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מאמרים קשורים
Decay of Planetary Debris Disks
We 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.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| 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.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This 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
| Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by
We present polarization spectra near maximum light for the stronglysubluminous Type Ia supernova SN 1999by that show that the supernova isintrinsically polarized. SN 1999by has an observed, overall level ofpolarization of ~0.3%-0.8%, a rise of the polarization P redward of 6500Å, and a change in polarization across the Si II λ6150feature of about 0.4%. The presentation of the polarization at differentwavelengths in the Q-U plane is shown to be a powerful tool to determinethe overall geometry and the interstellar component. The distribution ofpoints with wavelength using this empirical Q-U plane method revealsthat SN 1999by has a well-defined axis of symmetry and suggests aninterstellar polarization (ISP) vector with PISP=0.3% andposition angle Θ=150deg with an error circle in the Q-Uplane of radius about 0.1%. Synthetic non-LTE spectra for axisymmetricconfigurations based on delayed-detonation models have been computedassuming ellipsoidal geometry. The input ejecta structure andcomposition are based on a Chandrasekhar mass delayed-detonation model.The parameters of the explosion are chosen to reproduce the timeevolution of IR spectra of SN 1999by without further adjustments.Spherical models are then mapped onto ellipsoidal geometries and theaxis ratio, viewing angle, and ISP adjusted to provide the bestagreement with the polarization spectra. Both flux and polarizationspectra can be reasonably well reproduced by models with an asphericityof ~20% observed equator-on. The general properties of the polarizationcan be understood as a consequence of the structure of subluminousmodels. Best fits are obtained for the theoretical models withPISP=0.25% and Θ=140deg, consistent with theempirical method. We discuss our results for this subluminous Type Ia inthe context of ``normally bright'' Type Ia supernovae. For normallybright Type Ia, the photosphere is near the inner iron-rich layers atmaximum light and the ubiquitous iron lines give a rapid variation tothe model polarization spectra. In subluminous models, the photospherenear maximum is in the silicon layers with fewer lines and a smootheroverall polarization spectrum, as observed for SN 1999by. Though dataare sparse, the low upper limits for polarization determined for manynormal events in contrast to the high polarization in SN 1999by maysuggest a relation between the asymmetry we observed and the mechanismthat produces a subluminous Type Ia. Among various mechanisms, rapidrotation of the progenitor white dwarf and/or an explosion during abinary white dwarf merger process are likely candidates to explain theasphericity in SN 1999by.
| 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
| Absolute declinations with the photoelectric astrolabe at Calern Observatory (OCA)
A regular observational programme with a photoelectric astrolabe havebeen performed at ``Observatoire du Calern" (Observatoire de laCôte d'Azur, OCA, phi = +43() o44′55.011″; lambda =-0() h27() m42.44() s, Calern, Caussols, France) for the last twentyyears. It has been almost fully automatized between 1984 and 1987. Since1988 the photoelectric astrolabe was used without any modification. Inaddition to determining the daily orientation of the local vertical, theyearly analysis of the residuals permits to derive corrections to theused star catalogue \cite[(Vigouroux et al. 1992)]{vig92}. A globalreduction method was applied for the ASPHO observations. The new form ofthe equations \cite[(Martin & Leister 1997)]{mar97} give us thepossibility of using the entire set of the observing program using datataken at two zenith distances (30() o and 45() o). The program containsabout 41648 stars' transits of 269 different stars taken at``Observatoire du Calern" (OCA). The reduction was based on theHIPPARCOS system. We discuss the possibility of computing absolutedeclinations through stars belonging simultaneously to the 30() o and45() o zenith distances programmes. The absolute declination correctionswere determined for 185 stars with precision of 0.027arcsec and thevalue of the determined equator correction is -0.018arcsec +/-0.005arcsec . The instrumental effects were also determined. The meanepoch is 1995.29. Catalogue only available at CDS in electronic from viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational 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)
| Spectral energy distributions and physical parameters of the atmospheres of main-sequence A stars with infrared excesses: Comparison with normal stars
For eight stars with infrared excesses greater than 0.5m in the IRASbands, we obtained the energy distribution over the range 3200-7600Angstroms and measured their infrared JHKLM magnitudes. Similarobservations were performed for 21 stars of various subclasses of thespectral type A with excesses no greater than 0.3m. Based on the energydistribution derived from the response curves of the photometric UBV andWBVR systems, we computed the synthetic B-V color indices, which are ingood agreement with the photometric observations. A comparison of thenormalized energy distribution in the spectra of A stars with infraredexcesses greater than 0.5m in the IRAS bands and the mean energydistribution for stars of the corresponding subclasses shows that themaximum difference does not exceed 10% both in the ultraviolet(3200-4000 Angstroms) and in the near infrared (up to 7600 Angstroms) Inall of the stars studied, we detected no excesses in the JHKLM bandsthat were greater than the accuracy of our measurements. Using themethod of infrared fluxes and comparing the observed energy distributionwith theoretical models of Kurucz, we determined the effectivetemperatures and angular diameters of the stars. For three stars, weestimated the parameters of their dust shells.
| Stellar spectrum scanner.
Not Available
| The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&db_key=AST
| 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 Sun
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..101..551L&db_key=AST
| Secondary spectrophotometric standards - Mean energy distribution in the spectra of A stars
Forty-nine A stars of IV and V luminosity classes are used to obtainmean spectral energy distributions of stars of different subtypes.Forty-six are in the catalog of 238 secondary spectrophotometricstandards, which are characterized by good agreement between thespectrophotometric data of the Moscow and Alma-Ata catalogs. Three starsare primary standards. Mean energy distributions for all the subtypes ofthe A type in the range 3200-7600 A are presented. The effectivetemperatures obtained from a comparison with the theoretical modelatmospheres of Kurucz (1979) are in satisfactory agreement with theT(eff) scale constructed on the basis of total flux determinations.
| Secondary spectrophotometric standards
Energy distribution data on 238 secondary standard stars are presentedin the range 3200-7600 A with 50 A step. These stars are common to theCatalog of the Sternberg State Astronomical Institute and the FessenkovAstrophysical Institute. For these stars, the differences betweenspectral energy distribution data of the two catalogs do not exceed 5percent, while the mean internal accuracy of both catalogs data in thisrange are about 3.5 percent. For 99 stars energy distribution data inthe near infrared (6000-10,800 A) obtained at the Sternberg StateAstronomical Institute are also presented.
| An optical search for Beta Pictoris-like disks around nearby stars
A 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.
| The stellar temperature scale for stars of spectral types from O8 to F6 and the standard deviation of the MK spectral classification
Empirical effective temperature of 211 early-type stars found in aprevious investigation (Kontizas and Theodossiou, 1980; Theodossiou,1985) are combined with the effective temperatures of 313 early-typestars from the literature. From these effective temperatures of a totalnumber of 524 early-type stars of spectral types from O8 to F6 a newstellar temperature scale is developed along with the standard deviationof the MK spectral classification.
| Spectroscopic calibration of the magnitudes of B-stars of the Galactic disk
The present work is based on a catalog of the equivalent widths ofhydrogen and neutral helium lines in the spectra of 500 B-stars. Thecalibration relationship of absolute magnitude versus equivalent widthsis derived from spectral observations of B and A stars in open clusters.The zero point of this dependence is determined from the trigonometricparallaxes of the nearest A and B stars. By comparing values ofW(H-gamma) and the beta-indices of the Stromgren system, an M(V)(beta)calibration is obtained which differs substantially from the Crawfordcalibration.
| A new calibration of the Geneva photometry in terms of Te, log g, (Fe/H) and mass for main sequence A4 to G5 stars
A new semiempirical calibration of the d, m2, and B2 - V1 parameters ofGeneva photometry is given, which allows Te, log g and (Fe/H) of A4V-III to G5 V stars to be estimated. The calibration is based onKurucz's grid of atmosphere models, corrected by means of fundamentalstars. Recent evolutionary tracks for stars with a metal content Z =0.02 are used to calibrate the d vs. B2 - V1 diagram in terms of massfor solar-composition stars.
| Third preliminary catalogue of stars observed with the photoelectric astrolabe of the Beijing Astronomical Observatory.
Not Available
| An observational study of the Herbig AE star VV Serpentis, and of stars with reflection nebula associated with it darks cloud
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1988A&A...197..151C&db_key=AST
| The early A type stars - Refined MK classification, confrontation with Stroemgren photometry, and the effects of rotation
The MK classification system for the early A-type stars is refined, anda parallel system of standards for the broad-lined stars is introduced.With this improved system, stars may be classified with significantlygreater precision than before. It is shown that spectral types in thissystem are not systematically affected by rotational line broadening. Atotal of 372 early A-type stars are classified, and a confrontation ofthese spectral types with Stroemgren photometry reveals a number ofsystematic photometric effects of rotation. In particular, high v sin istars are systematically redder than low v sin i stars of the samespectral type, and the beta index is weakened by rotation. It isconcluded that precise spectral classification in conjunction withStroemgren and H-beta photometry can potentially provide a valuablecheck and input to the theory of the atmospheres of rotating stars.
| Calibration in temperature of photometric parameters
In calibrating photometric parameters in temperature, it is very easy touse a Planck distribution to show that a color index is a temperatureparameter, but it is more difficult to calibrate such a color index interms of temperature because only a few effective temperature values aredetermined. A pioneering work is that of Kuiper (1938), who derived atemperature scale according to spectral type (from A0 to M2 for dwarfsand from G0 to M8 for giants) and a Becker index. The first study givinga relation between effective temperature and a photoelectric color indexis that of Popper (1959) in which Popper derives a relation betweeneffective temperature and B-V for the A and F stars and for G8 to K5(dwarf and giant) stars. Popper has shown the relation betweentemperature parameters of two photometric systems, R-I from thesix-color system of Stebbins and Whitford, and B-V. The intent of thepresent work is to define a set of stars which can be used to determinea calibration of a photometric parameter in terms of effectivetemperature.
| A new photometric WBVR system
A description of the new broad-band photometric system, WBVR, is given.It is shown that WBVR is close to the UBVR system but is defined bystrictly fixed response curves and the primary standard. Theconstruction of a network of secondary standards distributed uniformlyin the northern sky, as well as the coordination of the magnitudes ofthe standards to the self-consistent system are discussed. All standardsare examined for a possible brightness variability and it is found thatno light variability exceeding 0.015 mag in amplitude is present andthat their average magnitude shows stability on a scale of severalyears. The individual WBVR observations of the standards are presentedin tables.
| Catalogue of the energy distribution data in spectra of stars in the uniform spectrophotometric system.
Not Available
| The System of Highly Accurate Photoelectric Standards in WBVR Bands
Not Available
| Spectrophotometry of peculiar B and A stars. XVI - The derivation of indices by fitting the Balmer and Paschen continua of the normal stars and application to the mercury-manganese stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1984A&AS...55..479A&db_key=AST
| Picture gallery - A structured presentation of OAO-2 photometric data supported by OAO-2 spectrophotometric data and UBV, ANS and TD1 observations
Graphs are presented for the stellar fluxes of 531 stars in the5500-1330 A wavelength range, which have been divided into 52 categorieson the basis of spectral types. The merging of medium band interferencefilter photometry, UBV photometry, ANS photometry and TD1 fluxes, aswell as the ordering of the objects, should prove helpful in studies ofinterstellar reddening, luminosity effects, bandwidth effects, andcomparisons with model stellar atmospheres. The agreement between thevarious UV photometric systems for early-type stars is generally betterthan 0.10 mag. A list of stars whose photometric properties indicatestellar or interstellar anomalies is also provided.
| Observed and computed UV spectral distribution of A and F stars
An automatic and fast procedure was implemented to determineTe and log g from the comparison of the UV S 2/68 spectrawith the Kurucz models. The method has been applied to all A and F starswith (B-V)0 greater than or equal to 0 and luminosity classes III, IV,IV-V, and V, included in the Ultraviolet Bright-Star SpectrophotometricCatalogue. From the analysis, it appears that the models match theobserved fluxes fairly well and that the effective temperatures derivedfrom the UV data agree with those derived from Stromgren photometricindices within 200 K.
| Balmer-line equivalent widths in main sequence B-F stars, and comparison with model atmospheres
Equivalent widths of the Balmer H-alpha, H-beta, and H-gamma lines weremeasured for 175 main-sequence B-F stars and compared with curvescalculated from various model stellar atmospheres. When averaged overeach spectral subtype, the equivalent widths are well represented by thestandard models; this confirms the previous finding of Glushneva andDoroshenko (1980) that the models are fully consistent with theobservations.
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