IR spectroscopic observations of WY Cnc, SV Cam and XY UMa binarysystems of RSCVn type are presented. Systemic velocity and radialvelocity semi-amplitudes of primary components in each system arederived using cross-correlation between cleaned spectra and G2 noiselesstemplates. Secondary semi amplitudes are determined from the primarysubtracted spectra using the cross-correlation function maps and directcross-correlation. Masses of the stars are derived and system propertiesbriefly discussed. Primary components in all three systems seem to bemore massive than was earlier deduced from their spectralclassification. Regarding stellar masses, SVCam should be classified asF5V+K0V pair. Observations of XY UMa fully confirm results obtainedpreviously using a method for detecting faint secondaries in the lowsignal-to-noise spectra.
| Variable Radial Velocities Among Composite-Spectrum Binaries in the PG Catalog
Ferguson, Green, & Liebert (1984\markcite{FGL}; FGL) identified aset of stars from the PG catalog that are composite-spectrum binariesconsisting of a hot subdwarf star and a late-type (G-K) star. During twoseasons at Kitt Peak National Observatory (KPNO), we examined theFGL\markcite{FGL} sample for close binaries by searching for largeradial velocity changes among typically three or four differentobservations. Only one of the twelve stars sampled so far has shownsignificant radial velocity variations (greater than 3sigma ): DeltaV_max = 83+/- 11 km s(-1) for PG 1224+309. Two other stars showedmarginal velocity changes: PG 0825+428, with Delta V_max = 24+/- 16 kms(-1) (1.6sigma ), and PG 1210+429, with Delta V_max = 39+/- 29 km s(-1)(1.3sigma ). The other nine stars did not display variations larger than0.9sigma (Delta V_max ≲ 15 km s(-1) ). We have reclassified thecool components of these twelve FGL\markcite{FGL} stars and made newestimates of the fractional contributions of the cool components to thetotal flux in the wavelength interval 5320-6532 Angstroms, generallyfinding spectral types slightly earlier, and flux fractions less, thanfound by FGL\markcite{FGL}. Both of these effects lead to reviseddistances to the binaries, mostly larger than those estimated byFGL\markcite{FGL}. An interesting and unexpected result from this workis the discovery of several rapidly rotating secondary stars. Thesestars may be related to the wind-accretion induced rapidly rotatingstars (WIRRing stars) discussed by Jeffries & Stevens(1996)\markcite{js96}. At least two systems, PG 1309-078 and PG1502-103, have large positive mean radial velocities, possiblyindicating halo orbits. We also tabulate mean apparent velocities forthe hot subdwarfs, measured from the He I D_3 line (based on an adoptedwavelength of 5875.697 Angstroms) which may be useful in assessing thegravitational redshifts of these stars. Four of the hot subdwarfs didnot exhibit a measurable D_3 line.
| Disk Accretion and Mass Loss from Young Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...452..736H&db_key=AST
| Spectroscopic orbits of eclipsing binaries - AA UMa
High dispersion and time resolution spectra for the eclipsing binary AAUMa, obtained using an intensified Reticon, were measured bycross-correlation techniques. The spectra of the two components arefound to be very similar, and AA UMa is shown to belong to Subtype W ofW UMa binaries. The orbital elements of the system are determined andcompared with other W UMa binaries. It is found that the orbital periodis 0.48171 days and that the estimated mean mass center velocity for thesystem is -34.8 km/s. The mass ratio between the components isdetermined to be 0.54 and the magnitude difference in the wavelengthobserved is estimated to be 0.36.
| Standard Velocity Stars
Not Available
| Photoelectric radial velocities, paper 6 Heard's IAU standard stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975MNRAS.171..407G&db_key=AST
| The establishment of 21 new ninth magnitude IAU standard radial velocity stars
Not Available
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Рак |
| Прямое восхождение: | 08h53m49.94s |
| Склонение: | +26°54'47.7" |
| Видимая звёздная величина: | 8.463 |
| Расстояние: | 40.552 парсек |
| Собственное движение RA: | 13.4 |
| Собственное движение Dec: | -5.3 |
| B-T magnitude: | 9.389 |
| V-T magnitude: | 8.54 |
Каталоги и обозначения:
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