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The status of Galactic field λ Bootis stars in the post-Hipparcos era
The λ Bootis stars are Population I, late B- to early F-typestars, with moderate to extreme (up to a factor 100) surfaceunderabundances of most Fe-peak elements and solar abundances of lighterelements (C, N, O and S). To put constraints on the various existingtheories that try to explain these peculiar stars, we investigate theobservational properties of λ Bootis stars compared with areference sample of normal stars. Using various photometric systems andHipparcos data, we analyse the validity of standard photometriccalibrations, elemental abundances, and Galactic space motions. Therecrystallizes a clear picture of a homogeneous group of Population Iobjects found at all stages of their main-sequence evolution, with apeak at about 1 Gyr. No correlation of astrophysical parameters such asthe projected rotational velocities or elemental abundances with age isfound, suggesting that the a priori unknown mechanism, which createsλ Bootis stars, works continuously for late B- to early F-typestars in all stages of main-sequence evolution. Surprisingly, the sodiumabundances seem to indicate an interaction between the stars and theirlocal environment.
| On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of lambda Bootis type stars
Generally, chemical peculiarity found for stars on the upper mainsequence excludes delta Scuti type pulsation (e.g. Ap and Am stars), butfor the group of lambda Bootis stars it is just the opposite. This makesthem very interesting for asteroseismological investigations. The groupof lambda Bootis type stars comprises late B- to early F-type,Population I objects which are basically metal weak, in particular theFe group elements, but with the clear exception of C, N, O and S. Thepresent work is a continuation of the studies by Paunzen et al.(\cite{Pau97}, \cite{Pau98}), who presented first results on thepulsational characteristics of the lambda Bootis stars. Since then, wehave observed 22 additional objects; we found eight new pulsators andconfirmed another one. Furthermore, new spectroscopic data (Paunzen\cite{Pau01}) allowed us to sort out misidentified candidates and to addtrue members to the group. From 67 members of this group, only two arenot photometrically investigated yet which makes our analysis highlyrepresentative. We have compared our results on the pulsationalbehaviour of the lambda Bootis stars with those of a sample of deltaScuti type objects. We find that at least 70% of all lambda Bootis typestars inside the classical instability strip pulsate, and they do sowith high overtone modes (Q < 0.020 d). Only a few stars, if any,pulsate in the fundamental mode. Our photometric results are inexcellent agreement with the spectroscopic work on high-degree nonradialpulsations by Bohlender et al. (\cite{Boh99}). Compared to the deltaScuti stars, the cool and hot borders of the instability strip of thelambda Bootis stars are shifted by about 25 mmag, towards smaller(b-y)_0. Using published abundances and the metallicity sensitiveindices of the Geneva 7-colour and Strömgren uvbybeta systems, wehave derived [Z] values which describe the surface abundance of theheavier elements for the group members. We find that thePeriod-Luminosity-Colour relation for the group of lambda Bootis starsis within the errors identical with that of the normal delta Scutistars. No clear evidence for a statistically significant metallicityterm was detected. Based on observations from the Austrian AutomaticPhotoelectric Telescope (Fairborn Observatory), SAAO and Siding SpringObservatory.
| A revised catalogue of delta Sct stars
An extensive and up-dated list of delta Sct stars is presented here.More than 500 papers, published during the last few years, have beenrevised and 341 new variables have been added to our last list, sixyears ago. This catalogue is intended to be a comprehensive review onthe observational characteristics of all the delta Sct stars known untilnow, including stars contained in earlier catalogues together with othernew discovered variables, covering information published until January2000. In summary, 636 variables, 1149 references and 182 individualnotes are presented in this new list. Tables 1 and 2 will be accessibleonly in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Convection, Thermal Bifurcation, and the Colors of A Stars
Broadband ultraviolet photometry from the TD-1 satellite andlow-dispersion spectra from the short wavelength camera of IUE have beenused to investigate a long-standing proposal of Bohm-Vitense that thenormal main-sequence A and early-F stars may divide into two differenttemperature sequences: (1) a high-temperature branch (and plateau)comprised of slowly rotating convective stars, and (2) a low-temperaturebranch populated by rapidly rotating radiative stars. We find noevidence from either data set to support such a claim, or to confirm theexistence of an "A-star gap" in the B-V color range 0.22 <= B-V <=0.28 due to the sudden onset of convection. We do observe, nonetheless,a large scatter in the 1800--2000 A colors of the A--F stars, whichamounts to ~0.65 mag at a given B-V color index. The scatter is notcaused by interstellar or circumstellar reddening. A convincing case canalso be made against binarity and intrinsic variability due topulsations of delta Sct origin. We find no correlation with establishedchromospheric and coronal proxies of convection, and thus nodemonstrable link to the possible onset of convection among the A--Fstars. The scatter is not instrumental. Approximately 0.4 mag of thescatter is shown to arise from individual differences in surface gravityas well as a moderate spread (factor of ~3) in heavy metal abundance andUV line blanketing. A dispersion of ~0.25 mag remains, which has noclear and obvious explanation. The most likely cause, we believe, is aresidual imprecision in our correction for the spread in metalabundances. However, the existing data do not rule out possiblecontributions from intrinsic stellar variability or from differential UVline blanketing effects owing to a dispersion in microturbulentvelocity.
| HD 111786: a spectroscopic binary λ Boo star.
HD 111786 = HR 4881 is one of the most interesting λ Boo stars;in fact it is one of the few stars of this class which shows narrowfeatures that can be interpreted as remnants of the circumstellar matterfrom which star was formed. On the basis of optical observations andinspection of IUE spectra, we demonstrate that HD 111786 is, as a matterof fact, a binary system and that one of the component is characterizedby a narrow line spectrum; the main contribution to the narrowabsorptions is not that of circumstellar gas related to the λ Boophenomenon, but is due to the photosphere of the companion star.
| 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.
| A catalogue of variable stars in the lower instability strip.
Identifications, positions, photometry, spectra, some pulsationalfeatures, other astrophysical parameters and literature for 302pulsating variable stars in the lower instability strip, near the ZAMS,are given. About 185 stars have near homogeneous photometric informationin the Stroemgren's uvby-β photometric system. Thiscatalogue/database covers information published until November 1993.
| An atlas of southern MK standards from 5800 to 10200 A
An atlas of stellar spectra covering the wavelength range from 5800 to10,200 A is presented of 126 southern MK standard stars, covering theluminosity classes I, III, and V. Some peculiar stars are included forcomparison purposes. The spectra were obtained at a resolution of 4.3 Aper pixel using a Cassegrain-mounted Boller and Chivens spectrographequipped with a Reticon detector. The quality and utility of the dataare discussed and examples of the spectra are presented. The atlas isavailable in digital format through the NSSDC.
| δ Scuti stars: a new revised list
An extensive and up to date list of δ Sct stars is presented. Thiscatalogue is intended to be a comprehensive review of observationalcharacteristics of all the δ Sct stars known until now, includingstars contained in earlier catalogues together with other new discoveredvariables, covering information published until November 1993. Globalinformation in the form of histograms and diagrams are also shown.
| The 71st Name-List of Variable Stars
Not Available
| A catalog of stellar Lyman-alpha fluxes
We present a catalog of stellar Ly-alpha emission fluxes, based on newand archival images obtained with the IUE spacecraft. The catalogincludes 227 stars with detectable Ly-alpha emission fluxes, and upperlimits on the Ly-alpha emission flux for another 48 stars. Multiple fluxmeasurements are given for 52 stars. We present a model for correctingthe observed Ly-alpha flux for attenuation by the local interstellarmedium, and we apply this model to derive intrinsic Ly-alpha fluxes for149 catalog stars which are located in low H I column density directionsof the local interstellar medium. In our catalog, there are 14 late-Aand early-F stars at B-V = 0.29 or less that show detectable emission atLy-alpha. We find a linear correlation between the intrinsic Ly-alphaflux and C II 1335 A flux for stars with B-V greater than 0.60, but theA and F stars deviate from this relation in the sense that theirLy-alpha flux is too low. We also find a good correlation betweenLy-alpha strength and coronal X-ray emission. This correlation holdsover most of the H-R diagram, even for the F stars, where an X-raydeficit has previously been found relative to the transition regionlines of C II and C IV.
| The coronae of main-sequence A stars
We have undertaken a deep X-ray imaging survey of single, normal,mid-to-late-A- and early-F-type main-sequence stars using the ROSA TPSPC. Eight of the nine such stars in our fields went undetected at veryweak emission levels, with upper limits on the normalized X-ray flux ineach case of better than log R(x) below -6.3. The only A-F star wedetected was 83 Tau, a member of the Hyades that we now belatedlyrecognize as a possible binary star, whose emission may come from anunidentified companion. Our observations reveal a systematic decline incoronal X-ray brightness with increasing effective temperature, which weattribute to the presence of strong coronal winds. These new results addfurther weight to earlier suggestions that single A and early-F stars inthe field and in nearby older open clusters generally have intrinsicallyfaint coronae.
| Magnetic activity in dwarf stars with shallow convective envelopes
The magnetic activity in dwarf stars with shallow convective envelopesis addressed by studying the C II and soft X-ray surface flux densitiesin relation to the stellar rotation velocity. The C II emission from theacoustically heated basal atmospheric component increases withincreasing T(eff) up to at least B-V about 0.25, reaching a level ofabout 8 x 10 exp 4 erg/sq cm/s for early F-type dwarf stars. Arelationship between rotation rate and level of magnetic activity isfound to extend down to at least B-V about 0.25, although the C IIemission associated with the magnetic activity at B-V = 0.25 issuppressed by a factor of about 100 as compared to stars cooler than B-Vabout 0.60 with the same rotation rate. The suppression of activity isstrongly color-dependent below B-V about 0.60, with the dynamoefficiency leveling off for cooler stars. The radiative lossesassociated with magnetic activity do not show a noticeable dependence oncolor in diagrams relating fluxes from chromosphere and corona for F-,G-, and K-type dwarf stars, suggesting comparable atmospheric structuresfor the magnetic component. The observed mean rotational velocities as afunction of spectral type are shown to be consistent with theassumptions of (1) a reduced dynamo efficiency and (2) a comparablemechanism for angular momentum loss through a stellar wind for allmagnetically active cool stars.
| The onset of chromospheric activity among the A and F stars
Results are reported from a search for an upper boundary for the onsetof main-sequence star activity based on a quest for high-temperature UVline emission in a large collection of IUE spectra. It is shown thatstrong chromospheric emission is common among early F dwarf and subgiantstars. At its brightest, the emission is equal to that of the mostactive solar-type stars and is exceeded only by that of the spotted RSCVn and BY Dra variables. It is suggested that the emission from themain-sequence stars reaches a peak near B-V = 0.28, in the vicinity ofspectral type F0 V, before it declines to lower flux levels among thelate A stars. Emission is seen in some dwarf stars as early as B-V =0.25. It is demonstrated that the C II emission of stars earlier thanthe spectral type F5 is uncorrelated with rotation. Previous findingsthat the coronal X-ray:chromospheric UV flux ratio is lower for starsearlier than spectral type F5 than for those later than F5 areconfirmed.
| Study of Delta Scuti stars in the Geneva photometric system. I - New photometric data and period analysis for nine stars
Results are presented of the period analyses for nine Delta Scuti stars,of which six are new short-period variable stars. For some of thesestars, extensive data sets covering time bases of the order of two yearsor more were obtained. The results from year to year are consistent withthe multiperiodic solution derived from the combined data for thosestars with the largest data sets. In some cases of less frequentlyobserved stars, the results are not always compatible with one another.It is suggested that, generally, this is due to insufficient datasampling or too narrow time bases. The fact that many unevolved orslightly evolved Delta Scuti stars are multiperiodic with very smallamplitudes of a couple of hundredths of a magnitude is confirmed.
| The early F-type stars - Refined classification, confrontation with Stromgren photometry, and the effects of rotation
The classification for early F-type stars in the MK spectralclassification system presented by Gray and Garrison (1987) is refined.The effect of rotation on spectral classification and ubvy-betaphotometry of early F-type stars is examined. It is found that theclassical luminosity criterion, the 4417 A/4481 A ratio givesinconsistent results. It is shown that most of the stars in the DeltaDelphini class of metallic-line stars are either normal or areindistinguishable from proto-Am stars. It is suggested that thedesignation Delta Delphini should be dropped. The classifications arecompared with Stromgren photometry. The effects of rotation on thedelta-c1 index in the early-F field dwarfs is demonstrated.
| The Onset of Chromospheric Activity Among the A- and F-Type Stars
Not Available
| Absolute luminosity calibration of Stroemgren's 'late group'
A statistical parallax method based on the principle of maximumlikelihood is used to calibrate absolute luminosities for samples ofcooler stars constituting the 'late group' defined by Stromgren (1966).The samples examined include 415 stars of all luminosity classes and asubset comprising 86 main-sequence stars. Linear calibration relationsinvolving the Stromgren beta, (b-y), and bracketted c1 indices arederived which yield mean absolute magnitudes with an accuracy of 0.09magnitude for the overall sample and 0.13 magnitude for themain-sequence subsample. Several second-order relations are considered,and the results are compared with Crawford's (1975) calibrations as wellas with mean absolute magnitudes obtained from trigonometric parallaxes.The possible effect of interstellar absorption on the calibrationrelations is also investigated.
| Rotational Velocities and Spectral Types of Some A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972PASP...84..584L&db_key=AST
| Four-colour and H BET photometry of some bright southern stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972MNRAS.159..165S&db_key=AST
| K-Line Photometry of Southern a Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971ApJS...23..421H&db_key=AST
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