Rotational velocities of A-type stars Royer, F.; Gerbaldi, M.; Faraggiana, R. ...
Astronomy and astrophysics (Berlin),
01/2002, Letnik:
381, Številka:
1
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Within the scope of a Key Programme determining fundamental parameters of stars observed by HIPPARCOS, spectra of 525 B8 to F2-type stars brighter than V=8 have been collected at ESO. Fourier ...transforms of several line profiles in the range 4200-4500 Å are used to derive v sin i from the frequency of the first zero. Statistical analysis of the sample indicates that measurement error is a function of v sin i and this relative error of the rotational velocity is found to be about 6% on average. The results obtained are compared with data from the literature. There is a systematic shift from standard values from \citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisons with other independent v sin i values tend to prove that those from Slettebak et al. are underestimated. This effect is attributed to the presence of binaries in the standard sample of Slettebak et al., and to the model atmosphere they used. Based on observations made at the European Southern Observatory (ESO), La Silla, Chile, in the framework of the Key Programme 5-004-43K. Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or via cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105
UV flux distributions of γ Doradus stars Gerbaldi, M.; Faraggiana, R.; Caffau, E.
Astronomy and astrophysics (Berlin),
09/2007, Letnik:
472, Številka:
1
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Context.It seems that the recently identified class of pulsating stars, the γ Dor type-variables, includes objects with different metal abundances and a large percentage of binaries. Aims.We looked ...for indicators of metal abundance peculiarities and stellar binarity in a sample of 40 confirmed γ Dor stars. Methods.Absolute magnitudes from Hipparcos parallaxes and UV magnitudes, from the S2/S68 experiment on board the TD1 satellite, are retrieved from databases and compared with predicted values. A set of non variable normal stars is used to check the consistency of this analysis and also serve as reference stars. Results.Twenty-nine stars of the γ Dor star sample, which is 73% of it, are discovered having abnormal UV fluxes constantly showing UV flux excesses compared to those computed with the atmospheric parameters (Teff , log g, and metallicity) determined from calibration of the $uvby{\beta}$ indices. The reason for this UV excess of flux at 196.5 nm and at 236.5 nm, which was previously known only for HD 209295, cannot be ascribed to binarity alone. An extra source of UV flux or less UV absorption – yet unknown – must be present.
The heterogeneous class of λ Bootis stars Gerbaldi, M.; Faraggiana, R.; Lai, O.
Astronomy and astrophysics (Berlin),
12/2003, Letnik:
412, Številka:
2
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We demonstrate that it is arduous to define the λ Boo stars as a class of objects exhibiting uniform abundance peculiarities which would be generated by a mechanism altering the structure of their ...atmospheric layers. We collected the stars classified as λ Boo up to now and discuss their properties, in particular the important percentage of confirmed binaries producing composite spectra (including our adaptive optics observations) and of misclassified objects. The unexplained RV variables (and thus suspected binaries), the known SB for which we lack information on the companion, the stars with an UV flux inconsistent with their classification, and the fast rotating stars for which no accurate abundance analysis can be performed, are also reviewed.
λ Bootis stars with composite spectra FARAGGIANA, R; BONIFACIO, P; CAFFAU, E ...
Astronomy and astrophysics (Berlin),
10/2004, Letnik:
425, Številka:
2
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We examine the large sample of lambda Boo candidates collected of Gerbaldi et al. (2003) to see how many of them show composite spectra. Of the 132 lambda Boo candidates we identify 22 which ...definitely show composite spectra and 15 more for which there are good reasons to suspect a composite spectrum. The percentage of lambda Boo candidates with composite spectra is therefore >17% and possibly considerably higher. For such stars the lambda Boo classification should be reconsidered taking into account the fact that their spectra are composite. We argue that some of the underabundances reported in the literature may simply be the result of the failure to consider the composite nature of the spectra. This leads to the legitimate suspicion that some, if not all, the lambda Boo candidates are not chemically peculiar at all. A thorough analysis of even a single one of the lambda Boo candidates with composite spectra, in which the composite nature of the spectrum is duly considered, which would demonstrate that the chemical peculiarities persist, would clear the doubt we presently have that the stars with composite spectra may not be lambda Boo stars at all.
The identification of post-T Tauri (pTT) stars selected through spectroscopic criteria by Pallavicini et al. (CITE) and by Martín et al. (1992) among the candidates belonging to visual binary systems ...is revisited in the present paper by studying their position in the HR diagram. These stars belong to the so-called Lindroos binary sample (Lindroos CITE), i.e. to systems with early-type primaries and late-type companions. If these binaries are physical and not simply optical pairs, similar ages must be found for the early-type primary and the late-type component of each system. The ages of these systems have been derived by Lindroos in 1986, by using calibrations of the uvby ${\beta} $ indices. In this paper, we revisit these ages through the position of these stars among new evolutionary tracks in the HR diagram for pre-and post-main sequence stars. We derive new estimations of the ages of each system component, as well as their masses, using parallaxes of the early-type component derived from Hipparcos data and by forcing the late-type companion to be at the same distance. $T\rm_{eff}$ and $\log g$ of the early-type components have been computed using the calibrations of two independent photometric systems: the uvby ${\beta}$ photometry and the Geneva system. The $T\rm_{eff}$ of the late-type stars have been determined by using various calibrations of several photometric systems: uvby ${\beta}$, UBV and VRI, in order to determine the uncertainties and systematic errors on these parameters and consequently on the ages. Differences in the ages and masses obtained by using various sources of recent evolutionary models are considered and discussed. The consistency of the age of the late type component with that of its early type primary is examined; the validity of this criterion for a selection of physical pairs is discussed. The accuracy of the observational and theoretical data are not sufficient to assign stringent values to the age for several of the examined systems. Nevertheless, in spite of the large error bars, we have established that we could select a number of systems which, according to their position in the HR diagram, may be physically associated. The selection of possible physically bounded systems obtained with the present approach and that made by Pallavicini et al. (CITE) or Martín et al. (1992) on the basis of spectroscopic criteria are not always coincident. Spectroscopic criteria, for example the presence of a strong Li feature, are more stringent conditions than that of coherent ages of primaries and secondaries; however the Li I 6708 doublet is expected to fade in the latest stages of the pre-main sequence life of a star, so that the "oldest" pTTs may not be detected by spectroscopy only. The different results so obtained are discussed for each system and we conclude that the present approach may be used as a powerful criterion to select new pTT candidates in visual binaries to be observed and analyzed with high resolution spectrographs and to select candidates that have almost reached the main sequence.
With observations at the Canada-France-Hawaii telescope, photospheric abundances of two evolved field Am stars, τ UMa and HR 178, have been determined for Li, Al, Si, S, Fe, Ni, and Eu by model ...abundance analysis. Thanks to these new determined abundances and the HIPPARCOS mission, making it possible to better estimate the evolutionary stage of field stars, one can consider the influence of evolution on the abundances of Am stars. No abundance trend for Al, Si, S, and Fe is found during Main Sequence evolution, up to its very last phases. The abundance of Li is, generally, cosmic. Large Li deficiencies may be observed in the very last phases (the “hook” region of the H-R diagram) and, too, on the red side of the Am phenomenon region where the ρ Puppis-like stars lie.
High angular resolution observations have shown that some stars classified as λ Boo are binaries with low values of angular separation and magnitude difference of the components; therefore the ...observed spectrum of these objects is a combination of those two components. These composite spectra have been used to define spectroscopic criteria able to detect other binaries among stars classified as λ Boo. The application of this method to HD 111786 is presented: the contribution of 5 components to the observed spectrum is demonstrated by the shape of the O I 7774 Å feature. This result makes unreliable any attempt to perform an abundance analysis of this object which therefore must be definitely rejected from the class of the peculiar λ Boo stars. This approach allowed us also to recognize that the SB2 star HD 153808 is in reality a triple system.
We demonstrate that HD 174005, a star recently classified as belonging to the λ Boo group, is in reality a double lined spectroscopic binary; at some phases, the observed composite spectrum may be ...similar to that of a single star with weak metal lines.
The spectrum of the classical λ Boo star HD 64491 reveals that the object is a double-lined spectroscopic binary whose components have different $v\,\sin\,i$ values. From such a tangled spectrum, we ...show that large metal underabundances can be derived with the hypothesis that the spectrum is formed by a single object, while the abundance peculiarities are drastically reduced when the duplicity is taken into account.
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