45 research outputs found
Abundance analysis of Am binaries and search for tidally driven abundance anomalies - III. HD116657, HD138213, HD155375, HD159560, HD196544 and HD204188
We continue here the systematic abundance analysis of a sample of Am binaries
in order to search for possible abundance anomalies driven by tidal interaction
in these binary systems. New CCD observations in two spectral regions
(6400-6500, 6660-6760 AA) of HD116657, HD138213, HD155375, HD159560, HD196544
and HD204188 were obtained. Synthetic spectrum analysis was carried out and
basic stellar properties, effective temperatures, gravities, projected
rotational velocities, masses, ages and abundances of several elements were
determined. We conclude that all six stars are Am stars. These stars were put
into the context of other Am binaries with 10 < Porb < 200 days and their
abundance anomalies discussed in the context of possible tidal effects. There
is clear anti-correlation of the Am peculiarities with v sin i. However, there
seems to be also a correlation with the eccentricity and may be with the
orbital period. The dependence on the temperature, age, mass, and
microturbulence was studied as well. The projected rotational velocities
obtained by us were compared to those of Royer et al. (2002) and Abt & Morrell
(1995).Comment: 11 pages, 3 tables, 12 figures. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Scandium: A key element for understanding Am stars
{\rm Context.} Atomic diffusion is believed to cause the abundance anomalies
observed in AmFm stars. However, the detailed process has still not been
well-established. For instance, two possible scenarios for the diffusion theory
are presently envisaged. They differ mainly by the depth from which the
abundance anomalies emanate. The first scenario predicts that the abundances
are modified in the superficial regions of the star, just below the hydrogen
convection zone. The second scenario predicts that a much deeper extension of
the mixing zone exists due to the convection caused by Fe accumulation in
regions below the hydrogen convection zone. {\rm Aims.} We calculate much more
accurate radiative accelerations of Sc than previously, to better understand
the observed abundance anomalies of this element. We believe that it is a key
element to use as a diagnostic tool for understanding AmFm stars.
{\rmMethods.} The method employed to obtain these radiative accelerations is
based on an interpolation from the parameters of the so-called SVP parametric
method.
{\rm Results.} The radiative accelerations, shown here in a typical Am
stellar model, are discussed in light of the observed anomalies of Ca and Sc.
Our results suggest that the deeper mixing scenario is not entirely
satisfactory: the mixing zone should be deeper than what is predicted by recent
models to account for observed Sc underabundances. Our results seem more
compatible with the scenario where the abundances anomalies are created in the
superficial regions. However, only detailed evolutionary modelling with mass
loss and diffusion of all important species, including Ca and Sc, with accurate
radiative accelerations, will be able to give more insight into where the
source of these anomalies occur in AmFm stars.Comment: 6 pages, 3 figures, accepted for publication in A&
Atmospheric velocity fields in tepid main sequence stars
The line profiles of the stars with v sin i below a few km/s can reveal
direct signatures of local velocity fields (e.g. convection) in stellar
atmospheres. This effect is well established in cool main sequence stars, and
has been detected and studied in three A stars. This paper reports observations
of main sequence B, A and F stars with two goals: (1) to identify additional
stars having sufficiently low values of v sin i to search for spectral line
profile signatures of local velocity fields, and (2) to explore how the
signatures of the local velocity fields in the atmosphere depend on stellar
parameters such as effective temperature T_eff and peculiarity type.
For stars having T_eff below about 10000 K, we always detect local
atmospheric velocity fields indirectly through a non-zero microturbulence
parameter, but not for hotter stars. Among the A and F stars in our sample
having the sharpest lines, direct tracers of atmospheric velocity fields are
found in six new stars. The velocity field signatures identified include
asymmetric excess line wing absorption, deeper in the blue line wing than in
the red; line profiles of strong lines that are poorly fit by computed
profiles; and strong lines that are broader than they should be for the v sin i
values deduced from weak lines. These effects are found in both normal and Am
stars, but seem stronger in Am stars.
These data still have not been satisfactorily explained by models of
atmospheric convection, including numerical simulations.Comment: Acepted for publication by Astronomy and Astrophysic
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
Aims. We aim to provide observational constraints on diffusion models that
predict peculiar chemical abundances in the atmospheres of Am stars. We also
intend to check if chemical peculiarities and slow rotation can be explained by
the presence of a weak magnetic field.
Methods. We have obtained high resolution, high signal-to-noise ratio spectra
of eight previously-classified Am stars, two normal A-type stars and one Blue
Straggler, considered to be members of the Praesepe cluster. For all of these
stars we have determined fundamental parameters and photospheric abundances for
a large number of chemical elements, with a higher precision than was ever
obtained before for this cluster. For seven of these stars we also obtained
spectra in circular polarization and applied the LSD technique to constrain the
longitudinal magnetic field.
Results. No magnetic field was detected in any of the analysed stars. HD
73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to
have the abundances of a normal A-type star. Am classification is not confirmed
for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry
that is in good agreement with the observations. There is a generally good
agreement between abundance predictions of diffusion models and values that we
have obtained for the remaining Am stars. However, the observed Na and S
abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.
Li appears to be overabundant in three stars of our sample.Comment: Accepted for publication on A&
Chemical composition of A and F dwarfs members of the Pleiades open cluster
Abundances of 18 chemical elements have been derived for 16 A (normal and
chemically peculiar CP) and 5 F dwarfs members of the Pleiades open cluster in
order to set constraints on evolutionary models. The abundances, rotational
velocities and microturbulent velocities were derived by iteratively adjusting
synthetic spectra to observations at high resolution (R~42000 and R~75000) and
high signal-to-noise (S/N) ratios. The abundances obtained do not exhibit any
clear correlation with the effective temperature nor the projected rotational
velocity. Interestingly, A stars exhibit larger star-to-star variations in C,
Sc, Ti, V, Cr, Mn, Sr, Y, Zr and Ba than F stars. F stars exhibit solar
abundances for almost all the elements. In A stars, the abundances of Si, Ti
and Cr are found to be correlated with that of Fe, the [X/Fe] ratios being
solar for these three elements. The derived abundances have been compared to
the predictions of published evolutionary models at the age of Pleiades (100
Myr). For the F stars, the predicted slight underabundances of light elements
and overabundances of Cr, Fe and Ni are indeed confirmed by our findings. For A
stars, the predicted overabundances in iron peak elements are confirmed in a
few stars only. The large scatter of the abundances in A stars, already found
in the Hyades, Coma Berenices and the UMa group and in field stars appears to
be a characteristic property of dwarf A stars. The occurence of hydrodynamical
processes competing with radiative diffusion in the radiative zones of the A
dwarfs might account for the found scatter in abundances.Comment: 7 pages, 3 figures, accepted in A&
AmFm and lithium gap stars: Stellar evolution models with mass loss
A thorough study of the effects of mass loss on internal and surface
abundances of A and F stars is carried out in order to constrain mass loss
rates for these stars, as well as further elucidate some of the processes which
compete with atomic diffusion. Self-consistent stellar evolution models of 1.3
to 2.5 M_sun stars including atomic diffusion and radiative accelerations for
all species within the OPAL opacity database were computed with mass loss and
compared to observations as well as previous calculations with turbulent
mixing. Models with unseparated mass loss rates between 5 x 10^-14 and 10^-13
M_sun/yr reproduce observations for many cluster AmFm stars as well as Sirius A
and o Leonis. These models also explain cool Fm stars, but not the Hyades
lithium gap. Like turbulent mixing, these mass loss rates reduce surface
abundance anomalies; however, their effects are very different with respect to
internal abundances. For most of the main sequence lifetime of an A or F star,
surface abundances in the presence of such mass loss depend on separation which
takes place between log(Delta M/M_star)= -6 and -5. The current observational
constraints do not allow us to conclude that mass loss is to be preferred over
turbulent mixing (induced by rotation or otherwise) in order to explain the
AmFm phenomenon. Internal concentration variations which could be detectable
through asteroseismic tests should provide further information. If atomic
diffusion coupled with mass loss are to explain the Hyades Li gap, the wind
would need to be separated.Comment: 27 pages, 25 figures, accepted for publication in A&
Abundances and search for vertical stratification in the atmospheres of four HgMn stars
Using high resolution, high-S/N archival UVES spectra, we have performed a
detailed spectroscopic analysis of 4 chemically peculiar HgMn stars (HD 71066,
HD 175640, HD 178065 and HD 221507). Using spectrum synthesis, mean
photospheric chemical abundances are derived for 22 ions of 16 elements. We
find good agreement between our derived abundances and those published
previously by other authors. For the 5 elements that present a sufficient
number of suitable lines, we have attempted to detect vertical chemical
stratification by analyzing the dependence of derived abundance as a function
of optical depth. For most elements and most stars we find no evidence of
chemical stratification with typical 3\sigma upper limits of \Delta\log
N_elem/N_tot~0.1-0.2 dex per unit optical depth. However, for Mn in the
atmosphere of HD 178065 we find convincing evidence of stratification. Modeling
of the line profiles using a two-step model for the abundance of Mn yields a
local abundance varying approximately linearly by ~0.7 dex through the optical
depth range log \tau_5000=-3.6 to -2.8.Comment: 11 figures, 9 tables, table 6-9 (online material), accepted by MNRA
No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars
We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by
carrying out a deep spectropolarimetric study of Am and HgMn stars. Using the
NARVAL spectropolarimeter at the Telescope Bernard Lyot (Observatoire du Pic du
Midi, France), we obtained high-resolution circular polarisation spectroscopy
of 12 Am stars and 3 HgMn stars. Using Least Squares Deconvolution (LSD), no
magnetic field is detected in any of the 15 observed stars. Uncertaintiies as
low as 0.3 G (respectively 1 G) have been reached for surface-averaged
longitudinal magnetic field measurements for Am (respectively HgMn) stars.
Associated with the results obtained previously for Ap/Bp stars, our study
confirms the existence of a magnetic dichotomy among A-type stars. Our data
demonstrate that there is at least one order of magnitude difference in field
strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected
stars (Am and HgMn stars). This result confirms that the
spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring
detectable large-scale surface magnetic fields.Comment: 6 pages, 3 figures, accepted for publication in A&