Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful 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).
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| The Low-Velocity Wind from the Circumstellar Matter around the B9 V Star σ Herculis
We have obtained Far Ultraviolet Spectroscopic Explorer spectra ofσ Her, a nearby binary system with a main-sequence primary thathas a Vega-like infrared excess. We observe absorption in the excitedfine structure lines C II* at 1037 Å, N II* at 1085 Å, and NII** at 1086 Å that are blueshifted by as much as ~30 kms-1 with respect to the star. Since these features areconsiderably narrower than the stellar lines and broader thaninterstellar features, the C II and N II are circumstellar. We suggestthat there is a radiatively driven wind arising from the circumstellarmatter rather than accretion as occurs around β Pic because ofσ Her's high luminosity. Assuming that the gas is liberated bycollisions between parent bodies at 20 AU, the approximate distance atwhich blackbody grains are in radiative equilibrium with the star and atwhich three-body orbits become unstable, we inferdM/dt~6×10-12Msolar yr-1. Thiswind depletes the minimum mass of parent bodies in less than theestimated age of the system.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| The WR+OB Progenitor RY Scuti: Intensive Spectroscopy of Its Compact Double-Ring Nebula
We present a detailed spectroscopic analysis of the peculiar nebulaaround RY Scuti using data from the Space Telescope Imaging Spectrograph(STIS) and various ground-based observatories. This massive contactbinary may represent a rapid evolutionary phase in transition to ashort-period WR+OB system, and it is surrounded by a young nebula withunusual geometry. The ionized nebula is only about 1" across, so STIShas allowed us to spatially resolve the spectrum of the star from itscircumstellar nebula for the first time. Combining STIS data withground-based spectra at optical and infrared wavelengths solves somegeometric ambiguities posed by previous images and reveals the kinematicrelationship between the nebular geometry and RY Scuti's complexemission-line profiles. We offer a new determination of the systemicvelocity of 20+/-3 km s-1, and we estimate the distance to RYScuti as 1.8+/-0.1 kpc. Nebular lines show subtle phase-dependentvariability in addition to expected changes in equivalent width due toeclipses of stellar continuum. These changes in equivalent width yieldan updated ephemeris, but a previously suggested period change is stilluncertain after considering these new data. The nebula contains roughly0.003 Msolar of material that is He- and N-rich and probablyO-deficient compared to solar abundances, indicating that CNO-processedmaterial has reached the surface in at least one component of the binarysystem. Diagnostic line ratios suggest that the electron density andtemperature in the nebula are 2×105 cm-3 and~9500 K, respectively. There are concentrated regions of higher densityas well; emission knots seen around the rings suggest that commonenvelope mass loss during massive contact binary evolution may becharacterized by strong azimuthal asymmetry and sporadic mass ejectionevents. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gasdust shells and/ordiskshells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gasdustshells and/or diskshells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by freefree transitions of electrons and bythermal emission from dust grains in circumstellar gasdust shells(disks).
| Dust and Gas Around Young Stars
Asteroids, planets, and comets are believed to form within circumstellardisks of gas and dust around young stars (ages < 100 Myr). We haveobtained 10 - 20 μ m images, using the Keck I telescope and FUSEspectra of Herbig Ae and main sequence stars to characterize the dustand gas in these systems. We report the following results: (1) For zetaLep, we find that the majority of dust is located within 6 AU from thestar. Since the Poytning-Robertson drag lifetime of grains around thisstar is 104 years, significantly shorter than the 300 Myryear age of the system, we infer the presence of parent bodies with atotal mass 200 times that of the main asteroid belt in our solar system.(2) For the binary system sigma Herculis, which possess a Vega-likeinfrared excess, we observe circumstellar C 2*,N 2* and N 2**, blueshifted by asmuch as 40 km/sec. We propose that there is a radiatively driven wind,generated by sigma Her's high luminosity. In this model, the material inthe wind is created through collisions between parent bodies at 20 AUfrom the star, the approximate distance at which blackbodies are inradiative equilibrium with the star and at which 3-body orbits becomeunstable. (3) For the pre main sequence star AB Aur, we measure a dustmass 6x10-9 Mȯ , significantly less than the10-4 Mȯ inferred from millimeter photometry,suggesting the presence of a cold optically thick disk. We find thatmodels which include dust envelopes fit the data somewhat better thanmodels which incorporate flared circumstellar disks.
| Long-Term VRI Photometry of Small-Amplitude Red Variables. I. Light Curves and Periods
We report up to 5000 days of VRI photometry, from a robotic photometrictelescope, of 34 pulsating red giants, namely, TV Psc, EG And, Z Psc, RZAnd, 4 Ori, RX Lep, UW Lyn, η Gem, μ Gem, ψ1 Aur,V523 Mon, V614 Mon, HD 52690, Y Lyn, BC CMi, X Cnc, UX Lyn, RS Cnc, VYUMa, ST UMa, TU CVn, FS Com, SW Vir, 30 Her, α1 Her,V642 Her, R Lyr, V450 Aql, V1293 Aql, δ Sge, EU Del, V1070 Cyg, WCyg, and μ Cep, as well as a few variable comparison stars. V, R, andI variations are generally in phase. The length and density of the dataenable us to look for variations on timescales ranging from days toyears. We use both power-spectrum (Fourier) analysis and autocorrelationanalysis, as well as light-curve analysis; these three approaches arecomplementary. The variations range from regular to irregular, but inmost of the stars, we find a period in the range of 20-200 days, whichis probably due to low-order radial pulsation. In many of the stars, wealso find a period which is an order of magnitude longer. It may be dueto rotation, or it may be due to a new kind of convectively inducedoscillatory thermal mode, recently proposed by P. Wood.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The 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
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A 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
| On the Variability of Late B III-V Stars
We investigate the Hipparcos Satellite photometry of luminosity classIII-V B6-B9 stars. Most are relatively non-variable. Candidates forwhich further study is desirable are identified.
| Polarization measurements of Vega-like stars
Optical linear polarization measurements are presented for about 30Vega-like stars. These are then compared with the polarization observedfor normal field stars. A significant fraction of the Vega-like starsare found to show polarization much in excess of that expected to be dueto interstellar matter along the line of sight to the star. The excesspolarization must be intrinsic to the star, caused by circumstellarscattering material that is distributed in a flattened disk. Acorrelation between infrared excess and optical polarization is foundfor the Vega-like stars.
| Cosmic Microwave Background Anisotropy Constraints on Open and Flat-Lambda Cold Dark Matter Cosmogonies from UCSB South Pole, ARGO, MAX, White Dish, and SUZIE Data
We use combinations of 10 small-scale cosmic microwave backgroundanisotropy data sets from the UCSB South Pole 1994, ARGO, MAX 4 and 5,White Dish, and SuZIE experiments to constrain cosmogonies. We consideropen and spatially flat-Lambda cold dark matter cosmogonies withnonrelativistic mass density parameter Omega_0 in the range 0.1-1,baryonic mass density parameter Omega_B in the range 0.005-0.029 h^-2,and age of the universe t_0 in the range 10-20 Gyr. Marginalizing overall parameters but Omega_0, the combined data favor a Omega_0~=1 (1)open (flat-Lambda) model. Excluding the smallest angular scale SuZIEdata, an Omega_0~=0.3 (1) open (flat-Lambda) model is favored.Considering only multifrequency data with error bars consistent withsample variance and noise considerations, i.e., the South Pole 1994 Kaband, the MAX 4 iota Draconis, and the MAX 5 HR5127 data, anOmega_0~=0.1 (1) open (flat-Lambda) model is favored. For both open andflat-Lambda models and for all three combinations of data sets, aftermarginalizing over all other parameters, a lower Omega_Bh^2 (~0.005) oryounger (t_0~10 Gyr) universe is favored. However, the data do not ruleout other values of Omega_0 in the flat-Lambda model and other values ofOmega_Bh^2 in both models. At 2 sigma confidence, model normalizationsdeduced from the small-scale data are consistent with those derived fromthe Differential Microwave Radiometer data. We emphasize that since weconsider only a small number of data sets, these results are tentative.
| Averaged energy distributions in the stellar spectra.
Not Available
| Infrared Photometry of beta Pictoris Type Systems
We obtained small-aperture (4"-5" diameter) infrared (2-20 μm)photometry of 10 early-type main-sequence stars with infrared excessesfrom circumstellar dust. These systems possibly exemplify the betaPictoris phenomenon. We observed them with either the NASA MarshallSpace Flight Center bolometer array camera (``Big Mac'') or the InfraredTelescope Facility 2-30 μm single-channel bolometer system.Measurements were obtained in the KLMNQ filters and the narrowband(Deltalambda ~ 1 μm) 10 μm ``silicate'' filters. We fitted Kuruczphotospheric models to the photometric data to determine excess-emissionspectra. We report the nondetection of small-aperture circumstellar dustemission from HR 10 and 21 LMi. We confirmed previous nondetections ofnear-infrared or 10 μm excess emission from 68 Oph, alpha PsA, and HR4796A. We did not detect prominent silicate emission from any of thesources. The spectra of gamma Oph, sigma Her, HR 2174A, beta UMa, andzeta Lep show weak 10 μm excesses. We fitted simple models to thesedata, together with IRAS excess fluxes, to determine plausibledistributions of temperature and density of circumstellar dust grains.Significant quantities of these grains around HR 2174A, zeta Lep, andbeta UMa are at temperatures similar to terrestrial material in thesolar system.
| Mass determination of astrometric binaries with Hipparcos. III. New results for 28 systems
This paper is the third of a series devoted to the determination ofstellar masses from Hipparcos data. This is a continuation of Martin etal. (1997), who introduced the theory and assessed the performance ofthe method from simulated data, and of a second paper with the firstresults for 46 systems, ( te[Martin & Mignard 1998]{Mar98}). Theorbit file maintained by the CHARA group and new publications of orbitalelements made the processing of 70 additional candidate systemspossible, including 28 of the 145 systems already tested in the previouswork. Significant results were obtained on 22 systems, with relativeaccuracy better than 25% for the masses of 17 binaries. New estimatesare also given for 6 systems previously investigated, thanks to reliablevalues of the magnitude difference from the Hipparcos catalogue ( te[ESA1997]{ESA97}). New orbital elements are proposed for HIP 12623 (12Persei) from speckle/spectroscopic measurements. Results are discussedfor each system, alongside the mass-luminosity relation based onHipparcos magnitudes and distances.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| Infrared Space Observatory Mapping of 60 MU M Dust Emission Around Vega-type Systems
We mapped 60 mu m emission around five Vega-type systems. We utilizedthe PHOT-C100 far-infrared array camera on board the Infrared SpaceObservatory. The spatial resolution of the maps is ~30". The maps ofthree sources ( alpha CrB, sigma Her, and alpha Cen) do not show anyevidence of extended emission. The map of gamma Oph shows marginal hintsof extended emission ~30" (800 AU) from the star. The map of alpha PsAshows extended emission ~30"--80" (~210--560 AU) from the star. Thislocation, together with alpha PsA's characteristic IRAS temperature of58--75 K, suggest the observed dust grains around alpha PsA are up to~10 mu m in size. We estimate the mass of dust grains emitting the IRAS60 mu m excess of alpha PsA to be ~(2--6) x10^{-3} M_⊕ .
| Investigation of a group of rapidly rotating A stars having an emission excess at IRAS wavelengths.
Not Available
| ICCD Speckle Observations of Binary Stars. XVII. Measurements During 1993-1995 From the Mount Wilson 2.5-M Telescope.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1639H&db_key=AST
| Nonvariability among lambda Bootis stars
With asteroseismic techniques it is possible to investigate the interiorand the evolutionary status of stars via their frequency spectrum. Bothinformation would be very much needed for lambda Bootis stars, a groupof metal-poor Population I, A-type stars, since no conclusive theoryexists explaining the observed abundance anomalies. Geneva and Stromgrenphotometry place these stars inside the classical instability strip orat least very close to it. We therefore have started an extensivephotometric survey for pulsation in lambda Bootis stars and havediscovered so far 13 new variables. In this paper we present results forstars which presumably are constant, because we are able to establishonly an upper level for possible variability. A typical noise level of 3mmag for Stromgren b was achieved in the relevant frequency domain up to100 d^{-1}. Considering the given noise level of our survey, we concludethat at least 50% of all investigated lambda Bootis stars inside theinstability strip are pulsating, making this group remarkable comparedto stars with similar spectral types. This may suggest that a low(surface) metallicity has an influence on the pulsation Based onobservations obtained at ESO-La Silla, CTIO, SAAO, McDonald Observatory,Instituto Astrofisica Andalucia Observatory.
| Infrared Space Observatory Mapping of 60 MU M Dust Emission Around Vega-type Systems
We mapped 60 mu m emission around 5 Vega-type systems. We utilized thePHT-C100 3x3-pixel far-infrared camera on-board ISO, in the P32microscanning dedicated mapping mode. The maps are centered on the starsand span 6'15''x5'45'' (RAxDEC), at a spatial resolution of 15''. Themaps of 3 sources (alpha CrB, sigma Her, and alpha CenB) do not show anyevidence of extended emission when compared to a model point-spreadfunction obtained from a similar map of alpha Boo. The map of gamma Ophpossibly shows extended emission ~ 30'' from the star, slightly abovethe background noise ( ~ 4 MJy/sr). The map of alpha PsA shows extendedemission features ~ 30--80'' from the star. The extended emission peakbrightness in alpha PsA, after background subtraction, is ~ 13 MJy/sr,or ~ 4sigma , above the background noise. The S/N of our mappingobservations is not high but as a preliminary result we estimate somephysical parameters of the 60 mu m emitting dust around alpha PsA. Theinnermost ~ 30'' or ~ 210 AU from the star are relatively dust-depleted.The dust is found between ~ 210 AU and ~ 560 AU from the star, with peak60 mu m emission located at ~ 320 AU. We take the latter as acharacteristic location of the emitting dust. This location, togetherwith a characteristic IRAS temperature of 72 K (Backman & Paresce1993, Protostars and Planets III, ed. Levy & Lunine, 1253), suggeststhe observed dust grains are ~ 1.5 mu m in size. The mass of thesegrains emitting the total extended emission of ~ 11.9 Jy across ~ 3.4 x10(3) arcsec(2) is ~ 3 x 10(-3) M_⊕.
| Optical Interferometry: Breaking the Barriers
Not Available
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Hércules |
| Ascensión Recta: | 16h34m06.20s |
| Declinación: | +42°26'13.0" |
| Magnitud Aparente: | 4.2 |
| Distancia: | 92.678 parsecs |
| Movimiento Propio en Ascensión Recta: | -10.8 |
| Movimiento Propio en Declinación: | 53.8 |
| B-T magnitude: | 4.177 |
| V-T magnitude: | 4.193 |
Catálogos y designaciones:
|
