111 research outputs found
HD 174005: another binary classified as lambda Boo
We demonstrate that HD 174005, a star recently classified as belonging to the
lambda 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.Comment: Accepted by A&
Lambda Boo stars with composite spectra
We examine the large sample of lambda Boo candidates collected in Table 1 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 at all.Comment: Accepted for publication by A&A on June 3rd 200
Spectra of binaries classified as lambda Boo stars
High angular resolution observations have shown that some stars classified as
lambda 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 of the two components. These composite spectra have
been used to define spectroscopic criteria able to detect other binaries among
stars classified as lambda 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 Angstrom 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 lambda Boo
stars. This approach allowed us also to recognize that the SB2 star HD 153808
is in reality a triple system.Comment: Accepted for publication by A&
Chemical composition of a sample of bright solar-metallicity stars
We present a detailed analysis of seven young stars observed with the
spectrograph SOPHIE at the Observatoire de Haute-Provence for which the
chemical composition was incomplete or absent in the literature. For five
stars, we derived the stellar parameters and chemical compositions using our
automatic pipeline optimized for F, G, and K stars, while for the other two
stars with high rotational velocity, we derived the stellar parameters by using
other information (parallax), and performed a line-by-line analysis.
Chromospheric emission-line fluxes from CaII are obtained for all targets. The
stellar parameters we derive are generally in good agreement with what is
available in the literature. We provide a chemical analysis of two of the stars
for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm
implying a high lithium abundance. Its chemical pattern is not consistent with
it being a solar sibling, as has been suggested.Comment: To be published on A
Pulsation of the Lambda Bootis star HD 210111
CONTEXT. The Lambda Bootis stars are a small spectroscopic subgroup of
Population I A-type stars and show significant underabundances of metals. Many
are Delta Scuti pulsators.
AIMS. HD 210111 was selected for a detailed multisite pulsation study to
determine whether its pulsation properties differ from those of normal A stars.
METHODS. 262 hours of high-precision photometry were obtained at the SAAO and
SSO observatories.
RESULTS. 13 statistically significant pulsation frequencies were detected
with very small photometric amplitudes from 1 to 7 millimag in the visual. A
comparison with earlier 1994 measurements indicates a small increase in
amplitude. As a byproduct, one of the comparison stars, HD 210571, was
discovered to be a millimag variable with a frequency of 1.235 c/d and is
probably a new Gamma Doradus variable. The observed wide range of excited
frequencies from 12 to 30 c/d in HD 210111 can be explained with both the
single- and double-star hypothesis. HD 210111 is in a similar evolutionary
status to FG Vir, which also shows a wide range of excited frequencies with a
similar frequency spacing near 4 c/d. This is interpreted as successive radial
orders of the excited nonradial modes. In the double-star hypothesis previously
evoked for HD 210111, the low and the high frequencies originate in different
stars: here HD 210111 would resemble Theta^2 Tau.
CONCLUSIONS. The pulsation of the Lambda Bootis star HD 210111 does not
differ from that of normal Delta Scuti stars.Comment: 6 pages, 2 figures, 2 tables, submitted to A&
The Galactic evolution of phosphorus
As a galaxy evolves, its chemical composition changes and the abundance
ratios of different elements are powerful probes of the underlying evolutionary
processes. Phosphorous is an element whose evolution has remained quite elusive
until now, because it is difficult to detect in cool stars. The infrared weak P
I lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators
of the presence of phosphorus. The availability of CRIRES at VLT has permitted
access to this wavelength range in stellar spectra.We attempt to measure the
phosphorus abundance of twenty cool stars in the Galactic disk. The spectra are
analysed with one-dimensional model-atmospheres computed in Local Thermodynamic
Equilibrium (LTE). The line formation computations are performed assuming LTE.
The ratio of phosphorus to iron behaves similarly to sulphur, increasing
towards lower metallicity stars. Its ratio with respect to sulphur is roughly
constant and slightly larger than solar, [P/S]=0.10+- 0.10. We succeed in
taking an important step towards the understanding of the chemical evolution of
phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance
ratio is steeper than predicted by Galactic chemical evolution model model
developed by Kobayashi and collaborators. Phosphorus appears to evolve
differently from the light odd-Z elements sodium and aluminium. The constant
value of [P/S] with metallicity implies that P production is insensitive to the
neutron excess, thus processes other than neutron captures operate. We suggest
that proton captures on 30Si and alpha captures on $27Al are possibilities to
investigate. We see no clear distinction between our results for stars with
planets and stars without any detected planet.Comment: To be published on A&
Echelle long-slit optical spectroscopy of evolved stars
We present echelle long-slit optical spectra of a sample of objects evolving
off the AGB, most of them in the pre-planetary nebula (pPN) phase, obtained
with the ESI and MIKE spectrographs at Keck-II and Magellan-I, respectively.
The total wavelength range covered with ESI (MIKE) is ~3900 to 10900 A (~3600
to 7200A). In this paper, we focus our analysis mainly on the Halpha profiles.
Prominent Halpha emission is detected in half of the objects, most of which
show broad Halpha wings (up to ~4000 km/s). In the majority of the
Halpha-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles.
In ~37% of the objects Halpha is observed in absorption. In almost all cases,
the absorption profile is partially filled with emission, leading to complex,
structured profiles that are interpreted as an indication of incipient post-AGB
mass-loss. All sources in which Halpha is seen mainly in absorption have F-G
type central stars, whereas sources with intense Halpha emission span a larger
range of spectral types from O to G. Shocks may be an important excitation
agent of the close stellar surroundings for objects with late type central
stars. Sources with pure emission or P Cygni Halpha profiles have larger J-K
color excess than objects with Halpha mainly in absorption, which suggests the
presence of warm dust near the star in the former. The two classes of profile
sources also segregate in the IRAS color-color diagram in a way that intense
Halpha-emitters have dust grains with a larger range of temperatures.
(abridged)Comment: 68 pages, 14 figures, accepted for publication in ApJS (abstract
abridged
Rotational velocities of A-type stars I. Measurement of vsini in the southern hemisphere
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 A are used to derive vsini from the frequency of the first
zero. Statistical analysis of the sample indicates that measurement error is a
function of vsini 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 Slettebak et
al. (1975), which are 10 to 12% lower than our findings. Comparisons with other
independent vsini 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.Comment: 17 pages, includes 18 figures, accepted in A&
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