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Speckle Interferometry at the US Naval Observatory. XIII.
The results of 1424 speckle interferometric observations of doublestars, made with the 26 inch (66 cm) refractor of the US NavalObservatory, are presented. Each speckle interferometric observation ofa system represents a combination of over 2000 short-exposure images.These observations are averaged into 1053 mean relative positions andrange in separation from 0.36″ to 61.92″, with a medianseparation of 10.31″. This is the 13th in a series of paperspresenting measurements obtained with this system and covers the period2006 January 12-December 29. Included in these data are nine oldermeasurements whose positions were previously deemed possibly aberrantbut are no longer classified this way following a confirmingobservation. This paper also includes the first data obtained using anew "secondary" camera, designed and built at USNO.
| The heterogeneous class of lambda Bootis stars
We demonstrate that it is arduous to define the lambda Boo stars as aclass of objects exhibiting uniform abundance peculiarities which wouldbe generated by a mechanism altering the structure of their atmosphericlayers. We collected the stars classified as lambda Boo up to now anddiscuss their properties, in particular the important percentage ofconfirmed binaries producing composite spectra (including our adaptiveoptics observations) and of misclassified objects. The unexplained RVvariables (and thus suspected binaries), the known SB for which we lackinformation on the companion, the stars with an UV flux inconsistentwith their classification, and the fast rotating stars for which noaccurate abundance analysis can be performed, are also reviewed.Partly based on observations collected at the CFH Telescope (Hawaii) andat TBL of the Pic du Midi Observatory (France).Table \ref{tab5} is only available in electronic form athttp://www.edpsciences.org
| A study of lambda Bootis type stars in the wavelength region beyond 7000 Å
The group of lambda Bootis type stars comprises late B- to earlyF-type, Population I objects which are basically metal weak, inparticular the Fe group elements, but with the clear exception of C, N,O and S. One of the theories to explain the abundance pattern of thesestars involves circumstellar or interstellar matter around the objects.Hence, we have compiled all available data from the literature of wellestablished members of the lambda Bootis group redward of 7000Å, in order to find evidence for matter around these objects.Furthermore, we present unpublished ISO as well as submillimetercontinuum and CO (2-1) line measurements to complete the data set. Intotal, measurements for 34 (26 with data redward of 20 mu m) wellestablished lambda Bootis stars are available. There is evidence foran infrared excesses in six stars (HD 31295, HD 74873, HD 110411, HD125162, HD 198160/1 and HD 210111) and two are doubtful cases (HD 11413and HD 192640) resulting in a percentage of 23% (excluding the twodoubtful cases). Dust models for these objects show fractional dustluminosities comparable to the Vega-type stars and slightly higher dusttemperatures. ISO-SWS spectroscopy for HD 125162 and HD 192640 resultedin the detection of pure stellar H I lines ruling out an activeaccretion disk (as found for several Herbig Ae/Be stars) around theseobjects. The submillimeter measurements gave only upper limits for theline and continuum fluxes.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA; andobservations at the Heinrich-Hertz-Telescope (HHT, operated by the theSubmillimeter Telescope Observatory).Tables 2 and 3 are only available in electronic form athttp://www.edpsciences.org
| 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.
| The λ Bootis phenomenon: interaction between a star and a diffuse interstellar cloud
The λ Bootis stars, a group of late B- to early F-type PopulationI stars, have surface abundances that resemble the general metaldepletion pattern found in the interstellar medium. Inspired by therecent result that the fundamental parameters of these peculiar starsdiffer in no respect from those of a comparison sample of normal stars,the hypothesis of an interaction between a star and a diffuseinterstellar cloud is considered as a possible explanation of thepeculiar abundance pattern. It is found that such a scenario is able toexplain the selective accretion of interstellar gas depleted incondensable elements as well as the spectral range of the λBootis phenomenon.
| 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.
| The accretion/diffusion theory for lambda Bootis stars in the light of spectroscopic data
Most of the current theories suggest the lambda Bootis phenomenon tooriginate from an interaction between the stellar surface and its localenvironment. In this paper, we compare the abundance pattern of thelambda Bootis stars to that of the interstellar medium and find largerdeficiencies for Mg, Si, Mn and Zn than in the interstellar medium. Acomparison with metal poor post-AGB stars showing evidence forcircumstellar material indicates a similar physical process possiblybeing at work for some of the lambda Bootis stars, but not for all ofthem. Despite the fact that the number of spectroscopically analysedlambda Bootis stars has considerably increased in the past, a test ofpredicted effects with observations shows current abundance andtemperature data to be still controversial.
| The abundance pattern of lambda Bootis stars
Within a project to investigate the properties of lambda Bootis stars,we report on their abundance pattern. High resolution spectra have beenobtained for a total of twelve candidate lambda Bootis stars, four ofthem being contained in spectroscopic binary systems, and detailedabundance analyses have been performed. All program stars show acharacteristic lambda Bootis abundance pattern (deficient heavy elementsand solar abundant light elements) and an enhanced abundance of Na. Thiswork raises the fraction of lambda Bootis stars with known abundances to50%. The resulting abundances complemented by literature data are usedto construct a ``mean lambda Bootis abundance pattern'', which exhibits,apart from general underabundances of heavy elements (~-1 dex) and solarabundances of C, N, O, Na and S, a star-to-star scatter which is up totwice as large as for a comparable sample of normal stars. Based onobservations obtained at the Osservatorio Astronomico di Padua-Asiago,OPD/LNA, KPNO and DSO.
| The long-period companions of multiple stars tend to have moderate eccentricities
We examined the statistics of an angle gamma between the radius vectorof a visual companion of a multiple star and the vector of its apparentrelative motion in the system. Its distribution f(gamma ) is related tothe orbital eccentricity distribution in the investigated sample. Wefound that for the wide physical subsystems of the 174 objects from theMultiple Star Catalogue f(gamma ) is bell-shaped. The Monte-Carlosimulations have shown that our f(gamma ) corresponds to the populationof the moderate-eccentricity orbits and is not compatible with thelinear distribution f(e)=2e which follows from stellar dynamics andseems to hold for wide binaries. This points to the absence of highlyelongated orbits among the outer subsystems of multiple stars. Theconstraint of dynamical stability of triple systems is not sufficient toexplain the ``rounded-off'' outer orbits; instead, we speculate that itcan result from the angular momentum exchange in multiple systems duringtheir early evolution.
| Light element non-LTE abundances of lambda Bootis stars. II. Nitrogen and sulphur
One of the main characteristics proclaimed for the group of the lambdaBootis stars is the apparent solar abundance of the light elements C, N,O and S. The typical abundance pattern is completed by the strongunderabundances of the Fe-peak elements. In the first paper of thisseries, we have shown that carbon is less abundant than oxygen but bothelements are still significantly more abundant than Fe-peak elements.The mean abundances, based on a detailed non-LTE investigation, werefound -0.37 dex and -0.07 dex, respectively. As a further step, we nowpresent non-LTE abundances of nitrogen and sulphur for thirteen membersof the lambda Bootis group based on several spectral lines between 8590Å, and 8750 Å. Furthermore, LTE abundances for calcium inthe same spectral range were derived and compared with values from theliterature. Similar to the mean abundances of carbon and oxygen, nearlysolar values were found (-0.30 dex for nitrogen and -0.11 dex forsulphur) for our sample of program stars. Among our sample, onepreviously undetected binary system (HD 64491) was identified. From astatistical point of view, the abundances of the light elements rangefrom slightly overabundant to moderately underabundant compared to theSun. However, the individual objects always exhibit a similiar pattern,with the Fe-peak elements being significantly more underabundant thanthe light elements. No correlation of the derived abundances withastrophysical parameters such as the effective temperature, surfacegravity or projected rotational velocity was found. Furthermore, theabundances of the light elements do not allow us to discriminate betweenany proposed theory. Based on observations obtained at BNAO Rozhen andComplejo Astronómico el Leoncito (CASLEO), operated under theagreement between the Consejo Nacional de InvestigacionesCientíficas y Técnicas de la República Argentinaand the National Universities of La Plata, Córdoba y San Juan.
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| Hipparcos astrometry for 257 stars using Tycho-2 data
We present improved Hipparcos astrometry for 257 Hipparcos stars,resolved into 342 components. For 64 of the stars no astrometry wasobtained in the Hipparcos Catalogue, while for the remaining starsadditional components have been added by this solution or the positionshave been revised considerably. We have used the published Hipparcostransit data for the new solutions, together with results from thesecond reduction of the Tycho data for defining better initial values.Based on observations made with the ESA Hipparcos astrometry satellite.Table 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A search for circumstellar gas around normal A stars and Lambda Bootis stars
We have searched for interstellar or circumstellar absorption lines inthe center of Ca II K towards bright A-type stars that are mostly within80 pc of the Sun. Narrow absorption features are found in about 30 % ofthe 28 normal main-sequence A stars and 18 metal-deficient lambda Bootisstars studied. We have determined surface gravities and projectedrotational velocities. Most of the stars with detectable Ca K featureshave comparatively low gravities and high projected rotationalvelocities. This correlation with stellar properties implies that mostof the narrow absorption features are of circumstellar rather thaninterstellar origin. The preference of low gravity and rapid rotationfurthermore suggests that most of the gas shells around A stars developin the pre-main-sequence phase of evolution, and disappear largelybefore the star arrives at the ZAMS. Among the normal A stars studied,about 50 % are known to have dust disks. Unlike A stars withcircumstellar gas, these dusty stars do not prefer low log g and high vsin i. This results in an apparent lack of correlation between gas anddust, and indicates that normal A stars with gas shells and those withdust disks are not in the same evolutionary stage. We conjecture thatdust disks tend to develop after most of the gas has disappeared. Basedon observations collected at the European Southern Observatory, LaSilla, Chile
| The incidence of nonradial pulsation in the lambda Bootis stars
We have conducted a high-resolution, high signal-to-noise spectroscopicsurvey of the members of the peculiar lambda Boo stars accessible fromthe Canada-France-Hawaii Telescope to investigate the incidence ofhigh-degree nonradial pulsation (NRP) in these metal-deficient stars. Of16 objects observed more than once, 9 show conclusive evidence of suchNRP, which confirms that pulsational instability is a common phenomenonin the lambda Boo class. The widespread presence of NRP in the lambdaBoo stars indirectly supports the accretion/diffusion scenario for theformation of these objects, but unfortunately does not rule out otherpossible causes for the phenomena. However, extensive time-seriesphotometry and spectroscopy of several particularly interestingpulsating members of the class noted here should provideasteroseismologists with the eigenfrequency data needed to resolve thecurrent debate concerning the evolutionary status, and hence, the originof these peculiar stars. From our survey, we also identify a set of fourstars, including lambda Boo itself, with similar fundamental parametersbut remarkably different pulsation characteristic. These stars may offervital clues as to the physical excitation of lambda Boo pulsations.Figs.\,2--5, 7--10 are only available in the electronic version.
| How many lambda Bootis stars are binaries?
In the attempt to shed new light on the lambda Boo phenomenon weanalyzed the astrometric, photometric and spectroscopic characteristicsof stars out of a list of recently selected lambda Boo candidates. Weshow that the class is still ill-defined and discuss the possibilitythat some, if not most stars presently classified as lambda Boo, are infact binary pairs and that peculiar abundances may not correspond toactual values if the average values of the atmospheric parameters{Teff} and log g are assumed and the effect of veiling is nottaken into account. Partly based on data from the ESA Hipparcosastrometry satellite.
| Spectroscopy of Hot Stars in the Galactic Halo. II. The Identification and Classification of Horizontal-Branch and Other A-Type Stars
We discuss a spectroscopic and photometric technique that enables theidentification and classification of field horizontal-branch (FHB) andother A-type stars, even from relatively low signal-to-noise ratiomedium-resolution spectra. This technique makes use of broadband UBVcolors predicted from model atmosphere calculations and Balmer lineprofiles and Ca II K equivalent widths determined from synthetic spectrato estimate the physical parameters T_eff, log g, and [Fe/H] for starsin the effective temperature range 6000-10,000 K. A comparison of ourmethod with high signal-to-noise ratio spectra of standard starsindicates a scatter in the derived parameters of sigma(T_eff)=+/-250 K,sigma(log g)=+/-0.14 dex, and sigma([Fe/H])=+/-0.12 dex. This precisionallows for a separation of low surface gravity FHB and other, generallyhigher surface gravity, A-type (and somewhat later) stars. We alsodevelop a synthetic-template comparison technique, which is veryeffective in the identification of metallic-line and peculiar A-typestars. A detailed investigation of the influence of noise in the spectraon the determination of physical parameters shows that, for spectra withsignal-to-noise ratios in the range 10
| Light element non-LTE abundances of lambda Bootis stars. I. Carbon and oxygen
Abundances for the light elements of {lambda Bootis} stars are a mainkey to understand the astrophysical processes behind the so-called{lambda Bootis} phenomenon. These stars are characterized by a typicalabundance pattern (strong underabundances of the Fe-peak elementswhereas the light elements have apparently solar abundances) which isstill based mainly on LTE-calculations. Therefore we started aninvestigation to derive accurate abundances of the light elements (C, N,O and S). For this purpose a new oxygen model atom was implemented inthe Kiel non-LTE code. High resolution and high signal-to-noise spectrawere used. For each element only a single wavelength region with linesof the specific element was selected and observed in order to avoidcontamination from other elements. In the first paper we presentabundances for carbon and oxygen of a statistically significant numberof well established {lambda Bootis} stars. The second paper will dealwith nitrogen and sulphur. The most important result is that on averagecarbon is less abundant than oxygen but still both elements aresignificant more abundant than the Fe-peak elements. Furthermore theanticorrelation of carbon and oxygen with the silicon abundance isproven, which strongly supports the accretion/diffusion theory. Based onobservations obtained at ESO-La\,Silla and BNAO Rozhen
| The pulsation and evolution of lambda Bootis stars.
Not Available
| Pulsation in lambda Bootis stars
In this paper we present a further step in applying asteroseismictechniques to the group of lambda Bootis stars which can becharacterized as nonmagnetic A to F-type Population I dwarfs withsignificant (surface) underabundances of Fe-peak elements. Since noconclusive theory explaining the origin of the observed abundanceanomalies exists, an extensive photometric survey for pulsation in thisgroup has been initiated. Knowledge about the pulsational properties(most members are located within the classical instability strip) couldhelp to establish constrains about the overall abundance of these starsas well as on the evolutionary status. New photometric observations werecarried out for eleven stars. Variability was detected in four stars(e.g. lambda Bootis itself) whereas the remaining seven objects areprobably constant. In total, 52 members of this group have beenphotometrically investigated so far. With 22 pulsating and 30``constant'' stars, we derive a ratio of at least 50 % for variable tononvariable members inside the classical instability strip. This resultis based on high quality Hipparcos and new photometric data. Theobserved log /lineρ//lineρ_ȯ and log P values for thepulsating members are compatible with standard (solar abundant) deltaScuti models supporting the hypothesis that the found abundanceanomalies are restricted to the surface only. Otherwise the pulsationalproperties of this group are not outstanding compared to ``normal''delta Scuti stars, indicating that the mechanism driving the pulsationsis very similar. Based on observations obtained at ESO-La\,Silla, CTIO,SAAO, McDonald Observatory, Instituto Astrofisica Andalucia Observatoryand with the Hipparcos satellite
| On the evolutionary status of lambda Bbootis stars using HIPPARCOS data
Using the Hipparcos data, absolute magnitudes and thus the evolutionarystatus for the group of lambda Bootis stars were derived. The origin ofthis small group of non-magnetic, chemically peculiar stars, stillremains a matter of debate. Using new evolutionary tracks, an agedetermination could be provided to distinguish between the two competingtheories - the diffusion/mass-loss and the accretion. The resultsestablish the members of this group as objects which are very close tothe Main Sequence. This is supported by Pre-Main Sequence evolutionarytracks as well as by observational results. This contradicts priorconclusions that most of these stars are in the middle of their MainSequence lifetime. The new results strongly support the predictions ofthe accretion theory.
| The incidence of nonradial pulsation in the lambda Bootis stars
We have completed a high-resolution, high-signal-to-noise, spectroscopicsurvey of the northern members of the peculiar lambda Boo stars in orderto investigate the frequency of the incidence of nonradial pulsation(NRP) in these metal-deficient stars. Of 18 objects observed, 9 showconclusive evidence of NRP, which suggests that pulsation instability isa common phenomenon in the lambda Boo class.
| On the evolutionary status of λ Bootis stars using HIPPARCOS data.
We have used the Hipparcos data to derive absolute magnitudes and thusthe evolutionary status for the group of λ Bootis stars. Theorigin for this small group of nonmagnetic, chemically peculiar stars,still remains a matter of debate. Using new evolutionary tracks, we areable to provide an age determination to distinguish between the twoproposed theories - the diffusion/mass-loss and the accretion theory.Our results establish the member of this group as objects which are veryclose to the Main Sequence. This is also supported by Pre-Main Sequenceevolutionary tracks as well as by observational results. We thereforecontradict prior conclusions that most of these stars are in the middleof their Main Sequence lifetime. The new results strongly support thepredictions of the accretion theory.
| 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.
| A consolidated catalogue of lambda Bootis stars
lambda Bootis stars challenge our understanding of diffusion andaccretion processes related to stars and their circumstellarenvironment, and they are interesting components of the classicalinstability strip. Attempts to derive group properties with statisticalmethods are severely limited by the small number of unambiguouslyidentified lambda Bootis stars. In general, the subject appears to beobscured by incorrect memberships and it is therefore essential toprovide a sufficiently large catalogue of definitive group membersbefore modeling the lambda Bootis phenomenon. This paper describes thefirst steps towards this goal, based on our current knowledge of wellinvestigated members, leading to a concise definition of lambda Bootisstars: {Pop I hydrogen burning metal poor (except of C,N,O and S) A-typestars.} The definition does not depend on phenomenological features,like flux depressions, colour excesses, v sin i values, etc. Based on anew homogeneous catalogue with 45 lambda Bootis stars, we discussclassification criteria which can be used for a spectroscopic andphotometric all-sky survey for lambda Bootis stars in the field and inclusters of different ages. Based on observations obtained atESO-La,Silla, CTIO, Observatoire de Haute-Provence, OsservatorioAstronomico di Padua-Asiago, Univ. Toronto Southern Observatory,Observatorio do Pico dos Dias-LNA/CNPq/MCT (Brazil). Table 1 is alsoavailable 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.
| HD 193084: A new variable star
Not Available
| Nonvariability among lambda Boo Stars I.: ESO 1993 and 1994 Data
Not Available
| High-resolution spectroscopy of λ Bootis stars and `dusty' normal A stars: circumstellar gas, rotation, and accretion.
We present results of a high-S/N search for circumstellar Ca K lines intwo classes of A stars: (i) metal-deficient stars of the λBootistype whose surface anomalies are believed to be due to accretion ofdepleted gas, and (ii) stars known to possess circumstellar dust disks,including Vega and βPictoris. We derive T_eff_ and logg and combinemodel-atmosphere analysis with spectrum synthesis to determine projectedrotational velocities, calcium abundances, and the motion ofcircumstellar gas with respect to the star. Narrow components in Ca Kare present in five of the eleven λBoo stars studied. Thisexceptional high incidence suggests that the λBoo phenomenon isrelated to the presence of circumstellar gas. Specific topics discussed(Sect. 6) include: (1) a connection between rotation, the presence ofdetectable amounts of CS gas, and metal deficiency, (2) Vega andβPic as suspected λBoo stars, (3) λBoo stars asprobable pre-main-sequence objects, and (4) episodes of massiveaccretion in βPic and in the λBoo star HR4881.
| Mass and age determination for 21 λ Bootis-type stars.
Newly published theoretical grids of stellar models are used to evaluatemasses and ages of 21 stars with proven λ Bootis characteristics.Basic input parameters as effective temperature and luminosity for eachstar are calculated from their uvbyβ indices. Masses thus estimatedare in the range of 1.6-2.7 solar masses, the evolutionary ages arebetween 10^7^ and 10^9^ years. In terms of stellar evolution, most ofthe studied λ Bootis stars are in the middle of their mainsequence evolution. Only a few of them are young enough to fulfill therequirements of the "diffusion/accretion" model. Masses and ages of theexamined stars are compared with already published [Fe/H] data. Iron isless deficient in more massive λ Bootis stars. There is noevidence for significant change in the chemical composition ofconsidered λ Bootis type stars during their evolution on the mainsequence.
| Three other λ Bootis stars with a shell.
High signal-to-noise ratio spectra have been taken to detect a possiblecircumstellar shell around λ Boo stars. It is more and moreassumed that circumstellar matter can be found around such stars. Wehave considered the CaII K, Hγ and CaI λ4427 lines. Twospectra of λ Boo stars (HD 16955 and HD 104965) showcircumstellar shell features in the K-line. Such features could also bepresent in the spectrum of HD 220061 and we confirm the circumstellarshell recently found for 131 Tau by Bohlender & Walker (1994).
| Accretion from Circumstellar Discs and the Lambda-Bootis Phenomenon
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.269..209K&db_key=AST
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