15 research outputs found
The light variability of the helium strong star HD 37776 as a result of its inhomogeneous elemental surface distribution
We simulate light curves of the helium strong chemically peculiar star HD
37776 assuming that the observed periodic light variations originate as a
result of inhomogeneous horizontal distribution of chemical elements on the
surface of a rotating star. We show that chemical peculiarity influences the
monochromatic radiative flux, mainly due to bound-free processes. Using the
model of the distribution of silicon and helium on HD 37776 surface, derived
from spectroscopy, we calculate a photometric map of the surface and
consequently the uvby light curves of this star. Basically, the predicted light
curves agree in shape and amplitude with the observed ones. We conclude that
the basic properties of variability of this helium strong chemically peculiar
star can be understood in terms of the model of spots with peculiar chemical
composition.Comment: 11 pages, accepted for the publication in Astronomy & Astrophysic
Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis
CU Vir and V901 Ori belong among these few magnetic chemically peculiar stars
whose rotation periods vary on timescales of decades. We aim to study the
stability of the periods in CU Vir and V901 Ori using all accessible
observational data containing phase information. We collected all available
relevant archived observations supplemented with our new measurements of these
stars and analysed the period variations of the stars using a novel method that
allows for the combination of data of diverse sorts. We found that the shapes
of their phase curves were constant, while the periods were changing. Both
stars exhibit alternating intervals of rotational braking and acceleration. The
rotation period of CU Vir was gradually shortening until the year 1968, when it
reached its local minimum of 0.52067198 d. The period then started increasing,
reaching its local maximum of 0.5207163 d in the year 2005. Since that time the
rotation has begun to accelerate again. We also found much smaller period
changes in CU Vir on a timescale of several years. The rotation period of V901
Ori was increasing for the past quarter-century, reaching a maximum of 1.538771
d in the year 2003, when the rotation period began to decrease. A theoretically
unexpected alternating variability of rotation periods in these stars would
remove the spin-down time paradox and brings a new insight into structure and
evolution of magnetic upper-main-sequence stars.Comment: 5 pages, 3 figure
Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Vir
The spectral energy distribution (SED) in chemically peculiar stars may be
significantly affected by their abundance anomalies. The observed SED
variations are usually assumed to be a result of inhomogeneous surface
distribution of chemical elements, flux redistribution and stellar rotation.
However, the direct evidence for this is still only scarce. We aim to identify
the processes that determine the SED and its variability in the UV and visual
spectral domains of the helium-weak star CU Vir. We used the model atmospheres
to obtain the emergent flux and predict the rotationally modulated flux
variability of the star. We show that most of the light variations in the vby
filters of the Stromgren photometric system are a result of the uneven surface
distribution of silicon, chromium, and iron. Our models are only able to
explain a part of the variability in the u filter, however. The observed UV
flux distribution is very well reproduced, and the models are able to explain
most of the observed features in the UV light curve. The variability observed
in the visible is merely a faint gleam of that in the UV. While the amplitude
of the light curves reaches only several hundredths of magnitude in the visual
domain, it reaches about 1 mag in the UV. The visual and UV light variability
of CU Vir is caused by the flux redistribution from the far UV to near UV and
visible regions, inhomogeneous distribution of the elements and stellar
rotation. Bound-free transitions of silicon and bound-bound transitions of iron
and chromium contribute the most to the flux redistribution. This mechanism can
explain most of the rotationally modulated light variations in the filters
centred on the Paschen continuum and on the UV continuum of the star CU Vir.
However, another mechanism(s) has to be invoked to fully explain the observed
light variations in the u filter and in the region 2000-2500 A.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Structure of the magnetic field in the Ap star HD 187474
We reconstruct the complex magnetic field in the Ap star
HD 187474 within the frame of the point field source model,
where virtual magnetic charges are distributed in the stellar body.
The best-fit model describes sufficiently
well the observed nonsinusoidal variability of the mean magnetic field
modulus and the sinusoidal behaviour of the mean longitudinal magnetic
field with the phase of stellar rotation.
The best fit provides discrepancy on the
level of for all the analyzed data.
We show that in HD 187474 the magnetic dipole is displaced
from centre of the star by 0.055 . The dipole
has a size ~0.035 .
The angle between the stellar rotational axis and the magnetic dipole
is \beta=37\degr
Lithium and metal abundances in long period Am binaries
High signal-to-noise CCD spectra of three bright, long period Am binaries
(HD 108651, HD 116657 and HD 138213) have been obtained
and the atmospheric abundances of identified chemical elements were studied.
We found HD 116657 to be a new candidate as a
Li deficient, long period Am binary, in addition to 16 Ori. While HD 108651 and
HD 116657 have pronounced Am characteristics, HD 138213 is just a mild
Am star.
These stars were further studied in a more general context related to
the role of
binarity in CP phenomenon. It was found that both Li abundance and the
equivalent with ratio Ca 6718/Fe 6679 are more peculiar in
systems with more pronounced eccentricities. Such behaviour is the opposite
of what was previously found in Ap binaries and their anomalies. It
indicates that tidal effects are of crucial importance in driving CP phenomena.
Properties and nature of Be stars
Photometric and spectroscopic monitoring of the B star resulted in
the finding that this object is a new bright Be star with a clear positive
correlation between the brightness and emission-line strength.
The emission-line episodes are relatively short and seem to repeat
frequently which makes this star an ideal target for studying
the causes of the Be phenomenon.
The general character of the light variations, the low v sin i = 70 km s-1 and
the very pronounced line asymmetries of the \ion{He}{i} 6678 line, seen both outside
and during emission-line episodes, are all attributes which make
phenomenologically very similar to the well-known Be star ω CMa.
Radial velocities of the deepest parts of the metallic and \ion{He}{i} 6678
absorption lines vary with a strict period of 261507 over the whole
time interval covered by the observations, the velocities of the broad
outer wings of the same lines varying in anti-phase and with a lower
amplitude. This periodicity could not be found in the radial-velocity
variations of the sharp core of Hα. There is some indication of
variability on a time scale of 24–29 days but our data are insufficient
to prove that conclusively.
A comparison of the line spectrum obtained outside emission episodes
with synthetic spectra, standard dereddening of magnitudes
and Hipparcos parallax all agree with the conclusion that is a star with
the following basic properties:
Teff = 17 000 K, log [cgs], mass of 5 and radius of 11 .
The strong emission-line episodes may appear regularly,
in a cycle of 630 days but with different durations of individual
cycles. is probably one of the first B stars for which the Be nature
was predicted on the basis of the character of its light and colour changes