156 research outputs found
Theoretical analysis of the atmospheres of CP stars. Effects of the individual abundance patterns
Context. See abstract in the paper.
Aims. See abstract in the paper.
Methods. See abstract in the paper.
Results. We present a homogeneous study of model atmosphere temperature
structure, energy distribution, photometric indices in the uvbybeta and Delta_a
systems, hydrogen line profiles, and the abundance determination procedure as
it applies to CP stars. In particular, we found that Si, Cr and Fe are the main
elements to influence model atmospheres of CP stars, and thus to be considered
in order to assess the adequacy of model atmospheres with scaled solar
abundances in application to CP stars. We provide a theoretical explanation of
the robust property of the Delta_a photometric system to recognize CP stars
with peculiar Fe content. Also, the results of our numerical tests using model
atmospheres with one or several elements overabundant (Si and Fe by +1 dex, Cr
by +2 dex) suggest that the uncertainty of abundance analysis in the
atmospheres of CP stars using models with scaled abundances is less than
plus/minus 0.25 dex. If the same homogeneous models are used for the abundance
stratification analysis then we find that the uncertainty of the value of the
vertical abundance gradient is within an 0.4 dex error bar.
Conclusions. Model atmospheres with individual abundance patterns should be
used in order to match the actual anomalies of CP stars and minimize analysis
errors.Comment: 18 pages, 9 figure
Exploring the magnetic field complexity in M dwarfs at the boundary to full convection
Based on detailed spectral synthesis we carry out quantitative measurements
of the strength and complexity of surface magnetic fields in the four
well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass
regime around the boundary between partially and fully convective stars. Very
high resolution R=100000, high signal-to-noise (up to 400) near-infrared Stokes
I spectra were obtained with CRIRES at ESO's Very Large Telescope covering
regions of the FeH Wing-Ford transitions at 1mum. The field distributions in
all four stars are characterized by three distinct groups of field components,
the data are neither consistent with a smooth distribution of different field
strengths, nor with one average field strength covering the full star. We find
evidence of a subtle difference in the field distribution of GJ 285 compared to
the other three targets. GJ 285 also has the highest average field of 3.5kG and
the strongest maximum field component of 7-7.5kG. The maximum local field
strengths in our sample seem to be correlated with rotation rate. While the
average field strength is saturated, the maximum local field strengths in our
sample show no evidence for saturation. We find no difference between the field
distributions of partially and fully convective stars. The one star with
evidence for a field distribution different to the other three is the most
active star (i.e. with largest x-ray luminosity and mean surface magnetic
field) rotating relatively fast. A possible explanation is that rotation
determines the distribution of surface magnetic fields, and that local field
strengths grow with rotation even in stars in which the average field is
already saturated.Comment: 15 pages, 8 figure
Stellar model atmospheres with magnetic line blanketing. II. Introduction of polarized radiative transfer
The technique of model atmosphere calculation for magnetic Ap and Bp stars
with polarized radiative transfer and magnetic line blanketing is presented. A
grid of model atmospheres of A and B stars are computed. These calculations are
based on direct treatment of the opacities due to the bound-bound transitions
that ensures an accurate and detailed description of the line absorption and
anomalous Zeeman splitting. The set of model atmospheres was calculated for the
field strengths between 1 and 40 kG. The high-resolution energy distribution,
photometric colors and the hydrogen Balmer line profiles are computed for
magnetic stars with different metallicities and are compared to those of
non-magnetic reference models and to the previous paper of this series. The
results of modelling confirmed the main outcomes of the previous study: energy
redistribution from UV to the visual region and flux depression at 5200A.
However, we found that effects of enhanced line blanketing when transfer for
polarized radiation takes place are smaller in comparison to those obtained in
our first paper where polarized radiative transfer was neglected. Also we found
that the peculiar photometric parameter delta_a is not able to clearly
distinguish stellar atmospheres with abundances other than solar, and is less
sensitive than delta(V_1-G) or Z to a magnetic field for low effective
temperature (Teff=8000K). Moreover we found that the back determination of the
fundamental stellar atmosphere parameters using synthetic Stromgren photometry
does not result in significant errors.Comment: 12 pages, 6 figures. The final version, Sect. 3.4 revised, typos and
mistakes correcte
Orbital parameters, chemical composition, and magnetic field of the Ap binary HD 98088
HD 98088 is a synchronised, double-lined spectroscopic binary system with a
magnetic Ap primary component and an Am secondary component. We study this rare
system using high-resolution MuSiCoS spectropolarimetric data, to gain insight
into the effect of binarity on the origin of stellar magnetism and the
formation of chemical peculiarities in A-type stars. Using a new collection of
29 high-resolution Stokes VQU spectra we re-derive the orbital and stellar
physical parameters and conduct the first disentangling of spectroscopic
observations of the system to conduct spectral analysis of the individual
stellar components. From this analysis we determine the projected rotational
velocities of the stars and conduct a detailed chemical abundance analysis of
each component using both the SYNTH3 and ZEEMAN spectrum synthesis codes. The
surface abundances of the primary component are typical of a cool Ap star,
while those of the secondary component are typical of an Am star. We present
the first magnetic analysis of both components using modern data. Using
Least-Squares Deconvolution, we extract the longitudinal magnetic field
strength of the primary component, which is observed to vary between +1170 and
-920 G with a period consistent with the orbital period. There is no field
detected in the secondary component. The magnetic field in the primary is
predominantly dipolar, with the positive pole oriented approximately towards
the secondary.Comment: Accepted for publication by MNRAS, 17 pages, 12 figure
Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB
We aim to understand how stellar parameters such as mass and rotation impact the distribution of star-spots on the stellar surface. To this purpose, we have used Doppler imaging to reconstruct the surface brightness distributions of three fully convective M dwarfs with similar rotation rates. We secured high cadence spectral time series observations of the 5.5 au separation binary GJ 65, comprising GJ 65A (M5.5V, Prot = 0.24 d) and GJ 65B (M6V, Prot = 0.23 d). We also present new observations of GJ 791.2A (M4.5V, Prot = 0.31 d). Observations of each star were made on two nights with UVES, covering a wavelength range from 0.64 - 1.03μm. The time series spectra reveal multiple line distortions that we interpret as cool star-spots and which are persistent on both nights suggesting stability on the time-scale of 3 d. Spots are recovered with resolutions down to 8.3° at the equator. The global spot distributions for GJ 791.2A are similar to observations made a year earlier. Similar high latitude and circumpolar spot structure is seen on GJ 791.2A and GJ 65A. However, they are surprisingly absent on GJ 65B, which instead reveals more extensive, larger, spots concentrated at intermediate latitudes. All three stars show small amplitude latitude-dependent rotation that is consistent with solid body rotation. We compare our measurements of differential rotation with previous Doppler imaging studies and discuss the results in the wider context of other observational estimates and recent theoretical predictions
Stellar model atmospheres with magnetic line blanketing. III. The role of magnetic field inclination
Context. See abstract in the paper.
Aims. In the last paper of this series we study the effects of the magnetic
field, varying its strength and orientation, on the model atmosphere structure,
the energy distribution, photometric colors and the hydrogen Balmer line
profiles. We compare with the previous results for an isotropic case in order
to understand whether there is a clear relation between the value of the
magnetic field angle and model changes, and to study how important the
additional orientational information is. Also, we examine the probable
explanation of the visual flux depressions of the magnetic chemically peculiar
stars in the context of this work.
Methods. We calculated one more grid of the model atmospheres of magnetic A
and B stars for different effective temperatures (Teff=8000K, 11000K, 15000K),
magnetic field strengths (B=0, 5, 10, 40 kG) and various angles of the magnetic
field (Omega=0-90 degr) with respect to the atmosphere plane. We used the
LLmodels code which implements a direct method for line opacity calculation,
anomalous Zeeman splitting of spectral lines, and polarized radiation transfer.
Results. We have not found significant changes in model atmosphere structure,
photometric and spectroscopic observables or profiles of hydrogen Balmer lines
as we vary the magnetic field inclination angle Omega. The strength of the
magnetic field plays the main role in magnetic line blanketing. We show that
the magnetic field has a clear relation to the visual flux depressions of the
magnetic CP stars.
Conclusions. See abstract in the paper.Comment: 10 pages, 5 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
Rotation, magnetism, and metallicity of M dwarf systems
Close M-dwarf binaries and higher multiples allow the investigation of
rotational evolution and mean magnetic flux unbiased from scatter in
inclination angle and age since the orientation of the spin axis of the
components is most likely parallel and the individual systems are coeval.
Systems composed of an early (M0.0 -- M4.0) and a late (M4.0 -- M8.0) type
component offer the possibility to study differences in rotation and magnetism
between partially and fully convective stars. We have selected 10 of the
closest dM systems to determine the rotation velocities and the mean magnetic
field strengths based on spectroscopic analysis of FeH lines of Wing-Ford
transitions at 1 m observed with VLT/CRIRES. We also studied the quality
of our spectroscopic model regarding atmospheric parameters including
metallicity. A modified version of the Molecular Zeeman Library (MZL) was used
to compute Land\'e g-factors for FeH lines. Magnetic spectral synthesis was
performed with the Synmast code. We confirmed previously reported findings that
less massive M-dwarfs are braked less effectively than objects of earlier
types. Strong surface magnetic fields were detected in primaries of four
systems (GJ 852, GJ 234, LP 717-36, GJ 3322), and in the secondary of the
triple system GJ 852. We also confirm strong 2 kG magnetic field in the primary
of the triple system GJ 2005. No fields could be accurately determined in
rapidly rotating stars with \vsini>10 \kms. For slow and moderately rotating
stars we find the surface magnetic field strength to increase with the
rotational velocity \vsini which is consistent with other results from
studying field stars.Comment: Accepted by MNRAS, 10 pages, 4 figures, 4 table
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