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Analysing the Hipparcos epoch photometry of λ Bootis stars
We performed a homogeneous analysis of the available Hipparcos epochphotometry for all members of the LB group. Besides the known δScuti like pulsation frequencies, we searched for eclipses due tobinarity or rotational induced variability. The already knownfrequencies of HD 15165 are confirmed with an additional lower singlefrequency, which might be caused by an accretion episode. But we are notable to rule out a possible aliasing due to the temporal sampling of thedata set. Furthermore, three candidates (HD 15759, HD 120500, and HD149130) for a possible eclipse were detected. Otherwise, the frequencyanalysis reveals a null result within our strict detection limits.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| On the detection of chemically peculiar stars using Δa photometry
We have summarized all Δ a measurements for galactic field stars(1474 objects) from the literature published over more than two decades.These measurements were, for the first time, compiled and homogeneouslyanalyzed. The Δ a intermediate band photometric system samples thedepth of the 5200 Å flux depression by comparing the flux at thecenter with the adjacent regions with bandwidths of 110 Å to 230Å. Because it was slightly modified over the last three decades,we checked for systematic trends for the different measurements butfound no correlations whatsoever. The Δ a photometric system ismost suitable to detecting magnetic chemically peculiar (CP) stars withhigh efficiency, but is also capable of detecting a small percentage ofnon-magnetic CP objects. Furthermore, the groups of (metal-weak)λ Bootis, as well as classical Be/shell stars, can besuccessfully investigated. In addition, we also analyzed the behaviourof supergiants (luminosity class I and II). On the basis of apparentnormal type objects, the correlation of the 3σ significance limitand the percentage of positive detection for all groups was derived. Wecompared the capability of the Δ a photometric system with theΔ (V1 - G) and Z indices of the Geneva 7-color system to detectpeculiar objects. Both photometric systems show the same efficiency forthe detection of CP and λ Bootis stars, while the indices in theGeneva system are even more efficient at detecting Be/shell objects. Onthe basis of this statistical analysis it is possible to derive theincidence of CP stars in galactic open cluster and extragalactic systemsincluding the former unknown bias of undetected objects. This isespecially important in order to make a sound statistical analysis ofthe correlation between the occurrence of these objects andastrophysical parameters such as the age, metallicity, and strength ofglobal, as well as local, magnetic fields.
| C, N, O in λ Boo stars and in composite spectra
The selective abundance of C, N, O (almost solar) with respect to thatof other elements (underabundant) in λ Boo stars has beeninterpreted as a characteristic peculiarity of these objects, whenconsidered as single stars. We show here that a similar selectiveabundance is predicted from the composite spectra resulting from twounresolved stars in the same temperature range as the λ Boostars.
| λ Bootis stars with composite spectra
We examine the large sample of λ Boo candidates collected inTable 1 of Gerbaldi et al. (\cite{Gerbaldi2003}) to see how many of themshow composite spectra. Of the 132 λ Boo candidates we identify22 which definitely show composite spectra and 15 more for which thereare good reasons to suspect a composite spectrum. The percentage ofλ Boo candidates with composite spectra is therefore >17% andpossibly considerably higher. For such stars the λ Booclassification should be reconsidered taking into account the fact thattheir spectra are composite. We argue that some of the underabundancesreported in the literature may simply be the result of the failure toconsider the composite nature of the spectra. This leads to thelegitimate suspicion that some, if not all, the λ Boo candidatesare not chemically peculiar at all. A thorough analysis of even a singleone of the λ Boo candidates with composite spectra, in which thecomposite nature of the spectrum is duly considered, which woulddemonstrate that the chemical peculiarities persist, would clear thedoubt we presently have that the stars with composite spectra may not beλ Boo stars at all.Based on observations collected at ESO (Echelec spectrograph) and at TBL(Telescope Bernard Lyot) of the Pic du Midi Observatory (France).
| 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
| The spectrum of the SB2 HD 64491 and the problem of metal abundances in lambda Boo stars
The spectrum of the classical lambda Boo star HD 64491 reveals thatthe object is a double-lined spectroscopic binary whose components havedifferent v sin i values. From such a tangled spectrum, we show thatlarge metal underabundances can be derived with the hypothesis that thespectrum is formed by a single object, while the abundance peculiaritiesare drastically reduced when the duplicity is taken into account.Based on observations collected at the Observatoire du Pic du Midi(France).
| 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.
| First orbital elements for the lambda Bootis spectroscopic binary systems HD 84948 and HD 171948. Implications for the origin of the lambda Bootis stars
We present high-resolution spectroscopic observations of the two lambdaBootis type spectroscopic binary systems HD 84948 and HD 171948. Bothsystems consist of two true lambda Bootis stars, as has already beenproven by a detailed abundance analysis taking into account the binarynature. Nevertheless, we have obtained non-LTE abundances for carbon andoxygen in order to investigate those important elements. The abundancesfit excellently into the typical pattern for this group, leaving nodoubt that all four components are true lambda Bootis stars. With thehelp of the derived orbital elements it is possible to establish theages of the two systems. For the first time we can estimate theevolutionary status of Galactic field lambda Bootis stars. The origin ofthose nonmagnetic, metal-weak, Population I, late B- to early F-typestars is still controversial. The two widely discussed theoriesexplaining the lambda Bootis phenomenon (diffusion together with eitheraccretion of circumstellar matter or mass-loss) predict significantlydifferent evolutionary stages for this group of objects (close to eitherthe Zero-Age Main Sequence or Terminal-Age Main Sequence). It is alreadyknown that very young members of the group exist in the Orion OBIassociation and probably in NGC 2264. Hipparcos data established sixfurther Galactic field lambda Bootis stars which are close to theZero-Age Main Sequence, whereas the evolutionary status for the otherobjects remained undetermined. The Hipparcos data reveal that theGalactic space motions of both the systems that we discuss here aretypical of those of Population I objects. The photometric data from theHipparcos mission confirm the pulsation previously discovered for HD84948 with a period of about 110 min and a V-amplitude of about 14 mmag.For HD 84948, we estimate from the mass ratio an age of about 1 Gyr,ruling out a possible Pre-Main-Sequence status. HD 171948 has an age ofabout 0.01 to 0.1 Gyr which is close to the Zero-Age Main Sequence. Wetherefore conclude that the lambda Bootis phenomenon can be foundcontinuously from very early stages to the Terminal-Age Main Sequence,suggesting that different mechanisms might work at different stages ofstellar evolution producing the same abundance pattern. Based onobservations at BNAO Rozhen; SAAO and with the Hipparcos satellite.
| On the Orbital Periods of Two Bona-fide lambda Bootis Stars HD 64491 and HD 141851
Not Available
| Spectra of binaries classified as lambda Bootis stars
High angular resolution observations have shown that some starsclassified as lambda Boo are binaries with low values of angularseparation and magnitude difference of the components; therefore theobserved spectrum of these objects is a combination of those twocomponents. These composite spectra have been used to definespectroscopic criteria able to detect other binaries among starsclassified as lambda Boo. The application of this method to HD 111786 ispresented: the contribution of 5 components to the observed spectrum isdemonstrated by the shape of the O I 7774 Å feature. This resultmakes unreliable any attempt to perform an abundance analysis of thisobject which therefore must be definitely rejected from the class of thepeculiar lambda Boo stars. This approach allowed us also to recognizethat the SB2 star HD 153808 is in reality a triple system. Based on datafrom ESO, Observatoire du Pic du Midi, IUE Final Archive and onobservations obtained with the Hipparcos satellite.
| A spectroscopic survey for lambda Bootis stars. III. Final results
In the third paper of a series dedicated to the spectroscopic survey fornew lambda Bootis stars, we present all new and confirmed members of thegroup as well as a detailed analysis of the observed sample. The natureof this small group of chemically peculiar stars of the upper mainsequence still challenges our understanding of processes like diffusion,mass-loss and accretion. The typical abundances pattern (nearly solarvalues for C, N, O and S whereas the Fe-peak elements are moderate tostrong underabundant) can still not be explained by any proposed theory.Hence, the significant increase of new members gives the opportunity toinvestigate the group properties in more detail. We report the discoveryof 26 new members of the group and the confirmation of 18 candidatesfrom the literature. This almost triples the number of known lambdaBootis stars. The existence of one member in the young open cluster NGC2264 and four members in the Orion OB1 association proves that thelambda Bootis phenomenon already works at very early stages of stellarevolution. Recent results from the Hipparcos mission have shown that thewell established lambda Bootis stars of the Galactic field comprise thewhole area from the Zero Age Main Sequence to the Terminal Age MainSequence (~ 109 yr for an A-type star). There is a continuoustransition between very young and rather evolved evolutionary stages. Wefind that the overall percentage of lambda Bootis type among all normaltype stars in the spectral range from B8 to F4 is 2% in the Galacticfield as well as in open clusters. Furthermore, 44 metal-weak objectsare listed which might be connected with the lambda Bootis phenomenon.Our biased sample (chosen by photometric boxes) is not distinguishedfrom all A-type stars in the corresponding spectral region by therotational velocity distribution. Only for the luminosity classes IV andIII (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature. 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).
| 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).
| A speckle interferometry survey of lambda Bootis stars
A search for duplicity of lambda Boo stars has been made by using thespeckle camera installed at the Telescopio Nazionale Galileo. Theoperation mode and the reduction procedure allow one to obtain not onlythe separation, but also the magnitude difference between thecomponents; the latter parameter is fundamental for determining thedegree of contamination from the secondary component of a binary systemand thus the importance of the veiling effect that produces absorptionlines weaker than normal. Two stars, HD 38545 and HD 290492, are closebinaries with values of the separation and of the magnitude differencesuch that only a composite spectrum can be observed. For another 15lambda Boo candidates, observed with negative results, the upper limitsof a possible companion separation are given. Based on observations madewith the Italian Telescopio Nazionale Galileo (TNG) operated on theisland of La Palma by the Centro Galileo Galilei of the CNAA (ConsorzioNazionale per l'Astronomia e l'Astrofisica) at the Spanish Observatoriodel Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| 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
| A-shell stars in the Geneva system
Among the various kinds of A stars having a peculiar spectrum, we findthe A-shell stars. Many questions are still open concerning these stars,including their evolutionary status. In the present study we have useddata from the Hipparcos catalogue to examine this point. We have foundthat the majority of A-shell stars are well above the main sequence. Nodifferences could be established between A-shell stars in luminosityclasses III and I and those in luminosity class V as regardsvariability, duplicity, or the importance of the shell feature.
| 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.
| Astrophysics in 1998
From Alpha (Orionis and the parameter in mixing-length theory) to Omega(Centauri and the density of the universe), the Greeks had a letter forit. In between, we look at the Sun and planets, some very distantgalaxies and nearby stars, neutrinos, gamma rays, and some of theanomalies that arise in a very large universe being studied by roughlyone astronomer per 10^7 Galactic stars.
| 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
| Atmospheric parameters and abundances of lambda Bootis stars
The results of the abundance analyses of six lambda Bootis stars arepresented. For three stars, the impact of individual ODFs and varioustreatments of convection in the calculations of the model atmospheresare investigated.
| On the new lambda Bootis-type spectroscopic binary systems HD 84948 and HD 171948
We report the discovery of two new lambda Bootis-type spectroscopicbinary systems (HD 84948 and HD 171948). High resolution spectra wereanalyzed in order to derive abundances as well as astrophysicalquantities. It resulted in extreme (HD 171948) and moderate (HD 84948)underabundances of Na, Mg, Si, Ca, Sc, Ti, Cr and Fe with respect to theSun, thus establishing all four individual stars as true members of thelambda Bootis group. These two systems are therefore the firstspectroscopic binary systems where both components are lambdaBootis-type stars. Furthermore, we have discovered delta Scuti typepulsation for one component of HD 84948 making this star particularlyinteresting for further observations. Since many authors have used therotational velocity for discriminating lambda Bootis stars(vsini>50kms(-1) ), we note that both components of HD 171948 are theslowest (vsini<20kms(-1) ) rotating lambda Bootis-type stars known sofar. Using the Hipparcos data, we are able to rule out thediffusion/mass-loss mechanism as origin for these two systems. Pre- aswell as Main Sequence evolutionary tracks for the age determinationyield consistent results with the predictions of the diffusion/accretiontheory, establishing both systems as being very close to the MainSequence. A determination of the orbital elements and/or anasteroseismological investigation of HD 84948 could provide anindependent proof of our results. Based on observations obtained at theOsservatorio Astronomico di Padua-Asiago, McDonald Observatory, theSierra Nevada Observatory and with the Hipparcos satellite
| A spectroscopic survey for lambda Bootis stars. I. Strategy, techniques and first results
In recent years, the chemically peculiar (CP) stars of the upper mainsequence have become a fruitful field for the testing of astrophysicaltheories. Processes such as diffusion, convection and mass loss havebeen developed theoretically and introduced into models. The group oflambda Bootis stars, however, is remarkable among the chemicallypeculiar stars as they are nonmagnetic, Population I, A to F-type dwarfswhich show significant underabundances of metals (except for C, N, O andS). Unfortunately, the small number of confirmed members of the lambdaBootis class makes a sound statistical analysis of their propertiesimpossible. Thus, it is still difficult to decide between the twotheories - mass loss with diffusion and the accretion theory - whichhave been proposed to explain the origin of these stars. We thereforehave started a spectroscopic survey to find new lambda Bootis stars inthe field as well as in open clusters and associations. The presence oflambda Bootis stars in open clusters and associations would permit adetermination of the ages of these stars, and thus would yield animportant test for distinguishing between the two theories. In thispaper we describe the selection of candidates using photometriccriteria, the basic requirements, spectroscopic follow-up observationsand results from the first three observing runs. Special care was takento avoid misclassification of our programme stars (e.g. lambda Bootisstars are often confused with intermediate Population II, He-weak orhigh v sin i stars), using a refined MK system. The discovery of atleast six new lambda Bootis stars (including three in the Orion OB1association) shows the efficacy of our selection criteria. Based onobservations obtained at the Observatoire de Haute-Provence and the DarkSky 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.
| An atlas of the infrared spectral region. I. The early type stars (O-G0).
The Atlas illustrates the behavior of early type stars (O, B, A and Ftype) in the near infrared 8375-8770A region at a resolution of aboutone A. Intensity tracings of 76 stars are presented. Of these 51 starscover the spectral range O to G0 and luminosity classes V, III, Ib andIa. The influence of the rotational velocity is also illustrated as wellas the spectra of 19 stars with spectral peculiarities. The completeAtlas is also available under catalog number 3183 from the CDSStrasbourg and other data centers.
| The behavior of the λ Bootis stars in the infrared.
We observed a sample of 20 λ Bootis (LB) stars in threewavelength regions in the near infrared between 7600 and 10800A. We findthat the Paschen lines correspond systematically to a spectral typewhich is later than the one attributed in the blue. By comparison with aconvenient set of MK standard stars we find that the O, Ca, C, S and Silines are weak in LB stars in varying degrees. An analysis of the 16 LBstars which we confirmed in the infrared shows that seven exhibitevidences of classical A-type shells in the spectroscopic sense. Ofthese seven, five are certain and two are dubious, giving thus apercentage of at least 30% of shells in LB stars.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Ursa Major |
| Ascensió Recta: | 09h50m05.88s |
| Declinació: | +49°04'31.8" |
| Magnitud Aparent: | 8.125 |
| Distancia: | 201.207 parsecs |
| Moviment propi RA: | -4 |
| Moviment propi Dec: | -4.6 |
| B-T magnitude: | 8.478 |
| V-T magnitude: | 8.155 |
Catàlegs i designacions:
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